Central Air Help?

[vicious6]

Does anyone know anything about Central Air in the Home? My A/C was working fine and now the fan will only come on with the heat! Thermostat control? Mine is the old mercury switch type.

 

[titan1]

i would start with the T-stat get yourself a nice programable one. They are not much money, and go from there. could be a relay

 

[HUMBLE-U2]

how old is the unit most times sounds like you have a bad start capasitor.try to spin the the fan with power on if it comes on the capasitor is bad. grainger has them range from 6 to 8 bucks.hopefully that will do it. if not maybe fan motor or compressor if its the compressor it will ether make a banging noise or be really hot.if thats the case time for a new hi/eff unit google how to change a/c capasitor.

 

[TurboDave]

Does anyone know anything about Central Air in the Home? My A/C was working fine and now the fan will only come on with the heat! Thermostat control? Mine is the old mercury switch type.

When it comes on, go outside and see if the compessor is running. Problem is likely out there since all your getting is warm air.

 

[faster]

I am an A/C contractor.

Is it a heat pump or gas/propane/oil furnace?

Some units have two relays for fan control, one for heat mode and one for cooling mode. If it has two relays the fan will come on in heat and when you switch the fan from auto to on as the unit runs the fan off the same relay for these two modes. Cooling mode may have its own relay.

The fan starts in heat so the capacitor is good.

You mentioned it is a mercury t-stat.
1. Take it apart and gently tighten all the wire connections checking to see if any of the rivets have worked loose causing it to open the circuit. Put the mercury bulb section back on again gently tightening the connections.

2. You did not say if the condensing unit outside was coming on when it called for cooling. If not jump the yellow and red wires in the t-stat and see if everything starts. If everything starts problem is the t-stat. If the outside condensing unit starts but the air handler inside does not start problem is in the air handler control and the relay for that unit is not making or the fan motor has a bad winding.

3. The air handler fan motor will be 2-3-4 speeds with one speed for heat/fan on and another speed will be for cooling mode. The wires off the fan motor will be black/red/blue/orange/yellow/white/brown and possibly a brown with white stripe.

If you have a heat pump the motor will be 220v;
white will be one phase of 110v.
brown will go to a capacitor
if you have a brown/wht stripe (you may not) it will go to a capacitor/hot lead (do not mix up the brown with the brown/wht stripe you will smoke the motor).
the other colors are different speeds connecting to the other phase of 110v creating the 220v and have the brown/wht stripe tied to this phase and are not all used.

If you gave a gas/propane/oil furnace the fan motor will be 110v;
white is neutral
brown goes to capacitor
brown/wht stripe to capacitor/hot lead (again do not mix the browns up).
the other colors adjust fan speed and go to the 110v hot leg.


In either case the colored wires will go to two separate relays if you have a multi speed system which you almost certianly do. If you have more than a two speed motor there will be the unused colored wires form the fan motor capped off or "parked" on the circiut board in the air handler unit. Try swithing the colored wires and see if it starts if it does the motor has a bad winding. If not then you have a bad relay.

Mikey