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Mark Henley
Mark Henley, Journeyman Technician
Category: HVAC
Satisfied Customers: 3217
Experience:  UA Journeyman Pipefitter , HVAC, Refrigeration, DDC controls. 26 years.Commercial & residential
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Rheem furnace blowing 24vac 2 amp fuse every 2-3 days. Is

Resolved Question:

Rheem furnace blowing 24vac 2 amp fuse every 2-3 days. Is this the contactor for the fan, the transformer or the circuit board?
Submitted: 5 years ago.
Category: HVAC
Expert:  Mark Henley replied 5 years ago.

Hi,

 

The fuse is to protect the transformer so everything that is on the low voltage circuit can be a possible problem, more than likely its the contactor in the outdoor unit that is causing it to blow or some pinched low voltage wiring out there.

 

You say 2 amp fuse typically they are 3 amps or 5 amps, what color is the fuse ?

 

Thanks

 

Mark

 

 

Mark Henley, Journeyman Technician
Category: HVAC
Satisfied Customers: 3217
Experience: UA Journeyman Pipefitter , HVAC, Refrigeration, DDC controls. 26 years.Commercial & residential
Mark Henley and 4 other HVAC Specialists are ready to help you
Customer: replied 5 years ago.
It's a 2 amp 1/4 by 1-1/4 fuse. The schematic shows it as a 2 amp, that was the first thing I checked. Wires come off the xformer and land on the IFC board. The fuse holder is between the xformer and the IFC.
Expert:  Mark Henley replied 5 years ago.

HI,

 

OK, its not an on-board fuse but it will serve the same purpose. You have something in the circuit that is overloading the transformer. It's more than likely the contactor in the outdoor unit.

 

Thanks

 

Mark

Customer: replied 5 years ago.
Changed out the contactor. Problem persists. I also removed the electric company interface. I think this went bad, causing the compressor to single phase.

Checking the compressor, there is continuity between the two legs. This thing is shot, right?
Expert:  Mark Henley replied 5 years ago.
<p>Hi,</p><p> </p><p>The interface can't single phase the unit unless it is a 3 phase power supply ( very rare in residential ).</p><p> </p><p>You may have a short in the low voltage circuit that is causing the fuse to blow. It can also be the reversing valve coil if it is a heat pump.</p><p> </p><p>Can you tell me what the VA rating is on the transformer ?</p><p> </p><p>Thanks</p><p> </p><p>Mark</p>
Customer: replied 5 years ago.
Not a heat pump - std R-22 condensing unit. VA rating is 250va on the transformer.

Def not 3 phase.

If I disconnect the contactor wires from the IFC board, the furnace blower will run. If I connect the wires onto the IFC, the power out of the transformer drops to 12v (normally about 26v).

Can't find the VA rating, other than it is rated at 24vac 2 amp nominal.

When I disconnect the compressor from the contactor, I get continuity across the 2 hot legs of the compressor.
Expert:  Mark Henley replied 5 years ago.

Hi,

 

250 VA ? Is that a 120 to 24 volt transformer ?

 

If it truly is 250 VA at 24 volts you could easily put a 5 amp fuse in there.

 

I think that 2 amp fuse may be on the primary side ? Could you recheck that please.

 

Thanks

 

Mark

 

 

 

 

Customer: replied 5 years ago.
I've taken the fuse holder out of the circuit altogether, wiring the transformer directly to the IFC.

The fuse holder was in-line between the xformer and the IFC board. 120v into the xformer, 24v out. The schematic clearly calls for a 2 amp fuse. However, even with the fuse holder bypassed, the unit will not start. The xformer output drops to 12vac when I try to start the furnace, if I have the control wires connected to the board and the condenser unit. If I remove the condenser low voltage control wires from the IFC, xformer output rises back to 24vac. Contactor has been replaced. Bad transformer?
Expert:  Mark Henley replied 5 years ago.

Hi,

 

Probably , its either that or the wires running between the heatpump and the furnace. The contactor is the only thing using the low voltage out at the unit and if you replaced it that should not be the problem. So either wires or transformer. I suppose you could try the transformer. 250 VA is a large transformer for this application. Generally they are 45 to 75 VA.

 

Thanks

 

Mark

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