No blower with ac but ok with furnace

[Nicky]

Hi, I have an old day and night system from the mid 60's and will be replacing after my additions done in the near future. The furnace works great but the ac which was added probably in the early 80's doesn't trigger the blower to come on. There's not much to the system,the transformer which seems to be ok and the fan relay which I replaced but still the condenser will come on and the blower will not start up. Any suggestions would be appreciated

[Rich]

The only thing I can think of is a bad control wire connection. Possibly over time it's either worked loose or something like that... could be a bad connection internally also. Stuff gets old and it wears out..

[mjpliv]

Double check the transformer. Put a meter on the secondary coil side to verify line out voltage. The relay you are talking about is probably the same type that starts the blower on built-in vacuums. Those transformers are notorious for loosing the secondary coil.

[roger g]

Try turning on the heat and see if the fan comes on. If it does when the heat comes on and doesn't when the a/c comes on I would highly suspect the thermostat. I haven't a clue what a day night system is. There is only 2 low voltage wires going to the condensor (terminals Y and C) and they only close the contactor. There is nothing about a central a/c unit that would affect the furnace blower but the t stat can.


roger

[Nicky]

Day and Night is a brand name of forced air units. The blower does come on with the heat system on and I already bypassed the thermostat and it is not faulty. The problem is probably in the wiring since it is old, I can't imagine the fan relay that I installed being faulty. Thanks for the reply

[roger g]

On older furnaces we used to install a fan relay centre when installing a central a/c. Without the relay centre the furnace fan wouldn't come on. This relay centre was installed usually in/on a 4 inch by 4 inch electrical box attached to the furnace. The relay unit itself was quite large about 3 inches by 4 inches and about 2 1/2 inches high. I've never heard of these failing but there is no reason why they wouldn't fail. They have about 4-5 wires coming from them and I remember that two are the same colour.

roger