ecman51` -
I've been in the t'stat to do the R to G terminal 'jump test' and found no fuse (or batteries). This is an older, non-programmable Carrier digital t'stat which holds the input settings on EEPROM during power outages.
The Real Deal -
Hey, not so fast! I've identified electrical problems plenty of times without a multimeter... sometimes plain 'ol process of elimination works wonders.
I may ultimately need a meter to run down the potential electrical issues, but I was asking for info on any other bugaboos such as your drain pan float safety switch suggestion, or other limit trips that might not automatically rest after re-powering.
Neighbor w/ meter returns from fishing trip tomorrow. In the interim, so far I believe (sorry, I'm a pilot and like check lists):
1.) since the jump test did not work, process of elimination so far suggests in all probability a bad t'former rather than t'stat.
2.) if a meter test across the R and C terminals at the t'stat comes up empty, doing the same on the low voltage terminal block on the main board in the fan coil unit confirms either the t'former is bad or (assuming all connections are ok) there is a line break from the terminal block to the t'stat.
3.) testing the t'former 24v out red and brown (common) at the low voltage terminal block on the main board in the fan coil unit confirms status of 'former, assuming 230v AC is present (it is).
Ah, and I did find all the original 1995 install info, dutifully saved by the prior owner. This is a Carrier Model 38YRA042301 heat pump, FK4BNB005 upflow fan A-coil and FKAEH0601F20 20kw heat pack. Got an Aprilaire Model 440 whole house humidifier and Space-Guard high efficiency filter. Ratings are 12.70 SEER, 8.40 HSPF.
Best regards.
[EDIT] Located another meter, no 24v @ t'stat or t'former out.