I've got a question that I'm a bit perplexed about and I'm hoping that I can get some comments.

I've got a 2850 square foot area where we will be installing an HRV (new contruction). Heating source is radiant heat on top of floor (1.5" gypcrete), so we have no ventilation or air conditioning system other than our desired HRV. There are 5 bedrooms, a media room, living room and dining room into which we want to push in clean air. For stale air returns, I was wanting to have one in the master bathroom, one in each of our two full bathrooms and one in our powder room. We will also install one at the top of the stairway for the final stale air pickup. We will also install regular (low noise) bathroom fans in each bathroom for those higher use times (bath/shower, smells, etc.).

The HRV contractor told me his supply house guys informed him that if we install stale air pickups into our bathrooms we WILL have smell transfer problems. He was told that WHEN THE HRV IS NOT RUNNING (e.g. summer time, when the HRV is on a 20-minute on - 40-minute off schedule, if the HRV breaks down, etc.), there is a risk of smells from one bathroom traveling to one of the other locations where there is a stale air pickup. When my wife heard this, she became adament that she does NOT want to install any stale air pickups within any of our bathrooms. My concern is that the bathroom air will become stale over time if the bathroom (and regular fan) remain unused and there is NO HRV stale air pickup for that bathroom.

Am I way off base or is the information the contractor received bogus? I asked him and he said that he has never installed stale air pickups into bathrooms for any of his previous jobs. However, he said he would install the system that way if we want it. This is a smart guy when it comes to calculating the installation flow and balancing the system, but I'm look for information from folks with experience having HRV stale air returns within bathrooms or for folks that may know where there may be information about this posted online (at least as far as smell transfers goes)? There is a lot of information saying to have the HRV pickup air from bathrooms, but nothing about a potential for smell transfer if the HRV is NOT running.

My question is this: when the HRV is NOT running, could smells from one bathroom travel within the HRV stale air return lines to somewhere else within the house? Remember, this would ONLY occur if the HRV were NOT running. If it's running, there really should be no issue as far as I can tell (logically anyway).

There really are only a few times that I could think of during which the HRV might not be running. For example in the summer we will most likely turn it off or put it on a very reduced schedule as my wife likes to open windows and doors for ventilation. Or if it breaks and has not yet been repaired. Other than that, I think we'll keep it on most of the time or keep it on a 20-minute ON - 40-minute OFF schedule (city requires 8 hours continuous running each day).

All questions or comments welcome!

Thanks,
1BadBoy

You (or your contractor) are in my humble opinion worrying too much, or.... have extreme bad gas 1BadBoy.
HRV bad air areas out,, bath room, laundry, kitchen, home theater...
HRV good air in Bed room, living room, hallways, home theater (stuffy air area, movement is good).

Dan

What part of the country is this home being built.Remember any room which has a door which maybe closed needs supplys , transfer or undercut doors to enable exhaust systems to work properly.

We use ERVs to better control humidity - usually RenewAire - and use them as bathroom exhaust fans. If you install push button controls in each bath, regular bathroom exhaust is redundant. You can and will use an ERV in summer, as it will transfer latent heat (humidity) instead of exhausting conditioned air you will have to pay to condition again.

Where fresh air is dumped, depends a lot on climate and customer, but generally goes to larger rooms and hallways.

Naturally an experienced professional should size any ERV or HRV.

I second ERV and replacing bath fans with exhausts/boost switches.

I appreciate all responses so far. A couple of things I didn't think about:

1) Location is Seattle, WA

2) The house is radiant heated with no air conditioning, so my understanding is that ejecting humid air rather than capturing in the Seattle area is fine (HRV rather than an ERV).

3) We have 2 full bathrooms, 1 powder room, 1 master bath (jacuzzi tub, separate shower, separate water closet), so we would start to stretch the limits of even the largest manufacturer's HRV limits which would slow down evacuation of steam/odors.

A call to the contractor's product manufacturer (LifeBreath) indicated with their largest unit (DCS-300) that we would get to adequate ventilate for our 2,850 sq feet area for stale air removal (steam/odors), but that the boost would have to run for longer periods of time (e.g. 20-30 minutes) to completely clear all steam/odors. Plus a local product representative agreed that adding a dedicated vent in each bathroom would allow that room to more quickly evacuate steam/odors. So we opted in design to install a bath fan into each location (Panasonic - whisper quiet) with timers. The fan timers would be for boost control and we would use the HRV for scheduled running (20 min on/40 minutes off or simply run constantly at low speed).

I originally thought we'd use only the HRV to evacuate steam/odors, but based on feedback up to this point, we've decided to plan a dedicated fan as indicated above.

Am I way off base here? On some other forums folks have said they wished they had done this same setup, but it's not easy to retrofit after the fact.

50 cfm of exhaust per bath may not immediately evacuate steam and odors but will do so in a fairly timely fashion. that's a fairly regular and large amount of exhaust to lose your exchange efficiency on, plus it introduces potential negative pressure issues.

There is a 'vent zone' system by aldes that can allow for higher exhaust rates in less bathrooms at a time, as well. I would certainly do that in preference to separate fans.

ERVs are, simply, more efficient and typically more comfortable. without one, with seattle's average RH in january (80% at 15 degree F), I would say you could maintain about a 30% RH indoors at 70? that's acceptable, but 50% is better. but HRV would be acceptable in seattle IMO, if you like.

For a 3000 sq ft house for typical air exchange you'd need significantly less than 100 cfm continuous... probably only 60 or 70 cfm there tops. only for boost would you need higher rates. Typically we use a low speed continuous and boost from the bathrooms for 20 minute intervals.

Posted By NRT.Rob on 18 May 2011 08:53 AM
50 cfm of exhaust per bath may not immediately evacuate steam and odors but will do so in a fairly timely fashion. that's a fairly regular and large amount of exhaust to lose your exchange efficiency on, plus it introduces potential negative pressure issues.

There is a 'vent zone' system by aldes that can allow for higher exhaust rates in less bathrooms at a time, as well. I would certainly do that in preference to separate fans.

ERVs are, simply, more efficient and typically more comfortable. without one, with seattle's average RH in january (80% at 15 degree F), I would say you could maintain about a 30% RH indoors at 70? that's acceptable, but 50% is better. but HRV would be acceptable in seattle IMO, if you like.

For a 3000 sq ft house for typical air exchange you'd need significantly less than 100 cfm continuous... probably only 60 or 70 cfm there tops. only for boost would you need higher rates. Typically we use a low speed continuous and boost from the bathrooms for 20 minute intervals.

Huh?  

Rumor has it that average Seattle wintertime humidity (scroll down or use the search function) is a heluva lot higher than that!  (Try 100% RH @ 36F!)

The average winter months outdoor dew points in Seattle are in the 36-38F range (close to the average air temps, which is why it's constantly cloudy/rainy for months on end.)  That's also about the dew point of 70F 30% RH air, so only rarely, and only by overventilating would an HRV result in indoor RH under 30%, and even getting much below 35% would require a lot of ventilation, most of the winter, and in a tight home with standard recommended ventilation rates, 3-5 occupants 35-40% is more likely.

In summer the outdoor dew points are almost never over 60F (or 60% RH @ 75F), with monthly average dew points under 55F (or 50% RH @ 75F).

Also, wintertime indoor RH of 50% @ 70F is a max, not a min for human optimal human health, since dust mites can thrive only at RH>50%.  The health experts say anything between 30-50% RH is optimal (ASHRAE say 25%-65%).  But human health is only part of the picture...

Wintertime humidity that high can be a DISASTER for building materials in typical stick-built stackups (even in mild-winter Seattle, but worse in cold climates) without air-tight class-I vapor retarders (true vapor barriers) on the interior, or rigid foam on the exterior of the sheathing, since the dew point of the interior air is then ~50F, and the location which have that average temp will occur well inside the cavity of a studwall that has protective exterior foam. Any tiny air leaks or even vapor permeation through a kraft facer on a batt will cause condensation and moisture accumulation on the studs & sheathing.  At 30-35% RH/70F the average January condensation layer in a Seattle building is in the siding or near the outer surface of the sheathing, where the drying capacity to the outdoors is adequate, and the penetration of interior air is limited.

Rather than 50% RH, Keeping the December-February humidity between 30-40% would be best for both the occupants and the building.  An ERV would be working against your humidity purging, requiring higher ventilation rates to get/keep it under 40%, even in winter, with zero payback in summer.  This is in distinct contrast with most of the right-half of the US.

I noted that using the HRV wouldn't overdry significantly, and my design data was straight from NOAA. Did you miss that I agreed that using an HRV would keep him in range? I just disagree that the bottom of the range is where you want to be.

50% has additional benefits, including toxicity to flu virii. the target for indoor humidity has changed a lot over the years but having been in a humidity controlled environment I can report that 50% is pretty darned good for comfort as well, and drier air is harder on the lungs. I do not think 50% humidity is dangerous.... if it is, then he and everyone else in seattle without dehumidification are positively screwed in the summer where it stays over that naturally at least half the time (no deep analysis there, just average dew points and temps)

the problems you're talking about for the building are related to positive pressurization only. with a good fan system that should not be an issue. I would focus on health and design appropriately for that.

Remember ERVs are only about 50% efficient on humidity exchange. typically a constant exchange scenario is adequate without bumping up air exchange rates to dehumidify.

Again: if he's not cooling, the HRV will work, as I already noted. I do disagree that it's optimal, though.

Something it totally amiss with your dewpoint data source, Seattle is in no way that dry for a January average. (I lived there for 20 years, and I'm tellin' ya there was rarely a WEEK that averaged air that dry during my tenure there.) All sources I've looked at online show high mid-30s for a mean temp, including this one that uses NOAA data:

http://weatherspark.com/#!graphs;a=USA/WA/Seattle

Over on the right pull down the "select graphs", turn off temp, precip, etc, turn on only "dew point", and set the display to "monthly". Scroll & zoom to any year/week/day you like. It only goes back to 1948 though. January 1950 might have averaged that dry, but that was before my time. ;-)

To dehumidify down to under 35% @ 70F via dilution with exterior air that has a 36-38F dew point takes awhile even with an HRV.

Sure 50% is better than 30% on virus control, 30% is better on fungus, mold, & dust mite, control, almost all health professional recommendations are that 30-50% is an optimal range. The difference in comfort between 30-50% is pretty minimal too (whereas the difference in comfort between 20% & 30% is dramatic.) Over 50% isn't dangerous for most people- mostly just those with dust mite allergies. ( Over 60% those with mold allergies suffer, and comfort starts to slip when it's over 75F. Over 70% RH wood becomes mold & rot susceptible at room temp.)

But 50% interior RH is hell on buildings in cold climates (not nearly as bad in Seattle as in Minneapolis.) Almost all building-science humidity simulations & studys put a ramp on "normal" humidity that bottoms out a 30% during the cold weeks, and "high" humidity sim runs similar ramps that bottom out at 40%, with a 10% offset from the "normal" ramp. If one is intentionally running 50% all the time it would require different building designs/codes to fully protect the building excessive moisture accumulation within the assemblies during the winter.

Even if he IS cooling, the summertime dew points are low enough that there's no advantage to ERVs, unless he's actively dehumidifying to 40% or something. It's never muggy there.

I wasn't looking at mean temp, I was looking at cold temps so see what the minimum expected maintained RH would be. But I gather your point and mean is probably a better approximation for this discussion.

I will defer to your superior fact checking in this matter!

Rob is right.

You can go for the big horsepower bath exhaust fans, but you will likely pull a negative air pressure (won't get into all the bad things that might happen) unless you have a means of relief air...funny the horsepower guys rarely bring say where all that exhaust air will come from. This is particularly true of new "air tight" contruction.

Posted By 1BadBoy on 17 May 2011 11:10 PM
A call to the contractor's product manufacturer (LifeBreath) indicated with their largest unit (DCS-300) that we would get to adequate ventilate for our 2,850 sq feet area for stale air removal (steam/odors), but that the boost would have to run for longer periods of time (e.g. 20-30 minutes) to completely clear all steam/odors. Plus a local product representative agreed that adding a dedicated vent in each bathroom would allow that room to more quickly evacuate steam/odors. So we opted in design to install a bath fan into each location (Panasonic - whisper quiet) with timers. The fan timers would be for boost control and we would use the HRV for scheduled running (20 min on/40 minutes off or simply run constantly at low speed).

If your house is 2850sqft on two floors rather than a basement, or a fairly spread out floor plan, you might want to consider two regular or smaller HRV/ERV units.

Between the cost for long duct runs and labor, you might come out ahead with a small unit at each end of the house. Just some food for thought...

Our 3000 sq ft home has 4 bathrooms with an ERV running continuously and boosts in the 4 bathrooms. The boosts aren't pulling 50 cfm's. Any thoughts on why. Could the MERV 13 filters effect this?

If the filters are not internal to the unit, and were not accounted for in the duct design, yes.

Thanks. I thought that might be the problem.