[adamcgeiser]

Our A/C for the last week has had some strange issue. The thermostat will cut off intermittently (by that, I mean the screen goes blank, no power to thermostat). It seems to work fine in the evening and morning, but often when we come home from work, we'll find it off, or having just kicked back on. Yesterday, it was in the 80's and pretty hot in the attic, and we had this issue. I am not certain it's heat related, just a theory.

When it works, it works great. We have a new compressor and evaporator coil, but the furnace we did not replace. Is it possible the circuit board would intermittently cause power loss to the thermostat? When the thermostat is off, the light on the board goes off, so it seems none of it is getting power.

Before we go and replace the entire furnace, I'd like to try replacing just the circuit board, transformer, switch, etc, but am not sure what the most likely culprit is. It's going to get hot soon! It's Texas! Any ideas?


Thank you!

edit: thermostat, when it cuts off, cuts off completely. No power, blank screen. Just clarifying.

 

[BayouRunner]

Does the thermostat use batteries? A lot of them that's exactly what will happen when the batteries go out

 

[supers05]

Does the thermostat use batteries? A lot of them that's exactly what will happen when the batteries go out

Especially without a common on the power stealing versions. (when they can't get though power) Which stat do you have?

The fact that it doesn't stay off means that stuff like fuses are out of the equation. I've seen circuit breakers do this though. (both the low 24v ones and the 120/240v versions) Vibrations are a particularly nuisance. However, your are unlikely to have a 24v breaker so that's also out of the equation.

You'll have to check for incoming power when it does stop working.

Intermittent problems are the hardest to track down. You'll need patience.

Cheers!

 

[roughneck]

Do you have a condensate overflow switch?
Why would you randomly start replacing parts on the furnace, or the entire furnace, for an issue that wasn't diagnosed?

 

[adamcgeiser]

The thermostat is a Honeywell TH630wf1005

2/3 of the system is less than 2 years old (everything but the furnace was replaced). The flow switch was recently replaced. The guy who replaced it said that if that doesn't do the trick (because they had issues with that particular part before), then it's likely something within the furnace, or possibly the circuit board. However, the thing was running when he came out, so he couldn't troubleshoot.

He said with a 15 year old unit, to just replace the entire furnace. It seems funny to me that it will run just fine when it does run, and then periodically just shut off.

There is no battery in the thermostat, it's wired in somehow. I was told that the power is triggered by the unit itself.

If I can spend $100 and replace the circuit board, or replace a $20 transformer, I'd like to start there. However, if in someone's experience those items going out would cause a permanent issue (or not be able to produce an intermittent issue), then I'd skip over that component.

I am pretty handy but typically shy away from electrical, but direct replacement is no problem for me. We are in the middle of paying for tonsil removal, 2 semesters of college and a tax bill, so anything to lighten the bills. If I could reproduce the problem, I'd just call up my A/C guys again.

 

[yuri]

If the light on the board is off that means there is no power to the board. Rarely does the board fail and have no light. Post the brand and model and serial # of the furnace.

It could be something as simple as the door switch in the furnace (behind the blower door) not closing properly and cutting off the power.

If you catch it with no light give the blower door a whack and see if the light comes on. Most likely the problem will get worse and it will fail totally but you hate to have that happen on the hottest day.

If you have a regular 120 volt light switch for a furnace disconnect you may want to replace it. They cost $2 and can fail from old age and make poor contact.

 

[roughneck]

You never answered my question, of if there's an overflow switch.
If there is, you could replace the whole furnace and you'll have the same issue.
There's no reason to guess and throw random parts at it, this would likely be a simple issue to fix.

 

[yuri]

No disrespect but wouldn't a overflow switch be tied into the 24 volts not 120 which would kill the power to the board/light. I have never seen them tied into anything but the thermostat wiring.

 

[roughneck]

I've seen them wired into the transformer secondary, so it cuts power to everything if it opens.
And I don't know what the light means on the board, that the OP is talking about.

 

[yuri]

Most newer integrated circuit boards for furnaces have a diagnostic light/LED. Used to tell you a limit or press switch etc is open. When the power is on it glows or flashes a heartbeat (lennox).

 

[roughneck]

Most newer integrated circuit boards for furnaces have a diagnostic light/LED. Used to tell you a limit or press switch etc is open. When the power is on it glows or flashes a heartbeat (lennox).

Yes, but it is unknown what the OP has and many brands have the board and control transformer separate. There are many things that can cause a loss of control power that should be checked before making any conclusions.
I'd still do some diagnostic work well before I ordered a single part for that furnace.

 

[beenthere]

I would check for a weak high limit, they can trip from the heat of the attic. Also, the new float switch may be defective. And it could be that the condensate drain is draining slowly.

 

[yuri]

The open limit should give a flashing open limit code on all the boards I have run into. Not sure about some brands I don't see in Canada.

 

[adamcgeiser]

Too much going on yesterday so did not have a chance to get up there when it went out; assuming it goes out today, I'll check the float shutoff and the switches, since those would be best case scenario. I'll update when I get it figured out.

I appreciate the ideas; not experienced with HVAC and don't know all of the angles to approach with.

 

[beenthere]

A lot of Carriers(older ones) the limit breaks power to the R terminal, so no code. The blower relay on the board is a normally open, so the blower runs until the limit closes.

 

[yuri]

Yeah, I don't see many old older Carriers as Lennox and a local brand ICG sold 85% of the furnaces where I am.

Most Carriers I see are the ones with the limit code.

ICG became/was Canadian Keeprite and got bought by ICP.