HVAC question

kdebello

kdebello

Well-Known Member
I'm having a furnace issue and my knowledge is just enough to get me in trouble. Hopefully, someone can point me in the direction of what I can check myself or if I need to call a pro.

12 year old Tempstat DC90 furnace. Has run fine all winter.

On 1/31 I installed a NEST thermostat. Used the existing wiring config. Heat, Cool, Fan and Power. Ran fine for 5 days.

The issue that we started having is that the tstat would call for heat, furnace fires up, heats house to temp then heat shuts off, but the fan stays on. Checked the reset button on the furnace and it was not popped. The display on the NEST shows HEATING, but the furnace is off, fan on and blowing cool air out of the registers. Temp drops and fan never shuts off and tstat shows heating.

If I turn off the heat at the tstat, wait for the fan to turn off, then turn the heat on again, all works again. This can last for a day or so, or 5 hours, but eventually the problem pops up again.

The NEST has a consistent voltage of 3.8-3.9v, never falling below the required voltage. I read that some issues can be solved with a Common wire even if not needed for voltage issues. Connected a common to C on tstat to C on the furnace board. This worked for a day or so, then same issue.

Next, I disconnected the Fan wire from the tstat and furnace. Again, same issue. Worked fine for a while, then problem.

Reset the NEST and all worked for about 30 hours. Then same thing.

Reconnected the old tstat, thinking this is a NEST issue. All looked good for about a day, then problem came up again.

Back to the NEST, since I can at least keep an eye on it while at work. If I see the status as HEATING, but the temp dropping, I can shut off the heat from the app. Let it stay off for a while, then turn on the heat back on.

I looked around and can get a new board for the furnace which doesn't look too complicated to install. If there is something else, it's probably above my pay grade. I just don't want to be changing out parts just chasing the problem.

If anyone can point me what to look for next, it would be greatly appreciated.

Thanks!
 
hook up an inexpensive thermostat and check if it is the nest.

is the firmware up to date?

my guess is the tstat if it starts to happen after a couple days.

since you have a combined system with A/C - some of the heat systems don't use the fan control in heat mode - just heat causes the fan to come on, cause it could be bad if it didn't -

check the wiring is what you think it is....at both ends. wouldn't be the first time the yellow wire was hooked to the wrong terminal or bridged.
 
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I hooked up the old tstat and the same thing happened after a while. I thought it was the NEST, but then the old tstat did the same thing. Now, I'm thinking this is going to cost some $

Yes, it combined with AC. I disconnected the fan wire to let the furnace control the fan in heat mode and to see if maybe the Nest was sending a signal to run the fan. Same issue.
 
Maybe the fan relay is sticking....contacts getting welded together.
No alarms from the board...green led always?
 
I was thinking relay sticking intermittently. When I get home, when it does it again, I'll lightly tap the relay and see what happens.

Not sure about the LEDs. I've been killing the power before opening the furnace. I'll check that out.

Thanks.
 
perhaps a sensor is telling the board that there is an overheat problem - heat off, fan keeps running, won't let heat come back on.....

What happens when turning the power to the furnace off? Seems like cycling the thermostat - while it is calling for heat, also offers a clue - perhaps this would too.

missed the note about changing tstats - woops.

i still have a problem with cycling the thermostat to fix the problem. not sure what is buggin me yet.

that voltage seems low - aren't these 25vac controls?
 
I haven't tried killing power to the furnace while the fan was running. Didn't think that would be a good idea. Each time, I've just turned off the furnace at the tstat, and let the fan shut off.

When I do turn off the power, its after the tstat is set to off and the fan has stopped. Killing power to the furnace shows a no power message on the Nest.

I don't like the idea of shutting off, waiting for fan to stop, then turning on again either. At some point, it's just not coming back on again. If I don't track this down this evening, we'll have to call someone.

The 3.8-3.9volts for the NEST is for the battery. If the battery goes below a certain voltage, it goes offline to save battery. No wifi, no auto display. On some system, low voltage will also not send the correct signal back to the furnace to do its thing. They say to connect a C wire will correct some issues. Tried that and no difference.

I'm going to pick up a cheap tstat from the Depot before going home, just to try another one.
 
when connecting the c wire - make sure there is 25-40vac across the terminals. it could be the wrong C ??

gl with this.

if the nest is picking up the no-power from the furnace correctly, then shutting the switch won't hurt - like cycling the power on a computer. just give it 10 seconds in the off position.

i got nothing else. stupid controls.....
 
Does nest have a replaceable 9v battery? Or is it an internal that charges off the board voltage? If it's the latter I wonder if the power stealing fried the board just enough to cause some weirdness. Or maybe low voltage arced the relay causing it to stick as previously mentioned.

Also does the nest have an option to let the furnace control the fan in heating mode? If so make sure that's on. I had unexpected results with my ecobee set to run the fan, changed it to furnace calls for fan and all was well afterwards. Not sure how this would explain problems with both stats, but it's worth a look.
 
I lost heat for a day because I'm too cheap to call someone. I notice my evaporator pump was running and the box was filled with water. The exhaust line goes outside causing it to freeze so I bought 6ft of heat tape, emptied the pump and we were up and running again. Yes it took me 4hrs to figure that out.


Come summer I will run the water line into the washing machine exhaust so it's not exposed to cold weather.
 
A friend of mine knows an HVAC guy who was kind enough to stop by last night. He did a quick look over things and in his opinion, it's the circuit board.

The current board is an outdated Honeywell with no LEDs. I ordered the new board that replaces this one. Should be here tomorrow. Doesn't look too complicated to install. Looked over the docs from Honeywell and it should be just set the dip switches to the fan delay I need for cool/heat and move the wires from old to new.
 
kdebello said:
A friend of mine knows an HVAC guy who was kind enough to stop by last night. He did a quick look over things and in his opinion, it's the circuit board.

The current board is an outdated Honeywell with no LEDs. I ordered the new board that replaces this one. Should be here tomorrow. Doesn't look too complicated to install. Looked over the docs from Honeywell and it should be just set the dip switches to the fan delay I need for cool/heat and move the wires from old to new.
Click to expand...

When you install the new board scab a few 3 Amp fuses into the RC-RH line and if you're using the C terminal (B) insert a 3 Amp there also. The Nest thermostat is somewhat problematic with low voltage issues.
 
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