Providing Outdoor Combustion Air for a Wood Stove
We brought ducted outdoor combustion air to our wood stove by punching a hole through the back of the unused fireplace
In November 2012, I started on a deep energy retrofit of my 1976 raised ranch in northwestern Vermont, in the shadow of Mount Mansfield. As a Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. consultant, I wanted to make my leaky (8.25 ach50) house with fiberglass-filled 2x4 walls and a tuck-under garage much more energy-efficient.
My hopes to make the renovation hit the EnerPHit standard (a standard for Passivhaus retrofits) were dashed because of some of the basic realities of my house. The orientation was all wrong, and I had already replaced my home's old single-pane windows with what I was told by my contractor were the best windows on the U.S. market — namely Andersen super-low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. double-hung sash windows, not the Passivhaus windows I would have chosen had I known better at the time. Ugh!
The inability of the house to hit the EnerPHit numbers didn't deter me from superinsulating the house and getting as close to Passivhaus goals as I could. In the end I decided to remove the existing vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding and wrap the whole house in 6 inches of repurposed (used) rigid polyisocyanurate insulation above grade. Below grade, I installed 6 inches of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. on the outside and 2 inches of EPS on the inside.
There were lots of little improvements I made in addition to these changes; I've written about how I managed these improvements in a blog I have been keeping on the retrofit. Today I'd like to share details about the decisions I made concerning my wood stove.
A chimney is a thermal bridge
Wood is a very common heating fuel in Vermont. More than 50% of Vermonters use wood as part of their heating fuel mix. Wood is plentiful and cheap. It’s even free if you have land to get it from, and most of us in my neck of the woods do. So we will be keeping the chimney and the wood stove.
When it comes to thermal efficiency, chimneys have lots of problems. They are huge thermal bridges (especially if they are on the outside of the house like mine is); they are hard to air seal; and most wood stoves are atmospherically vented. This means that the air that the wood stove uses for combustion comes from the room it is sitting in.
To burn or not to burn wood?
Wood stoves are great. OK, maybe they aren't great. The efficiency sucks (usually less than 75% for older models), but the fuel is cheap, readily available, and renewable. I'm not saying that burning wood doesn't produce CO2 and CO and NOx and SO2, but it is a way to heat your house that is very popular here in Vermont.
The cheap part is also part of fuel independence. I can go out into the woods around my house and cut down enough wood to keep my house warm for a whole season. Just saw, split, stack, and use.
For a Passivhaus, a wood stove is in most instances a deal-breaker. For starters, a wood stove usually has a rated output that is way more than any Passivhaus can use. A 55,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h stove in a house with a 7,000 Btu/h load is just overkill.
Secondly, most wood stove installations introduce air-sealing problems that make it hard to get a home's airtightness down to 0.6 ach50.
My old Pendelton Avalon wood stove dates back to the 1990s. I bought it from the fellow I bought the house from. It is a good little stove with lots of years of use still in it. My wife and kids love the fire and it gives off 55,000 Btu/h when it is running hot. It is also an atmospheric combustion stove.
An atmospherically vented stove (one which gets its combustion air from the room where the stove is located) is fine if your house is leaky, but becomes a problem when your house is tight. If your house is tight, you need to bring in outdoor air for the combustion process.
I thought that my wood stove had the ability to be outfitted with a duct to bring outside air into the house, but upon further investigation I found this wasn't so. I ended up buying a new Hearthstone wood stove with a larger firebox (and with a ten-hour burn time instead of six hours) and the ability to add the sealed combustionCombustion system for space heating or water heating in which outside combustion air is fed directly into the combustion chamber and flue gasses are exhausted directly outside. fresh air duct I needed.
If you are going to do this type of retrofit on your wood stove and chimney, make sure that your wood stove can be outfitted with the outdoor air supply first!
A tight home’s wood stove needs to breathe from somewhere
We then need to figure out where to get this fresh air from. A friend of mine suggested that there are plenty of people who run the fresh air duct up the chimney and pull the air supply down from the top of the chimney. I could use this approach, because my chimney has two flues, one of which I’m not using for heating. I could just break through to the other flue and pull air from there.
The only problem with this scenario is the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.. Hot air rises. The warming of the chimney shaft next to the fresh air channel could cause an updraft that could make the “draw” of the fresh air difficult, which could decrease the efficiency of the wood fire.
My next idea was to just cut a hole in the back of the chimney and run a pipe through the chimney directly out to the outside. (I planned to install the wood stove as a fireplace insert.) This approach would give us a short horizontal duct with access to fresh air. A short run decreases the resistance caused by the inside surface of the pipe and the horizontal run would eliminate possible problems from the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season..
As I said before, one problem with outside chimneys are that they are horrible thermal bridges. To help cut down on these losses, I had 6 inches of rigid rock wool board put on the outside of the chimney. To learn exactly how I did this, you’ll need to check out my blog. The rock wool gives me about an R-24 and means that I had an additional 6 inches plus to get through after I was through the bricks of the chimney.
To ensure that all of the work would be covered by my homeowner's insurance, I hired a professional chimney company named Brickliners to do the installation.
They came and made a hole in the back of the chimney, and drilled a small hole through the Roxul. I widened this hole to allow for the 3-inch stainless-steel duct to come through. Getting the duct to come through the hole was no problem once the hole was big enough.
Air sealing details
Chimneys are difficult to air seal. If you are installing the type of wood stove that is designed to fit in an existing fireplace, you have a big hole just above the insert area up into the flue. At my house there was now also a hole at the back of the chimney to bring in fresh air. Both of these holes needed to be air sealed.
To air seal the bottom of the fireplace flue I made a baffle, which is just a piece of sheet metal cut to the shape of the chimney flue just above where the wood stove sits. This needed to be screwed into place and caulked. Again, I left this job to the professionals. I could have done the work, but I would rather that their liability insurance cover the work, not mine.
The fresh-air duct going out the back of the chimney was a bit different. I first needed to fill the space between the duct and the walls of the hole. I filled this with leftover Roxul insulation that I had lying around.
The black membrane you see in many of the accompanying photos is Mento 1000. This is a special European membrane that has a variable vapor-permeance rating. When the relative humidity is low, it has a perm rating of 4. When the relative humidity is high, which might happen if moisture gets into the wall, it has a perm rating of 34.
This variable permeance allows any moisture in the wall to migrate out of the wall while still preventing vapor and bulk water from entering the wall. Compared to Tyvek or Typar, Mento 1000 is expensive. But unlike Tyvek or Typar, it is a "smart" vapor retarder.
Since I had worked so hard to get the airtightness of the house down from 8.25 ach50 (that was the air leakage rate shown by the blower door test that was performed before the renovation began) to 2.0 ach50, I needed to make sure that any penetration through the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. was properly sealed.
Fortunately, there are some great products available for this purpose. I used Tescon tape, which has an acrylic-based adhesive. This acrylic actually penetrates the top molecules of the substrate it is being applied to and cures there to become one with the substrate material. This means that the adhesive doesn’t dry out like other tapes. The bond actually becomes stronger over time. These tapes have a 60-year guarantee!
I used this tape in conjunction with a silicone gasket which I got from 475 High Performance Building Supply in Brooklyn, New York. They also supplied the membrane and the tapes I used for this project. The photos below show how I sealed the gaps around the outdoor air duct.
Once the duct was properly sealed, I needed to add a piece of 3/4-inch plywood to make the plane of the PVC plate flush with the plane of the siding. The plywood has the same thickness as the strapping.
We hit our airtightness target
After the siding was installed and the new stove was in place, we performed a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. and found that we had hit the 2.0 ach50 target. We also found that the chimney workers had failed to seal two important seams in the chimney baffle plate. We invited them to come back and they fixed the problem. I still have to do another blower-door test to see what kind of change that made.
I’ve used the stove a few times — more to try it out than because I needed to for heating the house. It worked fine. It will be putting out way too many Btu/h for the house. I'll be using less energy over all.
For more information about my retrofit please visit my blog.
Chris West has a background in mechanical engineering and lived in the Netherlands for ten years during the 2000s. There he learned about advances in building science; he brought this knowledge back with him when he moved back to the U.S. in 2010. He is a Certified Passive House Consultant and is currently president of the Passive House Alliance of Vermont, husband and father of two wonderful children who love living in the wilds of Vermont.
- Chris West
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