older gas furnace standing pilot
temp star model NUGG10DF03

furnace will light after reset of switch and thermostat.
runs, but the burners come on for couple minute then shut off and repeat til reaches temp of stat.
fan runs thru whole process and shuts off when exchanger is at correct temp.
but after couple hrs of use it will not restart and I have to reset it again.
thermostat has new batteries. new furnace filter.

any ideas to check?

 

The burners usually cycle when the heat exchanger is over heating. That is cause by lack of airflow thru the furnace. Have you checked/replaced the filter ?

Also check for blocked returns and supply registers that are closed off.

 

temp star model NUGG100DF03 forgot 0 in first post.

It has new filter.
the dial is set at 200 for high temp
the big problem is at some point the furnace wont relight and I have to power off and start it over again.
I got 28 volts to the gas valve
pilot will stay lit but

 

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When the furnace overheats, the high temperature limit switch shuts off the burners while the fan continues to run to get rid of the excess heat. That's what your furnace is doing here.

While this could be caused by something else, odds are it is caused because the furnace is overheating. You should use an AC voltmeter to check to see if the limit switch is opening when the burners shut off, and closing when the burners turn back on again.


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As described above, the fan continues to run when the limit switch opens.

Describe in more detail what is happening at this point. Does the fan continue to run? If you have 24 VAC to the gas valve, the gas valve should turn on.

If you have 24 VAC to the gas valve and the gas doesn't turn on, you may have a gas valve that is sticking and failing to open when it should. That would likely require replacement of the gas valve, which is not a DIY task.

 

thx for replies

It does over heat and causes the burners to shut off.
it hits the 200 degree mark on the dial when it shuts down.
Im not sure what could have caused this. Nothing has changed in furnace.
Is it possible chimney plugged??
any possibility of cracked heat exchanger?

 

Do you have an A/C coil in that furnace ?
How is the air coming out of the registers ..... strong ?

Allowing the furnace to constantly bounce off the high limit can crack the heat exchanger.

Try running the furnace with the blower door open (safety switch held in) and see if it still overheats.

 

no a/c unit.
I will try the door tomorrow and post results.

 

Ok I ran furnace with doors open. It ran constant and did not overheat the dial for exchanger. Registers are all open

 

If it operates with blower service door open..... you have a return issue. Undersized duct work is one problem.

 

Nothing has changed with returns. Is it possible it was this way when installed ? It's been there for years.
Would bad high temp switch act like this ?

 

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You have a defect with the air circulation system that you need to identify and correct.

Issues like insulation in the duct that has come loose and plugged things up can be an issue.

You need to check the entire system to identify what is wrong. If you can't do that, call in a repairman to help you with that.

 

It could but that's an expensive part to try on a hunch.
If you are actually watching the dial turn until it hits 200° then it's most likely ok. Usually it would get stuck if it was bad.... not read over temperature.

You may have dampers in the supply ductwork that may have inadvertently moved. I would start by checking every register for equal air flow.

If not that then like SP said... there is something restricting the return side.

 

originally, furnace would run after working door switch, but only enough to heat house once. then not relit pilot would stay lit.
Also noticed cycle problem while looking in to this.
installed new gas valve, furnace is working again. problem 1 solved.

I also did some duct work tracing, I know previous owners did a good bit of remodeling so they called it, anyways they stopped or moved some duct work that now goes to nowhere. I am going to try to add a vent or 2 for return which they eliminated to see if this works.
thanks for all help.

 

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The furnace is overheating when it gets to the 200 degree level, and at that point the high temperature limit switch shuts off the burners. The fan continues to run to cool off the furnace, and when it cools off enough, the burners turn back on.

That seems to be your description of what is happening, and it's a classic description of a furnace which is overheating.