Duct arrangement allows reverse flow

Heating · Furnaces and Boilers · printed page 178 (PDF page 179)

Book index entries

Ducts – Arrangement for Wood Furnace Ducts – Allows Reverse Flow Duct Arrangement Allows Reverse Flow Reverse Flow through Duct Arrangement for Wood Furnace Wood Furnace

Text in the illustration

cold air return
fan
hot air plenum
heat
add-on wood furnace
existing downflow furnace (oil)
reverse flow of heat
floor
below floor ductwork
if the blower in the existing downflow furnace stops working, heat cannot rise out of the supply ductwork by natural convection
it flows backwards through the cold air return of the downflow furnace
this can damage the blower motor and belt as well as the furnace wiring

Generated explanation

The diagram shows an add-on wood furnace connected to an existing oil-fired downflow furnace and below-floor supply ductwork. The wood furnace's hot air plenum connects to a duct that descends through the floor to the supply trunk. A large arrow shows heat flowing backward from the wood furnace through the downflow furnace and out its cold air return. The annotations explain that if the existing furnace blower stops, heat cannot rise out of the supply ductwork by natural convection, and this reverse flow can damage the blower motor, belt, and furnace wiring.