Bryant 383kav, installed 2001

installed 2gig ct100 smart thermostat few months ago seemed to be working fine with heat. today coming back from camping set to cool with smartthings app. got home and still 79 in house. thermostat was showing cooling and fan would run periodically for 5 min but no cool air.

installed old simple thermostat and getting similar results. it shows snowflake but no cooling is happening.

CT100:
C
R 27v
W 0v
G 27v when fan is running, fan switched on works
Y 0v

cheap Honeywell thermostat:
C
R 24v
W 6v
G 24v when fan forced on
Y 0v

so both thermostats know and show cooling from display but neither is putting 27V to Yellow

any ideas.

 

You need to have someone stand by the thermostat while you stand by the compressor. Have them turn to a/c and if the compressor doesn't kick in you need to find of why. Maybe no Freon, maybe an electrical part has fail, etc.

 

wouldn't the thermostat still put 24v out....essentially they are just switches right? old mechanical relays just closed contacts when the temp got too warm or cold

also after leaving fan to on for a while we noticed several very brief moments where the fan stopped then started again. no codes flashing on hvac control board, solid red

can short R to G and Y to force fan and ac on?

 

shorted R G Y , fan runs, no cool air, no ac compressor running, no error codes flashing.

guess I'll be calling hvac guy tomorrow first thing because I love my ac.

 

You need to find out if Y at the control board has 27V or not, Trace Y from T-stat to control board, and see if you can find where it stops.

 

You tried directly connecting power to the condensor and you tried changing the stat. It would appear that the problem is out at the condenser.

Tripped breaker for condensor unit.
Defective contactor
Low or out of refrigerant.

Are all possible problems but you'd need a voltmeter to test for voltage at the required locations.

The furnace control board doesn't control the A/C. It won't show a fault LED flash on A/C failure.

 

Y wire goes through the control board (at least on some model, T-stat's Y does not directly go out to the condenser).

 

Y goes to the control board.... not thru it. That Y connection usually sets the cooling blower speed.

 

You may be right, but a while ago, I saw a board has two Y terminals. Right side Y receives wire from T-stat, left side Y has a wire goes out to the condenser. The 2 Ys are not connected by any wire. That is why I said the Y goes thru the board internally.

 

Do you have a full secondary pan under the indoor coil tripping a float switch?

I have seen a Trane air handler interrupt Y to the condenser before. I believe it was a TEM model that has a built in float switch or freeze protection. (I forget which).
Most systems do not interrupt Y in this manor. The float switch must be externally installed.

 

Had HVAC guy come out to fix it yesterday. The condenser was getting 220V but not the 24V control voltage when it should have. He was able to force the condenser to run.

He thought it was thermostat at first until I reminded him two thermostats behaved the same. Then back at the HVAC in basement, he found a "singed" wire (I didn't see it, slight possibility previous HVAC guy bumped/broke it while performing cleaning last fall). I think it was the 24V line exiting the board to signal the condenser. He and I were surprised the 3A fuse was NOT blown but allowed a wire to fry.

After he fixed the wire (I didn't see what he did, cut off singed portion of wire and stripped?), working fine again. Now on back of thermostat R G and Y are 27V as I would expect while calling for AC and fan.

I probably(????) fried it while changing out thermostats and not turning off the HVAC power supply. The first CT100 I bought had problem where the display flickered. Thinking about it more, is it possible to fry something like this with shorting thermostat wires?

Shorting:

R to C seems bad (assuming C is ground?), would blow fuse? or fry 24VAC supply in HVAC?

W, Y or G to R is normal connection to call for heat/ac/fan

W, Y or G to C ... would these shorts be bad?

Sorry for all the crazy questions and thanks for the help!