HVAC question, help needed.

[jamesk]

Hoping someone might be able to help?
Have a Tempstar Forced air gas furnace (20 yr old)
Goes into heat mode (flame is on) for about 3-5 minutes and than shuts off...including the Thermostat power.
The blower and or fan continue to run about 30 seconds.
Thermostat power comes back on and little later the heat starts again.
Keeps happening until it reaches the set temp.
I can run the unit with heat off and fan ON, no problem.
Pulled out and cleaned the Flame sensor, no change.
Thinking this had been going on for some time now (months) now that I think about it.
Hoping for an easy fix before I call a pro in.
Any ideas?
Thanks Much!

 

[FSH 210 Sport]

Low voltage transformer going out?
change batteries in thermostat?
Is there a flash / trouble code somewhere?
If using ultra high efficiency drop in filter could be causing excessive heat in plenum.

 

[Coult45]

I just had a similar issue (different furnace and thermostat), and it was the batteries in my thermostat.

 

[jamesk]

New Filters already and I have 2 and plenty of flow.
Thermostat has no batteries, has c wire power off 24v from system and when it is running I am getting 28 volt to all the wires.
System has no display panel to show error codes.

 

[Floridaman79]

Does it have a float switch? I've had some small leaves get sucked into the drain line and lift the float. That could kill power to the thermostat depending on how it is wired.

 

[Hoppy]

New Filters already and I have 2 and plenty of flow.
Thermostat has no batteries, has c wire power off 24v from system and when it is running I am getting 28 volt to all the wires.
System has no display panel to show error codes.

So do you lose your 24v power when it acts up? I would suspect something in the 24 volt circuit to be causing a problem. Like a bad transformer, a self reseting circuit breaker, or a short pulling the voltage down like a gas valve heating up and shorting out. Or are you losing the 120v to the 24v transformer? There may be a wiring diagram inside the furnace to troubleshoot with otherwise try googling the model number.

 

[Beachbummer]

You said 28v but did you measure while it was running? If not hook it up voltmeter and leave it in place while the furnace comes on and observe voltage while issue occurs to confirm the power supply is not it.

 

[jamesk]

It runs for about 5 minutes.
Turns off the flame and turns off power to the thermostat.
The furnace still has power, as the fan continues to blow for about 30 seconds.....I believe this is a typical safety feature....The fan runs after the flame is off to remove the left over heat.
And the cycle starst all over with about a 30 second pause in run time as it restarts and the power comes back to the thermostat.
I believe something is causing it to reset this like, I was hoping I could find an easy fix but will call a pro this week.
I have called in the pros in the past to find it was a simple problem and than felt like an idiot but this seems like something more.
Thanks for trying to help.

 

[TimW451]

It sounds like the flame sensor. You said you cleaned it, but does it work? Find the specs for your model and test it with a multimeter.

I agree that the unit is killing power to the thermostat to prevent you from bypassing the powering off of the unit due to the TBD fault. I had that happen to me when a clogged drain set off a safety switch that turned off the AC and killed power to the thermostat.

 

[buckbuck]

Remove your flame sensor again and get it shiny using emory cloth. See if that fixes it.

 

[dan144k]

At the furnace. Take a picture of the thermostat wiring amd take notes.
1. Turn off power at furnace
2. Disconnect the thermostat wiring (all of them).
3. Use jumper or spare wire amd test furnace. This will seperate the thermostat from the problem/caise 100%.
3a. Turn furnace ON, no thermostat wires connected. Nothing should happen.
3b. Turn off power, install Jumper between R to G = main fan will run. Turn on power and test.
3c. Turn off power, remove jumper from R to G.... now connect R to W (or W1)= turning ON heat...turn on power and test
....
See if you still have problem. Or thermostat is the problem.

 

[dan144k]

Flame sensors (sorry Buck) never use sandpaper or emery cloth to clean. You will actually etch the metal amd cause it to rust. This is ok in an emergency, but you should replace the flame sensor before it rusts.
......
I was taught to use green scotchbrite. Or a kitchen sponge with green scotchbrite on the back of it. (Almost every kitchen has one)
The scotchbrite will clean the metal but should not etch it.

 

[buckbuck]

@dan144k Thanks for the information. No wonder I had to clean my flame sensor every year.

 

[jamesk]

It can't be the thermostat. It's been doing this for several months. I just became aware that it was a problem last week when I added a smart thermostat that is non battery powered. That's when I noticed the power going out to the thermostat and put 2 and 2 together and starting thinking that it seems like it has been starting and stopping often (sort cycle) for awhile. It runs almost 5 minutes, so it's not that noticeable. I actual thought it was the new thermostat and took it back and got a new one. I also then, ran a new 5 wire to it as well, so that answers that part of it being the problem. If the unit would shut off right away or only run a few seconds that would be one thing but I think if it was a flame sensor it would not run almost 5 minutes before shutting off, but I don't know.

 

[jamesk]

Going to try a new flame sensor, guy at HVAC store told it is possible that the unit could run for 5 minutes before shutting off with a bad flame sensor. Not feeling confident that what it is but will give it a shot. Thanks!

 

[Brad_Ct]

It could be several things. I would first watch the burner flame and compare what it looks like before and after the blower is running. If you have a cracked heat exchanger you will most likely see that the flame pattern changes and most time will pull away from the flame sensor, newer furnaces also have rollout switches that look like a small disc that sense if the flame is coming out of the heat exchanger. It could also be a high temperature limit, without seeing the wiring diagram of the unit it is harder to determine the issue but it definitely sounds like it is on the low voltage side since you are loosing power to the thermostat.

 

[dan144k]

It could be (maybe) a timer (setting) in the thermost.. I would still recomend bypassing the thermostat. Just to 100% prove it not the cause. Also sometimes the wires in the wall to the thermostat get knicked and cause weird problems.
( most people don't know too expose 1/2 inch of wires with stripper amd tear the insulation covering back to expose the multiple wires)
.....
A wiring diagram may explain some things.
......
How you say it is at 5 minutes. Have you timed it? Or guesstimate? Asking because if it am exact time that it failures each time that is valuable/useful information.
....
Do you know if the unit is 2 stage heat? And is your thermostat wired for 2 stage heat? On the control board in the furnace is there a W1 and a w2.
...example. thermostat may only be wired to W1, and switch or jumper on furnace control board tells the furnace you are only wired with single stage thermostat. Then furnace board may internally turn on W2 (2nd stage heat) after 5 or 10 or 15 minutes. Depending on manufacturer. .... you can turn of power and move the switch or jumper to tell furnace board you have a 2 stage thermostat. And then run it on w1 (1st stage heat) and see if you fault.
......
Also if high efficiency furnace with pvc pipe. Clean and flush out the drains internal in furnace. These can block water, fill up ventor motor cavity, slow down (drag) the ventor motor. And cause intermittent faults.
- ventor motor drains
- heat exchanger drains
- pvc exhaust has drain (these can fill and get blocked with pvc shavings)
- intake may have a drain
--- all these drains go to a common point in the furnace and then through a trap. Clean the trap too.
....
Did you ever give us a flashing code from the control board.

 

[dan144k]

@jamesk
Did you ever get your unit fixed?