Furnace turns off power to the thermostat


Limits are only designed to open when there's a problem, so the first step here is to find out what the problem is and fix it.

The most common cause for a high limit to continually trip, is a dirty or improperly installed return air filter. Make sure you change the filter regularly, and that you're using one with a similar density to the one recommended by the manufacturer's documentation.

Restricted return and supply ducts can also reduce air flow through the system, which can lead to the furnace overheating. Make sure all return air vents are clean an unobstructed by furniture and other obstructions. Also make sure that all supply registers are open, and unobstructed.

Check the blower compartment for dust/dirt/debris. It's not common for the blower to get dirty, but it does happen from time to time. This is especially common, when a furnace is run without a filter, or with an improperly fitted filter.

A dirty blocked up evaporator coil, could also...

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This is old but still applies today.

How many mail list subscribers does it take to change a light bulb?
Answer: 1,331

1 to change the light bulb and to post to the mail list that the light bulb has been changed

14 to share similar experiences of changing light bulbs and how the light bulb could have been changed differently

7 to caution about the dangers of changing light bulbs

27 to point out spelling/grammar errors in posts about changing light bulbs

53 to flame the spell checkers

41 to correct spelling in the spelling/grammar flames

156 to write to the list administrator complaining about the light bulb discussion and its inappropriateness to this mail list

109 to post that this list is not about light bulbs and to please take this email exchange to another list

203 to demand that cross posting to other lists about changing light bulbs be stopped

111 to defend the posting to this list saying that we all...

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Before you assume your furnace or A/C is malfunctioning, try these simple steps to make sure it isn't a problem with your thermostat. Thermostats are simply a switch; they turn the furnace on and off. The only big difference from a light switch is that they have a mechanism that measures the temperature and when it reaches a predetermined point, it activates.

If you turned on the thermostat and the heater (or A/C) didn't come on, it may be a problem with the thermostat and not the furnace. First things first, make sure that all the settings on the thermostat are as they should be. Make sure the "ON" switch is on, make sure it is set to "HEAT" or "AC" as appropriate, make sure there are no programming overrides. I've turned up the heat and had nothing happen because I failed to notice that someone turned it to "AC".

If your thermostat is battery operated, try installing a fresh battery. Even if the display is lit, a weak battery may be a culprit.

If the...

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Though HVAC professionals should repair most furnace problems, you can take care of a few furnace repair tasks yourself.

Is your furnace not working? If your heater doesn’t heat or works poorly, this guide will help you troubleshoot and fix typical problems.

Need furnace help NOW? Get a Local Heating Pro Fast!

If your home is heated with a forced-air heating system, a furnace or a heat pump is at the heart of it. Here we look at how to handle furnace problems from a furnace working poorly to a furnace not working at all. For heat pump problems, see Heat Pump Troubleshooting & Repairs.

Though forced-air furnaces are normally quite reliable, they can break down. Because of this, it pays to take care of a furnace and know how to fix it when something goes wrong. Inevitably, a furnace stops working when you need it most. Consequently, fixing it is urgent. The following instructions will help. With a little...

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Turn the power to the furnace off. The back of the thermostat will typically have little letters corresponding to the wire color, and if you are installing new wire and have to connect the furnace, the furnace terminal board will be the same or there will be stubs of wire in the proper color you can connect to.

If there is any confusion consult the furnace manufacturers installation manual for the connection points. It will still refer to the thermostat wire colors.

Red (R) is the 24 V power up from the transformer in the furnace. White (W) is the furnace burner circuitry.

Green (G) is high fan speed on, used for AC and when the thermostat is switched to “fan on”.

Yellow goes outside to the compressor switching relay outside on AC units.

Check and double check your wiring before turning on power, then test operation by turning up the t-stat all the way to see the furnace come on, then all the way down to make sure it goes back off. That’s not a...

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Common Thermostat Problems...more advanced to follow, but start with the basics first.Work with the power off whenever possible.When a furnace does not work it may be caused by problems with the furnace or the thermostat.

Before troubleshooting your thermostat, check to see if all filters are clean and all hoses to and from the units themselves are free of wear.
If these are not causing problems, then it is time to look at specific thermostat issues and possible solutions.

Your thermostat could have aging wiring that is faulty or the transformer may require replacing.

A thermostat may need cleaning to eliminate erratic operation.

Accumulated dust is a common culprit.

First, you should have a small paintbrush, a screwdriver, and a voltmeter.

anticipator is a small metal tab positioned in front of an arcing printed scale. Give it a light push in both directions. This easy step may solve the

Give the thermostat's...

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The average furnace cycles on and off three to six times every hour. When a furnace turns on and off for very short periods of time, it’s called short cycling. Short cycling is a symptom of a larger problem, and if it’s not noticed and then corrected right away, it can cause permanent damage to your furnace, not to mention lead to higher utility bills. Learn more on gas furnace short cycling here.

Common causes of short cycling:

Dirty Air Filter

This is the most common cause and the most easily fixed. Warm air can’t pass through a dirty filter. Since it has nowhere else to go, it backs up within the unit. This sends the incorrect message to your furnace so your furnace shuts off. When your home becomes cold a short time later, your furnace starts up again. To help keep your furnace from constantly turning on and off, make sure that you change the air filter at the start of each new season and check it monthly.

Faulty Thermostat

You should...

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The furnace thermostat controls the temperature in your home during the winter heating season. Thermostats normally deliver trouble-free service over many years of operation. But like any manufactured product, thermostats eventually fail and may leave you without heat. If your furnace fails to turn on, there are some simple things you can do to check out whether the thermostat is bad. The test procedure is the same for natural gas, oil, and propane furnace thermostats.

Check that the thermostat is set for heat if it controls both the furnace and air conditioner.

Check that the thermostat's set point is higher than the room temperature and that nothing is blowing hot air on the thermostat.

Check that the furnace's power switch and circuit breaker are turned on. If your furnace has a pilot light or condensate reservoir, check that the pilot is lit and the condensate reservoir is empty.

Remove the thermostat cover to expose the inner workings and blow or...

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Furnace starts, then shuts off.

Rheem Criterion model.

It begins its power-on cycle normally. First the updraft blower turns on for a bit, then the HSI glows bright orange, then the gas turns on and the flames ignite fully and the big blower turns on.
About 3 seconds after the gas ignites, it shuts off. That is the gas shuts off, the blower continues for about a minute then it shuts off. This cycle will repeat a few times, then usually after the second or third time everything stays on and works normally.

Is there some kind of flame sensor I can check/clean/replace?
Anything else?

Would any pics be of help?

Last work done was the HSI was replaced two years ago.

How long is it after the big blower cuts on that the gas shuts off?

to dm19891

My guess would be that the pressure switch is opening.

It may not necessarily be faulty, there could be an underlying cause. When the blower kicks on, it's pulling vacuum or...

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Two nights ago my furnace began short cycling. Short cycling is when the furnace will kick on for a few seconds, then shut off for a few seconds, then back on, then back off, repeat indefinitely. It’s loud and it doesn’t heat anything. For me, the timing couldn’t have been worse: a polar vortex slipped down that afternoon bringing with it the coldest temperatures we’ve had in decades. I ended up tinkering, but couldn’t make any progress. I moved the kiddos into the one room in the house with baseboard heating, and called a service technician in the morning. With his help and the help of Nest phone support I was able to get everything back up and running. I couldn’t find this solution online anywhere, so I’ve posted it here hoping it’ll save someone else a frigid night.

I’m not a heating and cooling expert, so I’ll only claim this solution is helpful for my system as configured. It’s a common configuration, but if yours differs, you may have to do a little more digging. My...

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