Problem: Furnace, air conditioner heat pump will not come on. Nothing happens. I have a blown fuse on the control board. No power between R and C (Com) on my control board. Why did this happen?
Solution: Use a Volt Ohm meter. Set the volt meter to “Volts AC,” to check and see if you are getting between 24-28 volts AC between your “C” and “R” terminals on your furnace’s control board, or between “R” the red low voltage thermostat wire and ground. You might have to tape the blower door safety switch, to keep the voltage on so you can perform this test. Turn your power back OFF after completing this test. Check for fuses on the furnace control board to see if the furnace has a low voltage protection fuse. Below we have a YouTube video made by Grayfurnaceman which shows how to locate a fuse on a furnace control board. If the furnace has a fuse pull the fuse out and see if it is blown. If the fuse is blown check all your low voltage wiring to make sure it is not grounding out anywhere. I have seen pinched wires that are stuck between furnace doors, animals that have chewed through wires and just weathered low voltage wiring that has lost its insulation due to the hot sun over the years. Any wires that are touching together can cause the low voltage fuse to blow. The fuse protects the expensive furnace control board from getting burned up because of a short to ground. If the fuse is blown then I would go to the local hardware and purchase 5 to 6 new fuses. If you do not find the problem that is causing the fuse to blow right away then you will need more than one fuse for testing or you could purchase the Lil Popper 5 amp control circuit breaker tool that we sell on the following page: Please click here if you would like to see the Lil Popper 5 amp circuit breaker tool we sell. This device allows you to reset the fuse by pushing a button instead of having to replace the fuse. If your fuse is OK or your furnace does not have a fuse, and you are not getting low voltage between the “C” and “R” terminals then you might need a low voltage transformer. I have seen some of the transformers just go bad. Below I explain the job of a transformer. We have a picture of an International Refrigeration Products Universal Low Voltage Transformer below. The air conditioner or heat pump not coming on could also be caused by bad or weak capacitors, thermostat problems, circuit breaker thrown (not in on position) and shorted wire problems. We discuss these other problems on the following page: Please click here if you are interested in seeing more information on the problem: The Air Conditioner outdoor unit will not come on. Nothing Happens.
We have two really good YouTube Videos below that show and discuss furnace control board fuses by Gray Furnace Man and How to Find a Short in a furnace or air conditioning system by Word of Advice TV. Many thanks to Gray Furnace Man and Word of Advice TV of making these excellent informative troubleshooting videos!
What is a Low Voltage Transformer? The job of a low voltage transformer is to take 110 volts AC on the primary end, and transform or lower the voltage to 24 volts on the secondary end. That is why on the transformer label, below it has “PRI” 120, 208, 240 and SEC 24V 40VA. The transformer that we sell can be used with multiple voltages either 120 volts, 208 volts or 240 volts. You would hook up the right color coded wire to use the voltage that you have. The color coded voltage wiring directions are on top of the transformer. For example: The white and black wires would be used for 120 volts for most furnaces. The White and Orange wire would be hooked up if you were using the transformer to replace a bad transformer on an outdoor air conditioner or heat pump that uses 240 volts. Please click here if you would like to see the low voltage transformers that we sell.
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