# Power to thermostat from HVAC is intermittently cutting out



## pdp76 (Mar 14, 2011)

My thermostat seems to be occasionally losing voltage on the power (R wire) coming from the HVAC. It seems to happen only when the heat is on and running, only if it's been running for a good 10+ min. After the power is lost, it will turn back on again, usually within a couple minutes. When this happens, the thermostat will turn the heat back on. But, like a most HVAC systems, it will not turn the heat back on until it throttles down and wait a minute or two. After that, the cycle repeats, but the next time it turns off, it happens much quicker than 10 minutes. I don't think it happens when the heat is off because the thermostat still turns on the heat consistently. The only two explanations I can think of are:

1) The control board is going bad
2) The 120V->24V transformer is going bad

I don't know how likely #2 is since the problem is intermittent; can a transformer only not work intermittently?

I can say for sure the 120V source is good.

Basically, I want to know if there are any more troubleshooting items I can perform before going out and getting a new control board? I have an older Payne/Carrier unit, I'm not sure what model number it is, but I attached a picture of the serial number plaque (sorry about the quality) as well as a picture of the control board packaging. Thanks!


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## how (Feb 26, 2011)

Do you have an diagnostic led on your board that is flashing a code?
Is the furnace overheating..Dirty filter/coil/blocked air return????
Have you done a temp rise test?


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## beenthere (Oct 11, 2008)

Furnace may be over heating. Check your air filter.


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## pdp76 (Mar 14, 2011)

Unfortunately, there are no diagnostic lights on the control board. This house was a rental in the past, and I just checked, there was no filter at the intake/return vent in the hallway!

Can the filter be located inside the HVAC furnace itself?

Anyway, thanks for the tips so far, is it still possible to clean out the furnace? Obviously I'm not experienced with this, but is there anything I can look for myself and clean out? And what about the coil mentioned previously?


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## roughneck (Nov 28, 2014)

Yes, it is possible for the filter to be in the unit itself. There should be access doors to the evap coil to clean it. If it's really dirty, it's best to remove it and clean it with chemical and water. 
Some units break 24V to the thermostat in the event that a safety opens.
Is a safety opening in your unit that could cause this?


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## pdp76 (Mar 14, 2011)

roughneck said:


> Yes, it is possible for the filter to be in the unit itself. There should be access doors to the evap coil to clean it. If it's really dirty, it's best to remove it and clean it with chemical and water.
> Some units break 24V to the thermostat in the event that a safety opens.
> Is a safety opening in your unit that could cause this?


The safety switch behind the access panel to the control board was my first suspicion. I temporarily taped it down to make sure it wasn't coming loose and opening the circuit as a test, but the problem still persisted. I suppose the switch can still be broken, but it doesn't seem likely, it's such a simple mechanical switch.

I'll Google "furnace coil" to see what one looks like and I'll try to clean it. I'll also try to find the filter inside the furnace (hopefully there is one!)


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## pdp76 (Mar 14, 2011)

I soon realized that if I have a gas furnace, there should be no heater coils right?

Anyway, after looking inside the furnace some more, I found the filters. First, they were filthy, they were nearly 3 years old according to the install date written on the filter. Second, I'm pretty sure they were installed backwards. The arrow was facing away from the blower. Also, all the dirt was accumulated on the side of the filter that the arrow was pointing to.

I vacuumed up the insides as much as I could (pretty tight quarters) and tried to remove as much dirt/dust from the blower motor itself.

Hopefully this does the trick, we'll see tonight when it gets cold! Thanks again everyone.


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## roughneck (Nov 28, 2014)

If you have an air conditioner, the evaporator can collect dirt and become clogged


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## GoodToFind (Sep 6, 2017)

I had this problem: Heat Pump HC/AV unit with Honeywell Thermostat experiencing intermittent power outages. Air conditioning intermittently on, with increasing inability to keep the house cool. Had electrician check out the system, and found that the transformer on the HV/AC unit had solid 24-volt power, but saw that the electronic thermostat was sometimes blank indicating no power.

Counseled with HV/AC people and learned that my drain line was probably clogged. Service man could not get out until tomorrow. I poured 1/2 cup of Clorox into the drain line on the HV/AC unit in my closet. Left for the day.

When I returned, it was to a cool house, with the A/C running perfectly. From now on will do this every other month or so.

Hope this helps others.


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## KHouse75 (May 14, 2008)

pdp76 said:


> I soon realized that if I have a gas furnace, there should be no heater coils right?
> 
> Anyway, after looking inside the furnace some more, I found the filters. First, they were filthy, they were nearly 3 years old according to the install date written on the filter. Second, I'm pretty sure they were installed backwards. The arrow was facing away from the blower. Also, all the dirt was accumulated on the side of the filter that the arrow was pointing to.
> 
> ...


The filters being backwards is not much of a concern. They have a direction arrow because manufacturers put strengthening material on the side that faces the furnace to keep the media from being pulled from the frame. Sometimes, manufacturers shape the frame so the suction for the blower seals the edges better. Some filters supposedly have a more porous side and installing them backward can supposedly restrict air flow.

The filters not being changed for that long certainly could cause issues. The blower being dirty means the evap coil is probably pretty dirty. This would indicate filters weren't installed for a while or unfiltered air is being pulled in from somewhere.

This reminds me of a crazy filter story...Many years ago, I stayed at one of the budget Disney World resorts. The room wouldn't cool so I opened the a/c cover and there was 5 years of dust built up about 2 inches thick on the filter. Cleaned it off and it cooled off very quickly. Told the front desk and the next day, there was a cart stacked with filter and they were going room to room swapping them all out. Every time I stay at one of their resorts, the first thing I check is the filter. They've all been within 1 month of the date written on them.


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## SouthernCA (Dec 12, 2020)

Looks like I have the same problem. My ECOBEE keeps rebooting, ften when heater is turned on but not always. 

Please help me with some diagnostic steps.


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## beenthere (Oct 11, 2008)

Have you checked or changed your air filters.

What brand and model number furnace is it.


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## roughneck (Nov 28, 2014)

Probably a limit issue. 
What kind of heat is this? Make and model?


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## SouthernCA (Dec 12, 2020)

beenthere said:


> Have you checked or changed your air filters.
> 
> What brand and model number furnace is it.


I have ECOBEE and I now find out that it has been happening for the last two years without me knowing it. 

I taped the safety switch closed and so that was not the issue.

It is gas fired furnace about 25 years old. There is no filter in the furnace. Just a filter on the return path at the wall. I have changed filter recently with a merv13 filter. In any case, tomorrow, I will run the furnace without any filter and see if it stops.

I have shaken all the wire to see if there are any loose connections and the control board.

What else to check?







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## SouthernCA (Dec 12, 2020)

Photo of my furnace.


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## beenthere (Oct 11, 2008)

MERV 13 ait filter is most likely too restrictive to air flow.


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## SouthernCA (Dec 12, 2020)

beenthere said:


> MERV 13 ait filter is most likely too restrictive to air flow.


Since the problem has been happening for the last 2 years and i put in Merv13 only a month back, that nat not be a problem. Also I have removed the filter to check if that is the problem. It was not.

How to check the limit switch on the gas furnace?


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## beenthere (Oct 11, 2008)

Check temp rise of furnace, see if it is within allowable limits. Check for voltage across it when thermostat has no power.

What MERV rating was the old air filter. High limits become weak if they end up being an operating control, instead of a safety device.


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## SouthernCA (Dec 12, 2020)

been there said:


> Check temp rise of the furnace, see if it is within allowable limits. Check for voltage across it when the thermostat has no power.
> 
> What MERV rating was the old air filter. High limits become weak if they end up being an operating control, instead of a safety device.


How would I check the temperature rise in the furnace? Where in the furnace? And what would the allowable temperature limits?

The old filter was a non-Merv filter and it's Merv rating is not known

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## beenthere (Oct 11, 2008)

The furnace is the thing you took a pic of the label.

A temp probe in the return, and then put into the supply when the furnace is running, is how it is done.


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## SouthernCA (Dec 12, 2020)

beenthere said:


> Check temp rise of furnace, see if it is within allowable limits. Check for voltage across it when thermostat has no power.
> 
> What MERV rating was the old air filter. High limits become weak if they end up being an operating control, instead of a safety device.


I did. When the heat was just turned on, it was 70 F to 112 F and after some time it was 80 F to 133 F using a infrared thermometer. The house was getting too hot and so I turned off the heat. The power to t-stat had not interrupted by then.

What do you think? Is it too much of a difference?

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## SouthernCA (Dec 12, 2020)

beenthere said:


> The furnace is the thing you took a pic of the label.
> 
> A temp probe in the return, and then put into the supply when the furnace is running, is how it is done.


I did. When the heat was just turned on, it was 70 F to 112 F and after some time it was 80 F to 133 F using a infrared thermometer. The house was getting too hot and so I turned off the heat. The power to t-stat had not interrupted by then.

What do you think? Is it too much of a difference?

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## beenthere (Oct 11, 2008)

IR don't work well enough to get an accurate temp reading. And the supply and return temps must be taken at the unit, to be accurate, and not influenced by duct temp loss.


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## SouthernCA (Dec 12, 2020)

beenthere said:


> IR don't work well enough to get an accurate temp reading. And the supply and return temps must be taken at the unit, to be accurate, and not influenced by duct temp loss.


Ok. Thanks. I do have a wired thermometer as well. Tomorrow, I will try to find a place where my thermocouple can measure hot air temperature close to the furnace.

Do you know where that could be?

And what should be the max temperature there? 

Again thanks in advance.

Shailendra 

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## SouthernCA (Dec 12, 2020)

SouthernCA said:


> Photo of my furnace.


I did check my transformer. It is providing the 24v but can not say if it cuts off intermittently.

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## beenthere (Oct 11, 2008)

Might have to drill holes.

1 in between the air filter and the furnace, and 1in the supply duct right at the plenum.


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## SouthernCA (Dec 12, 2020)

beenthere said:


> Might have to drill holes.
> 
> 1 in between the air filter and the furnace, and 1in the supply duct right at the plenum.


Ok. Thanks. I will try.

What should be the max temperature for the hot side?

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## SouthernCA (Dec 12, 2020)

From the internet: When the combustion occurs and air is first heated, the temperature is between 140 degrees F and 170 degrees F. 

So it seems that the limit switch will be set above 170F or may be 200 F.

Is that about right?


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## SouthernCA (Dec 12, 2020)

Here is the problem I am trying to solve. It seems like the power to the thermostat is cut off intermittently. Many times it is when the heat is turned on but at other times, it does not seem to be the case.

The pic above (from the ECOBEE server) shows that the thermostat is rebooting every time the power is cut off. And I can see it on the ECOBEE faceplate as well.

My hypothesis was that the furnace was heating too much and the limit switch was shutting down the furnace. Question: Does the limit switch also cut off power to the Thermostat as well?

If not, why is the power to the thermostat shutting off intermittently?

Thanks in advance. I will check the furnace temperature later today.


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## beenthere (Oct 11, 2008)

Many Carrier/ICP models do.


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## SouthernCA (Dec 12, 2020)

SouthernCA said:


> View attachment 636724
> View attachment 636724
> 
> 
> ...


Ok. So I have removed the filter and open all registers. The thermostat is still getting intermittent power or there is something else wrong with the thermostat.

Unless the limit switch is defective, there is no reason for power to be shut off.

I will try to check the temperature at the furnace tomorrow or the weekend.

I just realized that there is only one return path in the hallway and registers are all in bedrooms.

That means if we sleep with the door closed, the room will not get much heat. Is that correct?

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## SouthernCA (Dec 12, 2020)

So here are the latest updates.

1. The thermostat is still losing power about 4 to 10 min after heat is called when No filter is installed and the switch to the furnace door has been bypassed

I have not been able to find a suitable place to check the hot air temperature.

The spec says 55 degree temperature rise and 175-degree max temperature.

I think this is the limit switch sensor. See pic below. Please confirm. I will check the voltage across and report. What voltage should I see across the leads?













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## roughneck (Nov 28, 2014)

You shouldn’t read any voltage when it’s closed.


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## SouthernCA (Dec 12, 2020)

Thank you everyone for suggesting that I need to do furnace temperature measurements. So here they are.

1. With no air filter in the furnace, the furnace hit air temperature is between 158 and 163 degrees. The furnace ran for more than 10.min. Without the limit switch tripping.

2. With Merv 13 filter in place, the furnace operating temperature was between 172 and 176 degrees.

The max limit switch temperature is set at 175 degrees.

While I did not wait long enough for the limit switch to trip, it was clear that the furnace is running close to the limit switch temperature.

Here are the options we have

1. Get another filter that has much less resistance. But I feel even with no filter, the furnace was operating too close to the limit switch temperature. The furnace should ordinarily operate at 130 degrees with a filter.

2. Adjust furnace heat so that it operates at 55-degree (furnace spec) temperature rise (75 plus 55 = 130 degrees). 

I want to use a Merv 13 filter as it traps 85 percent of the Covid virus.

Now a few questions:

Can I adjust the furnace temperature myself or do I need to hire a technician?

Is there an obstruction in the furnace fan or the supply ducts that are making the furnace run so hot?

Since this switch has been tripping for the last two years, this problem may be present at least for that long.

And before then, the thermostat had a battery backup and so even if the limit switch was tripping then we could not have come to know about it.

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## roughneck (Nov 28, 2014)

Your furnace filter is NOT going to trap COVID. It’s NOT going to protect you from COVID. It’s not even there to filter the air in the home. It’s there to protect the equipment. Not to filter the air in the house. Use a cheap throw away. 
You’ll need a tech to check gas pressure, and adjust if possible. And check and adjust airflow.


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## beenthere (Oct 11, 2008)

Might want to add more return so it moves enough air to stay within allowable temp rise.


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## SouthernCA (Dec 12, 2020)

roughneck said:


> Your furnace filter is NOT going to trap COVID. It’s NOT going to protect you from COVID. It’s not even there to filter the air in the home. It’s there to protect the equipment. Not to filter the air in the house. Use a cheap throw away.
> You’ll need a tech to check gas pressure, and adjust if possible. And check and adjust airflow.


 The latest research shows that Merv 13 does filter out 85+ percent of the covid droplets..

But in my case, even with no filter, the furnace is running (162 degrees) outside the specifications (130 degrees).

I will get a tech to check if the fan is operating correctly. 

Thank you.

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## SouthernCA (Dec 12, 2020)

SouthernCA said:


> The latest research shows that Merv 13 does filter out 85+ percent of the covid droplets..
> 
> But in my case, even with no filter, the furnace is running (162 degrees) outside the specifications (130 degrees).
> 
> ...


For some reason, for the last three days, power to thermostat stopped cutting out intermittently.

Ordinarily it will be a good news.

But since I don't know why it fixed itself, I may have to put my temperature probe again inside and check the temperature at which this furnace is running. By the way, MERV 13 filter is still in place. As you recall, earlier, even removing the filter altogether did not fix furnace going above the limit switch temperature.

The only thing I did since then was to remove and reinstall the same limit switch. It is not tripping anymore. Any guesses why?

I will do a recheck of furnace temperature again today.



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## moonspell2000 (12 mo ago)

SouthernCA said:


> For some reason, for the last three days, power to thermostat stopped cutting out intermittently.
> 
> Ordinarily it will be a good news.
> 
> ...


What was the outcome/resolution? I'm having the exact same issue. Thanks.


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## supers05 (May 23, 2015)

moonspell2000 said:


> What was the outcome/resolution? I'm having the exact same issue. Thanks.


It's best to start your own thread next time. Merv 13 if far too restrictive for most furnaces without ductwork modification. Open all of your vents and make sure that nothing is blocking the returns. You may have to use a higher fan speed.


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