DIYbanter - View Single Post - Strange noise coming from furnace after AC has shut off. Normal?

wrote:
I just moved into a new place and it's the first I've ever had central
air. The furnace is a Heil
single stage unit. When the AC has been running and turns off, either
manually or automatically by the thermostat, the blower shuts off and
everything seems fine. However, then about an hour or after it's been
off, the furnace makes a loud 'whir' sound that lasts two or three
seconds and then cuts off. Sometimes it'll happen one or two times
after the first time, about 5 minutes apart. Again, this only seems
like when the AC has been running. It seems like a distinct 'whir'
sound and not like when the blower turns on.

Is this normal? I'm not used to a central AC system. If it's normal,
what's it doing? Thanks.

It might be the condensate pump. Look around the outside of the
furnace, probably near the floor for a small square-ish looking pump.
There should be a drain hose or tube leading down from the AC coil to
this, then another tube leading from this to a drain or sump nearby.
They're usually float-controlled, so you're hearing it pump as its
reservoir fills up.