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Old 05-03-2011, 12:59 AM   #1
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A/C short cycle yesterday and today won't shut off....


Good Evening. I'm a Brit so I know nothing about air conditioning so really appreciate all help.

Last week we had a power cut in the middle of the night. Since then I noticed a few issues with the A/C. Firstly the upstairs unit wouldn't power on without me going in to the attic and turning the power switch up there, the thermostat wouldn't control it. Then it worked fine for a few days. Last night it started going through short cycles, coming on for twenty seconds and going off repeatedly.

Tonight I came home from work and it's running, thermostat won't shut it off. It's blowing cold air but given it's cold outside tonight it doesn't feel as cold in the house as it could with a/c on constant.....

I've taken thermostat off to change batteries and a/c kept running. I can hear thermostat turning on and Off but fan is constant.

The downstairs unit is working no problem (knock on wood) and we didn't have any other electrical problems after power outage / surge....

Thanks a lot!

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Old 05-03-2011, 02:17 AM   #2
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A/C short cycle yesterday and today won't shut off....


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Originally Posted by charliej View Post
Good Evening. I'm a Brit so I know nothing about air conditioning so really appreciate all help.

Last week we had a power cut in the middle of the night. Since then I noticed a few issues with the A/C. Firstly the upstairs unit wouldn't power on without me going in to the attic and turning the power switch up there, the thermostat wouldn't control it. Then it worked fine for a few days. Last night it started going through short cycles, coming on for twenty seconds and going off repeatedly.

Tonight I came home from work and it's running, thermostat won't shut it off. It's blowing cold air but given it's cold outside tonight it doesn't feel as cold in the house as it could with a/c on constant.....

I've taken thermostat off to change batteries and a/c kept running. I can hear thermostat turning on and Off but fan is constant.

The downstairs unit is working no problem (knock on wood) and we didn't have any other electrical problems after power outage / surge....

Thanks a lot!
It sounds like the fan-timer board of the furnace has got damaged from the power surge.

There are only two control boards, one in the thermostat, and the other in the furnace. If the AC keeps running with thermostat disconnected then the board in the furnace is acting flaky.

Note that these boards are pretty standard, as there is one Honeywell fan timer board that works for many many different furnaces.

If the furnace itself is short-cycling, then the outside AC condensor unit will not be running very much at all, as there is a startup delay to protect the compressor motor, as its very hard on a compressor to short-cycle.

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Old 05-03-2011, 07:22 AM   #3
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A/C short cycle yesterday and today won't shut off....


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Originally Posted by Robo_geek View Post
It sounds like the fan-timer board of the furnace has got damaged from the power surge.

There are only two control boards, one in the thermostat, and the other in the furnace. If the AC keeps running with thermostat disconnected then the board in the furnace is acting flaky.

Note that these boards are pretty standard, as there is one Honeywell fan timer board that works for many many different furnaces.

If the furnace itself is short-cycling, then the outside AC condensor unit will not be running very much at all, as there is a startup delay to protect the compressor motor, as its very hard on a compressor to short-cycle.
Thank you, I've had someone replace the board on the upstairs unit before and felt it was very expensive for the time taken, so I'll go and have a look at it myself today. I guess it's just a case of marking the wires and placing them in the right place on the new board.... I'll have to order the board online so I'm going to leave the AC running for the next two days, I don't want to go up and turn it off by the power switch and then not have it come back on, so apart from a higher bill this shouldn't damage the unit any further, I assume? I'm in Texas so better to be cold than hot!

Thanks again....
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Old 05-03-2011, 07:49 AM   #4
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A/C short cycle yesterday and today won't shut off....


Ok, sorry to be SO dependent! By taking the furnace cover off the blower stopped anyway so I'm not going to restart it again and just hope the cooler weather continues down here. I had a good look at the control board but can't see a brand or model name on it. Are they generally the same? My unit is an Armstrong that's 15 years old + going from the age of the house. Obviously I'll google, but can someone recommend a particular board I should buy as a replacement, please?

Thanks again!!!!
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Old 05-03-2011, 09:44 PM   #5
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A/C short cycle yesterday and today won't shut off....


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Originally Posted by charliej View Post
Ok, sorry to be SO dependent! By taking the furnace cover off the blower stopped anyway so I'm not going to restart it again and just hope the cooler weather continues down here. I had a good look at the control board but can't see a brand or model name on it. Are they generally the same? My unit is an Armstrong that's 15 years old + going from the age of the house. Obviously I'll google, but can someone recommend a particular board I should buy as a replacement, please?

Thanks again!!!!
Go onto eBay and search for Honeywell Fan Timer. About 90% of these boards are made by Honeywell. You really need to replace what's there with the same thing or it will be very difficult to install.

http://www.comfortgurus.com/product_...oducts_id/8284

http://www.patriot-supply.com/produc...93&startRow=41
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Old 05-03-2011, 10:14 PM   #6
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A/C short cycle yesterday and today won't shut off....


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Originally Posted by Robo_geek View Post
Go onto eBay and search for Honeywell Fan Timer. About 90% of these boards are made by Honeywell. You really need to replace what's there with the same thing or it will be very difficult to install.

http://www.comfortgurus.com/product_...oducts_id/8284

http://www.patriot-supply.com/produc...93&startRow=41
Thanks I actually found it on HVACBESTBUY website for about $200 delivered, when I had another look at the board I noticed the R1 "conductor" was very hot and appeared burnt so I'm confident this is the problem and hope to be running again by Thursday.

Thanks for the help all!
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Old 05-03-2011, 11:37 PM   #7
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A/C short cycle yesterday and today won't shut off....


I had the same thing happen a few years ago to a Lennox system. Cost 600 to have board replaced.

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