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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
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Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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I'm troubleshooting a Rheem heat pump system and I was

Customer Question

I'm troubleshooting a Rheem heat pump system and I was experiencing a problem with the 3 amp fuse blowing which inops the system. Found defective thermostat and replaced with Honeywell non-programmable Digital Thermostat Model RTH3100C . With system on there is no heat. The blower is on as well the heat pump. Notice the indoor line going to indoor condenser is cold as if heat pump is in AC mode.
Submitted: 1 year ago.
Category: HVAC
Expert:  Brian HVAC Guy replied 1 year ago.

Hi I'm Brian and I'm here to help!

On Rheem heat pump units, the reversing valve needs to be energized in heating. This means you should not have a connection at O but instead move the wire from O to B. (I'm pretty sure the RHT3100C has BOTH an O and a B terminal). If the thermostat has an O/B terminal then the reversing valve wire should be connected there and the thermostat setup will have to be changed so it energizes the reversing valve in heat instead of cool. That should take care of your problem! (unless of course your reversing valve coil is burnt up and that was the reason the fuse was blowing!)

Let me know if you have further questions on this subject. Otherwise, please remember to rate before leaving (3 stars or higher please!). Bonuses are always appreciated! Thanks for allowing me to help and thanks for using Just Answer!

Thank you,

Customer: replied 1 year ago.
Hello again. I removed wire from O terminal to the B terminal. I turned system on and after 5 min delay system turned on and fuse blew. Disconnected wire at air handler which inputs to the defrost board outside which outputs to reverse valve signal and found a short. Disconnected outside at heat pump(defrost board) and confirmed this wire is shorted. Switched to a Different wire (spare). Turned system back on in heat mode and found the condenser was still operating in cool mode. The indoor condenser and piping was cold. I gone to the defrost board and visually confirmed the output signal actually comes out from the defrost board. I gone to the terminals marked reverse valve and found no voltage. Would it be a safe consumption to say the defrost board is bad? I did find one that's available but wanted to make sure if I troubleshooted to the fullest every possible fault.
Expert:  Brian HVAC Guy replied 1 year ago.

Hello again Al, I'm sorry it took so long to respond. I was away for the weekend.

It sounds like you have a volt meter so let's diagnose the system with that and a few logical steps as well. From your explanation of the symptoms, it sounds like it's either a shorted reversing valve solenoid, a defrost board that's failed or both. It sounds like you have your thermostat wired up right now with the reversing valve wire to B.

I would first disconnect the reversing valve itself from the defrost control board, I'm referring to the 'output' side of the defrost board, not the B wire thermostat connection to the 'input' side of the defrost board. Then ensure you have a good fuse in the air handler and turn power back on. Put the system in heat and measure the voltage at the outdoor unit from C to B at the defrost board. You should have ~24 volts ac. Now measure from C to the 'output' side of the defrost board reversing valve connection, you should have ~24vac there as well. If those checks pass, your defrost board is fine, if the checks don't pass, you have a failed defrost board.

Now with power off to the system and the two reversing valve coil wires disconnected, you should measure the resistance between the two reversing valve wires with your meter set to its lowest resistance scale (Rx200, Rx400 etc..). Let me know what you find. Alternatively, if the defrost board showed failed in the voltage check above, you could bypass the defrost board reversing valve relay by disconnecting the thermostat B connection and connecting it directly to the reversing valve coil wire that was on the output side of the defrost board. The other reversing valve coil wire should be connected to 24 volts common somewhere in the the circuit. If you can give me the model number of the outdoor unit, I can give more specific instructions. When you bypass the defrost board to reversing valve coil, turn all power back on and see if the fuse blows. If it does, you have a shorted reversing valve coil and will need to replace it. If the fuse doesn't blow and the system goes in to heat, you just have a failed defrost board.

Let me know how that goes and we'll go from there.



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