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A New Way to Duct HRVs

Instead of pulling exhaust air from bathrooms and the laundry room, consider exhausting stale air from bedrooms

Posted on Apr 22 2011 by Martin Holladay

UPDATED June 27, 2013 with an author's postscript

Nine years ago, I co-authored a Journal of Light Construction article with David Hansen on HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. installation. The article noted, “Stale air is exhausted from bathrooms, the laundry, and the kitchen. (An HRV is not intended to handle grease or smoke, so a range hood should be separately exhausted to the exterior.) Fresh air is supplied to the bedrooms, living room, and other living areas.” This advice is consistent with the long-standing recommendations of most HRV manufacturers.

The advice is logical: after all, it makes sense to exhaust air from the smelliest rooms in the home and to supply the fresh air to the rooms where people spend most of their indoor hours.

Although this traditional ventilation duct layout works well, I’ve begun to rethink the issue lately. It may be time to experiment with different ducting methods for HRV systems — especially for homes with single-point heating systems.

Ductless minisplits come of age

There is a strong trend among designers of superinsulated houses to use ductless minisplit systems for space heating and cooling. These systems can work well even when outdoor temperatures drop to -17°F or -20°F. Moreover, the systems are inexpensive, easy to install, and very energy efficient.

What type of ventilation system is best in such a home? Since there is no heating or cooling ductwork, a central-fan-integrated supply ventilation system is obviously out. The usual choices: either an exhaust-only system (for example, a bath exhaust fan that runs for much of the day) or a heat-recovery ventilator (HRV) with dedicated ventilation ductwork. (Throughout this article, I’ll use the term “HRV” to include energy-recovery ventilators as well as heat-recovery ventilators.)

Two problems with homes with single-point source heat and HRVs

As more HVAC engineers and builders wrestle with the details of designing a house with a single-point heat source (for example, a gas-fired space heater or a single ductless minisplit unit), they worry about two questions:

  • Will the bedrooms be cold?
  • Will the delivery of fresh ventilation air cause comfort problems?

Evidence on the cold-bedroom question is beginning to accumulate from a variety of projects, including R. Carter Scott’s net-zero-energy house in Townsend, Mass.; the Elakim’s Way project developed by the South Mountain Company on Martha’s Vineyard; and RDI’s Wisdom Way project in Greenfield, Mass.

It’s hard to generalize, but it seems safe to say:

  • Compact superinsulated homes heated by one or two heat sources have been successfully built, and haven’t resulted in many comfort complaints.
  • When bedroom doors are open, the bedroom temperature is usually within 1 or 2 degrees of the living room temperature.
  • If you live in such a home, remember to keep bedroom doors open unless you need privacy.
  • Monitoring studies show that the bedrooms in these homes are often several degrees cooler than the living room, but most homeowners aren’t complaining — so it’s possible that homeowners are deliberating keeping their bedrooms cool or are indifferent to lower temperatures.

Fresh ventilation air is still cool

At a presentation at the March 2011 Building Energy conference sponsored by NESEANorth East Sustainable Energy Association. A regional membership organization promoting sustainable energy solutions. NESEA is committed to advancing three core elements: sustainable solutions, proven results and cutting-edge development in the field. States included in this region stretch from Maine to Maryland., Robb Aldrich, an engineer with Steven Winter Associates, explained why there were few comfort complaints at the Wisdom Way project in Greenfield. “The winter heat loss from one of these bedrooms is in the range of 60 to 120 watts,” said Aldrich at the Boston conference. “If you’re sleeping or have one light on, you probably are generating enough heat to balance the heat loss. A sleeping person emits 60 watts. If the person is awake and has a laptop and a lamp, you might be up to 120 watts.”

Aldrich did note a problem in a house in Colrain, Mass., that is heated by ductless minisplit units. “There were comfort problem in the hallway where the fresh air was being distributed,” said Aldrich. Since the fresh air register was far from the space heater, the hallway stayed cool. Aldrich advised, “ERVs or HRVs have to dump cool air somewhere, so it’s a good idea to dump the fresh air near where the heater is.” In addition to his advice on fresh-air delivery, Aldrich suggested a novel way of moving warm, conditioned air toward remote bedrooms: why not locate exhaust grilles in the bedrooms?

Fresh air to the living room, exhaust air from the bedrooms

I think Aldrich’s suggestion is a promising avenue for experimentation. I can imagine a house with a single ductless minisplit unit on the living room wall. The HRV ductwork could direct all of the home’s fresh air to a ceiling diffuserIn a forced-air heating/cooling system, the diffuser is a register or grille attached to ducting through which heated or air conditioned air is delivered to the living space. In a tubular skylight or an electric light fixture, the diffuser is a cover plate through which scattered light is delivered. near the ductless minisplit unit, and all of the home’s exhaust air could be pulled from the upstairs bedrooms. Spot exhaust for the bathrooms and kitchen could be kept separate from the HRV.

This type of ventilation ductwork would set up a gentle air flow from the warm, conditioned living room toward the remote bedrooms. Every room gets fresh air, but by the time the fresh air reaches the bedrooms, it’s been tempered. And considering the way some unventilated bedrooms smell — especially those of some teenagers — there’s a certain logic to pulling exhaust air from bedrooms.

[Author's postscript: Dr. John Straube, a principal of the Building Science Corporation, has challenged the logic behind the novel method of HRV ducting advocated here by Robb Aldrich. In an article titled "Choosing HVAC Equipment for an Energy-Efficient Home," Straube is quoted as saying, “Ventilation air doesn’t do much to move around heat. ....Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.”]

Last week’s blog: “More Passivhaus Site Visits in Washington State.”

Image Credits:

  1. Martin Holladay

Apr 22, 2011 6:34 PM ET

Interesting info, but...
by Carl Seville

Martin - what about the millions of who live in climates that need cooling for at least part of the year? I'd love to get some information on how effective single source cooling is at maintaining temperatures throughout a house. Remember, I hang around here for a reason, to remind all you unreformed yankees that warm climates do, in fact, really exist.

Apr 22, 2011 6:57 PM ET

right on Carl
by John Brooks

The body heat, lights and laptop are going to work against us in the "neglected climate"
I would not want to rely on leaving bedroom doors open for comfort ...
or a handful of customers who have "not complained" very much

Apr 22, 2011 8:15 PM ET

Martin will be back in a few more days
by Daniel Morrison

He is on vacation with his kids. I have been publishing according to his schedule this past week (sorry to get this post up a bit late).


Apr 22, 2011 8:34 PM ET

"It's not the heat, it's the humidity"
by James Morgan

Given that humidity control is a major function of air conditioning and that humidity doesn't tend to vary much throughout a well-insulated and well air-sealed home enclosure of reasonable size, I don't see why this principle shouldn't work just as well in a cooling climate - perhaps even better, as users also have the option of local fans (ceiling or otherwise) to fine-tune comfort conditions on an as-needed room by room basis with little energy penalty. The concept certainly seems sound enough to be worth field trials in a variety of climate conditions.

Apr 22, 2011 9:25 PM ET

call me skeptical for Mixed & Hot Climates
by John Brooks

Good to know that Martin is on vacation.....

Meanwhile I came across this blog
where David Butler describes a ducted minisplit system for a hot climate passivhaus that sounds like a good strategy to me.
Of course it is not as simple and cheap as what Martin is proposing.

Apr 23, 2011 9:46 AM ET

Stay with the source point HRV
by Doug McEvers

One of the great advantages of the source point HRV, exhausting from bathrooms, is the elimination of traditional bath fans and all of the associated holes in the building envelope. I have never had a customer complaint when using this system when combined with remote timers in the bathrooms.

Apr 23, 2011 11:12 AM ET

How dare he?!
by David Meiland

He is on vacation with his kids.

Since he hasn't been posting, I was starting to think he didn't love us any more.

Apr 23, 2011 12:05 PM ET

Zender ComfoFresh ducts would be a perfect way to experiment
by Kyle Rearick

Seems like Zender ComfoFresh ducts would be the way to go if someone wanted to experiment on the best strategy. I too am curious what is the best for areas with greater cooling loads. Zender would be nice because you could experiment a bit or change your ducting strategy seasonly. You would just need to mark the ducts at the register box end so you know which room is which. It would give you a way to fine tune things a bit.

Apr 23, 2011 12:09 PM ET

Call me Old Fashioned...
by Armando Cobo

I agree with Carl and John… That’s all good up nowth. I’ve reluctantly used minisplits in “casitas” or guesthouses, but I still would like to have a good ol’ high efficient furnace/ac running at low speeds, taking care of the temperature, moisture control and ventilation “all-in-one” with an IAQ thermostat. Even small houses (1,000-1,500 sf) in the south or humid country will benefit from such system. FYI, most all houses I design have >.05 NACH and >1ACH50… maybe not Passivhaus, but pretty darn good energy efficient and healthy homes.

Apr 23, 2011 4:02 PM ET

Vent Connection
by Frank R


How about using an indoor unit that can accept an vent connections? For example the ceiling cassette models accept a vent. Duct from the HRV directly to the vent connect. or if cost is a concern, skip the HRV and connnect the an outdoor air duct directly to the indoor unit. In a smaller home the 45-90 CFM of outdoor air is a energy penalty, but it would work.

Apr 24, 2011 6:23 AM ET

Edited Apr 24, 2011 6:26 AM ET.

On temperature uniformity from room to room during the summer
by Martin Holladay

So far, the data appear to support the conclusion that single-point cooling can work well.

At the NESEA conference in Boston on March 9, 2011, Duncan Prahl from IBACOS gave a presentation on monitored data from the Fairview 2 Passivhaus building that Katrin Klingenberg erected in Urbana, Illinois. Based on monitoring from January 2010 to December 2010, Prahl reported, "Single-point AC in summer provides reasonable temperature uniformity with continuous ERV operation."

One possible downside to note, however: Prahl pointed out that "ERV fan energy is not trivial; it represents 7.5% of annual energy use."

Apr 24, 2011 2:51 PM ET

Edited Apr 24, 2011 2:56 PM ET.

More on summer cooling
by Kevin Dickson, MSME

Imagine a two story home with a point source of A/C on the first floor. On a hot day, the A/C runs in the evening to cool the kitchen and living spaces. That cool air won't diffuse to the upper floors unless the HRV is running flat out and drawing from the upstairs bedrooms.

Another way to cool the bedrooms just before bedtime is to simply open the windows upstairs and downstairs. In much of the US, outdoor temperatures drop to 70F or lower by 9pm. In those climates, the air in the bedrooms might be 75-80F. With the windows open, the stack effect will exhaust the warmest air and quickly pull the cool air on the first floor up to the bedrooms, without overheating the first floor.

Even with those fairly small temperature differences, hundreds of CFM can be moved this way. Lots of healthy fresh air, zero energy consumption, and zero first cost. That's a win-win.

Apr 24, 2011 11:18 PM ET

Reply to Kevin
by James Morgan

Stack effect cooling is great - if your house is properly insulated and has reasonable thermal mass, to ventilate at night and close up during the day is a time-honored strategy when the climate permits - but I'd never advise combining it with daytime a/c in the same diurnal cycle, if that's what you're proposing. Here in central NC we're not into a/c season yet, but when we are we we sure won't be opening windows at night and throwing away all our expensively dehumidified air. By the way, it's just getting down to 70° outside here at 11 p.m. - and it's still April.

Apr 25, 2011 10:31 AM ET

Edited Apr 25, 2011 10:33 AM ET.

Newbie Questions on Condensation, Comfort & Efficiency
by Mary Schultz

As an owner-builder thinking of using mini-splits for our new house I found your post very interesting and have few questions (from a novice) if you get a chance.
Hope you had a nice vacation!

You posted:
“…imagine a house with a single ductless minisplit unit on the living room wall. The HRV ductwork could direct all of the home’s fresh air to a ceiling diffuser near the ductless minisplit unit, and all of the home’s exhaust air could be pulled from the upstairs bedrooms. Spot exhaust for the bathrooms and kitchen could be kept separate from the HRV.
This type of ventilation ductwork would set up a gentle air flow from the warm, conditioned living room toward the remote bedrooms. Every room gets fresh air, but by the time the fresh air reaches the bedrooms, it’s been tempered.”

Condensation?: If the fresh (outside) air from the HRV is warm and humid (let’s say August in VA), and it dumps out near the minisplit that is cooling the air, then will there be a potential for condensation on the cool surfaces (dry wall?, minisplit itself?) near the minisplit?

Comfort?: If the fresh (outside) air from the HRV is cold (let’s say Feb in NY), and it dumps out near the minisplit in living room, then will the people sitting in the living room reading/watching TV be chilly?

Energy Efficiency?: Does dumping outside air near the minisplit run the risk of ‘fooling’ the minisplit into thinking the indoor space needs conditioning when it doesn’t. For example, in winter most of the house is comfortable and the cold outside air dumping next to the minisplit triggers it to keep running and some of the house actually gets too warm?


Apr 25, 2011 10:57 AM ET

Response to Mary Schultz
by Martin Holladay

In general, ventilation air flows are low -- in the range of 40 cfm to 60 cfm -- so the effects you worry about are unlikely.

However, if all of a home's ventilation air is being delivered in one spot, it's important to consider comfort. The diffuser should direct the air to a location near the heat source, but should not direct the air to a place where discomfort could occur -- in other words, don't aim the diffuser at your favorite armchair.

Apr 25, 2011 11:14 AM ET

Overventilation Penalty
by John Brooks

Concerning the Urbana example in post #11
In addition to ERV operational energy ....
What about the overventilation penalty?
When Boosting/maxing-out the run time on an ERV.....
True that the ERV will "recover" a percentage....
However, During the cooling season the "recovery" percentage is Not-So-Wonderful

Maybe not such a big deal in Urbana?
But as you move South and closer to the coast ....
The Overventilation(Hot Moist Air) "penalty" becomes greater and greater.

Apr 25, 2011 12:03 PM ET

Overventilation penalty?
by John Semmelhack

John Brooks -

I didn't see the presentation at NESEA. Was there a mention of "over"-ventilation? I'm pretty sure the idea is just to run the ERV in the typical fashion...24/7 at the lowest speed capable of delivering cfm required for fresh air for the occupants.

Apr 25, 2011 12:06 PM ET

Response to John Semmelhack
by Martin Holladay

I agree with you. I don't think there's any reason to believe that the Fairview 2 house is overventilated.

Apr 25, 2011 12:06 PM ET

Edited Apr 25, 2011 12:08 PM ET.

No insulation on ERV ductwork?
by John Semmelhack


There doesn't appear to be any insulation on the ductwork attached to the ERV in the photo. Was this a mid-construction photo?

Apr 25, 2011 12:09 PM ET

Second response to John Semmelhack
by Martin Holladay

John S.,
I took that photo of the ERV in the Passivhaus building owned by Margaret and Gregory Stanton (Urbana, Illinois). The photo was taken after all construction work was complete and the house was occupied. The ERV is in an upstairs closet.

Apr 25, 2011 12:13 PM ET

response to John Semmelhack
by John Brooks

John, I see what you mean
I may have taken it the wrong way.
I just assumed that ventilation had been increased or ramped up.

Apr 26, 2011 3:49 PM ET

2G H/ERVs change the conversation
by Barry Stephens

I have been following the blogs lately, and there is a very important factor in all of the conversations regarding heat recovery ventilation. The 2nd generation of H/ERVs will change the dynamic considerably. When you go from a delta T of 6-7 degrees to a delta T of 2-3 degrees, everything changes. The cool air you describe, and that people are dealing with when using H/ERVs, goes away. So does the noise factor. The basic and accepted method of heat recovery ventilation being used in Europe is proven, and reinventing the wheel is not necessary. We (Zehnder America) will have data from several diverse projects in North America shortly, and they will demonstrate the difference in efficiency, and what it means to these systems. Point source heating and cooling combined with 2nd generation, high efficiency heat recovery ventilation will be a very common model for high performance homes.

Jan 6, 2013 2:13 AM ET

Using existing ducts with this reverse flow idea
by L Buser

I think this is a novel idea and I am pondering an unusual reversal of the flow in my duct work.

I have a split level house with a newer gas furnace but I don't like the smell from running the furnace or air handler by itself. The cause I am working on, including the cold air return intake being under the stairwell -- when the furnace comes on, the depressurization there is high and likely draws air down from the attic and walls ( I will be sealing under the stairs); and unsealed ducts in the crawl space and to the upper floors (concrete floored and concrete block walled crawl space) that draw air from the crawl space and walls. Sme ducts are not accessible for sealing.

We have R50 in the attic now (Vancouver Island B.C.) and use the gas fireplace for heat, and some small electric heaters to supplement this. It provides all the heat we need without the furnace. (I am not a fan of ducted heating for a variety of reasons.)

Rather than having air delivered through this system to the rooms, this article made me wonder about pulling the air from the rooms via the floor vents and duct work, to an exhaust port on the hot air return, connected to an HRV. Fresh air would be supplied to two or three central dedicated vents, one for each floor.

I would not be able to use the furnace for heating with this setup I imagine. If the furnace ever needed to be used, I would likely have to disconnect the HRV.

I realize the flow is against the joins in the ducts but it would be at a low rate of just a few cfm and I would be able to seal about 80% of the duct work.

It isn't possible to provide new ducting to the bedrooms for a dedicated HRV system, so this is an intriguing alternative.

Can you tell me what pros and cons you see with this?

Jan 6, 2013 5:56 AM ET

Response to L. Buser
by Martin Holladay

L. Buser,
I don't recommend it. Ventilation air flow rates are quite low -- many homes require only 40 cfm or 60 cfm. HVAC systems have much higher air flow rates -- typically 900 cfm to 1200 cfm.

Since ventilation air flow rates are so low, a ventilation system requires perfectly sealed ductwork. It sounds like the ductwork in your home is so leaky that, when you use your ducts, you can't be sure where the air is being pulled from.

Nov 24, 2014 1:08 PM ET

Edited Nov 24, 2014 1:26 PM ET.

So has anyone tried this?
by Patrick Campbell

I'm thinking of doing a simple single intake/exhaust HRV with exhaust in upstairs hallway, fresh air coming out on first floor in vicinity of the output of a single point heat source (heat pump or Rinnai DV furnace). This would mix the colder air with the heated air and seems like by exhausting on the 2nd floor it would assist warm air going up the stairs. Bathrooms and kitchen are individually separately exhaust-only vented. Since there is no heat source upstairs, I agree it doesn't seem ideal to dump colder air upstairs. I suppose if it didn't work you could always reverse it.

Nov 24, 2014 1:20 PM ET

Response to Patrick Campbell
by Martin Holladay

I'm not sure what you mean when you wonder if this will work. Of course it will work. Fresh air will enter the house wherever you decide to put your fresh air vent; and exhaust air will be removed from wherever you install your exhaust vent. In that sense, it will work.

Here's what it won't do:
1. It won't deliver any fresh air to your bedrooms.
2. It won't help exhaust smelly or humid air from your bathrooms.
3. Because the air flows required for fresh air ventilation are quite low, it won't significantly affect heat distribution.

But you'll have a good-to-middling ventilation system -- better than the ventilation system in most U.S. homes.

Dec 7, 2014 9:43 AM ET

Edited Dec 7, 2014 9:44 AM ET.

Exhaust in each bedroom...
by Patrick Campbell

What if we were able to get an HRV ducted exhaust in each bedroom rather than just the hallway. Do you think that would that provide enough air movement to bring fresh air to the bedrooms by replacing the air that is exhausted?

Dec 7, 2014 1:21 PM ET

Response to Patrick Campbell
by Martin Holladay

If you install exhaust grilles in each bedroom, you will ensure enough fresh air circulation. The bedrooms will get enough air exchange. While this amount of ventilation will be fine, it won't necessarily equalize temperatures between rooms.

Feb 19, 2015 10:21 AM ET

HRV cost in Massachusettes
by Victoria Williams

I am building a Tier III home on Cape Ann and will be having a single minisplitheat source for my open kitchen/living/loft area with 2 bedrooms and a bathroom also downstairs (896 sf downstairs). I have read all the discussions regarding exhaust only ventilation vs HRV/ERV. There are many HRV systems out there at varing costs. I am trying to find the most decent cost effective solution to whole house ventilation. I was considering The Lunos fans but have heard that white noise could be an issue. I am looking for quiet and also cost effective to run over time as well. Any comments would be appreciated.

Feb 19, 2015 10:37 AM ET

Response to Victoria Williams
by Martin Holladay

If you are looking for an HRV that is "quiet and also cost effective to run," you should install a Zehnder HRV.

If you are looking for a ventilation system that has a low installation cost, install a Panasonic bathroom exhaust fan controlled by a timer.

Nov 23, 2015 5:25 PM ET

HRV in apartment with one supply/exhaust
by Zoe Brown

I have been reading your article with interest! I have a very poorly ventilated apartment in the UK. No ventilation whatsoever (no exhausts even in the kitchen or bathroom) and big condensation issues.
I have had a few companies advise a PIV unit bringing tempered air into the apartment but with no heat recovery. This is due to them being easier to retrofit and cheaper to install etc. however, doing my own research and I want to understand the possibility of having a HRV unit fitted. I have found a company who make a 'mini' version suitable for small properties and have an idea where it could fit. My suggestion would be to extract air from the bathroom and supply the fresh air to the lounge or hallway since they are close to the bathroom with easy access. My one concern is that by supplying fresh air close to the bathroom it will not allow the air to circulate throughout the 2 bedroom apartment and instead will only work to ventilate the bathroom and hallway/lounge. The apartment is around 60m2 in size. Although the company tells me it will work I just cannot get my head around the fact the air will go the route of least resistance - from the exhaust/supply back to the extract! I would appreciate any advice on this.

Nov 24, 2015 10:11 AM ET

Response to Zoe Brown
by Martin Holladay

Ideally, it's best if the supply air is introduced in the bedrooms rather than at a location near the exhaust grille. But in a retrofit situation, compromises are sometimes necessary.

It's fair to say that the proposed mechanical ventilation system will improve the situation in your apartment. It will be a lot better than what you have now, which is nothing.

Sep 17, 2016 12:02 AM ET

Ventilation aluminum ductwork source
by Jay Miracle

For a 3200 SF new construction project in upstate NY (near Albany), we've designed the ductwork layout for an ERV system (Broan ERV200TE) along the lines of this article (to help distribute conditioned air from five strategically located mini-split heads to remote rooms / locations). I'm now having trouble finding ductwork material that suits me for this application. Rather than galvanized vent pipe / fittings, I would much prefer medium gauge aluminum vent pipe and fittings, ranging from 6" dia. down to 3" dia., including tees, elbows, reducers, and possibly register boxes (or register grills that interface directly with 3" round duct pipe). I've found some online sources that supply subsets of these different aluminum venting components, but yet to find one good source for all of it. Am I being foolish with my determination to find all aluminum, small-ish vent components? My preference for aluminum is because 1) it's so nice and light to work with, 2) foil tape sticks to it beautifully for long-lasting joint seals, and 3) I've seen several older galvanized vent pipes corrode/rust... and have yet to see an older aluminum dryer vent corroded/rusted. My preference for necking the 6" trunks ultimately down to 3" at the registers is because A) 3" dia. pipe fits nicely within 2x4 interior walls and B) the air-flow rate is so much lower in ventilation than HVAC system which require larger duct sizes. Any suggestions for vent pipe sources and/or thoughts re. my approach would be much appreciated. Thanks!

Sep 17, 2016 5:21 PM ET

Response to Jay Miracle
by Martin Holladay

I can't help you with a source for aluminum ductwork.

I'm a fan of galvanized ductwork. Quality HVAC tape sticks well to galvanized steel, in my experience. Galvanized steel is also more rugged and less likely to dent or deform than aluminum.

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