Short cycling means a gas furnace is running through its full sequence of operation for heat, it is providing heat in the building, and then it is shutting off before the thermostat is satisfied. The furnace then immediately turns back on to heat the building and turns off again.
In order to troubleshoot a furnace you need to know the sequence of operation for heat. This requires that you know the type of furnace you are working on. In order to start the sequence of operation for heat you need to have twenty-four volts on the W on the control board. On a 90% efficient furnace the heat sequence is as follows:
· The inducer motor turns on.
· The pressure switch closes.
· The hot surface igniter turns cherry-red.
· The gas valve allows the full gas flow to the orifices.
· The control board senses the flame through the rectification process.
· There is a blower on delay on the control board that waits for the heat exchanger to heat up.
· After the delay, the control board signals the blower motor to turn on.
The four main reasons why a gas furnace short cycles are:
1. The thermal limit switch is tripping and opening up the electrical circuit:
· The blower motor is running at a speed too low for airflow to keep up with the heat that is generated by the flames.
· The water column pressure on the output of the gas valve might be set too high causing an over-firing of the system.
· The thermal limit switch is defective or broken.
2. The pressure switch is tripping and opening up the electrical circuit: (Even if the error code on the control board indicates the pressure switch is the problem, the pressure switch may be affected by a problem elsewhere in the system.)
· The condensate may be backing up into the inducer motor housing.
· The exhaust pipe may be closing off due an obstruction in the pipe.
· The exhaust pipe may be pitched incorrectly. (Correct pitch = ¼ inch per foot upward.)
· The intake may be blocked where the pressure switch tube is connected to intake at the combustion chamber.
· The inducer motor may not be running correctly due to damaged fins on the inducer wheel.
·The pressure switch itself could be the problem. (Test the pressure switch with a water common manometer that includes a pump or with a multi-meter and a manometer to determine if the pressure switch is the problem.)
3. The control board:
· The contacts in the relay on the control board may be pitted which affects the voltage flow.
· The relay on the control board is not working.
· The problem may be where a terminal soldered to the control board has become loose.
· A wire from the control board to the inducer motor may be bad or have a loose connection.
4. The flame proving process:
· On an outdoor unit, in the combustion chamber, rust at the flame retention head of the burner tube may be preventing the gas from getting across all the burner tubes to the flame rod.
· The flame rod sensor may be covered with a carbon dust.
· There may be a problem with the ground circuit for the flame proving process.
· There could be a voltage imbalance feeding the system or a bad ground that is negatively affecting the flame rectification process.
· The thermal pile or thermocouple may be the problem if equipped.
If you are looking for more information on what makes a Gas Furnace Short Cycle, be sure to check out our "Top 4 Reasons Why a Gas Furnace Short Cycles! HVACR Service Call Troubleshooting Tips!" video below!
Published: 1/8/2020 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel