Smallest ERV unit vs. RecoupAerator

Sheri,

In a small, tight, very efficient house, installing ductwork for a central ventilation system may be excessive and unnecessary. The Panasonic unit requires minimal ducting but it is quite inefficient in a cold climate.

All of my superinsulated houses use only Panasonic bathroom exhaust fans on programmable 24-hour timers with passive make-up air inlets (Aldes Airlet 100). Since the bath fans are required in any case for spot moisture removal, it is simple to use those highly efficient fans with their minimal ducting for whole house ventilation, coupled with strategically located (bedrooms and living spaces) passive inlets.

Additionally, an exhaust-only ventilation system is the only option that maintains negative pressure in the entire house when it's running. Negative pressure prevents any possibility of the exfiltration which causes condensation in the winter months (but requires a radon-proof basement and no air flow from an attached garage).

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thanks for your answers. I have read many of your posts on ventilation on this site, and now realize that there are no stock answers (unfortunately!) However, we are trying to keep our energy profile as low as possible -- mainly by building a small house -- which seems to be the thing that most people don't do so there are few answers for a small footprint. We have one story --an open floor plan - literally one big living/kitchen space with a bedroom on the north side (bermed) separated by an earth block wall that is open about 2 ft on the top. We have one other bedroom/office. And a small (300sq ft) basement for utilities and some storage. We are going to have a small woodstove by the earth wall for winter cloudy days. Obviously, we would love to not have to put in ductwork for an HRV with only ~ 40 cfms using the formula. Especially because everyone is telling us we also need a kitchen hood, bathroom fans and dedicated air supply for the woodstove. so, wouldn't that be an exhaust only ventilation system? And why would you need inlets if you are going for exhaust only? sorry I am so confused by all this...help is needed.

Sheri,

The entire purpose of the ASHRAE requirement for whole-house ventilation is to guarantee a minimum exchange of air to maintain a healthy indoor environment. Additionally, in a tight house, particularly a small one, indoor humidity levels will rise into the danger zone without regular air exchange.

So, in addition to a mechanism for exhausting stale and humid air, there must be some avenue for fresh air return, either through a heat-recovery system or simple fresh air inlets. If the house is very tight, then insufficient air can be drawn in through the envelope to replace exhaust flow and there will be no way to control where air leaks in.

If you were going to burn the woodstove most of the winter, then it could be the "exhaust fan". But, in a highly-insulated little home, you won't be running the stove enough to create enough air exchange - and, as you're already aware, you should have a dedicated combustion air supply to prevent backdrafting of the stove.

So, as I suggested earlier, the simplest system is to install a 24-hour programmable timer in parallel with a demand switch on a quiet, efficient bath fan (Panasonic is the best), and set it for enough hours per day to get an average of 40 cfm (e.g. an 80 cfm fan running 2 hours out of every 4). And then install four Aldes Airlet 100 fresh air inlets 6" below ceiling level, one in each bedroom and two in the open living space, as distant as possible from the bath fan to create cross flow.

wouldn't those 4 fresh air inlets bring in cold winter air?

wouldn't those 4 fresh air inlets bring in cold winter air?

Of course. That's their purpose. Cold FRESH air.

But they are designed to allow fresh air in only when there's negative pressure in the house, such as when an exhaust fan is running, and limit the flow to 5-20 cfm deflected up to the ceiling where it will mix with warm air. They have mylar reed valves that limit air flow from external positive pressure, such as a gust of wind. They also have open-cell foam filters in the duct sleeves to control dust and insects and further limit airflow.

Or you can pre-warm the air by installing a centrally-ducted HRV, which is far more expensive to install and requires regular maintenance.

The choice is yours. I've been installing this passive system in super-insulated houses for more than 20 years and have never been able to justify the cost and complexity of an HRV.

thanks so much- I really appreciate all the great posts that you both have given to this forum, (Martin- your energy musings are really are good resource). I've been reading some of your other posts on ventilation issues and it is starting to make sense. given our small home and open plan, yours is a very appealing idea that we will pursue. I checked out the panasonic and they have some very low energy use (DC) exhaust fans.

Martin - if this was your house, would you be ok with an exhaust only?

thanks so much guys! I really appreciate that those of you with actual experience are willing to share. I have read the rationale behind your differences of opinion and appreciate your thoughtfulness. (Now we just have to find a kitchen hood that doesn't suck every molecule of air and we'll be on our way!)

P.S. Martin - It was interesting reading your blog article on heating a small home. We haven't moved in yet but already are thinking the in floor radiant was overkill, especially because now everyone is trying to sell us $$ systems to heat the water for it!. there just aren't enough truly green folks out there helping average budget folks think these things through. This winter, Ii guess we'll find out if we really need it.

Sheri,

While it's true you can heat a small, very well insulated and tight house with almost anything, if you're pouring a slab-on-grade, I feel it's foolish not to install PEX - even if it's not initially hooked up to a heating source. You don't get a second chance to install the radiant tubing in concrete, and it's also much easier to install it in a wood-framed floor at the time of construction. The materials are inexpensive, and almost any hydronic heating equipment - including solar thermal or a woodstove coil - can be later attached to supply uniform, comfortable heat.

You may not feel a need for central heat at this point, but as you age it becomes more of a necessity than a luxury. If you read my article on passive solar slabs in the April/May issue of Home Power magazine, you'll see that I believe (contrary to some) that a solar thermal mass floor and a high-mass radiant floor make a good match, as the radiant heat tends to even out the passive solar heat in areas not in direct sunlight.

Have downloaded the article and will read! We are hoping that the floor needs little input, but we are looking into solar thermal b/c the inspector requires backup to move in. Thanks again for your resources.

I have to register a disagreement with Mr. Riversong as well. over the last 20 years the cost/benefit of HRVs and ERVs vs exhaust only systems has been questionable at many points. Not least of which because for years ventilation codes were laughably oversized. However, energy costs have risen and probably will rise again. ERVs and HRVs with low power drawings (like the ones already mentioned) exist. exchange efficiencies have risen greatly. and full ducting has benefits other than simply energy efficiency; you get fully controllable levels of fresh air in all rooms in a home, and help with thermal redistribution in the home as well. Also, properly designed systems will replace the bath fans he notes as requirements, so there is a cost savings offset in saving those fan units.

I've designed both types of systems , but these days in any real heating climate I do not see exhaust only as a wise choice. Just an inexpensive one. ERV/HRV systems are typically about twice the cost of exhaust only variants and more work to put in. but with 40 watt power draws and exchange efficiencies in the 80's and 90's, and the lack of additional negative pressure on a home (which typically suffers from too much negative pressure already via dryers, stack effects, and combustion appliances) they have some significant benefits. I've never considered negative pressure to be a positive design feature.

In wisconsin, your mean winter temp is somewhere around 12 or 15 degrees. if we assume a 65 degree home, that's a 50 degree difference for easy math, october through march. 180 days x 24 hrs/day x 25 CFM x 1.08 x 50 dt = 5.8 Million BTUs/winter. If you have natural gas in a 90% boiler/furnace, you're at around 70 CCF or therms/winter. And that's best case, other fuels are more expensive almost everywhere. I'd estimate a simple payback period of approximately 5-7 years or so, assuming electrical draw is a wash or close to it, at something like today's energy costs.

That's hardly unjustifiable, especially with the additional benefits involved.

Thanks for weighing in Rob. I'm not sure what you are getting at with the data on BTUs/winter. Could you clarify? What is the comparison referring to?

Also, an EKO HRV is 10 times more costly than a pansonic fan (not 2 to 3 times) and that is without the ducting, plus ducting is difficult for us given our northern and western sides are bermed (concrete). So, financially and practically, it is a difficulty and we need to be convinced that it is really necessary (our HVAC and home designer did not believe it was). Thanks again for weighing in.

Sheri,

I don't think your inspector will consider solar thermal an acceptable "back up" system. What he really wants. to see is a primary heat source, which solar can never be.

Check out the Panasonic ERV which can be found online for as little as $280, and needs very little ductwork.

Sheri,

What Rob is saying is that if you draw cold air into the house, you'll need to heat it up somehow in the depths of winter. By his math, you need to provide 5.8 million BTU of heat to warm up the incoming fresh air throughout winter, in addition to the regular heating load of the house.

You can either use your house heating system to provide this heat, a dedicated tempering system on the incoming cold air, or a HRV/ERV unit to transfer the outgoing heat to the incoming cold air.

If you're simply exhausting warm air, you're basically blowing heat out. If you've paid to heat that air up, it's nice (and efficient) to try to recover some of that heat in some situations with a HRV/ERV. While a HRV/ERV unit costs more, you often recover that additional cost in just a few years due by not simply blowing heat out of your house.

NRT. Rob,

You can "register a disagreement" but you should at least base it on accurate numbers.

The average heating season temperature (Oct - Apr) for Madison WI is a little over 30°, and the HDDs are only 7500 (a milder climate than where I am in north central VT). So the average delta-T is more like 35°.

Secondly, the volumetric heat capacity of air is 0.0182 btu/cf·F°, not 1.8 which overstates the heat loss by a factor of 100. So, even using the 7 month heating season I averaged above, the heat loss from ventilation at 25cfm continuous (which is inadequate for most homes, but that's the number you pulled out of your hat) would be 210day x 24hrs/day x 60 min/hr [you also forgot to convert cfm to cf/hr] x 25cfm x 35° x 0.0182 btu = 80,262 btu, or less than 50 ccf of natural gas - about $100/year.

A reasonable price to pay for fresh air and health. And, since the HRV might recover 75% over the heating season and has operating and maintenance costs as well as the higher equipment and installation costs, the payback might be something on the order of 20-25 years.

If you don't understand the benefit of uniform negative interior pressure in a heating climate, then you'd better review your engineering books. All houses in a cold climate will have stack effect pressure differentials, with positive pressure in the upper levels (typically the upstairs ceiling) and negative pressure in the lower levels (typically the basement). A "balanced" ventilation system does not change this pressure differential, except in so far as the incoming cold air gets warmed and expands and occupies more volume than the exhaust air, so it may enlarge the positive pressure zone. Positive pressure causes exfiltration, and exfiltration causes condensation, and condensation causes moisture accumulation, and moisture accumulation causes mold and decay and invites wood-boring insects.

If the entire house is under negative pressure, there can be no exfiltration. Infiltration of cold air can not cause condensation. Additionally, an exhaust-only ventilation system with passive make-up inlets - like a fully ducted system - puts the fresh air exactly where it's needed and also allows some minimal passive ventilation even when the power is out in the dead of winter.

Hence, not only less expensive and simpler to install, operate and maintain, but also makes the house both more durable and healthier.

Correction: the formula yields 48 therms which is, as I indicated, about 50 CCF of natural gas.

Another suggestion: You can duct or locate a passive air supply behind the refrigerator to preheat cold incoming air.

This thread like so many seems to push the reader to take sides.

I would love to finish reading an interesting thread and feel a list of proper and improper solutions was at the end, apparent to all.

Keeping the dream alive.

AJ: Sides? That is interesting. All I see is information being presented, backed w/ a little math. If any "sides" are being implied, it seems to me that it would be the usual ones: Do you want to spend more money now, or spend it a little at a time, over a longer period? I'm glad these folks are putting out the numbers; numbers don't have sides. There are always options, not sides, IMO.

I see options John like you do. But often I see a debate ended less clear instead of more clear is all I am saying. I appreciate your thoughts on the matter. HRV discussions are now to be with us forever, glad this thread is up. Entrepreneurism will I am sure bring us many more solutions that don't involve the $5,000 a typical system today costs.

13.5 watt HRVs... I like that number. Panasonic's $300 cost is hitting a home run too. Be nice to see an HRV that had both- 13.5 watts and $300.

AJ: Roger that. Sometimes the options are almost overwhelming, and, yes, by the time I get done reading various places, I wonder what I learned for certain. I guess you just keep reading and thinking until you are satisfied that what you are planing is at least intelligent. Sometimes here, like anywhere that science is discussed, I feel that some of the debates are almost starting to split frog hairs. I know that some of the cats on the hearth.com site are absolute (admitted) fanatics, so if one gets close to their setup, they are doing real well. How many standard deviations above average does one go before one says "Enough" ? Do I have a positive ROI for 14" of EPS under my slab??? Etc. But, were it not for folks pushing the envelope, the envelope would be just sitting. j

What a great--and informative--thread.

I've been in the planning stages of a new home for awhile now, and look to be moving forward by year's end to build a small (1,197 sq ft), well-insulated place in Maine. I've been leaning in the direction of an UltimateAir RecoupAerator DX200 system for awhile now, but just wish they offered a HEPA filter option due to some pretty bad seasonal allergies I've got. I've got a couple questions I hope folks may be willing to answer for me:

1) If I were to opt for a HEPA (MERV 16) system instead of a MERV 12 system, what types of additional contaminants would such a system be able to filter out of the incoming air?

2) Would 7 to 8 air exchanges per day be sufficient to keep the air in a home (with just two occupants) optimal?

The UltimateAir RecoupAerator (MERV 12) system would be capable of 70 cfm @ 40 watts, costing around $4/month here in Maine to run and producing approximately 10 ½ air exchanges per day. The HEPA option (MERV 16) I’d also consider—the Venmar True HEPA HRV 1.0—would run at a rate of 50 cfm and 110 watts, costing around $10.85/month and producing about 7 ½ air exchanges per day . The latter would be a noticeable jump in cost to run, but would offer a higher level of air filtration. So I’m also wondering:

3) What factors should one take into account in determining which system would be the best long-term investment? Is there enough of a health benefit from a HEPA system to justify the additional expense? Are there other variables that I’m overlooking?

Thanks for any input!

Litawyn

Martin:

Thanks for the input. It's much appreciated. It appears my planned ventilation rates are in line with the old ASHRAE rule of thumb, which would be 47.5 cfm in my particular situation.. With the new rule of thumb however, the requirements for the home drop to just 27 cfm. I'm curious why the radical shift with ASHRAE's recommendations? And short of going with a Panasonic FV-04VE1 WhisperComfort Spot ERV, would there even be ERV/HRV units that would run at 30 cfm or less?

I will check out the other threads you've referenced in case answers may be found in either. Thanks again.

Litawyn

Martin:

This may be a dumb question, but might it not cost about the same to run an HRV continuously on its lowest setting than to have the system running for 40 minutes every hour and flipping on and off 24 times a day? Isn't there some kind of a spike in electrical usage every time, say, a light bulb is turned on? If so, all things being equal, I'd rather pay to have a system running on low 24/7 than to have it cycling on a timer two dozen times a day if the net expense of each is going to be about the same.

Any ideas?

Litawyn

Martin:

Thanks for the clarification. It's helpful.

I work from a home office, so I'm home most of the day. But I will make sure the system is shut off when I'm not home.

Litawyn