Heat-A/C - typical schematic and wiring diagram

[RichK]

Hi,

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.

Rich

[Mikepier]

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?

[Matt]

For the furnace wiring:

For the AC wiring (Actually it's a fridge, but its close enough):

[udarrell]

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

[RichK]

----- 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

[hvactech2]

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

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