Even a very large 150-200KBTU/hr wood stove is drawing at most 25cfm for combustion air even at full throttle, which even at big delta-Ts is not very much heat. The specific heat of dry air at standard temperatur & pressure, by volume is only about 0.018 BTU per cubic foot per degree-F.

At 25 cfm at 0F outdoor air temps the "cooling rate" delivered by bringing in air colder than a 70F room temps is only about 70F x 0.018 x 60 minutes/hr = ~75 BTU/hr, which isn't exactly going to warp anything. Even if the incoming air is -50F, it's no big deal.

By design the air-intake manifolds on EPA rated woodstoves pre-heat the combustion air to be able to light off the secondary burners- it's not just a 2" hole in an iron box.

The notion that even 150 BTU/hr less heat is going to affect the emissions characteristics of wood stove cranking away at 150,000 BTU/hr is kinda out there, when you look at the napkin-math order of magnitude. The relative cooling factor is less than 0.05% of the heat output of the stove.

BTW: Where is the code regarding gas furnaces requiring pre-heating of the combustion air? (This was news to me, and I couldn't find it in a quick web search.)

James02: Houses are never really done, are they? I won't strip the shingles solely for the purpose of putting an air tight membrane & rigid foam over the plank roof decking, but you can bet I will when I re-roof in a few years. I'm not going to gut the upstairs just to make the walls air tight, but you can bet when I do the make-over to make it my kid's bedroom it'll be tighter and better insulated when I'm done. Nearly every project on an older home has opportunities for improving the thermal performance, opportunities that aren't cost effective for doing the project on energy use grounds alone, but well worth the trouble when getting into it. My house is currently 91 years old and in pretty good shape, but it's still a work in progress (just like most houses).

BTW: Where is the code regarding gas furnaces requiring pre-heating of the combustion air? (This was news to me, and I couldn't find it in a quick web search.)


Alberta Building Code 2006
9.32.3.4.5) An outdoor air supply duct shall be installed between the outdoors and the furnace return air plenum and shall be connected
a) not less than 3 m upstream of the plenum connection to the furnace, as measured along the length of the duct, or
b) through an acceptable mixing device installed in the return air plenum.

Any chance you might be a little light on the cfm given a 6" chimney, two storey stack and wind induced positive and negative pressures on the intake and stack?

Would the stack/wind driven pressures not be the same wither the stove is at full bore or just smothering?

If you look at the exploded view of the Jotul, do you see anything to prevent the combustion air from bypassing the grate if it is covered with ash/coals and rising right to the top? Yes, it has side baffles etc. but it doesn't look like they are directing the incoming air through the fire. Maybe brushing by the side baffles will rise the temperature enough but that really depends on what the actual volume and velocity of the air is.

Most outside air kits I have worked with in the past are between 3 and 5".

[quote]Even a very large 150-200KBTU/hr wood stove is drawing at most 25cfm for combustion air even at full throttle,[/quote]
I'd be very surprised if it is that little. Even at moderate burn, my 75KBTU stove is drawing quite a bit more air through the outside supply duct than my 80cfm bathroom fan moves. Quite a bit more. The 25 cfm number is more akin to the stoichimetric amount of air a stove of that size needs to support the chemical combustion. And, there has to be several times that amount present for efficiency. The way the stove gets it is by raising the temperature of all of it to 800K or more and exhausting it, thereby drawing more in at the bottom.

Any chance you might be a little light on the cfm given a 6" chimney, two storey stack

Exactly. Estimate the stack effect you get with a 8" stove stack and a temperature difference of say, 400F.

Posted By FBBP on 12 Nov 2014 07:16 PM
BTW: Where is the code regarding gas furnaces requiring pre-heating of the combustion air? (This was news to me, and I couldn't find it in a quick web search.)


Alberta Building Code 2006
9.32.3.4.5) An outdoor air supply duct shall be installed between the outdoors and the furnace return air plenum and shall be connected
a) not less than 3 m upstream of the plenum connection to the furnace, as measured along the length of the duct, or
b) through an acceptable mixing device installed in the return air plenum.

Any chance you might be a little light on the cfm given a 6" chimney, two storey stack and wind induced positive and negative pressures on the intake and stack?

Would the stack/wind driven pressures not be the same wither the stove is at full bore or just smothering?

If you look at the exploded view of the Jotul, do you see anything to prevent the combustion air from bypassing the grate if it is covered with ash/coals and rising right to the top? Yes, it has side baffles etc. but it doesn't look like they are directing the incoming air through the fire. Maybe brushing by the side baffles will rise the temperature enough but that really depends on what the actual volume and velocity of the air is.

Most outside air kits I have worked with in the past are between 3 and 5".

That isn't an outdoor connection to the combustion air, that's the return plenum of the conditioned space air, which has orders of magnitude higher CFM than the combustion air of a wood stove. 

I presume that part is inserted in the code to avoid depressurizing the house relative to the outdoors due to duct imbalance issues, since the intake end of the air handler would be the lowest pressure point on the system. By porting that directly to the outdoors, the air-handler & duct system can't depressurize any part of the house below the outdoor pressure, though it can still  positively pressurize parts of the relative to the outdoors.It guarantees where most of the air handler driven infiltration enters the house, but not where it exits.  It also guarantees that the air-handler driven infiltration air is heated before it goes into the conditioned space too, rather than, say blowing across a drain or potable plumbing and freezing it up. 

It doesn't have anything to do with combustion air though.

I've never seen an outdoor air kit for a wood stove requiring anything bigger than 3" I.D. (7 square inch cross section)  even for ~150K BTU/hr wood stoves.  I can believe 4 inchers may be necessary for some super-blaster stove, but 5? A 5 incher would have a cross section of 19.6 square inches, which would probably be enough combustion air serve a 500K BTU/hr behemoth. What wood stove out there has a 20 inch cross sectional air intake?

The cfm going up the stack is much larger than the incoming combustion air, due to it's very low density after being heated up by 300-600F, and the conversion of solid or adsorb/liquid materials into gases. The combustion air draw of woodstoves is much lower. This (not super data-driven not highly referenced) source places it at an average 10-25cfm (see the 3rd to last paragraph), Martin Holladay seems to think it's 30-50cfm (see first paragraph), but that's not too credible. Good old Mother Earth News is selling the 10-25cfm story too (would they lie to you!? :-) ) 

Even a wind gust driven surge of 300cfm (the blast of a pretty good sized range hood exhaust fan) is still be far less than a typical gas furnace air-handler's rate.

At typical forced draft tank type hot water heater runs 50-60 CFM. If your wood stove were sucking even that hard on a 3" intake kit would be quite a breeze, getting into bath exhaust & hair dryer type volumes.  With the 3" port on my ~50KBTU/hr woodstove I can't feel the air movement at the intake with a bare hand, but probably could detect it easily with a spider web or hair as a wind vane.

I've never looked closely at a recent model Jøtul but I've looked at several others. Most non-catalyic EPA rated wood stoves have a single operable butterfly valve as the draft control, placed at the input of a manifold that splits the combustion air into multiple paths, one or more of which traverses the fire box to feed superheated combustion air at the top of the stove to light off unburnt gases & particulates.  The geometry differs by manufacturer and stove type, but the principle is the same- some air-injection ports near the bottom of the firebox, more up top, usually through ports that are smaller than 1/2" in diameter.  Is there a good online diagram of the Jøtul model you can refer me to? 

While it's remotely conceivable that Jøtul is concerned about mechanically stressing the casting with large temperature differentials, but cast iron is pretty tough stuff. It's pretty common to have ~1950C flame fronts licking the bottom of a pan while cooking the contents at 100C, an 1800C temperature difference, and you can crank the flame up & down with abandon without stressing the pan.  Having combustion air 50C lower outdoor temp than indoor temp on an arctic cold day in Alberta is tame stuff by comparison.  If there is a rational engineering explanation, it's probably a different issue.

You are correct. It is not combustion air. That's why I called it "make up air". My point was that manufacturers don't want cold outside air anywhere near their metal heat exchangers. When make up air was introduce to the Canadian code in the '80's, it was followed by a decade of cracked and rusted through heat exchangers spewing carbon monoxide into homes. While not all agreed, most conceded they were caused by the cold mua. Subsequent codes inserted the 3 meter rule and it seems to have solved the problem.

The original mua requirement was introduced into the code due to the tightening of the building envelope due to the introduction of the R2000 program and the requirement of 6 mil poly vapour/air barrier. There had been an increased incidence of backdraft due to exhaust fans etc. The theory was that if the furnace is running and there is a negative pressure created, it will be satisfied by the mua, therefore no spill. At the same time, the requirement for combustion air was added to the code. The combustion air was to dump near the front of the furnace, and was sometimes regulated by placing a combustion air bucket at the end.

While cast will take a lot of high temperature differentials, it can be shocked quite easily by sudden cold temps. A cast frying pan can be cracked by taking it from the freezer to a hot stove. Once the whole pan is warm, the temps can be changed with little harm.
http://chowhound.chow.com/topics/657110
That said, cast iron is still pretty tough stuff but the more corners and thickness difference it has, the more likely it will be subject to shock.

http://www.woodheat.org/outdoor-combustion-air-in-the-canadian-national-building-code.html

I presume that part is inserted in the code to avoid depressurizing the house relative to the outdoors due to duct imbalance issues, since the intake end of the air handler would be the lowest pressure point on the system. By porting that directly to the outdoors, the air-handler & duct system can't depressurize any part of the house below the outdoor pressure, though it can still  positively pressurize parts of the relative to the outdoors.

I'd avoid this method whenever possible. It will definitely positively pressurize the rest of the house and this is a bad idea for energy and wall moisture reasons. A barometric damper will limit negative pressure to some reasonable value (they are adjustable) without such effects - very slightly negative is what you want in the Winter.

All wood stoves require a chimney, but chimney's are large thermal bridges. What is general solution for this?

All wood stoves require a chimney, but chimney's are large thermal bridges. What is general solution for this?

Sealed firebox?

ICF - I have heard you discuss sealed firebox before. I googled it but can't seem to figure out what a sealed firebox is exactly??

what a sealed firebox is exactly??

It's just a term I use to describe a stove in which air to the firebox can be controlled as opposed to the abominable open fireplace and cast iron stoves of the past. As long as you can seal off the firebox and thereby, access to the flue, you can stop a lot of that heat loss.

So if the fire is contained in a firebox, then you can use a different type of chimney or something that is energy efficient? B/c brick chemneys, like I had when i grew up, must of been a huge thermal bridge.

Wood stoves can use special fire rated insulated sheet metal vent piping, with sections for where it passes through attics and roofs that can be installed at zero clearance.

They can also be vented into brick chimneys, but that usually requires a stainless steel liner to narrow the flue to a proper size for the wood stove, with rock wool insulation blown between the liner and the chimney's brick or terra cotta liner.

must of been a huge thermal bridge.

Huge. We had a lovely brick fireplace and chimney that must have been at least 12 feet by 6 feet and went from the center of the living/family rooms straight up and outside. The worst thing might have been the "damper", though. It was about 15" by 8" and was intended to stop warm air in the house from rising up the chimney all year round. After the first season, it was so warped by the heat of the fire that you could see daylight through it when it was closed. Most often, when you went to open it before a fire, you would find that it had already been opened by the wind. During cold snaps, my father would really stoke the fire in the fireplace. What that did was draw cold air in the leaky bedrooms at the end of the hall and you had to go to bed with 4 or 5 blankets on the bed to keep from freezing. After that, we had electric blankets. Oddly enough, the bigger the fire in the fireplace, the colder it would be down at the end of the hall. We enjoyed fire-popped popcorn, though.

Posted By Dana1 on 17 Nov 2014 05:26 PM
Wood stoves can use special fire rated insulated sheet metal vent piping, with sections for where it passes through attics and roofs that can be installed at zero clearance.

Sounds like this reduces the thermal bridging versus a brick chimney.