Wood stove backdraft
I’m working with a contractor and home owner on a woodstove backdrafting issue in a new home. The homeowner has installed two Blaze King Chinook 20 free standing wood stoves. There is currently no make-up or combustion air ducting installed. I’m not a fan of telling the home owner to crack a window when operating the stoves. The home does have an HRV unit installed. The HVAC contractor changed the balance to slightly positive thinking this may help the backdraft. I’m going to suggest changing back to balanced. The first unit is located in the basement, 2-90 degree elbows and 24 feet of class A chimney installed on the exterior of the home. The second is installed on the main level, 14 feet of class A chimney with no elbows installed through the attic. Both stoves backdrafted when the bath fan was turned on. The home owner indicated that both stoves seem to operate correctly, no back drafting, not hard to start a fire, when the bath fan is off. I conducted a blower door test of the home which tested at 1.78 ACH @ 50 PA, 514 CFM. .11 ACH natural. The bath fan tested at 75 CFM. The home has a Steffes ETS electric storage forced air furnace located in a small utility room in the basement. This furnace is large, filled with ceramic bricks that are heated at night. Usually, this furnace is on an off-peak electric system with a reduced electricity rate, the electricity rate in my area is $.0495/KW, a good option in rural areas. The heat radiating off the furnace will raise the temperature in the mechanical room to close to 90 degrees if the door remains closed. I’m planning to suggest a fresh air supply near the ceiling of this room. With the door closed, a jump duct or vent cut in the door may be needed. I’m hoping this location at the ceiling will be near the neutral pressure plain in the stack effect. Is my thinking on this location for the vent correct? What size vent should be used and has anyone found an effective damper to use with fresh air vents? The location of the home is zone 7, northern Minnesota.
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Wood stoves in tight houses are tricky. The issue is pretty simple, and you clearly understand what's going on.
If the owner isn't willing to crack a window, this house needs an outdoor air duct to supply combustion air to the woodstoves. The usual place to terminate such a duct is near the wood stove, but in theory, you could terminate the duct in the basement mechanical room, as long as there is a clear air path between the mechanical room and the rooms where the wood stoves are located.
The advantage of terminating the outdoor air duct near the wood stoves is that you can include a damper of some kind on the grille or register -- so that the flow of outdoor air into the house can be reduced when the woodstove isn't on.
One other point: If the only exhaust fan causing problems is the bathroom exhaust fan, you could simply pull exhaust air from that bathroom into your HRV, and disable the bath exhaust fan.
Randy, this sounds like a very cool home. I have never heard of that Steffes thermal storage furnace before. I would like to see one of those up close.
Thermal storage techniques may be useful for your problem, more on that at the end of my comments.
Sharing my partially analogous situation:
I bring fresh air into just below the ceiling level of my ground floor mechanical room. This happens through a small gravity damper, can be seen in this picture at the far end of the room just above the HRV. I think this works for me because the outside wall location of this fresh air intake is relatively well protected from prevailing winds. (Prior to that gravity damper installation I had some backdrafting too.)
This damper is normally closed unless the clothes dryer or range hood run. (I do not have bath fans, my bathrooms are connected to the HRV.) The damper opens too if the HRV goes into defrost; I have posted recently about that elsewhere on GBA. It will also open slightly at full fire of our 1st floor masonry heater. However there is a difference between how my masonry heater burns wood and how many typical woodstoves operate.
The masonry heater is burned full tilt, maximum fire, lots of draft, and then the input and output dampers are shut off towards the end of combustion. From that point on the masonry mass slowly radiates heat, releasing it over as long as 24 hours. It's thermal storage.
More typical woodstoves are throttled back to a slow constant burn to control the heat level, and I imagine therein lies part of the problem in the home you are working in. In a tight house, I suspect that constant small amount of woodstove flue draft as the fire slowly burns/smolders isn't enough to overcome the exhausting action of a bath fan.
BTW, I took the picture from right underneath the louver where I pull combustion air for the masonry heater. As you can see it's in the ceiling area of that same mechanical room. So the combustion air is coming from about the same pressure plane as the fresh air input.
Lastly, and I think you probably realize this, the woodstove on the basement level of the home you are working on is a tough nut. It's at a low pressure point, feeding a cold outside flue, that is very tall. I don't know what the solution for that is with a traditional woodstove. I heated with wood 24/7 with a traditional woodstove in a home that had a similar outside flue. It was a constant battle of cleaning creosote. And if I didn't keep ahead of it, well let's just say the roar of a chimney fire is pretty unmistakeable!
Changing that basement woodstove to something like a stand alone Tulikivi masonry heater could be the answer. This way your homeowner could have strong draft at the times that the masonry heater is in operation, and no backdrafting other times. Plus there would be less worry about creosoting up that outside flue.
Basement wood stoves are even trickier, since the firebox is lower in elevation than where the combustion air has to enter the house, so there's a natural bit of stack pressure working against you during a cold start. Ducting the combustion air to the firebox could even constitute a code violation for that reason, since the combustion air duct behaves as a parasitic flue.
Thanks for the thoughts and advice. Martin-I'll remember the idea of not having a bath fan and using the HRV unit in bathrooms to eliminate the need for powered ventilation in some applications. The home owner is having an issue with the bath fan. He's using a central fan mounted in the basement joists, and the register is high on the wall in the main level bathroom. (only a basement with walk out and main level in this home.) Nice because you can't hear the fan, not so nice because the back drafting issue must be pulling some outside air through the bathroom ducts, forming frost right at the fan motor. If he uses the fan, or temperature warm, it starts dripping. The first 5 feet of ducting from the outside is insulated. I'm confident the make-up air will solve that problem. Unfortunately, the drywall is complete and I don't think he's open to a remodel to change the HRV ducting.
Andrew-You've done exactly what I'm looking to suggest. From the photo, it looks like you used a 4 inch duct. I was hoping a gravity damper would work, but have only seen them on fuel oil exhaust venting. I also wasn't sure what size to use, was thinking about a 6 inch and then necking down to a 4 inch. That way he would have the option for more air if needed. His duct location would be under a south facing deck that is a couple feet above grade. Should be well protected. I'm assuming you have a hooded outside duct termination that has a bug screen. The homeowner said it wasn't too tough to start the basement wood stove. A little extra paper goods is how he put it. I'll be sure to mention frequent chimney sweeping. http://www.steffes.com for more info on the electric storage furnaces. Neat unit and company. Family owned business in North Dakota.
Dana, good point, some thing else I need to remember. My suggestion is going to be for the make-up air to be located between the first and second floor in the mechanical room. The basement wood combustion air would end up with a north or west exposure. I'm concerned about the effect of the wind at this location.
Randy, you have a good eye. Yes, that is a 4" gravity damper, turned around backwards.
Similar to your thought of necking down a larger damper, I actually bought a better grade 6" balancing damper, cut off the balancing weight, and tried it too. Thought it would be more "free acting" with its better "bearings" or "gimbals". That was not the case. Seemed like the larger surface area caused too much inertia, drag, or something like that anyhow.
You are correct also that the damper is fed through a louver with screen, although it's not really bug tight screen. Attached a picture showing it and the HRV louvers in their protected below grade "well".
Left to right it's dryer vent, HRV exhaust, make up air intake, and HRV intake.
It seems to me that your below-grade well could be filled up with snow. All it would take is one good snow storm, and it could happen in a single night, as you slept.
Good point Martin, a possible snow storm is part of the reason we made the well as deep as it is.
You can use the three levels of TimberSIL 2xs to gauge the depth; the well is almost 3' deep. In fact that triangular center reinforcement section was purpose built in that shape, so a trapped possum had a fighting chance of escaping.
In nearly four years my biggest problem has been scooping an inch or two of leaves out in the fall and spring. Snow has never been an issue, so far. This well is on the leeward side of the house, I live on a windy hill, and that area is slightly protected by the overhanging eave above. Those were important "details" that I didn't mention!