Home AC thermostat

[Jimmy9190]

I think the thermostat in my house is bad. When I set the AC temp on 78 and the room temp thermometer on the thermostat reads 72, the AC still runs. At night it will cycle on and off more frequently but during the day the AC runs all day long. It never reaches the setpoint. If I have it on 72 the room thermometer reads about 75 and the AC still runs constantly.

Any ideas here would be helpful. All I can find online is how to clean the thermostat. I don't think dirt is the problem here but I can't find anything on diagnosing a defective thermostat.

 

[brianl703]

Is the thermostat level on the wall (assuming this is the old mercury bulb kind)?

 

[Jimmy9190]

yes it is level, and it is the old style with a mercury bulb. I am thinking of buying a new digital one today and replacing it to see if it helps. If it doesn't maybe I can get a refund and try to find some other cause.

Thanks.

 

[labman]

There are 2 switches in a thermostat. One is the mercury bulb or whatever they use now. It switches power from the red wire to one or the other of the other 2 terminals. The other switch connects the terminal hot when below the set temperature to the white wire to the gas valve in the heat position. In the cooling position, it connects the terminal hot when above the set temperature to the yellow and green wires to the AC and fan. It sounds to me like that switch is bad. I think they are double pole, double throw switches just using 4 of the 6 terminals. You may be able to find a new one at Radio Shack.

If you buy a digital thermostat, make sure you have a 5 wire cable to it if it needs one.

 

[Julian]

Originally Posted By: labman
I think they are double pole, double throw switches just using 4 of the 6 terminals. You may be able to find a new one at Radio Shack.
Is Radio Shack still around? The ones we had here were a big ripoff. The only thing they were good for were those electronic parts you couldn't find anywhere else, and then they charged you an arm and a leg for them. Probably why I never went there that often. Probably why I'm not being much help right now.

Good luck with your thermostat Jimmy9190.

 

[Gary Allan]

Originally Posted By: Jimmy9190
I think the thermostat in my house is bad. When I set the AC temp on 78 and the room temp thermometer on the thermostat reads 72, the AC still runs. At night it will cycle on and off more frequently but during the day the AC runs all day long. It never reaches the setpoint. If I have it on 72 the room thermometer reads about 75 and the AC still runs constantly.



What I see missing here is that it indeed shuts off when you set it to the current temp. That is, suppose your system can NEVER reach the setpoint at 100% duty cycle at that ambient temp? Your indicating that it cycles at night sorta points to a capacity issue and not a thermostat as the problem.

Or did I really miss something (I may have).

 

[labman]

Originally Posted By: labman
There are 2 switches in a thermostat. One is the mercury bulb or whatever they use now. It switches power from the red wire to one or the other of the other 2 terminals. The other switch connects the terminal hot when below the set temperature to the white wire to the gas valve in the heat position. In the cooling position, it connects the terminal hot when above the set temperature to the yellow and green wires to the AC and fan. It sounds to me like that switch is bad. I think they are double pole, double throw switches just using 4 of the 6 terminals. You may be able to find a new one at Radio Shack.

If you buy a digital thermostat, make sure you have a 5 wire cable to it if it needs one.


I am afraid I misread the question. Having reread it, I don't understand at all. If you set it on 78 when the temperature is 72, it shouldn't run at all. Even if it does run, it should eventually cool the house off enough to shut off if the A/C is working right. Sounds like you may have more than one problem.

Try disconnecting the wires from the thermostat. If the A/C still runs, you have a problem elsewhere.

 

[Gary Allan]

I misread it too

I inverted the numbers. He sounds like he has a bent needle. If it was "switched" wrong (trying to prevent reaching a setpoint (cooling) and trying to maintain it (heating) ..then it would explain the observations, but it should never cycle in that condition. It would always be cooling when below the setpoint ..and never reach it EXCEPT when the cooling system ran out of capacity.

 

[doitmyself]

A thermostat is just an on/off switch. If you have a multimeter and can figure out which wires go to what, it isn't too hard to test this low voltage circuit for on/off function when you turn the adjustment up/down. Almost don't need a multimeter.

It could be a bad/stuck relay in the air compressor.


try googling "troubleshoot thermostat".....lots of articles.

Good Luck.

 

[duaneb9729]

it really sounds like the thermostat isnt level, but on old honeywell round thermostats there is a resistor that goes bad once in a while, sort of like a cooling temperature anticipater.

you have to remove the top portion of the thermostat to get at the screws to level it. first you pop off the ring around the outside edge, then you unscrew three screws, these screws do not come out, just loosen them, you will feel when they are disconnected.

take the top portion off, what you have left is the sub base, the top of the sub base is flat and you just set a level on it, and if needed you just loosen the screws holding it to the wall slightly and adjust it till its level.

if you find the thermostat level and the sensing bulb not heavy with dust, you could have other system problems

 

[Jimmy9190]

Wouldn't you know, the AC is cycling tonight when I came home. I am planning on taking the thermostat down for inspection as soon as I can. The outside AC unit and the inside air handler both seem to be working OK. I cleaned the coils on both and removed the squirrel cage fan and cleaned the blades. The ductwork in the attic is OK, I taped up a few leaks. The air filter is clean too. The AC always runs non-stop most of the time during the day. I am in Florida so it's still over 90 every day here right now. I don't know why the AC runs constantly. And I have yet to locate an honest AC company that will give an honest and/or fair estimate on the cause and the repair. No offense to HVAC guys here, but most AC guys here are really good at ripping you off.

I am going to keep Googling "troubleshoot thermostat" and see what I come up with.

Thanks for the help here.

 

[greenaccord02]

http://www.hunterfan.com/product_detail.aspx?id=7394

I got that one when my old one did some similar stuff to what you describe. It solved all my problems. It's touchscreen and the backlight is indaglo (sp?) red or blue depending on if the A/C or heat is active. It switches between cold and heat automatically and you can set separate thresholds for each at four different points during the day every day of the week. And, believe it or not, despite all the features it's really easy to operate. (OK, the reviews on the site say it's hard to operate, but I assure you those people have a mental disability. I never even read the manual.)

 

[labman]

If the A/C is coming on and staying on, but the house isn't cooling, the thermostat is doing exactly what it should. As mentioned, it is nothing but an on/off switch.

Try to get a look at the A coil in the air handler after the system has been running for a while. If it is iced up, likely you are low on freon.

Note, I was given a Hunter Thermostat. I took it back because you have to set the cooling temperature 4-5 degrees above the heating temperature to make it work. I took the trash back and excanged it for a LUX, a fine product.

 

[greenaccord02]

If the house is at 72 but the thermometer is set at 78 why is the air still blowing? The switch should be off, but it's not - right? If I understand this correctly, the house is cooling but it's not stopping when it should. You sure it's not a thermometer problem?

 

[mechtech2]

I can't ignore the fact about the AC running all day and not getting to the setpoint.
This indicates an AC problem, not exclusively an thermostat problem.

 

[labman]

+1 A new thermostat won't fix that.

 

[greenaccord02]

Originally Posted By: mechtech2
I can't ignore the fact about the AC running all day and not getting to the setpoint.
This indicates an AC problem, not exclusively an thermostat problem.


Oh, I didn't get that part. In that case, it's probably not exclusively a thermostat problem, if at all.