# ERV exhaust in bedrooms

| Posted in Mechanicals on

Hello GBA,
thanks to everyone that has helped with my questions in the past.  My current confusion is around exhausting the bedrooms vs supplying them through the ERV.  I realize there is an old article on this and a few QA posts, but I was not able to find an answer that really made it clear or people actually doing this.  Has anyone ever done such a setup before?  The original GBA article below introduces the idea, but no examples.  A later QA post answer by Martin says this has been “debunked”.  Malcom also posted to a recent post that this was false, but I’m having a lot of trouble with the math.  Supposedly the reason it is debunked is that the flow rates are too low to affect temperatures in a measurable amount.  So let’s setup a scenario:
1.  Outdoor temp of 32F.
2.  Indoor temp of 72F in greatroom.
3.  Considering a bedroom with closed door.
4.  80% efficient ERV supply or exhaust at 20CFM continuous ventilation.
5.  For 20CFM, ACH = CFM * 60 / volume.  For a 10x10x10′ bedroom, this would be 1.2 air changes per hour.

In this scenario, if I were to supply the bedrooms with outdoor air, every ~45 minutes the room is being replaced with 64F air (72-(72-32)*.2).  If I were to exhaust the bedroom, the air in the bedroom would be recirculated with 72F air every 45 minutes.  That’s a temperature delta of 8F just from the ERV which seems huge.  Also, why not exhaust the bedrooms?  That should still help mitigate CO2 levels in the bedroom and in a way that should be warmer and less likely to blow on occupants.  If the air you’re pulling in from the rest of the house is stale or smelly it seems like you have other issues that need to be resolved.

Martin stated “The idea that unusually creative ways of ducting HRVs might help equalize indoor temperatures has been debunked, because HRV air flows are too low to move much heat around your house. For more information on this issue, see:

Ducting HRVs and ERVs

A New Way to Duct HRVs

Does a Home with an HRV Also Need Bath Fans?”

But none of the linked articles seem to debunk this from what I read, I may have missed something.

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1. | | #1

I don't have anything scientific to add, but I have supplies in the bedrooms and there is no perceptible difference in temperature compared to any other rooms, and I cannot feel the air blowing on me even when the HRV is on boost mode (~40cfm per outlet).

If I had to guess as to why this is, it's probably due to the fact that the air in the room has a much smaller heat capacity than the physical objects in the room (including walls, floor and ceiling). Another factor is that even though you've introduced enough air to "replace" the room air in 45 minutes, that's not actually what's happening. It's not like a hose where the stuff at the end gets pushed out as the new stuff comes in. The incoming air mixes with the old air and very quickly a thermal equilibrium is found, so the air going out is not the 8F differential you describe, it's a tiny fraction of that. This same phenomenon explains why nighttime cooling is only moderately effective. It can drop down to 5C/40F overnight, and even leaving all your windows open all night is unlikely to drop the indoor temperature more than a few degrees C.

2. Expert Member
| | #2

You are complicating the process.

Your 64F feed at 20CFM will deliver 18BTU worth of cooling into a 72F room. That falls into the category of noise for energy use. Most bedrooms would have around 1500BTU cooling load with two people in it.

This is also why the "free cooling" benefit that some ERV manufacturers promote doesn't work. There is just not enough CFM for the small delta T to transfer any real energy.

I have stale air pickups in my bedroom with the fresh air supply in the living space. Works quite well. According to my IAQ meter, the air is pretty much the same in both places.

I don't have a CO2 sensor, but my guess is the CO2 levels would be ever slightly higher as the bedroom is now fed with diluted fresh air from the living space instead of fresh air from the outdoors.

1. | | #6

Last time I ran the numbers, a bedroom receiving diluted fresh air required 2X the airflow for the same CO2 level. But of course it depends on the details. Similar applies to other pollutants (fresh air dilutes better than partially polluted air).

1. Expert Member
| | #8

When you are in the bedroom, unless you have a huge indoor garden, there is no CO2 source in the living space. Once the air there gets flushed out after a couple of air changes, the CO2 concentration will be the same as outdoors, so will any air drawn into the bedroom by the exhaust pickup.

This process is not instantaneous, so there is a slight delay, but CO2 levels in the living space with decent ventilation can never get high enough to matter much.

Overall, the goal is to circulate fresh air through the entire house. As long as the layout allows for this, you'll have decent IAQ. Certainly better than a home with no balanced ventilation.

1. | | #9

There may be people in the other space. And more importantly, flushing the CO2 is far longer than "slight delay". If your non-bedroom space never sees CO2 levels significantly more than outside, you are wildly over-ventilating. Run the numbers .... they are surprising (especially with code min ventilation).

1. | | #10

I don't know how to run those numbers, but I've never lived in a house with continuous ventilation for the bedrooms and I assume it must be better than that . For a bedroom, I think you could assume that the majority of the time there are not a lot of people in the main house when the door is closed and someone is sleeping.

2. Expert Member
| | #11

People breathe about 0.4CFM and breath is 4%CO2 so 40000 ppm.
Outside air is 400 ppm CO2.

Say a typical house 2000sqft with 150CFM balanced ventillation and 4 people.

During steady sate, people are breathing out 1.6CFM which is diluted by the 150CFM outdoor air. The steady state indoor CO2 would then be

(1.6*40000+ 150*400)/152=815 PPM.

So when you go to sleep, the living space is at 815 PPM. The CO2 levels would then drop exponentially based on ventillation rate and building volume. After:

1h ~580PPM
2h ~480PPM
3h ~430PPM

So the fresh air supplied to the bedrooms is higher CO2 initially, but approaches outdoor air at a reasonable rate.

So assuming 25CFM bedroom flow with 2 people sleeping (0.3CFM breathing rate sleeping/person).
1h 1480ppm
2h 1380ppm
3h 1340ppm

Provided my math is fine, it doesn't seem to me there are any issues with this setup.

3. | | #12

Is 1300 ppm good? I searched and according to https://www.greenbuildingadvisor.com/article/ensuring-fresh-air-in-bedrooms
you want 1000 ppm. Then again even houses with hrv/ervs and I'm assuming supplies to the bedrooms seemed to go a bit over 1000 ppm in the bedrooms. My ERV would be ~100 CFM, but the bedroom rate should be 40 CFM. I was actually thinking of just using the continuous bathroom exhaust from the attached master bathroom as the bedroom exhaust, then doubled it to 40 CFM. This would save me from having to put more soffits into the home for the ducts, but also save time. I was thinking this would also allow for better whole house ventilation vs a short cycle from the master bedroom to the master bath.

4. Expert Member
| | #13

The 1000ppm target is good while active. I doubt going over it matter much when sleeping (my math could also be wrong).

I run around 25CFM in my bedroom and I never find the air stale except when very cold outside and the ERV starts running at lower rate to prevent core freeze up.

If you have central air, you'll also get much more mixing from the HVAC system, which would bring the CO2 levels down significantly. Plus there is always a bit of air leakage even in a tight building.

I'm not sure I follow your setup. I would stick to standard ventilation specs, they seem to work.

5. | | #15

"I'm not sure I follow your setup. I would stick to standard ventilation specs, they seem to work."
Setup is master bedroom connected to master bathroom. Since I need to exhaust the master bathroom anyway, I figured I would just put in a bigger vent for the bathroom and use that to vent the bedroom instead of putting in two vents.

6. | | #16

Over 1000ppm is bad, even for sleeping. Your brain continues to perform important cognitive functions when you sleep. It's not like you're going into suspended animation. I suspect your math might be off. 25cfm for two people is within the recommended range by most ventilation standards, so I'd be surprised if that allowed for regular excursions to 1300ppm and up.

I wouldn't count on building leakage for anything. Even in a leaky house, the amount of leakage is variable. In a tight house, the amount of fresh air being delivered via leaks is analogously down below the level of noise the ventilation air is contributing to heating. My whole house gets 85cfm at 50Pa. A single bedroom, in relatively still air, is probably something like 1cfm; perhaps far less.

7. | | #17

Here is what I get for an example bedroom CO2 ppm. Notice that even 2x isn't quite enough to make up for the decreasingly stale air from the rest of the house being pulled into the bedroom. Yes, > ~1000 ppm is a health hazard (although it won't kill you).

fresh air, 25 CFM into bedroom (supply)
CO2 ppm at 0h= 400 (same as outside)
CO2 ppm at 1h= 895.593
CO2 ppm at 2h= 1130.79
CO2 ppm at 3h= 1242.4
CO2 ppm at 4h= 1295.37
CO2 ppm at 5h= 1320.51
CO2 ppm at 6h= 1332.44
CO2 ppm at 7h= 1338.1
CO2 ppm at 8h= 1340.78
bedroom CO2 avg = 1144

stale air, 50 CFM into bedroom (exhaust)
CO2 ppm at 0h= 1253.33 (same as the rest of the house steady state)
CO2 ppm at 1h= 1503.46
CO2 ppm at 2h= 1435.82
CO2 ppm at 3h= 1324.64
CO2 ppm at 4h= 1225.04
CO2 ppm at 5h= 1144.6
CO2 ppm at 6h= 1081.33
CO2 ppm at 7h= 1031.94
CO2 ppm at 8h= 993.467
bedroom CO2 avg = 1221.52

Assumptions: 75 CFM of ventilation air, 9' ceilings, 15x15 bedroom, 4 people in house, 2 in bedroom, others sleep at the same time, .3 and .4 CFM respiration rates.

8. Expert Member
| | #18

It is hard to get consensus on what is "bad" for CO2 levels. Workplace levels are allowed to be silly high for 8h. High CO2 levels by themselves don't pose any significant health hazard.

Even older leaky homes will spike well above 2000ppm in bedrooms and we've been living in those for decades without issues.

I would look at CO2 more of as a proxy for air quality. Much above 2000ppm the air starts feeling stale, which is what you want to avoid.

Ventilation costs energy, at one point you have to say that it is good enough.

9. Expert Member
| | #19

Akos,

CO2 may not be a significant health problem, but being in a high CO2 environment can make you dumb as a stump:

10. Deleted | | #23

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2. Deleted | | #21

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3. Expert Member
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4. | | #4

though it's still hard to get my head around replacing a room's air with 8 degree cooler air not being really uncomfortable. I guess from Trevor's point, the air would be warmed by radiant transfer faster than the convection from the ERV. From the calculation in the article, the BTUh transfer from the ERV would be BTUh = cfm of the fan • delta-T • 1.08, which would be 173 BTUh from incoming air 8 degrees cooler.

5. | | #5

"pre-supposes the doors will remain open or you have installed affective jumper ducts. otherwise there is pretty big impediment to effective air movement." I assumed that 20-40CFM would be easily drawn from under the door. Would I not get decent enough ventilation given this setup to reduce CO2 levels in a closed bedroom?

1. | | #7

According to https://www.greenbuildingadvisor.com/article/when-are-door-undercuts-sufficient-for-return-air, door undercuts should be fine up to around 80 cfm if I follow it correctly.

6. | | #14

For what it's worth, I'm planning to exhaust the bedrooms in my new build. Exhausts will be in the closets killing two birds with one stone; drawing fresh air into the bedrooms from below door undercuts, as well as exhausting any odors/contaminants from the closets (leather shoe smell, dryer sheet odors, dirty laundry, formaldehyde in new clothes, etc.)

Supply air will be into high ceiling living areas where it has plenty of time/distance to mix before reaching occupants.

Supporting my position is the testimonial from the previous owner of a used HRV I just purchased. They used it for a year and then shut it off, claiming it caused cold drafts. I didn't ask specifically which rooms were concerned, but he did mention one of the complaints came from his Mother in-law who had moved in with them, so I assume that means a bedroom. It's a VanEE 90H, purchased used for \$200 with a year worth of run time on it. It looks like new inside and out.

1. | | #20

Lance, great to hear. My setup is actually a linear rectangle with master bedroom, walk through closet, and then master bath. I'm planning on just doubling up the ventilation in the master bath and undercutting the closet doors. This is killing 3 birds with one stone I guess:) And why not?

7. | | #22

I looked more into it. The original published study on co2 levels is here: https://www.nxtbook.com/naylor/ENEB/ENEB0217/index.php#/p/30. In it, Fig. 3a shows 3 homes with balanced ventilation, but only one with supply to the bedroom (and I'm assuming no exhaust in the bedrooms either). They are practically identical, with the supplied bedroom being roughly the median between the three. It doesn't say how many occupants/etc are in each of these limited samples, but that's at least something to go off of. Also, from my calculation below of 180BTUh, if I include the master bath exhaust that should come out to about 400 BTUh. Seems like a pretty big win on cold nights.

Jon, how did you get over 1200 ppm for steady state in the main house? I'm using a 100 cfm panasonic erv. According to http://www.energy.wsu.edu/Documents/CO2inbuildings.pdf, 1200 ppm would be roughly equivalent to 12cfm per person and I think that includes air leakage rates. The main house should be roughly 800 ppm according to the table.

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