control board fuse blowing

[ernie]

Greatings HVAC techs-
First off, I am not a HVAC tech myself, rather someone who enjoys (mostly!) fixing things myself.  I am tryin to troubleshoot a problem on a system which is bowing the 24v fuse on the AHU control board.  The system is a Rheem heat pump.  I initially thought the problem to be a bad transformer so I installed a new one, no fix.  The new transformers output is at 28.6v, same as before.  Does this seem unusually high to anyone?  I inspected all stat wires for grounding problems,  both indoors and out, and checked for continuity, all seem good.  I even went as far a replacing the control board, no dice.  Any help would be appreciated-

[Admin]

I wouldn't worry about the 28V output from the transformer.
When does the fuse blow?  Heating or cooling or both?
If nothing has changed inside the house, renovations etc, I would look closely at the low voltage wiring outside.
I would disconnect all the low voltage wiring on the control board coming from the thermostat.  Using a jumper wire I would test the heat by jumpering W and R.  Next test the continuous fan by jumpering G and R.  Test the cooling by jumpering R and Y.  I'm not sure what heat pump model you have but you need to energize the reversing valve as well, usually terminal O or B.  If the fuse still blows then we have ruled out the indoor low voltage wiring and thermostat as causing the short.  We can't really disconnect the outdoor wiring from the AHU control board or our outdoor unit won't run.

If you determine the problem is with the outdoor low voltage wiring I would just replace it.

You could also buy some 3A fuses and 1/4" female stakeons and install the fuses throughout the low voltage wiring going to the outdoor unit.  This will pin point the short.

[ernie]

I am quite sure it ran fine (approx 1 hr) on heating, it was when I switched to cooling that it blew again.
Nothing has changed with the house as I have been working there for about three years now.  This fuse situation strarted about a year ago, new fuse lasted for a year before it came back, now they dont last 15 min.
I swapped t-stats with anohter in the house , no change.
I have the board number 47-21517-14, I willl look for model tomorrow.
I will do jumper test with wires to the AHU removed, are you refering to a fuse on the control board of the outside unit that may blow?
I like the in-line fuse check, thanks so much-

[Admin]

I was talking about the AHU control board fuse.  Your outdoor unit may have its own transformer and control board with fuse.  If you haven't changed a fuse outside then it's likely not a problem with that circuit.  However I believe that part number is for a defrost control with no fuse protection.  Did you inspect that board and the contactor for signs of corrosion and shorting?

I also forgot to mention, when you said you tested the wiring for continuity, I would suggest you test for no continuity as well.  If in fact you have a short then you could have continuity even after you separate the 2 wires you are testing.  I always twist 2 wires together and check for continuity, then separate them and make sure I no longer have continuity.