Thermostat loses, then regains 24v power


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Old 05-22-22, 10:00 AM
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Thermostat loses, then regains 24v power

I have a Lennox furnace, A/C and controls which were installed new in early 2016. The thermostat is a 10F81.

Two days ago I noticed that the tstat did not "wake up" when touched. It would not disply. The screen was black. It was working earlier that day. Sometimes it takes two or three tries to make it display but but on this day it would not respond to repeated attempts.

Here is what is happening. If I remove the tstat from it's wall mount and leave it off for awhile and then re-install it the display comes back on and I can operate the furnace or AC as normal. After another short while it will seem to "die" again. No display and hence, no operation. Remove the tstat again, wait awhile (a few hours), re-install and everything works as normal until the display "dies" again.

I've done this several time over the last few days and it has repeated this behavior. Just today I had it working as normal, then the display went black again but the heat was still on.

After the first failure and re-installation of the tstat I was advised elsewhere online to locate a float switch that may have been stuck. I was told what to look for and where, but if this switch exists, I haven't found it. So I can't rule that out as a cause. But from the way it is behaving I tend to think this is not the problem.

I was also advised that it could be a fuse for the 24v transformer but the fact that it stops working and then starts again indicates that this is not the problem.

I have also read that this particular model tstat malfunctions at some point and needs to be replaced. This is a discontinued item and the ones that are still available are pricey. The Lennox replacement for this unit is very expensive.

Any advice would be appreciated.

 
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Old 05-22-22, 02:58 PM
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My guess is there is a loose connection. Best approach is to place a multimeter on the 2 t'stat wires providing 24 vac at t'stat base. Make sure all connections on the t'stat base are tight. If the voltage is steady, the t'stat needs replacing. If not find the other end of cable attached to t'stat. Should be near/at furnace. Make sure all connections on t'stat cable are tight. Place a multimeter on the 2 t'stat wires providing 24 vac at cable termination. If the voltage is steady, the t'stat cable needs replacing. If not you are into the control board which is usually not a DIY diagnosis.
 
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Old 05-22-22, 10:53 PM
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You need to go to the furnace. See if there is a sight glass in the blower door. You need to verify that the fault light is blinking. If it is blinking..... count the flashes as it will give you a code. You can open the blower door after you have obtained the code to see a list of faults.

It sounds like your furnace is going into fault. A fault in the limit circuit will remove power from the thermostat. Removing the thermostat or turning off the furnace power should reset it instantly. Some furnaces will reset automatically in one hour.
 
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Old 05-23-22, 06:29 AM
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Hi, I am sure you have replaced the batteries!
Geo 🇺🇸
 
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Old 05-23-22, 11:08 AM
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That's a comfort wifi thermostat.
It appears it may have inherent defectives that the manufacturer is aware of.
It's an expensive thermostat.
Apparently Lennox may offer some replacement help. Give them a call.... 800-953-6669.
 
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Old 05-24-22, 07:03 AM
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I have checked and re-checked the control voltage and it is constant at 23.8v.

I contacted Lennox at that 800 number but the tstat is out of warranty.

So I guess I need a new one. Any suggestions on that?

 
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Old 05-24-22, 01:40 PM
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Yes.... I'm sure it's out of warranty but they don't offer any help ?

Standard single stage gas furnace ?
Single stage A/C ?
Are you looking for a wifi replacement ?
A picture of the wiring on the thermostat would be helpful too..... How to insert pictures.
 
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Old 05-25-22, 08:08 AM
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Help? No. They sent me a list of local Lennox dealers. What kind of help were you thinking of? Maybe I should call back.

I presume single stage in both cases.

Can I use something other than a WiFi? I don't control the heat or AC remotely.

This is a poor pic but it's the best I can do right now. Four terminals. Red wire to R; white to i+ ; blue to i- ; yellow to C. There is a green wire which is not connected.

I thought blue was common and yellow was AC. But anyway I used the R and C terminals when checking for 24v.


 
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Old 05-25-22, 09:06 AM
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Hi, any 2 stage stat will work, most list 2 H & 2 C but you will only use one of each, depends on how much you want to spend, try to find one that does not need batteries, but it will require a common connection which I believe you have “C”,
Geo🇺🇸
 
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Old 05-25-22, 12:27 PM
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@ Geo..... you are incorrect. That is a communicating thermostat.
It cannot be directly replaced with a standard thermostat.

Unfortunately I'm not a Lennox dealer so I'm not sure what's available for a basic communicating thermostat.
I will contact my forum partner to see if he knows.
 
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Old 05-26-22, 05:31 AM
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Hi, can you post a clearer pic of the thermostat connections and also at the furnace, as Pete mentioned I may have given bad advice.
Geo🇺🇸
 
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Old 05-26-22, 07:26 AM
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Geo

Sorry. I wish I could help you but that's as good a pic as I can provide at this time.

I described it in my previous reply. What can I tell you? What do you need to see?
 

Last edited by Axminster; 05-26-22 at 08:32 AM.
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Old 05-26-22, 08:21 PM
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Communicating thermostat......
Red wire to R
White to i+
Blue to i-
Yellow to C.

Wow.... after sticking it to everyone they discontinued that thermostat.
It is replaced with a 12U67 iComfort s30 system. Still expensive.
I'm still checking around for other alternatives.

Lennox 12u67 iComfort s30 system
 
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Old 05-30-22, 08:47 PM
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I'm going to have to get something pretty quickly.

I intend to call Lennox again and try to talk to someone a little more knowledgeable.

But it looks like I will have to pay through the nose. It's getting warm here and I need that AC working.

 
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Old 05-31-22, 10:55 AM
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Are any of these communicating thermostats?

Lennox ComfortSense 7500, ComfortSense 5500 and ComfortSense 3000.
See below.

Lennox customer service says they will probably work in place of the iComfort Touchscreen. Not too sure about the 3000. It looks pretty basic.

https://www.lennox.com/products/comf...ls/thermostats

I do not need or want the WiFi or voice control features of the iComfort S30, E30 or M30, not to mention the cost.

I can upload the installation manuals if you need to see the wiring.

Edit: now that I have asked, I see elsewhere that this is not a communicating thermostat so I guess I will have to bite the bullet unless someone has an alternative.
 
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Old 06-24-22, 04:26 PM
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It took awhile but I was actually able to have this thermostat repaired.

I had to send it back after the first repair attempt because it continued to exhibit some of the same bad behavior.

It cost a pretty penny but it was still the least expensive solution.

So far it is working as it should.


 
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Old 06-24-22, 08:44 PM
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I had hoped a Lennox tech would have stopped in.
I had no idea they were servicing those thermostats.
 
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Old 06-25-22, 02:37 AM
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I had hoped a Lennox tech would have stopped in.
I had no idea they were servicing those thermostats.
They're not. Lennox was no help in this matter. All they wanted was for me to buy the S3 and have it installed by a Lennox dealer.

Tstat was repaired by Radwell International at their St Charles, MO location.

 
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Old 06-25-22, 06:54 AM
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Wow... pricey repair service. I may have to look into that.

For those following along or considering repair..... Radwell service.
 
 

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