Glossary · Heating, cooling & ventilation

Natural-draft (atmospherically vented) appliance

Also called: atmospheric vent · draft hood · natural-draft water heater · natural-draft furnace

A furnace, boiler or water heater that takes combustion air from the room and relies on hot exhaust rising up a chimney, so house pressure can reverse it.

What it means

These appliances, including many older furnaces and standard gas water heaters, have an opening called a draft hood and depend on the chimney's natural draft to carry exhaust away. If exhaust fans, a dryer or a fireplace depressurize the house, or the chimney is cold or blocked, exhaust can spill back indoors. Pairing one with a powerful kitchen exhaust fan without make-up air is a classic cause of backdrafting.

Why it matters in an audit

Natural-draft appliances are the ones most likely to backdraft, so auditors test them before and after air sealing and may suggest a sealed-combustion replacement.

Typical numbers

Reference values with their sources. Your auditor’s report gives the figure for your own house.

What	Value	Source
Testing (DOE Building America)	Natural-draft furnaces, boilers and water heaters need combustion safety testing, including a spillage check at the draft hood with exhaust fans running	basc.pnnl.gov ↗

Sources

Related terms

Sealed-combustion (direct-vent) appliance A furnace, boiler, water heater or fireplace that brings in combustion air from outdoors through its own pipe and vents exhaust straight outside.
Backdrafting and spillage When exhaust from a furnace, boiler, water heater or fireplace comes back into the house instead of going up the chimney or vent.
Draft (chimney draft) The upward pull in a chimney or vent that carries exhaust out of the house; weak or reversed draft lets combustion gases spill indoors.
Combustion safety test A check that furnaces, boilers, water heaters and stoves burn cleanly and vent their exhaust outside, even with fans running.
Worst-case depressurization test A combustion safety check run with exhaust fans on and doors set to create the lowest pressure the furnace room is likely to see.
Make-up air Outdoor air deliberately brought into a house to replace air removed by big exhaust fans, range hoods, dryers or fireplaces.

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