HELP! Came home to HVAC system running in fan mode, thermostat is powered off. UPDATE

[The Stigenator]

I am not sure what happened over night, I wasnt here...

I just walked into the house, noted the blower was blowing.. the house is frigid...

problem: the HVAC thermostat is off..there is no display and no lights on it.. no response from it. Its on the same circuit as the rooftop mounted lennox.

Any idea what it may be. Is this a failsafe for the system, thermostat blows, causes the fan to go on...? NO idea how long this has been running... it was fine when I left around 2:30 yesterday afternoon.

UPDATE: 12/31/2012

Tech came out, pulled panel, no power in system. Blower running. Runs upstairs to roof, finds no heat system going. Manually turns on the system by overriding it, works for 5 minutes fails.

Turns out a sensor failed, its an overheat sensor that failed. He's taken the sensor and will be back later in the week when he gets a replacement part. A 5 minute job for him then but yeah he was really nice.

Turns out there was some mickey mousing in the wiring. The thermostat is supposed to get power, but it turns out in cool mode if the sensor fails the AC will continue to work and the thermostat will continue to work, however in heat mode, if the sensor fails, it all stops working.. why.. well turns out the system is is wired funny.

Update: 1/12/2013

Tech came in, put a new sensor, 12 hours later it fails on over heat.. I think there is a bigger problem here. Hope that the sensor by pass did not cause my heatexchanger to crack / fail due to overheat issues. I believe its now over heating for another reason, the sensor failing was the first obvious issue, but now there is something else that needs to be looked at.

 

[wirednuts]

check for error blink codes on the main pcb. record findings, turn off circuit and turn back on. record error blink findings again and proceed as needed.

 

[The Stigenator]

where is the main PCB... on the roof control board where the thing is located if I remember right.. you have to remove a panel to find the stuff..

Its a single packaged air handling unit.. heat/ac/blower all in one.. its on the roof of the town house, the only thing down here is the breaker and the thermostat..

I can kill the power thats about all I can do right now, its raining hard right now outside, so I'd have to wait to go up..

 

[The Stigenator]

what I dont understand is that the blower is still blowing, but no heat / thermostat power.. so something failed some where.. question is that is the fan blowing a fail safe?

Obviously the thermostat isnt getting any power for whatever reason.

 

[The Stigenator]

ok killed power, waiting 20 minutes before i turn on the breaker again..

 

[spidey07]

Batteries in thermostat?

 

[The Stigenator]

Batteries in thermostat?

possible.. lemme check if there are any but most thermostats are powered by the system.. i remember in the past when i killed power in that circuit, the thermostat would power off.. so i doubt there is any but let me grab the model of the totalline thermostat and see what comes of that

 

[The Stigenator]

nope this model apparently does not have a battery backup, its built in for a max of 48 hours.. its a totalline P0441 model (as per the sticker)

 

[The Stigenator]

okay killed power, restarted power 10-15 minutes later, blower comes on again, no display in thermostat.

killed power again.. its either the thermostat that failed or there is more going on here.

 

[wirednuts]

well definitely check if stat uses batteries. many do, and they can last years without replacement. they can also have the compartments hidden pretty well, so check well.

any blink codes on furnace during startup?

an easy check too is power on furnace, wait for the blower startup to finish (usally 90 sec then it shuts off). if you jump the R and W contacts on the furnace, blower should turn back on and fire should light. that would mean your stat is broke.

 

[BUTCH1]

Possible failure of the 24V transformer that steps down the voltage to the thermostat??

 

[dud]

OP, do you have a furnace of a heat pump?

 

[Flammable]

check furnace pitot/restart furnace

 

[The Stigenator]

okay so I called a tech out to look at it.

As it stands this model does not have a battery, it has a capacitor or something for 48 hours of memory (checked the manual online), there is no battery associated with it.

As for the thing, if you power off and on the breaker in teh garage, it immediately turns on the blower, but not the thermostat.. which is dead as an doornail.

My suspicious is that the transformer on the roof died and since there is no power to the stat, its gone into safety and turned on the blower only.. since the stat provides control logic to turn on heat/fan/ac etc.

I hate issues with home ownership!

 

[OrByte]

your furnace has anywhere from 6 to 12 little thermostat "fuses" for lack of a better word. They are safety requirements...in case the furnace begins to get too hot..the whole thing will shut down

sometimes when one of these goes bad...you get the result of the furnace shutting off but the blower continuing to run.

Its best to have a tech come out and take a look at the furnace.

 

[DCal430]

Most thermostats use batteries and require a battery to work.

 

[Rubycon]

Most thermostats use batteries and require a battery to work.

Yes but failing with G-R connected is odd. What if there was a fire?

Most residential controls use 24V so it won't blow your balls off if you "do the wrong thing". (just don't lick the wires - they really won't taste good and 24V will do a little more than tickle!) Function can be verified by removing thermostat and checking points on the sub base. If the blower continues to run with the thermostat off the sub base (measure across G-R terms, should NOT be close to 0 ohms if it is then fan switch is ON! Glubbers!) the problem is topside. Rooftop package units...

OP if you feel comfortable going to the roof and pulling panels off and troubleshooting with a multi meter it's probably an easy find/cure. If not get a tech out there.

 

[The Stigenator]

Tech is going to come out on monday, so i am without central heat but i do have the small room heaters that i am using for now.. tonight i may go crash in my office, the heat works there.. or my friends place.

The tech called little while ago, he ran through the diagnostics with me on the phone, he suspects a short in the system, but he is at another job, if its done early he will come by. Because the system is on the roof, he wants to be here at day light not night time to dig around the electricals...

 

[spidey07]

Should be a 24 hour emergency service if you need it.

 

[The Stigenator]

UPDATE:

Tech came out, pulled panel, no power in system. Blower running. Runs upstairs to roof, finds no heat system going. Manually turns on the system by overriding it, works for 5 minutes fails.

Turns out a sensor failed, its an overheat sensor that failed. He's taken the sensor and will be back later in the week when he gets a replacement part. A 5 minute job for him then but yeah he was really nice.

Turns out there was some mickey mousing in the wiring. The thermostat is supposed to get power, but it turns out in cool mode if the sensor fails the AC will continue to work and the thermostat will continue to work, however in heat mode, if the sensor fails, it all stops working.. why.. well turns out the system is is wired funny.

 

[jupiter57]

I had something quite similar a couple of years ago.
August, middle of a heat wave, fan would not shut off, but we noticed that the temp & humidity were rising.
Went to the HVAC closet & discovered that the drain piping was stopped up (due to an improperly installed trap) and had flooded the Control Board, shorting it out.
I cleaned the drain, took out the board, dried it with compressed air & alcohol, then re-installed it.
It worked good for a day or so, then the fan started starting & stopping at random, sometimes just for a couple of minutes & the compressor quit working again also.
So, I called the one HVAC Co. I trusted. Tech. came out, found the board was, in fact, fried (which I was sure of), but he was able to jumper it so that the AC worked properly, though the fan ran continuously. he called his office & gave them the model number of the board, they checked and said it would cost ~$300.00 & would take 3-4 days to get it. I OKed this. (This would make the cost of the repair: $75.00 for the service call, $300.00 for the new board, plus another $75.00 to install it= $450.00)
Meanwhile, I found the exact same board on e-bay for $80.00! Aghhh!

The office called me several days later to tell me they could not find the board in stock, but would keep trying. I told them I had already found one, just send me a bill for the service call, which they did.
Got the new board in a few days, all was well!

I know this won't help you, OP, but I learned to check prices online for suspected bad parts. Next time, I will research any replaceable parts before approving the Contractors price!

 

[The Stigenator]

Grrr.. they have to come back.

SO last time the guy had bypassed the switch / sensor and got my heat going. This afternoon they got a new part in finally and set it back up and running.

12 hours later it fails... now my house is miserably cold on the coldest night of the year in CA. I have portable heaters for tonight but I know that they have to come back, I already sent them an email and will call them first thing in the morning.

I think its not the sensor, something else perhaps.. how can a new sensor fail so easily?

okay reading http://inspectapedia.com, I think I now know what is causing the problem. So the overheat sensor is supposedly to protect the heat exchanger / plenum. After the system runs and thermostat says 'heat reached', it shuts off the heat and then the blower continues to run until the pleunm temp reaches below safe zones and it shuts off.

Well whats happening is that the overheat sensor trips and then because of the wiring way it is, thermostat loses power and then the blower keeps blowing because its control logic board has shutdown due to the overheat sensor.

I went up today and noted that one of the labels on teh side was coming off (peeling off), it wasnt as bad before (when i last went up) the guy bypassed the sensor, but this time it was really coming off.. so it now tells me that there is a problem with the heat exchanger or something.

I'll check the filter that I have in the house, but I doubt that is the problem, replaced it in October and I was gone for November.

Hope they can find the problem .. since now I know its the damn overheat sensor due to something else going on, I understand it better, hopefully can get the tech to think a bit more.. i am tired of this BS>

 

[mcurphy]

UPDATE:


Turns out a sensor failed, its an overheat sensor that failed. He's taken the sensor and will be back later in the week when he gets a replacement part. A 5 minute job for him then but yeah he was really nice.

Turns out there was some mickey mousing in the wiring. The thermostat is supposed to get power, but it turns out in cool mode if the sensor fails the AC will continue to work and the thermostat will continue to work, however in heat mode, if the sensor fails, it all stops working.. why.. well turns out the system is is wired funny.

So did everything turn out OK?

 

[The Stigenator]

So did everything turn out OK?

nope.. they replaced the sensor today, it went out 12 hours later... the over heat sensor tripped, cold air blowing just like last time. I learned a lot through http://inspectapedia.com

but i think the problem is else where.. hope the heat exchanger hasnt cracked / etc when i ran it when the system sensor was bypassed before.

 

[BigBarney]

pitot as mentioned before.

little rubber hose could also be plugged with water and needs clearing.