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Heating, Air Conditioning, Fridge, HVAC/carrier heat pump blows 5amp fuse


QUESTION: Carrier heat pump blowing fuses.  So I have read too much, poked and prodded too much and am still at a loss.  History of things that matter:  I replaced the old thermostat with a “non aux” thermostat (my bad), after the first 5amp fuse blew (didn’t know about the fuse).  Still blew fuses, but quit blowing fuses after I moved some wires around trying to find the short.  Cooled fine all summer Sat off for a month, turned on heat and blew fuse.  I found a bare spot on a wire in the outside unit and repaired it.  Still blowing fuses.  Could the "non aux" thermostat cause this?  .  I can hot wire the red and white wire and heat and fan work….no blown fuse.  If I incorporate the "C" at all, fuse blows.  Can't find a short on any of the stat wires.  I am sure it is inside…. not out.  Outside unit is isolated.  Transformer shows 29v.  Nothing shows to ground.  I have checked it so many ways, I really don’t know where I’m at.

ANSWER: C is common, IE neutral and is wired to the ground.  If you touch C to R or anything else while it is powered up, you will blow the fuse. C is used to power up some thermostats and C also goes out to the condensing unit or heatpump to power things out there.  What is blowing the fuse is likely a inductive load of some sort (a coil).  Finding the problem can be a challenge since coils can go bad and one side is connected to ground.  


---------- FOLLOW-UP ----------

QUESTION: Thanks Craig!  I actually figured it out.  It was partly self induced problem.  I have read several posts regarding this issue, all involving the thermostat type I used.  None of the HVAC guys get what is going on when the DIY'ers post the problem because the HVAC guys would know to use the correct stat.  
 A short in Rev valve was the original fuse blower.  I replaced the stat because I did not know about fuse or the short.  My first mistake.  The stat is "single stage" only, which works fine in "cool".  Still blew fuse due to short in rev valve lead.  I didn't find the short, but did move it away from rev valve, copper line when searching.  Short was gone and "Cool" worked fine all summer.  

Changed to "heat" and the "R" and "C" were united by the stat.
The thermostat is not meant for 2 stage.  What happens is the location for the common "C", is included in the circuit of the thermostat when "heat" is selected and not when "cool" is selected (shorts and blows fuse), as you stated.  The "G" green wire (can't remember at the moment, maybe compressor), is not included in the circuit during "heat" but is during "cool".  I switched the "G" to the "C" location to avoid running to get another thermostat as well as to get the satisfaction of the win.  Voilà, I righted my wrong and learned something (No, not call someone who knows what they are doing).  I will get another stat before summer so I don't go through this again due to memory issues.  And I understand now why "C" was on my original "non battery", 24V stat.  I hope all of that makes since and helps another DIY'er.  Or like my son says  "RIG'er".  Thanks Again!

The G term on the stat is for the indoor fan.  Old school needed the G to be connected for heating and cooling.  New school with electronic controls in the A/H do not worry about the G and turn the fan on with a call for heat or cool normally with a delays built in.  You need to pay attention to the air handler and cond. unit wiring.  In some cases the t-stat does all the work and in others the control boards in the units do all the work based on the t-stats signals.  FYI the rev valve can energize on heat or cool depending how it is set up.  I sounds like in your case the valve energizes on heat, so cooling is a non issue with a shorted rev. valve coil.


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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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