I just installed a smart LUX thermostat and I know it's right but even with my old thermostat it was doing the same thing. My temp will be set at 70 yet its 62 in my house. I have to go to the basement and turn the power to the gas furnace off then on. It kicks on with in a few seconds and runs fine the later in the day I'll have to do it again. Wtf is going on with this thing? It just started doing this with in the last week it is a heat and air unit that's gas and I know its wilted right because it will work fine once I power cycle it.
Why does the furnace need power resets all the time
furnaceheating
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I had the exact same symptoms with my natural gas furnace, and it turned out to be the flame sensor.
It's an L-shaped metal bar (but can also be straight, depending on model) that detects if there are flames actually coming out, and turns off the gas if not. It had a white high-temp wire connecting it to the control board. Taking it out, cleaning it with some steel wool fixed the problem for me.
If yours is like the above, you'll only see the back part of it and the bracket, as the L-shaped end is inside the heating unit.
If the high limit switch was bad, the furnace should not fire. When the high limit switch trips, the flame/heating element will be shut off and the blower will continue to run to clear the heat. The blower will continue to run until either the system is reset, or the limit switch closes.
I believe high limit switches are normally closed, and they should always fail open. Which means when they go bad, or the temperature gets too high they open the circuit. To test a limit switch, you'll want to check continuity to determine what state the switch is in.
- If there is a serviceman switch, turn it off. If not, shut the power off at the breaker panel.
- Open the access panel and locate the high limit switch. It should be somewhere on the hot box.
- Disconnect the wires from the switch (don't forget which goes where).
- Using a multimeter, test for continuity between the leads on the switch.
There is an easier test to check limit switches, but it should only be performed by HVAC techs. So I will not discuss it here.
If the circuit is open, you'll want to replace the limit switch.
If the limit switch is adjustable, make sure it is set properly according to the manufactures specifications.
If the limit switch tests OK; or you've replaced it and still have the issue, you'll want to focus on the thermostat.
From your description, it sounds more like a problem with the thermostat. Most safety mechanisms in furnaces do not let the furnace fire if there's a fault, so if you're getting flame I would not suspect a limit switch. I would suspect either the thermostat has gone bad, or it is miswired. If the fan is always on I would suspect either the fan switch has failed closed (is always in the ON position), or the fan wire was incorrectly bonded to the voltage feed line.
Thermostats are basically switch boxes. They'll have one live wire [R] (usually 24v in the US, or line voltage in other parts of the world), one heat call wire [W], one blower fan call wire [G], and optionally a cool call [Y], and/or C wire. When the thermostat wants to warm up, it will connect the R to W and G which tells the furnace the thermostat is cold and needs heat. If it's hot it connects the R to Y and G, to tell the furnace it's hot and wants to be cooled down. If you have a FAN control and you turn it to ON, the thermostat connects R to G and the blower fan turns on.
The easiest way to rule out the thermostat, is to replace it. Troubleshooting thermostats is a pain in the ass, so it's much easier to simply switch it out.
If you've changed the thermostat and you're still having the problem, call an HVAC tech.
Best Answer
It sounds like a safety circuit is telling your furnace not to turn on again until it is serviced. A dirty air filter can be the cause, your control board usually has LED’s that indicate what the fault is by a sequence of flashes. It is best to find the cause of the fault not just powercycling. The brand and model of the furnace and the sequence of flashes could tell us what the fault is. A dirty filter is a first thing to check as they are most often overlooked.