Weird issue with Lennox A/C + Honeywell t-stat, please help!
The warm season has started and I went out few days ago to start up my Lennox A/C (XC16), and it doesn't work, something really strange happens.
I have a Honeywell visionpro iaq thermostat.
My furnance is also a Lennox G61MPV90.
When I switch the system setting to cool and wait for the 5min delay, what happens is the thermostat shuts off, blank screen and my furnace (if set to FAN ON) shuts off as well. Then the unit resets and started the 5 min delay again.
These are the things I've isolated so far to test what's wrong, to my knowledge. I'm a QA software engineer, so I can only approach things to this degree.
1. Swap thermostats, same thing happens to another honeywell t-stat I own.
2. Check A/C breaker and outside pull switch is passing current. No problem there
3. Check any shorts in the Y1 Y2 or C wire to the A/C unit or from t-stat to furnace PCB. Nothing, tested with multimeter.
I tried searching and the only other thing I could find was something about the condensate safety switch triggering the shutoff. But when I run the heat cycle, it runs fine, unless there's a condensate switch for the condenser? I don't see any wires coming out or going anywhere from the drain and there's nothing but a pvc elbow coming out of the condenser.
The only switch is the one for the furnace and that seems fine, otherwise the heat won't work?
Is there some sort of safety switch or breaker on the A/C unit somewhere that would trigger shutting down my t-stat? If there is, how is that possible when only 3 wires go from the furnace to the A/C unit (Y1 Y2 C)
I guess I can measure the voltage across those terminals and see what is going on but I'm really confused why it's behaving like this.
Oh, and my A/C Unit is about 1 year old, the furnace is about 1.5 years old.
There is a short somewhere on the control circuit that occurs when the call for cooling is made. The voltage drops due to the short causing the thermostat to reset (which is fortunate as it is preventing damage to a fuse or transformer). You might check to see if the dog has chewed the wires going out to the condensing unit, but either way you will need a service call.
thanks, which control circuit would it be? the one in furnace or ac?
Originally Posted by cjpwalker
the short would be between what though? y1 and ground?
I can't get very technical on this site, but you are on the right track. The condensing unit, furnace, and EIM (white "honeywell" box near the furnace) are all tied together. I don't recommend putting your hands into any of these as damage to equipment or yourself could occur. There could be other issues as well with a circuit board or other component, but this kind of thing usually turns out to be wiring.
Why can't you get technical?
Well it should be something in the condensing unit since I haven't touched anything in the house since I tested the A/C
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Originally Posted by FlashEngineer
Like I said, it could be anywhere, without having seen the system myself I still think you are on the right track.
Call a pro.
You have a low voltage short. This is not a DIY scenario. Sometimes issues like this can throw off even the most seasoned professional.
Did you by chance install a new thermostat yourself at some point?
ENGINEER run for the hills.
It's not if your doing it right it's whether your doing the right thing that is important.