The Day and Night furnace air blower runs for 20 seconds every minute. This happens even when the thermostat is completely disconnected. The only way to stop this is to unplug unit for several hours( 3-6 ) then it will operate normally for 6-8 hours before starting the blower every minute. Any ideas?

What is the model of the furnace?

I believe you will be replacing the circuit board unless it has a separate motor timer device. If it is a newer model with a DC motor hold on to your hat when you get the price.

Model # is: 398AAV036040A
Product # is: 398AAV036040AAKA

Blower motor is 120 VAC

I agree with hvac1000, I'd say it's a bad blower relay on the control board.

Do you need to replace the control board or can you just replace the relay?

More than likely the relay is part of the board. You will have to look on your furnace for the wiring diagram.

What brand is the furnace? I'm not familiar with that model number.

Okay, I have the control board in my hands. The blower relay is part of the circuit board. Looks like I would need to use a soldering iron to remove Now can you guys tell me where I can get a new board? I would call a tech in, however, where I live (far north Sonoma coast of California) it very difficult to get services up here. By the way thanks very much for your help.

Try here

http://www.expertappliance.com/partfinder.php

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Carrier, Bryant, Payne, Day & Night

If you can get the part number you can always check Ebay.

Check all your limit switches first. If you have a bad spade or wire connection going to a limit switch it will do what your saying. If the 24 volt circuit is broke at any time heating or cooling the blower will run. Sounds like a loose 24 volt wire connection to me.

This is what is wrong with your unit it is old and the old it gets the more time you use it so get a NEW UNIT That's ENVIROMENTLY Friendly