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Trane XR-12 Heat pump blows the 5amp control fuse when switched

Customer Question
to the heater mode, works...
Trane XR-12 Heat pump blows the 5amp control fuse when switched to the heater mode, works fine in AC mode, what would cause this.
Submitted: 6 years ago.Category: HVAC
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Answered in 9 minutes by:
11/10/2011
HVAC Technician: Matthew, HVAC Technician replied 6 years ago
Matthew
Matthew, HVAC Technician
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Matthew :

Hello, My name is XXXXX XXXXX I will do my best to help you today.

Matthew :

This would have to be something in the heater relay. This includes the thermostat W2 ( some label it Aux) terminal, the wire running from this W2 terminal to the air handler, and the heat relay that this wire hooks into at the air handler. This wire and it's relays (thermostat and heat relay in air handler) are the only thing that energize different in heat that do not energize in cooling that would cause the 5 amp fuse to blow. It could be this wire is bare somewhere from point A to point B and is touching something it shouldn't, could be the relay is shorted out or it could be a short in the thermostat.

Matthew :

The w2 circuit is energized in two instances during a heat call.

Matthew :

1. When you turn the demand temperature 3 or more degrees above the ambient temperature of the thermostat the w2 (electric heat kit) energizes to help the heat pump.

Matthew :

2. When the demand is less then 3* of the ambient but the system has run for around 8 mins without reaching the demand temperature.

Matthew :

I take that back there is a third way it is energized in a heat call.

Matthew :

When the outdoor unit goes into defrost mode the unit sends a signal to the heat relay in the air handler to bring on the heat kit. This almost never happens though as if it is cold enough out for the system to go into defrost then the heat kit is almost always already on because the system has been running for over 8 mins.

Customer:

Unit has a new thermostat, doesn't explain why it would kick on in a rapid cycle, but not after sitting. Tried a 10 amp fuse and doesn't blow but still wont start the unit unless it is already running in A/C mode. Could it be a weak capacitor at the unit itself?

Matthew
Matthew, HVAC Technician
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HVAC Technician: Matthew, HVAC Technician replied 6 years ago
the outdoor unit energizes the same exact way in cooling or in heating the only difference is that in cooling the O terminal is energized which causes the reversing valve in the outdoor unit to slid into the cooling position. When O is not energized the unit is in heating position. Don't use a 10 amp fuse if you go bigger then 5amp the short will not blow the fuse and it will take out your air handler's transformer. The capacitor is a high voltage part not low so it would not cause the fuse to blow the only thing that can cause the fuse to blow in heat and not in cooling is this W2 wire or the heat relay in the furnace that's it since you say the thermostat is new.
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