Problem with central air handler

Old 08-06-04, 09:01 PM
John Ross
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Question Problem with central air handler

I have a Carrier Comfort Zone, Four Zone home AC 3 1/2 Ton system that was installed in 1992. The unit cools fine, but after the heat pump and air handler shuts down like it should the blower fan in the air handler will still come back on. Sometimes it will just stay on and other times it will come on for about one or two seconds and then back off again, back on again and then off again. This happens until the thermastat turns the AC back on again to cool the home, and then the same problem again, the air handlers blower fan will turn on and off, on and off. The blower runs fine like it should when the unit is running. This even happens when the switch is turned off on the thermastat. The only way I can stop it is to pull the electrical connection inside the electrical box in the attic.

I went in the attic and disconnected the low voltage thermostat wire connect to the air handlers terminal G and nothing changed. The unit will stay off for a while and then it comes on and off, on and off and sometimes it will just stay on and then turn off after a while.

I did notice that the circuit board directly behind terminal screw G had one of the looked like resistors that was burnt on each end of the circuit board. The resistor looked OK but the board is brown on each end where the resistor is soldered.

This is a four zone system that has a master digital programmable thermastat that has three additional sensors connected to it. These three sensors are located in other areas of the home. These other sensors report the temperatures of those areas back to the master thermastat that will open or close dampers to adjust the temperature.

Carrier 3 ton AC air handler
Model Number FB4ANFO42
Product Number FB4ANFO42000AAAA
Serial Number 00692H00943
Motor is HP

Last edited by John Ross; 08-06-04 at 09:34 PM.
Old 08-07-04, 01:53 AM
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Hi John:

Good troubleshooting! Most likely the circuit board that you're seeing adjacent to G terminal is an indoor fan time delay relay. They are also called fan post purge relays. Basically they are a delay on break type delay relay that keeps the indoor fan running for about a minute after the thermostat shuts off the cooling [delay on break means that when the operating circuit to the relay is de-energized, the power circuit contacts remain closed until the time delay elapses. IOW- the load, which in this case is the indoor fan motor, stays on until the time delay times out.]. The theory is you squeeze out a little more cooling by expelling the resdiual cool air in the cooling coil area and the ducts. When the manufacturer's design engineer's run their computer models, this post purge helps them move higher in the SEER ratings game. There are two types of setups. The first is where the delay relay is pilot duty and it operats another standard high power relay which turns the fan motor on/off. The second type is a delay relay and high power contacts all in one package and it controls the fan motor directly. If you search thru the posts here, you'll see that these relays malfunctioning is a common failure. To see what generic replacements look like go to Highlight products, then click on fan blower controls. At each model number place your curser over the picture to click and see more details about each one.
Old 08-07-04, 11:16 AM
John Ross
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Arrow Thanks for responding

BigJohn thank you for replying you sound very knowledgeable. I take it that you have worked on Carriers Comfort Zone systems in the past? I looked at the website and none of the circuit boards match mine.

I understand the Carriers Comfort Zone is a very complex system with multiple circuit boards. My unit has a extra set of circuit boards in the laundry room on the other side of the wall from the thermostat. I believe heat pump and air handler are controlled by these boards along with the thermostat?

1. Should I be concerned about the blower fan running all the time,
would not want something getting hot and starting a fire?

2. Can I disconnect a wire from the thermostat after the air handler
shuts down to turn off the blower until I get this problem fixed?

3. I take it that you think the problem is the circuit board and if so do
you have an idea where I can purchase one possibly over the

4. Also, is there someway to completely troubleshoot this board
because I would not want to purchase a new one to find out that
the problem is with another component?

Thank you again for your help,

John Ross
Old 08-07-04, 02:38 PM
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Well, that's my bad. Somehow I didn't realize that the zoning system was also from Carrier, I was thinking it was an aftermarket system. Teach me to pay better attention! It sounds like you have the old Carrier/Parker controls which I do not have much experience with. [and they are complex as you say] I'm afraid you're going to have to call the Carrier dealer, but make sure you tell them in advance what type system you have and to only send a tech who has training on your system otherwise you're not paying. I don't think that the fan running all the time will cause a problem. The motor has an internal protector that will shut it off if it overheats for some reason.

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