A/C problem

[Jan Philips]

I've had a problem with A/C at my parent's house, currently unoccupied. We've had some temperatures over 95F and some power outages from storms. The thermostat has a few-minute delay.

The thermostat was set on 78F. When I went over there late Friday afternoon, the compressor was not running and it was 89F inside. I cut off the curciut breakers and turned them back on to see if it would reset it. After a few minutes, nothing had happened.

I checked the pan under the air handler and it was dry.

Then I saw that the batteries in the thermostat were low. I cut off the unit at the thermostat and replaced the batteries. I turned it back on, and in a few minutes it came on and started cooling.

This made me suspect the thermostat, but read on...

I went back Saturday afternoon and again the compressor was not running and it was 90F inside. I cut the circuit breakers off and back on and switched the thermostat to off and then back to cool. In a few minutes it came on.

I went back Sunday and again today (Monday) and it is still working.

Any ideas of what the problem is/was? (At first I suspected the thermostat, but now I don't know.)

[Tony Hwang]

Hi, Is it programmable 'stat? Can you tell the model? If program is still as programmed, did you check? If not sure, you can reprogram it or set to default program setting. If battery low was displayed from that time battery is good another couple weeks B4 it really goes dead.

[Jan Philips]

It is not programmable. I can't remember the brand or model, but I think it is Honeywell or White. I can find out in a day or two.

N/A

Well, one of the batteries was leaking, so I thought it was best to replace them.

[Tony Hwang]

Hi, Of course. I use Lithium ones and mine is wired so that battery is on stand-by mode. Some 'stats can draw power from 24V AC transformer. One extra wire run to 'stat. If it is digital model, it could be going bad working intermittent. Or can be even loose connections at either end ('stat and control board in the air handler) When it is hung up. just try to tap 'stat gently and see if it starts working again.

[gfretwell]

Have you found the operating manual for the thermostat and looked at what all the indicators mean. Your answer may be there.

If not, I would start by carefully cleaning the battery contacts. If the batteries were leaking, it could have corroded a contact or two.

Usually there are status indicators in the thermostat that say whether it is calling for cool or not, whether the program is active and perhaps some totally bogus state that could shut it down.

I have had to do a total reset of my thermostat to get out of one of those. I really miss my old T-87

[Jan Philips]

I just had the control board and motor in the air handler replaced about 2 months ago.

[Jan Philips]

I'll have to look up the model, but it is a simple one. When my parents upgraded their A/C system, they first had a very sophisticated programmable thermostat. But you had to go through a menu to do everything - even on/off and up/down! They never could learn how to use it, so we replaced it with a simple one.

We cleaned one of the contacts, but didn't have what we needed to do it properly.

Yes, it was calling for cool (it is a heat pump).

[J Burns]

Was the blower running? If neither the blower nor the compressor was on, I'd get a volt meter and see if there was 24 VAC on the cooling terminal of the board that controls the heat pump. I believe that's usually the yellow wire.

I once had voltage at the thermostat end but not the control board end. The wire had broken about 4 feet from the control board. I located the break by moving the cable at various points, starting at the control board.

[Stormin Mormon]

To isolate the problem, going to take some trial and error, and some testing. The corroded contacts inside the stat are good place to start. Do you want a few ideas? I'm much less skilled with heat pumps, but happy to share what I know.

Was the readout on the stat saying it was calling for cooling?

[Stormin Mormon]

I think you are on to something, here. Of course may HVAC techs are not able to take VOM readings, so it's good to be careful encouraging a Usenet list reader to take readings.

Jan, are you skilled with a volt meter?

[NotMe]

Check to see if the leaking fluid has not dinged the hidden circuit.

[trader_4]

If it was me, I'd figure out the two wires that need to be connected at the thermostat to make the system cool. Probably red and yellow. Connect with a jumper and see if the system works OK during observation. If it does, then it's likely a bad thermostat, corrosion damage, etc. During testing, the system should be off for about 10 mins before restarting. Thermostats do that, by directly connecting wires, you would need to make sure if you stop it, you allow that period before restarting.

[Tony Hwang]

Hi, Yes. 'stat has built-in pause period to protect compressor for too many cycles without resting(like safety lock out) Think this is unique feature for heat pump controlling thermostat. Dumb 'stat won't have this important feature, only tjhing it has will be an anticipator.

[J Burns]

My thermostat is for conventional heating with air conditioning. I bought it to replace a mechanical thermostat. I was glad to read about the delay in the specifications.

When the display says "cool," that's supposed to mean the cooling relay is closed and there's 24 VAC on the terminal for the yellow wire. When I went to run the AC for the first time this year, I turned the thermostat from "off" to "cool." "Cool" flashed and the AC didn't come on. Seven minutes later I realized it was the delay. Now I know what it means when "cool" flashes.

I want to sue the manufacturer for my 7 minutes of anxiety! I hope he has deep pockets!

[hrhofmann]

On my Hunter thermostat, if I go directly from off to cool, there is a dela y in starting the indoor air circulating fan and the outside compressor and cooling fan, I don't remember now how long. If I go from off to heat for a couple of seconds and then quickly go thru off to cool, the compressor an d fan and indoor air circulating fan all start up immediately. I have been doing this for years, that's why I don't know how long the delay is when g oing directly to cool mode, but it hasn't seemed to have harmed anything.

[Jan Philips]

It was working again until today. The thermostat is a basic Aprilaire model.

I left it set on 74 and it was 77 when I got there - same as before, but not so hot, so it must have been working until a few hours ago.

I went through the procedure that worked the other two times. This time it did not work. The 'stat has an indicator that indicates that it is calling for cooling. It was flashing "cooling" when I turned it back on. Then after the minute or two delay, I heard the click that is made when the compressor starts. But the compressor did not start.

We are renting out this house starting sometime in September, so I need to make sure it is fixed, so I called my usual repairman.

Thanks for all of the helpful replies and ideas.

[J Burns]

What about the blower that blows air across the evaporator indoors? Does that come on?

If the contactor clicks but the compressor doesn't start, and it works normally other times, that sounds like a bad contactor. I've read about the opposite situation, where the contactor sticks and wrecks the compressor. It happened to me, but I had the great luck to catch it even before the blower turned off.

It seems to me there were 13 wires connected to my contactor. I had to get a clipboard and draw a diagram to replace it.

[gfretwell]

Be aware there may be a time delay relay in the compressor control box that prevents it from starting within 3-5 minutes after the call for cooling on a restart.

[trader_4]

Which could be a factor, especially since she said the thermostat was

*flashing* cooling. I think that's a sign it was enforcing it's delay before restarting, meaning the thermostat thought the compressor had been switched off within the last few minutes. As you point out, if the compressor also has it's own protective delay, it could just be that the delay at the compressor is longer than the delay of the thermostat.

The obvious questions is what happened after about 10 mins? If it didn't start then, she can rule the delay out. Also the fact that the temp was 77, while set to 74 suggests something isn't right.

[J Burns]

Switching the thermostat off could account for the two-minute delay while the display flashed. The subsequent click shows that the compressor was getting the "start" signal.

My guess that it was a bad contactor was ignorant. It's usually the capacitor. They can lose capacitance over the years. If the compressor starts sometimes but not others, the capacitance is probably marginal.

Replacing a capacitor seems like a DIY job. Buy the right size and switch off the breaker.