I have an old Lennox furnace (circa late 60's) with an A/C added on. The
schematic has not been marked up after an addition of the A/C. Had an air
blower relay failure recently and noticed that A/C compressor still switched
on, even without the blower running. That can't be right. When heat is
used, the blower does not come on unitl the air is hot.
Can someone point me to a "typical" schematic for a Furnace-A/C system.
You're right, the blower does not come on until the air is hot. But in
A/C mode, the fan should come on rightaway along with the compressor.
What happens when you turn the fan switch to "ON" on the T-Stat?
When I trurn T-Stat to ON, the fan does go ON. As it does in Auto. But the
fan running is not a permissive, either via a relay or flow switch, for the
compressor, at the present. I want to correct this and could devise my own
way, but am looking for a standard schematic, if there's such a thing :-)
Would be easier for someone else to recognize it in a future. I also want
to either draw a schematic, or print one out, if I find one, to which I will
For the furnace wiring:
For the AC wiring (Actually it's a fridge, but its close enough):
Well, they should have installed what is known as a "Sail Switch" that
will only close the 24-volt control circuit when adequate airflow turns
the sail and closes the switch.
The "Sail Switch" will work if the evaporator gets ice over, or return
filter gets overloaded/clogged.
There should always be a sensor that performs that duty to protect
Other sensors can be used too "that require an adequate amount of
airflow/or pressure." An air pressure switch needs to be down-stream of
the evaporator to leave the switch open if there is not adequate
pressure to close it. This protects against a lot of things and indicate
to a knowledgeable tech a lot of things.
They should be illustrated in the schematic and included in a trouble
shooting owner's manual guide. indicating the various causes for the
switch to not close!
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.