Top 4 Reasons Why a Gas Furnace Short Cycles
Short cycling means the gas furnace is running through its full sequence of operation for heat. It’s providing heat in the building and then it’s shutting off before the thermostat is satisfied. Then it’s immediately turning back on again and trying to heat the building again and then shutting off again.
To troubleshoot a furnace, you need to know the sequence of operation for heat. To start the sequence of operation, you have to have 24 volts on the W on the control board.
So, R sense 24 volts to the thermostat, and inside the thermostat, the R and W touch, then at the control board, you have 24 volts on W and that starts the sequence of operation.
In a 90% efficient furnace, you have the inducer motor turns on first and then the pressure switch close. Then the hot surface igniter turns cherry-red then the gas valve blows.
The full gas flow then the control board senses the flame through the flame ratification process then there is a blower on delay so it’s waiting for the heat exchanger to heat up then the blower motor turns on.
That’s a quick overview of the full sequence of operations for heat.
So, what we’re talking about is all of those things are happening and then all of a sudden, the unit shuts off.
- Thermal Limit Switch
The first reason for a furnace to short cycle is the thermal limit switch is tripping. There are different types of the thermal limit switch.
Its function is to turn the furnace off when it’s getting too hot.
They are a little bi-metal disk that’s going to trip due to heat. It’s going to pop and it’s going to open up the electrical connections and send the signal to the furnace to stop the ignition process or flame process to allow the blower motor to cool down the heat exchanger.
If you see on it that says “L200-40”, that means if it gets up to a temperature that is 200 degrees then this is going to open up and it’s not going to close back down again until it gets to 160.
These are normally closed electrical safety switches. It could be up at 180 or 200, some of these types of switches could be 220 – it just depends on the furnace.
Several things could be affecting the switches and one is the blower speed. The blower motor might be running at too low of blower speed and it’s not able to keep up with the heat that’s being generated by the flames.
In that case, the heat exchanger is heating up, and then finally this limit switch trips. That at least allows the furnace to run a little bit its full sequence of operation then it shuts off during the main run time.
So, you may have to check the blower speed and increase the blower speed so that your temperature rises. You can take a temperature measurement in your return duct and your supply duct. Check your temperature differential, you want to make sure that that’s stable and it’s not just increasing.
In a single-speed gas furnace, you may have let’s say 70 degrees on the return and then 50 degrees on the supply. You want to make sure that that’s not rising.
One problem could be the thermal limit switch itself like it’s got weak over time. For example, you’re reading the temperature in the supply and this normally closed limit switch should open up but it didn’t. By that, you know that the switch is the actual problem.
- Pressure Switch
Another reason the gas furnace could be short cycling is the pressure switch.
It is located near the motor and it shuts down the furnace if it detects negative pressure created by the inducer motor. This prevents the furnace from cycling if there’s mechanical failure.
Just because the error code on the control board is signaling there’s a problem at the pressure switch, it does not mean that the pressure switch is the actual problem. It could be a problem affecting the pressure switch.
You could have your condensate backing up into the inducer motor housing so you have the extra efficiency being taken from the water that’s created during the flame process. Then it’s draining the water out through its tube.
If the water level rises in the inducer motor housing, it’s going to shut the pressure switch off. So, the pressure switch is proving that the inducer motor is running and there are no issues while the furnace is running.
You could also have something like the exhaust pipe could be closing off. Maybe there’s a bird’s nest in it or something like that. You don’t have it pitched properly. what’s happening, is your furnace extracts so much heat that you have water.
Basically. your gas is condensing up in the exhaust pipe and then it’s trickling back to the furnace that’s why the exhaust pipe has to go a quarter-inch of pitch upwards as it moves away from the furnace.
So, what happens is your actual water that’s condensed will trickle back down into the furnace again. We’ve also seen this quite a bit where instead of pitched upwards, it’s pitched downwards and all of a sudden, your water is all building up inside the exhaust pipe filling up.
And then there’s a little pathway for the exhaust to go through then it runs for a little bit. Then shuts off, runs for a little bit then shuts off again because it’s pushing through this little pathway.
So that’s an issue, make sure that your exhaust pipes are supported. Support the pipe every 4’ to 5’ horizontally and that you’re going a quarter to pitch per foot upwards as you move away from the furnace.
You could also have a problem where your intake is blocked. The pressure switch tube is connected to your intake, your combustion chamber and if you have a problem with your intake, you could have the pressure switch tripping.
Another issue is your inducer motor itself like it may not be running. You could have maybe some of the fins on the blower or the inducer wheel are broken.
You could also have your actual pressure switch itself is the problem.
The pressure switch is reading a vacuum. There will normally be a water column reading on it. It could be as small as 0.1 eight-inch water column, so remember that this is a very small reading and you don’t want to be sucking on these tubes with your mouth because you can’t control how much pressure you’re exerting on that tube.
You can check the pressure switch with your multimeter and you should T-in a manometer while you’re reading it. Before you turn your furnace on, what you want to do is you’re going to either test the pressure switch with a resistance value across the two terminals with the electrical wires off.
But to keep your furnace completely running the whole time and the full sequence of operation happening, you’re going to want to put one alligator clip on the common terminal over at the control board or on the ground of the furnace, and the other terminal on the tap that is not normally supplied with 24 volts.
So, you can see if there are any hiccups while it’s running like maybe you are reading the correct water column level and then all of a sudden there’s a hiccup and the water column level lowers.
You can also test the pressure switch by itself using another tool such as SDMN6. It is a dual water comm manometer. It also has a pump built into it so you could turn the power off to the furnace and test the pressure switch by itself, independent of the actual furnace running.
- Control Board
To troubleshoot a control board, you need to know how the system works. So, for instance, when you have your 24-volt signal on the W, you want to know when you’re sending your 120 volts to your hot surface igniter, when you’re sending 120 volts to your inducer motor or when you’re sending power to your blower motor.
What happens is a lot of times, the relays (the black boxes), and the contacts can become pitted so you’re not sending the correct amount of voltage or you’re not sending voltage at all.
You have like a high resistance across the contacts when they’re closing and your output voltage all of a sudden doesn’t come out. You could also have a problem where the terminal was soldered onto the board and so that could be an issue.
You could also have a wire falling apart somewhere within your furnace. It happens a lot on out-package units but it can also happen on an indoor furnace where the terminal is just not tight enough on the spade connectors.
So, that could be an issue where you’re sending voltage and then all sudden, you’re not sending voltage. It just has to deal with maybe the vibration of the furnace, it could just be a loose connection or something like that so you want to be able to check your connection.
You turn your furnace off to check your electrical connections and you can tighten them down with your wire strippers and cutters. Then you can push it back into the spade connector again.
You just need to measure your output voltage, know the sequence of operation to know when the Control board’s supposed to be sending the voltage out on each of the terminals.
- Interrupted Flame Proofing
Another problem that could be occurring in a furnace is the flameproofing process could be interrupted. If you had the hot surface igniter turned cherry-red and then your gas flows coming, one thing on an outdoor package unit is it could become rusted shut and the gas isn’t making it through.
Sometimes, it’s making it crossed over to the flame rod. Your furnace may be running and sometimes not. What happens, in that case, is you need to take the burners out and clean them or purchase new ones because each time that you’re cleaning them out, it’s kind of the chambers are opening up more because the metal is lessening.
You could also be having some other problem such as maybe the flame rod itself is fully covered with carbon dust and you need to clean that off. So, you need to turn the furnace power off, take the flame sensor out and clean it with unsoaked steel wool to get that black carbon dust off of it.
That may be impeding the voltage from making it into the flame.
So, there’s AC voltage coming from the control board over to the flame rod and then from the flame rod into the flame and then it’s rectifying the voltage and you have a DC microwave signal being sent back over to the control board – that’s the flameproofing process and it’s called flame rectification.
Another flameproofing component is a thermopile or a thermocouple. These are used on furnaces that don’t have line voltage. They don’t have AC voltage to control the ignition process though they may have 120-volts going to a blower motor. These are typically used on furnaces that don’t require power so they’re usually older furnaces or freestanding stoves.
And what you’re doing is you’re involving this thermopile or thermocouple with a flame and you’re generating DC millivolts to power the gas valve.
The first thing is you have to have enough millivolts to have the pilot flame continuing to be lit. So, you’re opening up that first chamber, you’re actually holding open the first chamber within the gas valve but maybe you’re not supplying enough DC millivolts to open up the full gas flow when your thermostats powering for heat so that could be an issue.
Knowing the sequence of operation for the furnace that you’re working on is important. You might have one that does not have a sealed combustion chamber, it’s just open and maybe it’s a spark-ignition so it could be a pilot ignition or it could be a direct ignition. You could also have an 80% efficient furnace where you don’t have to worry about the condensate.
You need to know the furnace that you’re working on to diagnose it properly. When you’re at a furnace, you can look at the wiring diagram to determine what’s supposed to happen to the furnace that you’re working on.
There are many reasons why a gas furnace would short cycle and if you have different experiences, we’d love to hear from you. Write to us!
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