My central air unit outside comes on from thermostat control but the blower will not kick on.Will not kick on even if I turn the fan on manually from thermostat. If I turn on the furnace control the blower works fine.I just replaced thermostat hoping this will fix problem, but didn't.I have to go down to the basement and turn it on and off from a switch in the unit itself.Please help! I'm burning up.

Your fan relay is bad or the t-stat. Probably the fan relay. It's a $5.00 part and one of the fan wires leads to it. You must have a gas heater where the fan is controlled by a separate fan switch. You have two wires with 24 volts to coil on fan relay and one wire connected on normally open contact to high speed on fan, one on normally closed leading to fan switch, and one to L1 power supply. It's very simple, but don't mix them up. The symbol for normally closed is a set of contacts with a line through the contacts and no line on normally open contacts. Just look in control box in furnace where all wires lead. Most likely it is the only relay there. DrTom4444

How do you replace the fan relay on the air unit

I need to know what kind of unit you have on the inside and how it is set up. It could be in several places. You may IM me at drtom4444 on Yahoo Messenger and you may do a video call, as well. Then I can instruct you. DrTom4444

Thanks drtom. This was very helpful. It turns out that the relay slipped out of the socket apparently from years of vibration. I simply pushed it back in and works fine. Thanks again.

You are welcome. Glad to be a help to you. DrTom4444