Wood Stove with HRV
We live in an 1942 brick colonial (about 1,000 sq ft each level) in Minnesota. We have both a wood stove and a forced air furnace. We burn about 3 cords a year which covers most of our heating needs and the furnace fills in the gaps. We also rely on the furnace fan to circulate the warm air from the stove throughout the house. Not perfect, but it keeps things pretty evenly warm. Last year, we had a blower door test done and were told our house was too tight after new windows, doors and adding closed cell spray foam to the attic and rim joists. So we got an HRV installed which seems to be working exactly as designed; however, we noticed our room temps are very uneven when heating with our wood stove. It seems to me this is the problem: hot air from the stove goes down the cold air return in that room and is almost immediately directed to the HRV. In other words, the hot air has no chance to circulate throughout the house.
Two questions:
1) does my diagnosis sound reasonable
2) what can I do about it? (Ideally, inexpensively)
Thanks!
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Replies
Did you have draft issues with the stove or furnace before you installed the HRV?
Did the blower door tester give you any numbers to support the too tight claim something like xx ACH at 50 or xxxx CFMs?
I would be surprised if your home tested less than 5 ACH given its age and the work you described.
If that is the case one could say this house is too tight to operate a wood stove.
The most likely symptoms would be the stove wants to back draft when lighting it also it would back draft when operating the dryer and or vent fans.
Walt
Does your HRV have independent ducts or is sharing ducts with the furnace? If it's the latter, there might be ways to reconfigure it to fix the problem. A diagram of how it's connected now would be a big help.
Jonathan - no issues with back draft that I'm aware of; the furnace is a new tightly sealed unit which will back draft if the flue bypass is closed and the door is opened quickly
Walter - I dug up the report and here are the relevant comments
"ATTIC INSULATION AND AIR SEALING:
Attic Insulation: You have sufficient insulation in your attic.
Air Sealing: Blower door test result: 685 cfm50 at 3050 sq. ft."
"Recommendation: Add a controlled source of fresh air for your home by installing mechanical
ventilation. Mechanical ventilation, such as a continuously running bath exhaust fan, can improve
your indoor air quality by removing pollutants and reducing excess moisture, which can lead to
durability issues for your home as well as health issues for its occupants. If adding an exhaust fan,
the suggested continuous flow rate for your home is 94 cubic feet per minute. Refer to the ventilation
handout provided by the home visit crew for more information."
Charlie - the HRV shares duct work with the furnace. I'll sketch something up and post it.
If the hot air heat from the wood stove was being pulled out by the HRV return, that heat would also be recovered by the HRV supply (or 80-90% of it, anyway) being sent to all the rooms, so that's not likely to be an explanation for why the heat distribution changed. The cfm of an HRV is too low to provide much heat distribution, too low to really matter compared to the cfm of the air handler on a hot air furnace.
Even the furnace air handler is very inefficient and doesn't move a lot of heat when the intake air is only 5-10F warmer than the rooms it's trying to heat (compared to 110-150F air coming from the furnace). In many cases the power used by the air handler would be better applied to space heaters in rooms that mattered. In leaky houses even small duct imbalances drive air infiltration by pressurizing some rooms relative to others, with "the great outdoors" being part of the return path. The resulting infiltration can be more BTUs lost than the kwh used by the air handler.
A house that tests at 3ACH/50 (current code-max) is not a problem for running a wood stove, but it's tight enough to need an HRV. A typical wood stove need only on the order of 5-10cfm of combustion air, which is not a huge de-pressurization. It could be a problem if lighting a stone-cold stove with all of the bath & kitchen exhaust running, along with the clothes dryer, but that's pretty easy to fix.
For what it's worth, here is my rudimentary sketch.
Dana - so sounds like you are saying even if the warm air from the room with the stove bypassed the HRV and went directly to the furnace, we wouldn't notice much of a difference? Any other ideas why we're noticing this difference?
I'd do some well controlled tests. Try it with and without the HRV on and accurately measure temperatures.
A furnace connected HRV that is balanced with the furnace running will typically be unbalanced with the furnace off, causing increased in or ex-filtration.
Jon - clarifying question about your last comment...so, our furnace is never turned off. It's just that the thermostat is set to 66 so it won't kick on unless the temp drops below that. Given that , does your last comment still apply?
If your furnace fan is always running, then the HRV may or may not be balanced. There are various ways to check balance - garbage bags, temperature measurements, pressure measurements.
If you want to maximize the effectiveness of your furnace fan distributing stove heat, put the furnace return vent at the hottest spot you can find (probably somewhere above the stove).
Your CFM numbers translate to about 1.7 ACH50, assuming 8-foot ceilings, which is somewhat surprisingly tight for an old house, and low enough that you indeed should have mechanical ventilation.
Even though HRVs are very efficient at recovering heat and don't move a lot of air, the air they do provide is anywhere from "a few" to "several" degrees colder than the ambient air in the house, so the more the fan runs, the cooler the room will get, unless you leave doors open or have other means to distribute warm air.
I'm surprised that the HRV is having that much effect on the temperature distribution. It's true that it would be better have the exhaust air going into the HRV come from the far reaches of the house, away from the stove, rather than pulling out some of that nice hot air that you wanted to distribute. But the flow of the HRV is probably pretty small compared to the furnace fan. Unless you are running the furnace fan pretty low. Do you have any design number son the HRV flow rate the furnace flow rate?
It could be a different problem--for example, maybe the new ductwork isn't well sealed and the furnace is drawing cold basement air as much as warm living room air now.
You could measure temperatures of all the different air streams, with and without the HRV running, to get more idea of what's going on.
I think my ideal configuration for your situation would be to have since dedicated supply duct for the HRV, delivering the fresh air to the room with the wood stove. That mildly cool air gets warmed and then distributed to the rest of the house by the furnace fan. So you still get fresh air distributed everywhere. Then then dedicated exhaust ducts for the HRV run to the bathrooms and maybe the kitchen. That part is hard to install. But even if it was just whichever of those is easy to install, it might be better than you have.
But I'd want to have a better idea of exactly what the limitation of your current setup is before making changes.
Charlie - thank you. Looks like the HRV moves 66 CFM (I think...I attached a photo of the sticker on the unit which matches a table in the manual). I couldn't find any info on the flow rate of the furnace fan. Also, once the fire isn't going an de door and are closed it should be drawing only about 15-20 CFM for combustion (it's a fancy new one). Also, I'll measure temps in ducts around the house with and without the HRV running.
Your thought put the HRV supply near the stove would be relatively easy as the HRV unit is basically directly below the stove. A couple questions on that. 1) I assume you mean I would disconnect the HRV supply from the furnace...is that right? 2) Would we end up with a cool living room (where the stove is) when we're not burning? 3) Would it make sense to split the HRV supply between the furnace and living room especially since it appears the HRV supply CFM is much higher than the CFM draw from the stove? 4) Would it make sense to couple this with the living room return bypassing the HRV and going straight to the furnace so that hot air would have a chance to cycle through the house once?
The 66 CFM you see on the sticker is just the flow rate at which the unit was certified for HVI. It has a maximum of about 125 CFM. The installer should have left you with some documentation showing at what flow rate they commissioned it. The fact that it's only 60% efficient may make it more likely to be sending some heat outside before it can get circulated - depends on how close that cold air return is to the stove and what the overall flow of air is like. Just speculating, I have no experience with wood stoves and no degree in hvac.
That's great that in fact a duct from the HRV to near the stove is feasible. It might be tricky to split that between the furnace supply system and the living room, given that the pressure in the furnace supply system will be much higher than in the living room. With a lot of careful tweaking and measurement, you could get that to work. You might also be able to make the vent to the living room something you open and close according to whether the wood stove is running--when it is, you deliver the fresh air to the living room; when it's not, it goes through the ducts. But I wouldn't do that unless you or a good HVAC contractor is going do a careful commissioning job.
"3) Would it make sense to split the HRV supply between the furnace and living room especially since it appears the HRV supply CFM is much higher than the CFM draw from the stove?"
The purpose I had in mind for the supply to the living room is to essentially blow air across the stove to turn that slightly cool fresh air into warm air to have the furnace circulate through the house, not to supply combustion air for the stove. So it's good that it's a higher flow rate than the needed combustion air.
" 4) Would it make sense to couple this with the living room return bypassing the HRV and going straight to the furnace so that hot air would have a chance to cycle through the house once?"
Yes. I'd pull the return for the HRV from somewhere other than the living room, basically as far away as feasible, and preferably from multiple locations.
Fellow Minnesotans with a different question that applies to any house with both wood stove and HRV: we are late in the design stage of a tight, well-insulated house to be heated with air source heat pump, optional wood stove alternative because we like wood; it also would provide an alternative to electric resistance heat as backup for extreme cold, when the air source heat pump doesn't work.
(Design to include dedicated external air supply for the wood stove.)
BUT: do you ever encounter a situation where the wind blows your wood stove exhaust straight to your HRV intake and thus puts smoke in your house? I can imagine cracking a window and shutting off the HRV in that case. Have you needed to do that? If "yes," how did it work?
Thanks, Chuck & Anne