Wood Stove in a Tight House
I’m building a new house and have included a wood stove in the plans. My first thought was to provide an outdoor air supply directly to the stove, but have been reading conflicting opinions. My blower door test came in around 0.8 ACH50. I plan to install a central fan system from AirCycler to work with my forced air furnace and use the bath fans for the exhaust. The kitchen fan is low power and the only other competing exhaust is the electric clothes dryer. The wood stove chimney is insulated, internal, and a straight shot to the highest point on the roof.
Is there a particular brand or style of wood stove that will work better under these conditions?
Has anyone had success or issues with the outside air kit?
Any other suggestions?
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Replies
Hi SINARIT.
I'll let others comment on the particular wood stove as I don't have experience with them. In a tight house, though, makeup air should generally be dedicated and approached separately from ventilation. You may find these articles helpful: All About Makeup Air and Makeup Air for Kitchen Exhaust.
The combustion air requirements of a wood stove are pretty small- 10-15 cfm even at full fire for most of them. But backdrafting (particularly at low fire) when exhaust fans or clothes dryers can be an issue, especially at low fire.
With many outdoor combustion air kits for woodstoves the combustion air is just a proximity vent, not sealed to the firebox, in which case it won't improve the backdrafting situtation. No wood stoves are fully air tight- those door gaskets will leak a bit even under even fairly modest pressures, but they're still a lot tighter than an open air intake with a proximity vent providing the combustion air. The details of the combustion air kits vary from model to model, year to year- consult with the manufacturers (this isn't a market that I stay on top of.)
This topic has been covered here before, both in blogs and on the forum:
https://www.greenbuildingadvisor.com/article/how-to-provide-makeup-air-for-a-wood-stove
https://www.greenbuildingadvisor.com/article/providing-outdoor-combustion-air-for-a-wood-stove
https://www.greenbuildingadvisor.com/question/need-advice-on-air-intake-changes-for-backdrafting-wood-stove
I appreciate the quick feedback. I have read those posts and will reread them in case I've missed anything. It seems the recommendation would be to not have a wood stove in a tight house. If proceeding with the installation, there doesn't seem to be a clear recommendation other than to buy a few extra CO detectors. An indirect air inlet isn't very effective. A direct inlet poses a fire hazard. No makeup air can produce back drafting when the fire dies down overnight. I suppose trial and error will work. I would still be very interested to hear what people have done to succeed.
I would read through the blog posts from David:
https://www.greenbuildingadvisor.com/article/flatrock-passive-winter-update
To keep the wood stove from back-drafting, you are limited to around 5PA pressure drop. In a tight house, turning on a bathroom fan is enough to depressurise the house that much. A dryer or a range hood is definitely out of the question.
I think the only way to get it to go is with a very large make up air inlet that you open when you fire up the stove or open a window. With any makeup air inlet, you have to check that the pressure drop across it at the CFM of all the exhaust only appliances running together is bellow 5PA.
Our 1 yr old home has a small Morso 6143 stove with outside air kit. The air kit consists of a 4" duct plumbed directly into the stove and run through a nearby exterior wall. The air kit does not preclude us from needing to be aware of any exhaust fans (bathrom, dryer, range hood) that may be running while we're using the stove. After experiencing several instances where smoke was pulled into our home I began cracking a window near the stove whenever we use it.
My small, not that tight home has a wood stove I use all winter long. While starting a fire I cannot have any exhaust appliances going. I know pretty quick when I have forgotten due to the smoke.
I have also noticed if I have left a window open upstairs the stove will backdraft. My house is probably leaky enough that the draft created with an open upstairs window is greater than that of the chimney.
In my new build I am going to use a Blaze King. Users are reporting 20-30 hour burn times on low. Could easily be your sole heat source at that rate.
I crack a window when we start our stove.
I don't like the direct air supply stuff because in the unusual situation where someone doesn't know any better, I'd rather them smoke the house out than turn the direct vent into a chimney.
I'd also just say wait to see if you have a problem before you solve it.
Some manufacturers have starters built in, so you do not need to open the door to start the fire which may help with backdrafting when lighting the fire. I am considering this on the Apex 42 "fireplace" (really more of an in wall stove).
As Akos mentioned I have had a fair share of dealings with my wood stove. Our blower door test came in around 0.3 ACH50. The house is pretty darn tight. The woodstove operation was heavily influenced by the HRV. In fact, in a balanced state (air flow) the wood stove caused problems even with the outside air make up directly plumbed to the wood stove. I scratched my head about it for a long time until I was talking with an engineer that made it pretty clear that balancing air flows doesn't necessarily give you neutral pressures. The fact is, for two independently balanced fans, it is impossible for the return registers to be pulling air only from the supply registers. Its not like air short circuits and somehow communicates with the returns to say only pull air from here. It pulls air from the inside of the structure. If there are holes in the structure, air will be pulled in through them also, even the stove, regardless of the make up air attached to the stove. In fact, opening a make up air intake for the stove just creates another hole in the envelope providing another way for air to get in even if that is, lets say, through the stove and in through the stove pipe joints. Problems in an air tight house become amplified the tighter you go. My fix was to install a motorized adjustable damper on the supply inside the house. I purposely unbalanced the unit with the damper open using a manometer to measure the pressure difference between the inside of the house and outside (on a day with little to no wind). At this point I couldn't feel any air leaking in around the stove pipe joints or around the stove body (use a smoking match stick to detect the air flow, and the only air flow was going into the stove body or stove pipe joints). I knew at this point the balancing had gone beyond neutral pressure and slightly positive. At this point, I adjusted the damper to give me my balanced airflow. The solution worked like a charm. I push a switch on the wall before I start the fire, the unit pressurizes the house, and theres no smoke problem anymore. 3-4 hours after the fire is out, I push the switch and the damper closes to balance the air flows again. An extra mechanical part but the set up makes it much safer.
DG
Great news, thanks for the idea!
This is really helpful, thanks for all the detail !
I've noticed increases smoke spillage as I've tightened up my house, especially if the bathroom fan, or stove range hood is running. But zero issues with the door shut. The firebox is well sealed, it's welded steel with a substantial continuous gasket around the door. The only place air has a change to get in is the air inlet. I don't have an outside air kit but the Blaze King version is a direct attachment to a pretty well sealed air inlet path.
I have a Blaze King Ashford 30, I choose it because it turns down lower than many stoves. You can have a totally dark firebox with a active cat and lots of heat. On a full reload on low 30hrs is easily attainable. It has has a adjustable mechanical thermostat (bimetalic strip) to regulate the air based on firebox temperatures, this keeps the heat nice and even throughout the burn. If I had my father's Pacific Energy (AlderleaT5) stove I'd be heated out of my smallish house. It's a great stove but doesn't seem to turn down much.
Make sure you have a straight, insulated, and preferably interior run, chimney.
It sounds like pressurizing the house is a viable option. My ventilation strategy is not continuous, but I could use the outside air port that was intended for the stove, to run a small duct fan into the basement. It's a conditioned basement, but only used for utility and storage, so cold air won't be uncomfortable.
Another option may be to utilize the wood stove blower to pull air from the intake and pressure the house. The cold air would be heated by the wood stove on its way into the room. It might get a little cold when the fire goes out.
.43 ACH50 here and a RSF 320 zero clearance insert with a 4" ducted air supply. It's a definite learning experience dealing with these devices. First off, no stove hood exhaust when the fire is burning! Even though it's only 625cfm, it sucked smoke from the entire perimeter of the stove door. I have to open a window whenever I open the stove door. We also leave a window cracked above the dryer whenever it is running and a fire is going. The negative pressure would draw smoky air in from outside through the exhaust fans on occasion if we didn't open the window above the dryer. The bonus side effect with a tight well-insulated house and a woodburner is that it gets really hot inside. You get used to opening windows to cool off, despite having the HVAC fan running to mix air, and ceiling fans running, etc. According to my Manual J, my stove is way oversized for my entire house. However, there is nothing like the heat from burning wood so it is worth the extra effort.
Here is another data point for you, summarizing info posted on a couple of other GBA threads over the last decade.
Tight house (0.6 ACH50 first test, later 0.8 with range hood and woodstove connected but not operating. House is two levels on a 2,000 sqft footprint; lower level is walkout. Fresh air is by HRV. Bathrooms are vented via the HRV, not directly to the outside.
Small Quadrafire 2100 Millenium woodstove (tag said 11-28KBTU output), installed in the lower level, with air ducted directly to the stove from duct source at or slightly below level of the stove. Another stove was considered but rejected, since it did not allow direct connection of the air duct. The chimney is a straight shot up through a chase in the upper level and through the roof. The stove installed heated the whole house during the winter when the interior was being finished, burning part time.
The stove cannot be lit when the clothes dryer or range hood is in operation, or downdrafting occurs. I can tell by the feel of cold air coming out at the top of the door opening if either exhausting device is operating. Once the fire is started and the chimney filled with hot flue gas, the dryer and range hood can be used. I've never checked for downdrafting with the range hood on highest.
Just a quick follow up for future readers. I ended up splitting my wood stove budget with an HRV. I bought a fairly economical Drolet Columbia II wood stove, and a similarly economical LifeBreath RNC 95. The HRV works great with the wood stove. I don't have to adjust the HRV balance, I can leave it on or off, and the HRV recirculating defrost works fine. No issues getting a draft when starting a fire. If the wood stove is on and we want to run the bath fan or dryer, then a window needs to be opened a couple of inches. Failing to do so is immediately noticeable. Otherwise, it works fine.
The HRV made a really annoying high pitch noise when I first installed it. It was caused by the supply fan blade passing by the housing, and I was able to fix it by reducing the clearance between the fan blade and the housing at the outlet with a piece of adhesive backed semi-rigid foam.
A couple of lessons learned:
1. My family noticed the floor mounted fresh air supply register chilling the surrounding floor until I put an open ended box over it to raise the height by about 4 ft and improve air mixing. A vent located at the top of the wall would be unnoticeable in terms of comfort for my setup.
2. A recirculating defrost is a necessary HRV feature when using it with a wood stove.
3. The air smells much better with the HRV than it did when we were using the bath fan for ventilation. No more kitchen, basement, sheetrock, or paint smells. It's worth it for that alone.
4. Noise is an easily overlooked aspect of HRV performance. Even a fairly quiet squealing noise is annoying if it's on all day.
Thanks for the advise and feedback from everyone.
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SINARIT:
When you installed the stove, did you use any sort of special product where the flue penetrates the conditioned space ceiling/wall to limit air leakage there?
Also, is the takeaway from this thread that as long as you learn to manage the makeup air in your house (cracking a window), that any stove or fireplace should be manageable even in a tight house?
I have been searching for sealed stoves, and it leads me mainly to European manufacturers that don't seem available here in North America, or incredibly spendy Stuv products.
kenmoremmm,
We have the same problem others have described in that we have to crack a window to use the wood stove if either the range hood or clothes dryer is on. When you go to do that you can feel the house is depressurized, and cold air immediately flows in.
It's very manageable, but depending on where the window is can be uncomfortable. Dedicated sources of supply air near the appliances causing depressurization might alleviate the problem with the wood stove. Dedicated supply air to the wood stove can't always be counted on to do that.
I've got a blaze king chinook 20 on the first floor of my pretty-good-house (chimney is straight up through interior of house, no air kit). It heats the house well. Since getting the basement spray-foamed and improving air-sealing this year I have been concerned with the affect on the wood stove operation. No issues so far, but I appreciate all of this info and will read the articles. One thing I thought about doing at a later date is to replace my dryer with a heat pump dryer that does not need to be ducted at all. I'm also planning on removing my oil boiler which i think also sucks air (i think). I don't use my hot water rads anymore as they are just back-up, but the oil boiler does cycle to keep the flame/burner hot.
I have a near passive house home with 0.6 air tightness and have been researching the possibility of a wood stove. I'm not in favor of a ducted air intake. I don't like how it adds another hole in the house that creates a pathway for hot and cold air and humidity to go in all throughout the year. As of right now I don't have any penetrations in the house other than the 24/7 low cfm fully ducted hrv dedicated ventilation system. To start fires I'm in favor of cracking a window if needed. If I feel like I need to have continues positive pressure, I'm planning to attempt to put my hrv off balance for the winters. If that doesn't work well for me, I would consider putting in a through the wall Lunos Hrv instead of a duct going to the stove that may or may not work, but will be cold and leaky for sure. With a through the wall hrv tube, I feel like I'd have more control over envelope leaks and how cold the air would be entering the space and at what cfm?
User ...101,
Isn't a Lunos really just a spot HRV with balanced ventilation? It won't really help supply air.
The dedicated supply air duct can include a damper so that it is no air movement when the wood stove is not in use.
I have all the passive house boxes ticked minus a blower door test. I have a Kozy Heat wood fireplace with OAK. The OAK penetration is under the firebox and the vent pipe chase is a straight shut up to the roof and it is insulated so the only thermal bridging is the face of the fireplace itself. Hood vent is recirc, dryer is HP, house is all electric. No issues with burning, the OAK provides all the combustion air needed. My previous drafty house had a Quadrafire 7100 with OAK and depending on the weather conditions, I would need to crack a window, close all of the upstair's doors and make sure all the vented gas appliances were not running when starting the fire. Once the fire got hot, there were no issues.
The wood stove in our house would be going right in the middle of the house and in front of a big board formed concrete wall in slab on grade. So the other reason I'm against OAK in our situation is because it would be a longer run of about 12' across the room and there is no good way to hide it. I understand these pipes come insulated, but not very much and they still have joints, but I guess there is no perfect solution for a FP in a PH.
The lunos in wall HRVs I believe work best in pairs and as far as far as I know even when it's a single unit it cannot supply and exhaust air simultaneously. It comes with a remote control and I would think you would be able to simply put it into supply mode at desired speed while using a stove.