Best methods for ventilation with a wood stove
Tight construction and HRVs have been around for long enough to provide the empirical data needed to create a prescriptive ventilation standard for most house styles. That said, I have yet to find, or design, a system that I would consider ideal for use in a tight house with a wood stove. I built such a house 4 years ago and have an adequate system, but it’s far from ideal. I’m a not so rich engineering type and in my social circles, it’s common for a person to build a cost-effective house themselves and include a wood stove for a primary (or backup) heat source. For many of us, a rural home with a wood fire fills much more than a need for inexpensive heat. For me, it provides a sense of nostalgia and an unrivaled quieting of the mind. For my wife, it makes the cold winter tolerable, if not cheerful. To us, gas stoves are very nice, but not the same.
My current setup: I have dedicated bath fans (2 bathrooms), a ventless range hood, and a ducted HRV with a single exhaust point in the kitchen. I have a secondary merv 13 filter on the HRV inlet to help with smoke particles and will be moving the inlet to the windward side of the house this spring (based on the prevailing wind). My chimney construction is fairly optimal and my stove is rated at <2.5 g/hr.
Pitfalls:
– An east (non-prevailing) wind will bring some smoke back in the house until a new load of wood is burning hot enough. Recent studies indicate that wood smoke can have negative long term health affects and it would be nice to minimize the exposure.
– The bath fans will backdraft the wood stove if a window isn’t open (same with the dryer). Guests/children often forget to open a window when using a fan and fill the house with smoke. An open window is unpleasant when it’s 5 degrees F outside.
– Wall mounted bathroom exhaust dampers need to be lubricated with a silicon spray every year to prevent freezing shut.
– The kitchen HRV exhaust works fairly well, but my wife insisted on a gas range that would probably benefit from a vented hood and smoke detectors will go off if anything spills in the oven.
– Even the budget friendly HRV added significant installation expense and some maintenance without eliminating the need for any other systems except the range hood, which would have still been nice.
– Moisture from towels, the sink, and from the shower walls in the master bathroom continue to evaporate for hours after a shower causing more condensation to build up on the adjacent bedroom windows than in other parts of the house.
– HRV supply vents on the wall are ugly. Not the biggest problem, but still a consideration.
– Some say the west wall HRV inlet will be less than ideal due to vinyl siding off-gassing in the sun (not sure how much that will matter until I move it).
Alternative designs:
– Bathroom HRV exhausts eliminate the need to open a window, fix the frozen damper issue, lower installation cost, and can continue to remove moisture all day. That said, lower air flow rates can delay smell removal from the master bedroom which can “dampen the mood.” Boost mode and mechanical dampers can increase air flow to a specific exhaust point, but add cost, failure points, and complexity. Recirculating HRV defrost prevents depressurization for the wood stove, but will also recirculate bathroom smells. A preheater can fix the defrost issues, but adds installation and operation cost and complexity.
– A dual selectable HRV intake (one on the east wall and one on the west) could be used to minimize smoke re-entering the house based on wind direction. These motorized dampers would be hard to seal (condensation issue on the intake) and add cost and complexity. Direction would also be manually selected (ie. checking the weather and flipping a switch).
– OAKs for the wood stove are no longer recommended based on the potential fire hazard.
– A separate, heated, makeup air inlet would fix the open window issue, but adds installation cost, complexity, another hole in the house, and is expensive to operate.
What is the simplest, most cost effective compromise?
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Replies
Hi Sinaritt,
Woodstoves are one of the most difficult appliances to deal with in tight homes. I've personally struggled with them for years. Most people I'm able to talk into switching to a gas fireplace, but some insist on wood. My process is:
1. Set a goal for air tightness. If you are building new, this is an estimate that is confirmed with a blower door test after construction is complete. Existing structures just get tested. Be sure the test report includes the building leakage curve which will estimate the natural leakage rate at different pressures. I'm currently working on an article that discusses the building leakage curve.
2. Eliminate as many exhaust only appliances as possible. Using an ERV/HRV for bath fan replacements and moving to condensing dryers are two good options.
3. Limit the size of the vented range hoods.
4. Plan on installing a make-up air system when a home has a woodstove/fireplace and outside vented range hood. These systems need to be tuned to the exhaust rate of range hood and may require a heat option in cold climates. Fantech and Electro Industries both make powered makeup air systems.
5. Do not use any atmospherically vented appliances.
6. Choose a woodstove/fireplace with good sealing doors and one that used outside air for combustion.
7. Maintenance of all the systems is required. I've seen so many intakes for ERV's and HRV's that are plugged from debris, this causes a negative pressure inside the home unless the ERV or HRV are able to automatically maintain balance. These hoods should be cleaned a couple times a year.
8. Test and commission the systems. The home needs to maintain a neutral pressure with reference to the outside when woodstoves/fireplaces are present and there is an exhausting appliance.
No easy solutions and some expensive equipment are needed when a woodstove is included in a tight home. Hope this helped.
Randy
I was looking at this same issue while trying to put together a plan for a house I'd like to build. I posted the plans here: https://www.greenbuildingadvisor.com/question/roast-review-a-design But I’ve since revised them some based on some of the comments so I’ll post the updated ones to this post. My planned solution was to provide a dedicated outside air source near the stove. It wouldn’t be directly connected to the wood stove but would be nearby and could be opened in the winter and closed in the summer to allow air to flow freely in the winter. I also routed the outside air intake underground to temper it so instead of 5F air you’re getting 45F air or something like that. Anyhow, I don’t know if this is the best solution but it is possibly one solution. I’d provide a dedicated outside air source but I don’t think I’d want it directly connected to the stove as I see on some of the installations when I google OAKs (I can see why that might be a fire hazard). Routing yours underground may not be feasible as well.
krckadile,
I would check to make sure a proximity vent is allowed for y0ur stove. Our code has required the combustion air to be directly vented to the appliance for at least a decade. The reasoning behind that is it is less likely to back-vent into the house, and also represents less of an energy hit from introducing unconditioned air to the interior, rather than just to the firebox where it has little effect.
The dedicated combustion air intake is not a fire risk from back-venting ef embers if it is (as required) constructed from non-combustable ducting, and has the appropriate clearances.
What code/section of the code are you looking at? I know if differs by AHJ but I'm just looking at the 2021 IMC chapters 7 and 9 and they say is that combustion air must be provided in accordance with the manufacturer's instructions. I haven't been able to find anything requiring direct venting.
krackadile,
I am in Canada. By "our code" I mean the BCBC 2018. I brought it up not only to suggest you make sure proximity vents are legal where you are, but also to suggest they perhaps weren't a good idea.
Has the industry reached a consensus regarding directly connecting outdoor air to a wood stove intake? Aside from the fire hazard, I've also heard about possible condensation issues inside the box and that maybe the fire would end up running at lower than optimal temperatures for "complete" combustion due to the cold air. On the plus side, it seems like this could eliminate back drafting issues with bath fans and clothes dryers as long as the door was closed. Not sure what would happen if you opened the door with the house depressurized.
Randy - I would be interested in reading your article on the building leakage curve. I would think that this information would allow you to see what kind of flow differential you can have before depressuring a house to the point of back drafting certain types of appliances. If you think of it, maybe post a link when you’re done.
Sinaritt,
To be fair, I'm not sure there is consensus. https://www.woodheat.org/the-outdoor-air-myth-exposed.html
I would say though that part of the case against direct combustion air comes from conflating combustion and make-up air. Providing combustion air, whether directly or with a proximity source, doesn't negate the need for make-up air if there are other appliances de-pressurizing the house.
Air ducts connected directly to the wood stove should have a damper or valve on them so they can be closed when the stove isn't in use. That both reduces the risk of condensation, and improves energy losses.
Back-drafting of embers would be a risk whether those embers are expelled into a direct vent duct, or into the house interior from the air-inlet on the stove. It's not something I've ever heard of occurring , and may just be a theoretical problem?
I am very familiar with this subject. I have a tight house(4" exterior insulation, remote wall configuration), in Anchorage Alaska where it certainly can be cold, and primarily heat my upstairs with a Scan wood stove all winter. 1500 square foot house, HRV installed with 5 supplies and 3 exhausts. 2 bath fans that exhaust to the outside. The wood stove was set up with a 4" proximity air supply that terminated at the back of the stove. I only had backdraft issues when running a fan while trying to start a fire or when the fire was not very hot. During my latest phase of remodeling I added a range hood that exhausts outside (sub 400 cfm)and knew I might have more problems, so I bumped up the fresh air duct to 6" (stainless steel, has an adjustable damper). Has it cured all problems? I can't say for sure as I have not hooked up my range yet, but I feel better having the ability to have a bit more air available. BTW, having fresh air terminate at/near the wood stove is awesome - when running a bath fan I can feel the cold air coming in but only if I am within a foot of the stove. My version of pre-heating cold fresh air.