burner shuts off as soon as blowers turn on!

drptpgt

Hi, I hope you guys can help me out.

My furnace works fine first thing in the morning or when it isn't used for a few hours. After the first use, when it turns on again, the burner ignite, and after a few seconds, as soon as then flowers turn on, the burners shuts off. Anyone know what I can replace, or check? Can it be the thermostat?

Thanks in advance for any help.

Joey

JJboy

First check if the blower motor is working by select "FAN ON" on the thermostat. Also test in cooling mode.

__________________

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drptpgt

Yes, the blower works with "fan on" selection. Both on Heat and Cool.

av-geek

Do you have an older mechanical thermostat? If so, inside the thermostat is an adjustment called a "heat anticipator" It is designed to shut the system down early so that it doesn't get too hot. It is generally only needed on radiant systems and hot-water systems that can continue to heat long after the burner has been doused due to the thermal mass of the system. On a forced air system, the heat anticipator should not be on at all. It could be shutting your burner off early, before the furnace is warmed up if it is turned up all the way.

drptpgt

I have a digital thermostat. I don't think it has one of those switches. Should I try replacing the thermostat? I had a hunch it could be the thermostat. It's a cheap $20 unit.
Joey

Houston204

What's the make and model number of your furnace?

If it is ~ 20 years old it may have an old fan limit...

drptpgt

It's a Rheem, I'll check model number when I get home.

Joey

JScotty

If you have a digital stat they don't have heat anticipators, so that shouldn't be it. Does your unit have an ignitor or is it a pilot light system? If it has an ignitor how long do the burners stay lit before turning off? If it's less than 10 seconds your problem is likely a bad flame sensor.

When a furnace with an ignition system starts it allows the burners to run for about 7 sec. If it doesn't receive a signal from the flame sensor in that time it will shut the burners down. These are notorious for working sometimes & then not working at other times so that may explain why it seems to work in the mornings but not later on. If that's the problem the solution is simply to clean the flame senson off with a wire brush or something similar

Patrick Eubanks

Answer was already given. Its the fan limit.

drptpgt

Ok its a Rheem Criterion. And it doesn't have a heat sensor rod. I think the heat sensor is a different type. The burner usually stay on for a few seconds but is it a coincidence that the burners shut off right when the fans kick in?

Marty S.

Could be several things. Does your furnace have a standing pilot or a hot surface ignitor? That age of rheem/ruud with a hot surface ignitor used the ignitor as the flame sensor. The blower moving the flame could be enough to lose flame sense and shut down the burners.