Sealed Combustion Heating and the Appliance Hot Tub Party

by Erik North on July 24, 2013

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Sealed combustion systems are apt to become more central to efficient building as the new energy codes introduce air tightness standards. This implicitly means that you’ll need to pay attention to your heating system safety. And conveniently those codes also lay out explicit guidelines for combustion systems.

Building and energy codes often get adopted piecemeal around the country. In Maine, we’ve adopted the IECC 2009 (the 2009 International Energy Conservation Code) but exempted towns below a certain population level.

The IECC 2009 requires that new wood burning fireplaces get their combustion air from outdoors. The IRC 2006 requires that tightly constructed buildings get all their combustion air from outside the building enclosure. This is the trend in building codes; having combustion appliances get their air from the outside. That’s where sealed combustion comes in.

What is Sealed Combustion (finally)?

Sealed combustion are those heating system whose combustion is completely disconnected from the building’s interior. Traditional heating and hot water systems pull combustion air and dilution air for the flue gases from the building interior. As we build tighter, more efficient houses this becomes not to put too fine a point on it, increasingly dangerous and stupid.

What’s the problem? As the house becomes tighter, traditionally vented combustion appliances are increasingly apt to spill flue gases or backdraft.

Sealed Combustion and the Appliance Hot Tub Party

Why? With a tighter house, household ventilation appliances fight over a smaller pool of air and can begin fighting against one another. Think of it like a house party. Everything is fine when some folks are out on the porch, some in the kitchen, some in the pool. There’s more than enough merlot and potstickers to go around.

Everything is fine until some genius suggests that everyone should pile into the hot tub. And it is crowded. Folks would be jostling elbows, knocking over drinks plus you’d probably run out of guacamole. I…think this metaphor got away from me.

If the house is too tight and the ventilation appliances (oven hood, atmospheric hot water heaters, atmospheric heating systems, dryers, bathroom fans) run simultaneously, the appliances start fighting for elbow room. And if the other appliances are stronger than the heating system’s draft strength *BAM* backdrafting and nasty gases from the heating system are pulled into the house.

The point is that a tighter building envelope plus traditionally vented combustion appliances means potential problems. One solution – sealed combustion!

Tighter Buildings, Better Heating Systems

Sealed combustion is separated from the interior, eliminating the possibility of spillage and backdraft. There is an outdoor air intake and an outdoor exhaust usually combined as a single concentric pipe. This means that the combustion air and flue gases never interact with the interior. No problems! Everyone gets guacamole!

Folks sometimes say that a house shouldn’t be too tight. One reason is mold, another is moisture accumulation and another is concerns about backdrafting your combustion appliances. With gas burning sealed combustion systems, you still need to have your fuel lines regularly inspected by a gas contractor. But potential issues with backdrafting or spillage of flues gases won’t be a problem.

{ 2 comments… read them below or add one }

eric anderson July 28, 2013 at 12:05 pm

I found after I finished building my house that I couldn’t start the woodstove with the range fan on, unless I opened the front door about an inch. The good thing about wood stoves is that you know it is backdrafting because the house fills with smoke. the only other combustion appliances are the range and a sealed combustion munchkin boiler
I can avoid the problem through operator knowledge, but I need to change to a small wood stove with an outside air kit. I don’t think just adding a passive vent will do it. Any suggestions on a sub 30K btu woodstove with outside air? House has a design heat load around 15,000 btu’s/h. worst case CAZ is -37 pa, cfm50 is 458, ach50 is 1.35 for the house, I currently have a Waterford Leprechaun which works well. But I can’t light it without opening the front door if the rang fan is on.


Erik North July 28, 2013 at 2:09 pm


That is one tight house. And exactly the type of problems that the post was referring to.

I can’t recommend a specific wood stove, just not my area of knowledge (I deal with home assessments and retrofit work). My suggestion is to Google up some names then ring up 2-3 stove stores and pick their brains. Sorry I can’t be more specific.


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