A Little Help from My Friends

Brent S
Fan Relay
Have a Bryant central A/C unit that wasn't blowing cold, but otherwise worked fine. Had a tech come out to the house and he identified a loose Schrader valve, and recharged. Now blowing cold for a week.

Before the tech visit, the fan would blow continuously when the tstat was in the "on" position, and fan would only blow only when the compressor is running in the "auto" position. As expected.

After the tech visit, the fan behavior is reversed. The fan blows continuously in the tstat "auto" position, and only when the compressor is running in the "on" position. I checked the tstat with a multimeter and the fan is not running when 24V is sent to the Green fan wire, and it is running when there is no 24V to the Green fan wire. It's as if the fan relay that was normally open is now normally closed.

I didn't watch the tech when he was on the roof, but now I'm suspicious that he changed something electrically. I'm going to climb on the roof and take a look; any ideas on things to check?

Thanks

Saturn
Sounds like a t-stat setting problem he may have inadvertently changed some of the setup options while he was checking it. Get your t-stat manual out and go back thru the initial setup and set to match your unit. When you are checking for G terminal voltage be sure that you are going from G to a ground (C terminal maybe?) since G to R will show O voltage if the G terminal is energized as both have 24 volts keep in mind that the voltage meter shows voltage differential.

Brent S
Thanks for the advice. The tech was never in the house (i.e. he never touched the tstat). This is an old-school mercury (non-electronic) tstat so there are only 3 wires: R (24VAC power), G (fan), and Y (cooling). There are no settings other than fan ON or AUTO, temperature setting, and COOL/OFF. When I tested the tstat I disconnected the G and Y wires, put the tstat in ON mode, and put the multimeter probes between the R and G terminals. Got 0VAC, meaning the same voltage that was at R was also at G (i.e. fan relay should be energized). Then put the tstat in AUTO mode, and measured ~24VAC (i.e. fan relay should not be energized). After all that I swapped out the tstat with another identical model, and got the same performance. So the tstat(s) are working correctly, sending 24VAC to the G wire when in the ON position. But the fan does not run. When the voltage is 0VAC to the G wire, the fan runs.

So my conclusion is that the tstat is not the issue. The tech changed something on the roof, or something coincidentally broke.

mike n

Air Conditioning Repair
For the work that you described nothing should have been done to the control circuitry however since coincidences are rare he probably changed something and forgot to change it back or was just poking around in stuff he wasn't familiar with.
If you get on the roof (I am guessing we are talking package unit western U.S.) you will first want to look at the control wires as they enter the unit and since you shared type of stat if it is a heat pump you will have several wires depending on brand if it is straight electric or gas pack you will have 4 wires you will probably want to look for the green and yellow wires he may have done something with them if not he may have reversed wires on a fan relay although I can't imagine why, but hey at least he got you some cool air wait a minute I just saw that you already told me you had 3 wires so straight cooling no heat he has reversed the red and yellow wires so just switch them back and you should be fine


Brent S
I climbed on the roof this morning and sure enough found that the tech had switched the R (power) and Y (cool) wires. Why he would do this I'll never know. I'm not really even sure how this would have manifested the behavior I observed, but nevertheless I switched them back and now every thing works exactly as I expect. If anyone needs the name of an HVAC service in Albuquerque, NM that you should definitely avoid, let me know (there were other issues besides the tech re-wiring for me).

And thanks again for your help!

mike n

Air Conditioning Repair
He had them jumped out to make the unit run while he was working on it and just put them back wrong The reason it was acting funky is that your control voltage was being distributed through your thermostat in the wrong direction, instead of r to possibly y and g it was coming in on y to r and g and y and g are closed anytime your switch is turned from off to cool