I see there are conflicting recommendations on whether we should provide direct or indirect make up air to a wood stove in a tight home.

This is probably a dumb question but rather than putting a hole in the wall, couldn't we instead just adjust the HRV to intake more air than it exhausts when we're burning with wood?  Wood stove will be backup or ambiance heating , so not an everyday item.

HRVs do nothing to prevent backdrafting combustion products into the house, and the direction of the wind relative to the location of the HRV ports can deliver backdrafting forces. Unbalancing the HRV intentionally would be a real efficiency killer for the HRV, and wouldn't necessarily prevent backdrafting.

If you duct combustion air to a sealed firebox direct, when backdrafting conditions occur (which WILL happen occasionally unless your stack is extremely tall) the combustion gases go out the intake duct, not through the interior of your house.

Absolutely provide outside combustion air directly to a sealed firebox.

One key element here is whether or not the woodstove can have an OAK duct connected directly to it (sealed firebox, as ICFHybrid said). With some stove designs, you have the OAK duct dump raw air "into the vicinity" of the stove, rather than be directly connected.

Yea I am planning on something like a Jotul F3CB - these can hook up directly.

So why is it that in countries like Norway where the Jotul is produced, even in the case of tight and efficient homes, they do not use outside air kits?

Furthermore

Jøtul North America ONLY requires the use of the outside air kit where required by local code and does NOT promote or recommend the use of a directly connected outside air source

Proximity combustion air vents still put exhuast products into the conditioned space when they back draft.

Some manufacturers' outdoor air kits hooked up to their stoves are actually designed as proximity air kits, with an open gap between the connection point and the actual air intake of the stove.

It's not clear to my why it's done that way, or why Jøtul doesn't recommended direct combustion air connections.

It's not clear to my why it's done that way, or why Jøtul doesn't recommended direct combustion air connections.

Maybe there is a clue in the fact that when I tried to get direct connect, there was a lot of weirdness at the vendors. At first they claimed direct connect was illegal. When I pointed out the direct connect provisions in the product, they then claimed that was only for mobile homes. And so forth. It was like they had almost never done it before.

Direct connections are pretty standard fare at wood stove vendors near me. The installers aren't crazy about the extra step- there is some push back, but it's more along the lines of:

"Well, you know you don't actually NEED it- you have enough combustion air even if your house is pretty tight."

But there isn't much confusion about it being legal, and they all understand the concept and sell the kits. Then again, my sample size is small, and not Jøtul- specific.

How air tight are we talking here? <0.6ACH ? If you are at ~ 1.0 ACH would you need to worry about combustion for wood stove? 2nd Q: What is a sealed air box? I googled it and didn't see anything to describe what that is.

Adequate and efficient combustion is only part of the problem. If the home is leaky, all the combustion air comes from interior air that is already conditioned. It gets replaced with cold outside air which is undesirable from a comfort and energy standpoint.

Google 'wood stove outside air'
If you read the first item about "Outdoor Air Myth Exposed", you will see the kind of ignorance that makes people do stupid things in this respect.

Direct connections are pretty standard fare at wood stove vendors near me

I suspect your region is substantially more advanced in building airtight homes which would indicate the need for outside air.. Here, not so much.

Posted By ICFHybrid on 07 Nov 2014 12:23 PM
Google 'wood stove outside air'
If you read the first item about "Outdoor Air Myth Exposed", you will see the kind of ignorance that makes people do stupid things in this respect.

ICF - I read that article. So what you mean is that a leaky house to generate combustion is a terrible idea, correct? That article made sense to me but I wanted to be correct that we are on the same page.

a leaky house to generate combustion is a terrible idea, correct?

In most all cases, yes.

I hope you realize that parts of the article are very misleading and the overall conclusion is incorrect.

It doesn't take much leakage to provide combustion air, very little at all.

It takes a bit more leakage to completely eliminate backdrafting risk when the clothes dryer is running while someone has the range hood on full blast in the kitchen and someone else has the bath fan going.

But even at PassiveHouse levels of air tightness wind-induced backdrafting is possible, and if you're using the conditioned space as the the primary path, that backdraft dumps exhaust products directly into the house. With a reasonably tight ducted combustion air solution the backdraft goes out the combustion air duct, not into the house.

While no wood stove is 100% air tight and under some wind condition there is potential for blow-by at the door gaskets, the amount of exhaust getting into the house is quite limited compared to what it would be with no ducted combustion air, or a proximity-only solution.

I don't have the tightest house in New England (not even close) OR the biggest exhaust fans, but with both the kitchen range hood and bath fan going there is pronounced & obvious backdraft condition while cold-starting the wood stove. But during re-loads when the stack is already 300-500F those fans aren't a problem. As the house gets tighter we'll see if that continues to be the case. Through the glass door of the wood stove I've observed wind gust induced backdrafts on a very few occasions, but the exhaust backed out the air-intake, not into the house- nothing nose-detectable even at close range. YMMV.

Good stuff dana. I didn't know you had a wood stove for your place. You mentioned..." as your house gets tighter"... you doing updates/repairs to your place?


From reading all this, sounds like outside air directly fed into the stove for combustion is the solution.


I imagine that breaks the thermal barrier and some sort of gadget or gizmo is required to seal that up in the non-winter months. I hear chimneys are terrible from a insulation perspective as well. All that info and data confuses me. My personal house goal is a "pretty-good house" with a wood-stove.

So for background, I have a wood stove and a fairly small and tight house ~ 1.5 ach50. If the woodstove is already hot, I can run my clothes dryer, or bath vent fan or range vent on Low without a problem. If I light the wood stove and turn on my bath fan, the fire goes out, but does not really backdraft. If I turn on the clothes dryer or the range vent, I fill the house with smoke. This kind of sucks as the fire alarms all go off, the dog freaks out and the house smells like smoke for 3-4 days afterwards. Running the HRV on high- continuous clears the smoke smell in 1-2 days. The way to stop this is to open my front door about4- 6 inches until the fire is well lit or don’t use exhaust fans when you are lighting the stove.
As an experiment, a few years ago I tried turning all the fans on when the wood stove was fully hot and running, this corresponds to ~ -37 pa of depressurization in the house where the wood stove is. The wood stove does not backdraft, if it is very cold outside, but the rate of combustion slows down. If it is in the high 40’s it will backdraft with the range fan on the highest setting.
I have a Waterford Leprechan woodstove and the insulated metal chimney goes straight up~ 16 feet with no bends. So the flue is about ideal.
My conclusions are that you can be ok as long as you operate it intelligently. On the other hand about once per year I or a guest screw up and fill the house with smoke, so I guess that I would be a lot better off with a direct air supply.
The issue I face is that most of the woodstoves that I have found with air supply kits are grossly oversized for my house. The leprechaun is a ~28000 btu stove and my heat loss is about 14 K so I only have to run it a few hours unless it is really cold. This time of year I run it every second or third day. I have to be careful with it, because I can get the air temps into the 80’s if I wind it up.
I think eventually I will switch to a stove with an outside air kit. I would have done it already if I wasn’t such a cheap bastard.
Cheers,
Eric

Thanks for the info Eric! Good stuff!!

If you can't use outside air direct to the stove, then put in a barometric damper.

"Jøtul North America ONLY requires the use of the outside air kit where required by local code and does NOT promote or recommend the use of a directly connected outside air source"

There has always been some discussion on the suitability of outside air as combustion air.

Will it cool off the burning environment enough to cause lower after burn thereby increasing particulate matter up the chimney?

Will it produce thermal warp and or case hardening of the metal structure of the firebox when very cold airs a drawn into the very hot box? (this is why most codes to not allow make up air directly into a gas force air furnace but require it to be introduced far enough upstream to preheat the incoming air to prevent cracking of the heat exchanger.)

Will it produce acidic condensate at the mixing point?

Just some thought from the old school