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Flatrock Passive: A Winter Update

An investment in a root cellar and gardening pays off, but solving an indoor air quality problem proves difficult

Winter has settled over Newfoundland. The cold season amplifies some indoor air quality issues, and leads to some unexpected conclusions. (Image credit: David Goodyear)

Editor’s Note: This is one of a series of blogs by David Goodyear describing the construction of his new home in Flatrock, Newfoundland, the first in the province built to the Passive House standard. You’ll find his complete blog here.

In a previous post I talked a little about food security. I tried to focus some of the landscape in our yard on producing food — and it was a huge success! We had a steady supply of baby root vegetables and greens until the end of September. The garden was quite a bit of work but it was amazingly enjoyable to watch it flourish and be able to pick vegetables throughout the summer. I battled nature and learned a lot from my planting mistakes. I will put those lessons to work next year as I add new crops.

Getting ready for the fall harvest was no easy task. I had never owned a cellar before now, nor had I harvested or stored vegetables. A lot of effort went into researching the old methods of root cellaring vegetables to keep them fresh for a long time. Storage for most root vegetables is basically about creating underground conditions to keep the vegetable “alive.”

A low, constant temperature (less than 5°C, or 41°F) with high humidity (95% plus) is necessary and can be achieved by adjusting dampers to regulate air flow through the ventilation stacks inside the cellar. Luckily, my father and my father in-law have been around a few cellars. They grew up in a time when every family had a garden. It was a necessity.  They provided me with much of the knowledge needed to create good storage conditions.

Storage is pretty easy for potatoes. A large bin with slatted sides promotes air movement around the potatoes. I made some bins from leftover flooring, and they also proved useful for storing carrots, beets, and parsnips.

The vegetable garden was well worth the effort, providing produce through the summer and root crops that could be wintered over.

My fall harvest provided our family with a great bounty. Some of the root vegetables were ready for harvest through the summer so we picked away while the garden continued to grow. Nature provided us with turnip greens, lettuce, beet greens, and a variety of small vegetables, including carrots, rutabagas, potatoes, beets and onions.

By October the potato plants had completely died back and I let their skins “harden off” in the ground to toughen them for storage. By mid-October we pulled the potato plants. We left them to dry for a day, and moved them directly to the cellar. We ended up with about 300 pounds.

The cellar at this point was cool (less than 10°C [50°F]) but humidity was low. A perfect combination to promote drying for a couple of weeks to further harden-off the skins before closing up the door for winter. I left most the vegetables (carrots, parsnips, cabbage, beets) in the ground until the beginning of November while I waited for the root cellar to cool. Gently pulling them (on a cold day) to ensure minimal damage, cutting off the greens and leaving the dirt on them is all you really need to do for preparation.

They were then stacked inside of a burlap lined crate with sawdust between each of the layers. I paid careful attention to keep the roots from touching each other as that can promote rot in storage. One month later, the carrots had little green sprouts growing out the top, a  good sign that the vegetables are still alive and kicking!

A root cellar will keep root vegetables fresh over the winter with a combination of low temperatures and high humidity.

Although the snow-covered garden is a beautiful sight, looking out at an empty garden does lead to some heavy feelings. This being said, I get to enjoy the fruits of our labor (or should I say vegetables) for the whole winter and will start again in the spring. Creating food security does take work. Some of the work is easy. Some of the work is hard. The end game is all pure enjoyment! Going to your personal grocery store on a cold winter day to pick roots for a warm winter stew is priceless. Next year I plan on adding more perishable varieties of vegetables that lend themselves well to canning. Now that the days are short and the nights are long, I guess I’ll have plenty of time to research what comes next.

Airtightness and indoor air quality

It was a fairly cold fall this year with a noticeable drop in temperature around mid-September. At the same time, it was somewhat humid, as would be expected for our climate. Humidity inside the house was high.  After a purge with open windows on a dry day, humidity would drop dramatically in a very short period of time. After closing the windows humidity would continue to climb over several days until we would have to purge again.

Although the temperature inside the house was comfortable, the high humidity sometimes made it feel stuffy. Running a dehumidifier wasn’t much of an option because dehumidifiers just add sensible heat to the living space. Our heat-pump water heater helped a little, but our hot water usage was just too low for it to be effective.

I really should have listened to my Passive House designer from the beginning and installed a minisplit. Lesson learned! From what I recall, his primary concern wasn’t actually humidity. Instead, he expected the interior temperature to be an issue during the summer months, although the house has performed fine in that respect.

In any case, we opted to install a minisplit in order to address dehumidification. The minisplit, a Fujitsu 9RLS3H, made a huge difference up until the end of September and was able to keep the humidity around 50% while running in dry mode. As the exterior temperature dropped, I expected the interior humidity to fall as well, albeit more slowly than it would have with an HRV. It was much slower than expected (the opposite was true during the summer). It would take days to drop a couple of percentage points. Depending on the moisture load in the house, on some days relative humidity would increase, reach a new equilibrium humidity, then start falling again. I was concerned that as the temperature decreased, I would see a significant amount of condensation on the windows and doors. In response, we ran a dehumidifier continuously.

Construction contributes to high humidity

Many new airtight homes see high humidity levels. Where does all the moisture come from? Construction materials are filled with moisture. Drying those materials takes a long time. A concrete slab could take years to dry. However, the house had been closed in for almost a year so I suspected that most of that construction moisture would have dissipated. Given that we could purge the house of humidity quickly (it could drop 10-15 percentage points in 10-15 minutes just by opening the windows), I suspected there was some sort of ventilation issue.

I wondered whether it was the ERV, the airtightness of the house, or construction moisture. Was the current moisture load too high for ventilation to eliminate? I think the answer is a combination of things. Most houses are leaky (3-5 ach50). Even the ones that people call energy-efficient (less than 2 ach50) are still leaky when compared to the airtightness of this house — 0.36 ach50. At this leakage rate, the natural air infiltration is about 2% of the volume of the thermal boundary. At 3 ach50 the natural infiltration is about 20%. If 20% of the air in a house (under average weather conditions) is being exchanged from outside to inside I would think that the effect it has on interior humidity must be astoundingly greater than infiltration in an airtight house.

It is well known that gaining energy efficiency through airtightness can lead to indoor air quality issues if ventilation is not addressed properly. I started investigating every aspect of the ventilation system, from materials to installation methods and balancing. Then I started thinking, and here’s where it got complicated.

Tracking down the source of the problem

I lit our Walltherm stove when we moved in back in April. Smoke started leaking from the flue pipe joints. The joints are typically not sealed since the natural draft of the chimney should prevent smoke from entering the living space. When I opened the windows, the smoke infiltration stopped. The next day, I decided to do some testing. I placed my hands around the stove pipe joints and I could feel cold air leaking through them.

My first intuition was that the ERV was unbalanced, leading to depressurization of the house. I checked the airflow balance (using a digital differential manometer) according to the manufacturer’s test method and found that it was fine. After some additional experimenting, it became evident that the house was under negative pressure even though the ERV was balanced.

I increased the amount of fresh air supply to the building until the manometer read slightly positive. I tested this setup by burning a match near the stove pipe and the smoke was immediately sucked into the stove pipe joints. I had to unbalance the machine by almost 30 cubic feet per minute in order to achieve this. I left the ERV in this unbalanced state so I could use the wood stove. I decided at the time that I would revisit the issue in the fall when I had more time.

The depressurization and smoke issue seemed quite complicated and a solution eluded me for many months. I figured it was related to the ducting of the ERV, so when fall rolled around I started an investigation. My initial thoughts were that the depressurization was caused somehow by the duct configuration on the supply and return, so I switched them just to test the hypothesis. The good news was that the ERV basically remained balanced, meaning that the supply and return duct runs had nearly equivalent lengths.

I rebalanced the unit and once again could feel cold air leaking from the stove pipe joints. I decided to change the ducting at the outside hoods. I switched them and made a few other configuration changes and rebalanced the machine — with much disappointment. After rebalancing, the cold air kept seeping out the joints of the stove pipe. Each test led to the same result and further despair!

Creating neutral pressure inside the house

Then I thought that if the results are always the same it obviously has nothing to do with the unit. Instead, there must be some underlying fundamental physics going on. Then it came to me: Even a tight house is still a “leaky” plenum. Both fans in the ERV operate independently. Fresh air is being fed into the house by the supply. The exhaust has no way of differentiating where air is being drawn from. It simply pulls air from the space. It will pull some air from the fresh air sources but air will naturally start infiltrating through holes in the envelope since they offer a path of least resistance — unless they are plugged.

Plugging the holes can be done artificially by unbalancing the ERV so that the difference inside and outside the envelope is neutral. There are obviously more holes in the envelope then just the stove pipe so when pressurizing by using supply air, you have no control over which holes the supply air exits through. As a result, a large amount of air (in my case 30 cfm) could potentially be needed to effectively “plug” the holes and create neutral pressure.

I am guessing that with airtightness at 3-5 ach50, most homes are so leaky that there is little to no depressurization. I can almost bet that anybody in a Passive House who has balanced their ventilator would see depressurization if a manometer was used to measure the difference between interior and exterior pressure.

I am yet to find anybody willing to test this. If you do, please let me know! This effect is simply not unique to Flatrock! Physics is the same everywhere.

It seemed like a good idea to just continue operating the ERV in an unbalanced state to deal with the stove issue. What could possibly go wrong? The problem is that a solution to one problem can often lead to problems elsewhere. Needless to say, unbalancing the ERV had unexpected consequences. After a lot of research I determined that operating in an unbalanced state (supply greater than exhaust) will collect more of the moisture from the exhaust air stream than when it is operating in a balanced state.

When it is dry outside, interior humidity will drop much slower in this unbalanced state. In the summer, this unbalanced state will lower the moisture removing capacity of the unit and the interior humidity will climb much quicker than if the machine were balanced. The assumption during planning is that our ventilation would take care of exhausting some humidity during the winter but leave enough to be comfortable.  The assumption is fine as long as the machine is balanced.

The wood stove provides welcome heat, but also created an indoor air quality issue that took some time to sort out.

My solution for the wood stove was based on a lack of understanding about the mechanism of moisture migration from one air stream to another in the ERV core. I balanced the ERV airflows and interior humidity would drop dramatically quicker than in the unbalanced state.  So operating in an unbalanced state fixes the stove issue but leads to humidity problems! This also explained why humidity climbed so quickly during the summer when it was humid outside. So another solution had to be investigated.

Another solution emerges

I had read that wood-burning appliances in tight homes can be problematic… and they can be! Good quality assurance is absolutely necessary. This being said, I feel like this ongoing depressurization issue is beyond the understanding and scope of most ventilation installers. I do think that other people who are building super-airtight houses should measure to see if the house is depressurized even though the airflows are equalized. Equality of the supply and exhaust exchange rates does not imply that the house pressure is neutral and therefore could lead to dangerous backdrafting, flooding the living space with carbon monoxide.

I read some time ago that the Zehnder brand of ventilators have a function that helps prevent depressurization. I believe it’s called “ChimneySweep.” When activated, it unbalances the unit in order to provide 10% more makeup air, and therefore pressurizes the house to prevent back-drafting of smoke into the living space. My experience was that 10% would not be enough, and in fact I required almost a 30 cubic feet per minute differential between the supply and exhaust (on a ventilation rate of 95 cfm) in order to balance to neutral pressure.

By December I had concocted a strategy that would work. I decided that a normally open motorized damper on the supply duct would do the job.  I balanced the ERV unit (with the damper open) to pressurize the house,  checking pressure between the inside and outside using a differential manometer. Then I adjusted the damper’s closed position so that the supply and exhaust were equal.

Flipping the switch opens a damper and allows the ERV to pressurize the house, eliminating a back-draft of wood smoke into the house.

When I want to use the wood stove I would just flip a switch on the wall by the ERV control to force the damper to open, which would pressurize the house. It worked like a charm. If the damper motor fails, the spring-loaded damper opens and the wood stove can still be used. I feel this is a much safer solution and has redundancy built in. Unfortunately, it is another mechanical part that adds complication, but the safety factor makes it worth the complication. The added benefit of pressurizing the house is that when I add wood to the fire there are now no issues with smoke spilling into the living space. The stove now works like a charm!

Energy use

So how about energy use? I am pleased. I currently heat about 3,376 square feet of interior living space, including both the house and the garage. In December 2018, we depended purely on electricity since I wasn’t ready to use the stove until I fixed the ventilation issue. The temperature in the house and the garage (woodworking shop) are both set to 20°C (68°F). We used a total of 1,576 kWh of electricity.

I have found that the heat loads of the house and garage are definitely lower than the lowest heat output of the Fujitsu heat pumps. How do I know this? Simple: The heat pumps short cycle.

This can be easily confirmed by measuring the amperage at the breaker panel and tracking how long the machine is drawing power. At 0°C, the heat load for the garage is below the lowest modulated heat output for the Fujitsu unit (3100 Btu per hour). I calculated the load to be somewhere around 2800 Btu/hour, including air infiltration and heat loss through the envelope.

The heat pump short cycles every 10 minutes or so. Unfortunately, this does affect efficiency. How much I have no idea, but operating in a steady state would be more desirable. The heat pump in the house has short cycled less. It is the same size as the model in the garage and there is more heat loss due to a larger envelope, so that makes sense.

As the nights get even colder I have witnessed longer run times and less short cycling, which is great for efficiency. I think that the usage could have been less if the heat pumps short cycled less. I am still investigating what is going on but it appears to be a common thing with heat pumps operating under low-load conditions in airtight houses. So far, our bills have been about 38% of that used in our previous home which used electricity and propane.



A Garden and a Sun Shade

Air Sealing the Penetrations

Blower Door Test Comes Up Roses

Wrapping Up the Air Barrier

Firing Up the Heating System

Laying Out the Mechanical System

Framing and Insulating an Interior Service Wall

Insulation and an Air Barrier

Installing Windows and Doors

Foam Sheathing and Window Details

Framing and Air Sealing

A Well Insulated Slab

Footings and Frost Walls

A Final Design and Energy Modeling

An Introduction to the Flatrock Passive House


  1. natesc | | #1

    Don't forget the outdoor temp has huge effect on chimney draft. I would bet at this time of year you will not have issues with draft even with a depressurized house. It is mainly those cool Oct/April days where it is probably better to just run the mini split.

    1. DAVID GOODYEAR | | #2

      I wish this were the case, with the ERV balanced I can feel a trickle of air regardless of exterior temperature. Only on the windiest of days ie 50km/hr+ is there enough draft to overcome the effect.

      1. natesc | | #3

        That is interesting. I keep our house depressurized all winter. I crack a window to start the stove, and once it is up to temperature shut everything down and have not had a problem. Save once in the fall with some oddball weather we did lose draft at the very end of a fire and wound up with a chimney smell in the upstairs room the pipe runs through.

        Also just to clarify I am not implying your issue is some simple thing you've overlooked. Just giving my perspective living in a tight house.

        By the way, love your cold cellar, your self sufficiency mentality is similar to ours.

  2. JC72 | | #4

    Does the owner intend on capping the stove pipe during the shoulder/summer months?

    Perhaps a motorized damper?

  3. joshdurston | | #5

    When you say the ERV is balanced or unbalanced, are you referring to fan speed dial markings or, actual airflow measurements. I've found in commercial buildings that preset airflow ratings are often off significantly, even in very expensive lab grade equipment. The only way to balance accurately is with a some proper airflow test and balance equipment. That being said, turning off all your fans and closing the windows and then going for a appropriate target pascal difference might be as close as you can get. Keep in mind stack effect is going to change the height at which zero pascals should be expected.

    Can you hook up an outdoor air kit to get combustion air from outside? Most wood stoves have this an option. This would be preferential than forced makeup air for combustion. It would be good to have CO detectors on each floor and especially in your sleeping areas.

    1. charlie_sullivan | | #7

      That's my question as well.

      The faster drying when it's "balanced" seems backwards. If it's unbalanced, there's are the comes in, not through the ERV, that doesn't get to exchange humidity. So you'd expect faster drying with it actually unbalanced.

      1. DAVID GOODYEAR | | #17

        in this case, Moisture exchange is still dominated by airflow through the ERV. in my case I had unbalanced the unit so there was more supply than exhaust by almost 30 cfm. . The incoming dry air supply picks up more moisture from the exhaust air in the core then if it was balanced. it does require a lot of CFM to push all the air through the holes in the envelope in order to prevent backdrafting. At 0.3 ach50 I think the equivalent hole size in the envelope is 14 sq in or something like that. when you add an extra 30 cfm you have no choice where it goes. So there is only a small portion of the 30 cfm that actually makes it into the joins of the stove pipe. the rest escapes through the envelope...somewhere.

        as for faster drying, I am pretty sure that it would dry much quicker if the erv were balanced in the other direction. If the house supplyexhaust. This supports what I have seen at least and the effect would be opposite in the summer with humid air outside and dehumidified air inside.

        1. charlie_sullivan | | #23

          If you have 100 CFM supply air going through ERV, 70 CFM exhaust air through the unit, and 30 CFM through the envelope, you are right that you will capture a higher percentage of the moisture in that 70 CFM of exhaust air than you do in normal operation. But you capture 0% of the moisture in the 30 CFM that escapes through other leaks. So the net is that you recover less moisture than you would in balanced operation.

          1. DAVID GOODYEAR | | #28

            I understand what you are saying but the house will find an equilibrium with the airflows and humidity, and there is also mixing and we are adding moisture daily from expiration, cooking, cleaning, showers etc that adds to the situation. I am not sure that the argument is that simple...but maybe it is. this being said, it is not what I have experienced.

    2. DAVID GOODYEAR | | #10

      when I say balanced, i mean according to the static pressure tables on the unit for air flow across the core. I have also measured the air flows at each of the outside vents and in each of the rooms and both supply and exhaust add up to be the numbers that venmar states in the static pressure vs cfm tables for the unit. I have used calibrated equipment for this. a differential manometer and a anemometer with a custom hood. The house has no other fans and the windows/doors are closed while balancing otherwise you create another situation which will affect balancing. The stove is hooked up to an outdoor air kit and it is sealed appropriately. Having another duct connected to the stove really just provides another low resistance way for air to be sucked into the house from the outside. I am not actually forcing combustion air. I am just using the pressurization as a way to prevent smoke from being sucked into the house through the stove pipe joins and when I open the stove door to add wood.

      1. charlie_sullivan | | #16

        That cleared up a lot of my confusion--thanks!

        My current thinking is that the negative pressure in the neighborhood of the stove is in fact stack effect in the house (not the in the stove pipe--just the warm interior of the house compared to outdoors, and its height). And you are using the ERV to null that out at the level where the stove is installed.

        But a way to test that theory--which you may have already done, and maybe even told us--is to turn off the HRV. With the HRV off, is there still negative pressure at the stove?

        1. DAVID GOODYEAR | | #20


          i didnt actually turn off the erv, but when i run it on recirculate, the negative pressure disappears.

          1. charlie_sullivan | | #21

            I don't know enough about the duct system design to be confident that the recirculation mode wouldn't affect pressures--If I were you I'd want to test with the fans off to get a baseline.

          2. DAVID GOODYEAR | | #32


            Anything is possible I guess. THis being said, in order to unbalance and get neutral pressure there is a 30 CFM differential. This is pretty easy to measure a the exhaust and supply vent hoods and you definitly see that the unit is unbalanced in airflow. In any case, with the damper installed I have the best of both worlds, IAQ is much better regardless of the effect and interior humidity is now <45% and I have almost no condensation on the windows except for some in the master bath and other windows on the coldest of nights.

        2. DAVID GOODYEAR | | #25

          Sorry charlie,

          I was confused. I have turned off the hrv while i am measuring the differential pressure between the inside and outside by placing the hose out through a diaphragm taped around a slightly open window. if i turn off the unit, the pressure differential disappears.

          1. charlie_sullivan | | #29

            OK great, so you actually did exactly what I was hoping for! That means that it actually is induced by the HRV, and definitely not stack effect.

            In that case, I think it has to be that when you have it set to be "unbalanced" but with low pressure difference inside to out, you are actually setting it up to be better balanced than the configuration where you thought it was balanced. I know you say you totaled up the air flow in the two scenarios, and it was equal supply and exhaust, but those measurements are really hard to make accurately. Whereas the pressure difference measurement is much simpler and might be more accurate.

            That would also consistent with my comments on the moisture exchange. Yes, it's an equilibrium with the moisture being added--balanced or unbalanced, the ERV is only reducing the moisture content in the interior (assuming it's cold outside), but running such that it doesn't induced an inside-outside pressure difference will reduces the moisture content less rapidly than when it is driving leakage through the envelope in either direction.

  4. calum_wilde | | #6

    Is the stove in the basement? I'm wondering if stack effect could be causing a pressure differential within the house which is being highlighted by the position of the stove at the lowest pressure region of the house.

    1. JC72 | | #9

      IMO it sounds like:

      No 1: There isn't a damper to prevent air from back drafting down stove pipe.
      No 2: Makeup air ducting for the wood boiler is absent/insufficient.

      1. DAVID GOODYEAR | | #12

        #1 this is true, there is neither damper. this being said, the damper would't help when I go to light the stove.

        #2 the make up ducting is not absent. It was sized according to the manufacturers documentation and after discussion with the distributor. The stove works fine now with the current setup.

        1. JC72 | | #36

          Ok good to hear. Here are two the followup questions.

          No 1: Does the supply for the wood burning boiler have it's own dedicated damper?
          No 2: If not, you might want to look at where the supply hood located as it relates to the prevailing wind.

          Under certain circumstances I envision some outside air is flowing down the stove pipe if the supply port is located downwind. Due to the high efficiency of the wood burning boiler it just takes some time to generate enough hot exhaust air to overcome the pressure

    2. DAVID GOODYEAR | | #11

      there is no basement. the stove is on the main level.

  5. willempaynter | | #8

    Hey this project was designed by Passive Design Solutions out of NS:

    check them out!

    Great project David :)

    1. DAVID GOODYEAR | | #13

      It was! they were great to work with!

      1. Expert Member
        MALCOLM TAYLOR | | #15

        Perhaps we could ask Martin to add their names to the list of builders and consultants he has been compiling.

        1. GBA Editor
          Martin Holladay | | #18


          1. willempaynter | | #19

            hi Martin,

            that's great. What list is this referring to?

          2. GBA Editor
            Martin Holladay | | #37

            This one: Green Building Bulletin Board.

    2. DAVID GOODYEAR | | #26

      Hi Willem,

      your name wasn't there when you originally posted. I believe you may have calculated the heat load estimates for the house! Thanks! say hi to mike and natalie!

    3. calum_wilde | | #35

      Willem, do you guys do any retrofit type of work?

  6. Trevor_Lambert | | #14

    Where is the combustion air for the stove coming from? If it's coming from inside the house, then it makes sense that the house would get depressurized (and this is exactly why wood burning appliances are discouraged in this kind of house). If it's coming from outside, it's less clear what's going on.

    I'd be willing to do this test you describe, if you give me some instructions. (tml4873 at gmail).

    "Fresh air is being fed into the house by the supply. The exhaust has no way of differentiating where air is being drawn from. It simply pulls air from the space. It will pull some air from the fresh air sources but air will naturally start infiltrating through holes in the envelope since they offer a path of least resistance — unless they are plugged."

    I'm not sure I follow this logic. For the sake of argument, let's posit a scenario where there is little to no wind outside. If we balance the ERV, then by definition the only pressure differential that exists is what existed before. The holes in the house are not one-way valves. Without a pressure differential to begin with, they are no more likely to be allowing exterior air in than allowing interior air out. You could make the exact argument in reverse, and it's just as valid:

    'Stale air is being pushed out of the house by the exhaust. The supply has no way of differentiating where air is being drawn from. It simply pulls air from the outside. It will pull some air from the fresh air sources but air will naturally start exfiltrating through holes in the envelope since they offer a path of least resistance — unless they are plugged.'

    In both cases, the argument only holds if you have a state of pressure differential. Now, it may be that wind conditions tend to depressurize rather than pressurize a house. But that will hold true whether there is an ERV operating or not.

    I think a partial answer might be in the way your are balancing the ERV. By using a differential manometer, you are pressure balancing. This is not necessarily the same thing as flow balancing, which is what most people think of when talking about a balanced ventilation system; same amount of air coming in goes out. Anything that affects the pressure along the supply and exhaust paths (e.g. length, twists and bends in the pipe, wind conditions outside) will result in differing flows but the same pressure.

    1. DAVID GOODYEAR | | #22


      the combustion air comes from a duct that is plumbed to the walltherm stove and terminates at a vent hood outside the house.

      Regardless of how I connect the erv to the supply and exhaust ducts inside the house or the exterior vent hoods, if I balance the air flow, i can feel air coming through the joins in the stove pipe. Only on the windiest of days to I not feel the air leaking in. I understand your reverse argument. however regardless of the setup, I balance the air flows and I always feel cold air leaking through the pipes. If i measure the differential pressure between the house and the outside on a still day, the house shows negative pressure until the rebalance the unit to bring in more supply air.

      I have measured the air flow using the manometer by hooking it up directly to the unit. Static pressure measurements across the core for both supply and exhaust are correlated to air flows in the manufacturers tables. i balance the unit using their electronic procedure to give equal air flows and then measure the air flows in each of the rooms and at the exterior hood using an anemometer and a custom hood. the sum of the supplies and the exhausts add up to the tabulated values in the tables.

      Ill try to email you tonight with some details about how to measure this.


  7. joshdurston | | #24

    Beautiful house, my wife would love the colour palette you guys went with.

    Have you measured your draft on the chimney. Usually manufacturers have an acceptable max (not sure about min). But draft will be affected by flue gas temperature and outdoor temperatures, as well as chimney height. Typically chimneys are under enough negative pressure to not "leak" when hot. If you're leaking and house pressure is acceptable, I would suggest adding a couple feet of Class A so your chimney sucks harder.
    I have a Blaze King stove (a high efficiency catalytic model), when discussing the chimney with the manufacturer they have a min height requirement since the stove is capable of very low burn rates which might not generate enough draft on a short chimney. The first suggestion when dealing with smoke smell, is to add another 3' to the chimney.

    Is your chimney compliant with the 3/10/2 rule? (3 feet min, above roofline at base of chimney, and at least 2' higher than anything within 10'). Wind can do some funny things to pressure and if you chimney is tucked behind a roof it can affect thing in unpredictable ways.

  8. DAVID GOODYEAR | | #27

    Josh, thanks we love the colors also!

    I haven't measured the chimney draft but have thought about doing it. The chimney had no problems with draft once up and running. it was during the initial lighting that I would see smoke and would smell the stove once the flue started cooling down. the chimney is definitely above the peak on the roof and there is a there are no obstructions around the chimney 360 degrees. I have thought about adding an extra couple of feet.

    I am pretty sure it is compliant. We purchased enough chimney and it was all installed. the chimney as mentioned above above the roof and completely clear all the way around.

  9. GBA Editor
    Martin Holladay | | #30

    Interesting discussion.

    David Goodyear's analysis didn't make any sense to me. First, he noticed that smoke was entering the house through seams between the lengths of stovepipe. He also noticed that even when there was no fire in the stove, outdoor air was entering the house through seams between the lengths of stovepipe.

    To me, this means one of two things. Either (a) there is a big ceiling air leak, and the air is coming down the chimney due to the stack effect (this is unlikely in a tight house) or (b) a fan is depressurizing the house. If the only fan is an ERV fan, that means that the ERV is unbalanced, and the exhaust fan is moving more air than the supply fan.

    David Goodyear's explanation -- "Fresh air is being fed into the house by the supply. The exhaust has no way of differentiating where air is being drawn from. It simply pulls air from the space." -- is sort of getting close to what's happening, but it's phrased in a confusing manner. I would rewrite these sentences this way: "Exhaust air is being removed from the house by an unbalanced ERV. This depressurization pulls makeup air down the chimney."

    Trevor Lambert and Charlie Sullivan seem to share my confusion at David Goodyear's explanation. I agree with readers who have concluded that when David Goodyear thought his ERV was balanced, it was in fact depressurizing his house.

    1. DAVID GOODYEAR | | #31

      Hi Martin,

      I can only agree that the situation is confusing. I was dealing with it for almost 8 months! Would I put a woodstove in a super air tight house again...I might think twice! WRT (a) When the blower door was being done I also had thermal images taken through the whole house. My main suspect would have been the attic hatch. There was no evidence of air leaks upstairs or through the attic hatch. (b) I agree that the ERV is depressurizing the house. But something didn't add up. The ERV was balanced according to the manufacturers instructions to provide balanced airflow accross the core. After balancing the unit ( using their static pressure tables for supply and exhaust accross the ERV core) I also measured air flows at each of the supply and exhaust registers and at the exterior vents to verify that the air flows added up to what was set at the unit and I verified that supply == exhaust. I switched the supply and return ducts and the supply and return exterior hoods on the unit and carried out the procedure many times. Regardless of airflow balancing under different duct configurations I have always seen the same effect, cold air entering through stove pipe. So in short, yes the ERV depressurizes the house...but the ERV "appears" to be in an air flow balanced state and that balanced state has been verified using calibrated tools. Using a manometer between the exterior and interior of the house I have verified that when the ERV is in, what appears to be, a balanced state the house is, in fact, depressurized. If I turn off the unit, the depressurization disappears, If I compensate by adding supply air (and verify the unbalanced state by measuring the airflows) the depressurization dissappears. When I unbalance the unit to compensate for the depressurization it decreases the moisture removing capability of the unit which is inline with an unbalanced ERV where the supply>exhaust so I am sure it is has unbalanced airflows when I unbalance it. once I airflow balance, the moisture removing capability returns to normal. So based on current measurements I can only conclude that the ERV is actually airflow balanced even though the house depressurized.

      I tried to provide some analysis based on observations albeit confusing and I apologize for that. THis being said, the obversations are the obversations and that's the reason why I asked if somebody in a tight house would conduct the balancing measurements independently from me in another house. I do find it hard to believe that switching the supply and exhaust ducting at the unit to the interior ducting and the exterior vents and rebalancing would exhibit the same results....but it did, a total of 4 different ducting configurations were tested....everytime depressurizing the house with the machine airflow balanced. This lead me to think that there must be something more fundamental going on.

      At this point, regardless of the state of the ERV, I had to provide a way to run the woodstove safely while still maintaining IAQ when the stove wasn't being used. An airflow damper, adjust to provide balanced/unbalanced ventilation by the flip of a switch seemed like a simple fix and it works great! Interior humidity has been below 45% and much of the interior condensation has dissappeared, except on the bathroom windows!

      At this point, Trevor mentioned that he would be willing to do the measurements independently. I will contact him and report back with findings.

      1. calum_wilde | | #34

        Can you link to the manufacturers measurement instructions?

        You said a couple of time that you measured across the core. If the core is more restrictive in one direction than the other and that might be causing an error in your readings. The best way I know of to measure air flow in a duct is with a pair of pitot tubes, one pointing upstream into the air flow, the other perpendicular to the air flow. Plumb both to the two sides of a manometer and this will give a differential pressure. Make that differential the same for the supply and exhaust and your air flows will be balanced.

        1. DAVID GOODYEAR | | #38

          Yes this is is one way to measure if the manufacturer hasn't provided fan curves (ie SP vs CFM ) based on core resistance. I do have a dwyer differential manometer for this purpose with a differential pitot but it wouldnt work with my setup. the problem with the pitot measurement is that you really need a good straight length of duct to get laminar flow. Laminar flow will ensure accurate measurements. I believe the duct length needs to be somewhere in the range of 10x duct diameters if my memory serves me correctly. installation instructions (and balancing) as well as a video showing balancing can be seen here:

      2. Jon_R | | #39

        ... airflow balanced...something more fundamental going on

        You need to balance the mass flow, not CFM.

        Even with perfect mass balanced ERV flow, very few points in the house will have no pressure differential to the outside.

  10. DAVID GOODYEAR | | #33

    I know that the article placed a lot of weighting on the wood stove/ventilation issue. This being said, I didn't mean to detract from the other part of the article about growing food and our cellar. Of course I don't want to detract from the current discussion either but the cellar is an amazing piece of passive technology. Passed down through the ages this little piece of technology just works, no electricity required. simple ventilation and some psychometrics is all it takes. Cellars are making a resurgence at least here in Newfoundland amongst the middle aged crowd after generations of being ignored. Of course food security is a large issue here. NL produces less than 10% of the food we consume. Just 70 years ago we produced mostly everything necessary to live, except grains, and fruit although people had various plums, apple, and cherry trees to mention a few. local wild berries provided much of the winter fruit requirements and were bottled as preserves. Its a real sad state of affairs really...but this being said, growing food is not hard, it takes work, but its not hard and it is very satisfying. Rather than working for money you're working for food!

    1. charlie_sullivan | | #40

      Oh yes, I love the root cellar! As well as the design and look of the place.

      And also, don't take the people digging into puzzling of the pressure issue as a negative--some of us are here because we like puzzles and this was a good one. And we aren't going to let the fact that you have already solved the problem stand in the way of our fun trying to work out the puzzle, even if it makes our efforts unnecessary.

      1. DAVID GOODYEAR | | #41

        I’m glad people are digging. I’ve been digging for 8 months! At this point the system in place works and I get to use the stove. It’s a solution that works but the problem is still not fully understood on my side.

  11. AndyKosick | | #42

    Before I become thoroughly distracted by the pressure issue, I wanted to commend you on the root cellar. We manage to keep a small garden and compost bin, but the only thing we put up for the winter is jam from the raspberry bushes. I’m all for progress but I also feel we’ve forgotten many things that worked very well.

    I’ve had a real learning curve about the opportunistic nature of air over the past few years, especially with Habitat houses. They’re not at Passive ACH50s but they’re not very big either (8448cf) so they regularly blow around 200CFM. Things get really fickle under 200CFM50. I’ve paid attention to pressure across the envelope before because of dryers and rangehoods that remain a significant challenge, but without woodstoves intermittent negative pressures have been tolerated. Next house I test I will try to reproduce your results with the ERV.

    Couple things, I’m not questioning your testing skills, these are just things that have given me trouble. Having balanced many Venmar units with a DG700, I’m not convinced that the charts are always spot on. Measuring air flows at exterior hoods is finicky especially with wind and are technically subject to air density corrections in cold conditions. Although, the effects of density in cold weather would, if anything, seem to have the opposite of your problem. Lastly, when measuring at the inlets and outlets inside the house any back pressure from the measuring device will redirect air to other outlets, making for low reading overall. I’ve found this particularly problematic with the low air flows in ventilation systems. Balancing ventilators is what got me to drop the money on a powered flow hood.

    Also, even with the duct switching, I’m not entirely convinced it’s not a room to room issue. Are you convinced this couldn’t be solved by bringing all the fresh air into the room with the wood stove?

    That said, where’s the next designer Passive House product, “sealed induced draft wood stoves?” Isn't there some kind of “The Passive Image” catalog selling these yet? (only half joking here). Very interested if someone can make sense of this issue.

    1. DAVID GOODYEAR | | #45

      Andy, jam from the raspberry bushes is a great start! There are many things that work well especially when designed using passive principles.

      It would be great to keep this discussion in mind when you do another house. I would love to see some real world testing other than my own. More data would at least provide insight in some direction other than what I currently have i.e. more questions! This being said, more insight often leads to more questions....

      Although my background deals heavily with instrumentation and testing, I definitely can't say that I am an expert in HVAC testing. My experience is that the charts on the x24erv appear to be good. I did make sure to do my measurements on days when there wasn't any wind...a rare cosmic occurrence here in Newfoundland! They appear to be fairly accurate based on that. I have a differential pitot but the issue is that I don't really have a good length of duct to use it. I think they recommend a straight run of about 10 duct diameters upstream of the pitot and a couple feet behind in order to sample laminar flow otherwise accuracy can be affected a lot. it seems that any attempt to probe air flows can really screw with the system like in the case of using a flow hood. If you change the configuration even slightly you change the whole system. My hood was simple. made with cardboard with sloped sides and the cross section area of the opening for the anemometer was made to be much larger than what I estimated the open cross section area of the registers. my mode of thought was that if the sectional area was bigger than that of the register back pressure wouldn't be as heavily influenced. it seems to have worked ok but I'm sure more sophisticated tools exist like this powered hood you mentioned.

      No I am not convinced it couldn't be solved by bringing all the fresh air into the room with the wood stove but if i did I would think IAQ upstairs in the bedrooms upstairs would suffer as a result.

      Funny, this stove is said to have sealed combustion...I guess the stove does...but the components beyond the stove do not!

  12. dickrussell | | #43

    I would think there would be some concern about pushing 30 cfm of comfortable-humidity interior air out through the building shell, potentially accumulating moisture on the inside of the sheathing.

    I want to add support to the idea of adding height to the woodstove chimney. I have a small stove on the lower level of my house, with about 25 feet of chimney and a directly connected outside air duct. The house, while not as tight as David's, still came in at 0.8 ACH50 on the final test. I can't light the woodstove if either the range hood or clothes dryer is running, or the downdrafting results in smoke into the room (quick - open a window, run upstairs and kill the dryer!). If I'm paying attention, I can feel the downdrafting produced by the dryer as I prepare the wood for lighting, so I can wait for the dryer cycle to end. Once I get the fire going well, the dryer can be operated without problem, even if I open the stove door to add wood. A properly hot woodstove fire will keep the flue gas in a chimney of adequate height sufficiently hot that the depressurization effect of the upward draw overcomes whatever depressurization the dryer causes. However, I imagine there are limits to this, particularly for a house built to the tightness of David's. The air pushed outside by a dryer has to come from somewhere.

    1. DAVID GOODYEAR | | #44

      Dick, yes there is some reason for concern. This is why I implemented the unbalancing damper, so that I would only pressurize the house when I use the stove. A typical burn takes about 3-4 hours and then I wait about another couple before flipping the switch back to balanced operation.

      My experience was that once the stove was hot enough there was no smoke issue until it started to cool down and then I would start to smell it again. The erv damper fixes this issue.

  13. ethan_TFGStudio | | #46

    Could a small upgrade on the wood stove chimney have been made so as to avoid such leaky joints?

    1. Resilient_Kevin | | #47

      Tons of smart people on here. Impressive! I wonder the same thing. My assumption is this is double wall pipe for the stove. If so, all the double wall pipes I've installed in HRV/ERV houses that have natural gas stoves, I've sealed the outer joints with color matching silicone. Unfortunately, you do need to make sure these joints stay nice over the years though. These joints are never perfect just by twisting them together. Probably redundant, but seems like the stack effect of the inner chimney is likely pulling air through the outer chimney and thus from the room.

    2. DAVID GOODYEAR | | #52

      the problem is that I needed to use slip lengths in order to get the stove pipe to fit properly so there werent any options for upgrade.

  14. metamerman | | #48

    I'm confused: It seems this model wood stove has a provision for an external air supply (up from the bottom), but you're not using it? If so, I'd have to say "There's your trouble": It's insane to put a wood stove in a tight house without this key feature: You'll never be able to balance ventilation without it because the differential varies with the firing rate. I'd also suspect the chimney pipe in this case: Good quality stainless pipe has double seals on it and should admit almost no air (smoke), even with negative inside pressure.

    Our setup: Lennox (BIS Nova) stove/fireplace with external air kit, a custom dual 6' heat up-ducting system (don't bother with the fan kit or the factory heat duct kits), and a custom interior air duct with an intake near the ceiling (runs parallel to the flue pipe in stone-covered chase) with an in-line duct fan connected to a thermostat at the intake. Thermostat is set at 30C (it doesn't do F) and gets up to about 50C at full burn: All that heat would just sit up in the vaulted ceiling without this duct (well we could turn on a ceiling fan, but that would just cook the great room when it's the bedrooms that need the heat).

    The duct runs through the sealed crawl space (I also *strongly* recommend that design over a slab). The outlet of that duct is in the floor of the master bath, at the opposite end of the house. The fireplace will heat the whole 4500 square foot house up to 75F on a 0 degree (F) day if kept loaded (reloading softwood every hour or two), but will go 4 hours once heated up if you don't mind a few degree temperature swing. We only need to make sure the range hood and dryer are off when first starting up, and of course never have any air leakage or smoke issues because the entire system is sealed from the interior air and closing the damper prevents any air from circulating from intake out the chimney. We also don't fret about burning crap (scrap lumber, wet wood, etc.) because it's EPA II (we can use it on "no burn" days) without a catalytic converter. Burns hot and clean, even with suboptimal fuel because of the much more open airflow design than catalytic (or the Wallterm) designs: Fresh combustion air is injected at multiple points, ensuring complete combustion without any tendency for things to condense out and stick to the flue.

    The house also has a full radiant floor system (solar with tankless gas water heater as a backup), but that never even comes on when the woodstove is fired up (conversely, the solar takes care of about 80% of our heating needs, so we generally only use the fireplace on fully overcast days). Very tight house (ICF walls/SIP roof) with a 200CFM ERV (in Boulder, CO). The only negative pressure situation we ever see is with the huge (900CFM) range hood on high (which causes a good draft through the ERV and some leakage from the dryer duct despite the damper on that). Humidity runs around 50% during the winter, which would be ideal except for the Marvin Integrity windows which tend to sweat around the edges on very cold days: The main regret we have on this build is not spending more on better windows...

    1. DAVID GOODYEAR | | #51

      We are using the intake. Its plumbed to the back of the stove with an air tight gasket. In any case, my current setup is now working and I run the stove every second or third day with no issues.

  15. David Murakami Wood | | #49

    I love the place, and no doubt you get much more consistently low temperatures in winter in Newfoundland than we do in Ontario, but this all really confirms for me that we made the right decision not to install the Walltherm hydronic wood stove that we bought (same model as yours). We had lots of discussions about ventilation, pressure, air-tightness and temperature and in the end decided that a stove would actually overheat the place and create other problems. The only issue we were left with was that the company wouldn't take the stove back... but luckily we found a happy buyer with an 'almost passive house' who was looking for exactly what we had, and who came up all the way from Pennsylvania to collect it. I hope you resolve your problems!

    David MW.

    1. DAVID GOODYEAR | | #50

      Hi David,

      I am quite happy with the system now that I have worked out the quirks. We run the stove for about 3 hours on cloudy days or evenings. The temperature during a 3-4 hour burn increases from about 20 C to 24 in the living room...about 22 C in the dining room and it has little effect upstairs. So it does get a bit warm in the main living area. this being said, we have the insulation kit installed. It was a good move. If I hadn't purchased that option the stove would not be useable. The problems are now all resolved and I love the setup but I wouldn't recommend it unless you are self building and you have lots of experience with chimneys, ducting, hydronics, etc.

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