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Yourair, HVAC Technician
Category: HVAC
Satisfied Customers: 7
Experience:  Over 30 years of experience, licensed, Refrigeration, pipefitting, gas fitting and Burner tecnology. CM member of RSES
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How to tell if my thermostat is bad?

Customer Question

How do I know if my thermostat is bad?

Submitted: 5 years ago.
Category: HVAC
Expert:  airheatman replied 5 years ago.


To test to see if the problem is with the thermostat, do this. Disconnect the wires at thermostat (write down where each wire goes so you will know how to reconnect). Then touch R and G wire and see if blower comes on. Then touch R and Y together and see if outdoor unit comes on. If the blower and outdoor unit come on, the issue is thermostat and will need to be replaced. If not, the thermostat is not the issue and you will need to troubleshoot the unit to determine the issue. If you have a volt meter, I will be glad to assist if you need me to.

Customer: replied 5 years ago.

My son does have a volt meter and he just got here. We actually think it may be the central air unit but it is fairly new. I'll have him check the wires first.

Expert:  airheatman replied 5 years ago.

Great. Try what I said with thermostat, then let me know and we can go from there.

Customer: replied 5 years ago.

I have 5 ports - RH, RC, G, Y, W

Red is on RH; red is on RC; yellow is on G; white is on W; nothing is on Y

but... yellow is tagged G

No fuse but there is a resister

What wires need twisting together to bypass the thermostat?

Expert:  airheatman replied 5 years ago.

That is a bit strange. Here is what is normal. Red is to RC and if only one transformer used, a jumper from there over to RH. (That provides 24 volts power to the thermostat.) Then there is usually a green wire to G (That controls blower). Next is white to W (Which controls heat) and then a yellow on Y (To control outdoor unit).

If you have nothing going to Y, your outdoor unit would never come on. Has this been wired like this and worked in past and is this the only thermostat you have in the home?

Customer: replied 5 years ago.

Yes - it has been working, I've been here for 10 years and it has been this way.

Expert:  airheatman replied 5 years ago.

If I were there, I could figure this out, but since there was no wire to Y , I am a bit at loss for telling you which wires we need to jump to by pass thermostat. As mentioned, Red is power. When thermostat calls for fan, contacts close between R and G and send message to blower. On call for cool, contacts close between R and Y and send message to outdoor unit and on call for heat, contacts close between R and W and send message to heat to come on. Since you had nothing to Y, I have no idea what was bringing on the outdoor unit. Can you locate the wires from the outdoor unit and tell me what they connect to?

Customer: replied 5 years ago.

Thank you.

Expert:  gotoman for ac replied 5 years ago.

What is make and model number of equipment.

Customer: replied 5 years ago.

It is a Honeywell. I do not see a model number on it. Atleast one that is clearly marked. It is not digital. It is a mercury thermostat.

I checked the Honeywell webpage and they say the model number will start T, TH, RTH, C, and CT. I have not seen any of those markings.

Expert:  gotoman for ac replied 5 years ago.

What is the model number of the air conditioning equipment.

Customer: replied 5 years ago.

Coleman P1RA series.

Also... have checked it with a multimeter and it has approximately 25 volts.

Expert:  gotoman for ac replied 5 years ago.

Go to outside unit and take off the access panel. 2 things to check.

First make sure unit is calling for cooling, check to see if the contactor (large black relay is energized). The center will be pulled down. If it is energized feel motor on top of unit to see if it is hot. You have voltmater so check voltage on one side of contactor. You should see where power wires go into terminal. Check across the two terminals for 220 volts.

Water safety switch maybe?

Customer: replied 5 years ago.

I opened up the unit. I tested the transformer. I'm getting 240V on one side and 24V on the other. I checked the relay. Across L1 I'm getting 240V. Across L2 I'm getting 0V. I checked both sides of L1 to ground and got 120V. I checked both sides of L2 to ground and got 120V. I checked across the coil and got 0V. I checked one side of the coil to ground and got 7V and checked the other side of the coil to ground and got 7V. This was all done with the unit energized and the thermostat calling for cool. I never hear the relay click as though is it trying to switch. I'm assuming from this information that either the relay switch is bad or the relay coil is bad. Either way the relay needs to be replaced. Am I correct in this assumption?

Expert:  gotoman for ac replied 5 years ago.

Transformer is good. Does blower work? I will try to pull wiring diagram.

Customer: replied 5 years ago.

Nothing on the outside unit turns on. The fan doesn't move and the blower never turns. I can hear the transformer hum. The relay never clicks.

Expert:  gotoman for ac replied 5 years ago.

On outside unit you will have wires Red ,green, white and yellow. Jumper between the Red and Green to see if fan comes on. We are checking wiring and thermostat now.

Customer: replied 5 years ago.

On the outside unit I have 2 wires going into the house. White and Red.

Expert:  gotoman for ac replied 5 years ago.

What is throwing me is the Y terminal not connected. This may be one you have to be onsite to figure out.

Do you have an indoor electric coleman furnace.

Customer: replied 5 years ago.


My son solved the problem.

Expert:  Yourair replied 5 years ago.

I am also curious to know if your indoor blower control through your thermostat operates your a/c outdoor unit.

Here is why I ask. Your thermostat wiring having 2- reds, 1 on RC and 1 on RH usually indicates 2 separate 24volt power supplies, one from the outdoor condensing unit and one from your heat source, boiler, furnace etc. As the Airheatman indicated if there is a jumper between RC and RH then its only one 24 volt supply.

As stated above the Y terminal controls the cooling at the thermostat, the G terminal controls the indoor blower and also makes when the Y terminal makes.

So not having a wire on Y indicates that the cooling circuit is probably being made through G.

If you have a switchable subase (an optional component) it will have two switches

1 will be fan + auto this allows you to let the fan (indoor blower) cycle with the cooling, or you can operate just the fan continuously by selecting on.

The other switch is cool, off, and heat it allows you to switch between heat and cool.

If you do indeed have a switchable subase, switch the fan to on and see if the outdoor condensing unit operates, it will operate if it is wired through the fan circuit of the subase regardless of temperature setting or mode selector setting.

If this is the case, it will work, but is not right and should be corrected as it could cause a failure of your compressor.

Also if you could share with us how your son solved the problem it would be greatly appreciated.

Stay cool and thank you.

Customer: replied 5 years ago.

What I did was replace the contactor. Once that was done I was able to open the contactor and press it down manually. When I do that the system turns on and the fan spins. However, the unit ONLY WORKS when I do that. Otherwise the unit still will not turn on. I have been told by a few people that the unit may require a charge since about a week before it broke it froze up due to someone setting the thermostat at 40 degrees for an extended period of time. What you said about the wires and the different thermostat setting is exactly what she has in her home. But even if I turn it to fan on and set it to cool it will not work. So is the problem fixed? Is the only thing left to charge the freon? Or do you think it is a compressor issue?

Expert:  Yourair replied 5 years ago.

Many things can cause it to ice up, one being set too low, two being low airflow, etc.

You will need to get charge checked to see if the charge is low. I have pulled wiring diagram and some units are equipped with low pressure controls.

Check for voltage at the red and white wires outside at the condenser. If you have 24 volts there then the problem is with the controls in your outdoor unit.

If you have no voltage, then take the thermostat off, measure voltage between

RC and G.

RH and G

RC and W

RH and W

Was the contactor you replaced stuck in the open position?

What are the complete model numbers.