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Community and Q&A

Wood stove in a tight house

SINARIT | Posted in General Questions on

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

  1. User avatar GBA Editor
    Brian Pontolilo | | #1

    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.

  2. User avatar
    Dana Dorsett | | #2

    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

  3. SINARIT | | #3

    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.

  4. Akos | | #4

    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.

  5. stickerbush | | #5

    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.

  6. Joe Norm | | #6

    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.

  7. Nathan Scaglione | | #7

    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.

  8. CarsonB | | #8

    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).

  9. User avatar
    David Goodyear | | #9

    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

  10. Josh Durston | | #10

    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.

  11. SINARIT | | #11

    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.

  12. Kevin Spellman | | #12

    .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.

  13. Dick Russell | | #13

    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.

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