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.

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