Heat-A/C - typical schematic and wiring diagram

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?

For the furnace wiring:

Hi RichK, 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 equipment.

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! - udarrell

----- Original Message ----- From: "Mikepier"

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 rewire it.

Rich

It is correct. in A/C mode there is no fan proof on a residential system.