bryant home heat pump

[mke]

here is one for you gurus.....
ok let me take it from the start, a couple weeks ago i had a 5v fuse blow on my handler after running a series of tests i figured out there was a problem with my low voltage...the tests i ran were simply taking of the stat and twisting red and green together fan runs, no blown fuse= fan relay fine., next i twist the yellow and red together and it blows a fuse,next step replace the fuse then i take the coil side off the contactor no blown fuse...ok simple i replace the contactor well it works for a few hours then blows another fuse.so i throw in the towel and call someone to look at it 95 dollars later he says it's the board ok board costs 149 dollars well,my old boss does lots of work for a/c companies so i got a great price on the board 65 bucks back in action right....wrong it works for a few more hours and blows the fuse again so... i can bypass all the safety switches and the cooling works fine by running 24v to the fan and the reversing valve with the board bypassed... any suggestions on what might be cause this intermittent wierdness please help ps sorry for run ons

[Tradewinds]

Hi Mike,
Just wondering if you have any worn wiring that might be grounding intermittantly?

[mke]

i looked for worn wiring as did the technician neither of us found any.....i am surely stumped

[frank]

Have you checked the current draw when the fan is running at full speed?

Have you then checked the current draw 30 minutes later? Any increase in current draw at this time?

It may be a case of the fan bearing starting to go and the motor current draw becoming greater than the fuse rating as the bearings hot up and the power needed to drive the motor increases in line with this

[monkey spanners]

Is the fuse in the low voltage circuit or in the mains supply feeding the transformer? Could be the transformer going bad.

I have in the past with these sorts of intermitant problems put fuses in the feeds to each possible component and waited. You will need to decide whether this is a safe thing to do on your system. I have not tried this with low voltage (24v?) control circuits as all our stuff in the Uk tends to be 240v including control circuits in this size of equipment, i have seen automotive style blade fuses being used with crimps on low voltage circuits, would think the lowest amperage you can get as its only running a few relays etc.

It would be worth disconnecting both ends of the low voltage cable, keeping the ends seperate and checking between them with a continuity checker for shorts.

Jon :)

[mke]

we have checked all the low voltage circuits and seem to be getting normal readings while hooked up and also open against ground and it seems like the reversing valve wants to be the culprit but the strange thing is when we bypass the board and hook it to the 24v side of the contactor and run continuous powers it runs and cools fine this is why i am stumped

[yorkie10]

Its the timing of the fault I'll be looking at, why does the fuse fail after 2 hours, possibly its going into defrost and energising a solenoid coil on the 4 port valve and this coil is the culprit, another avenue for you, good luck