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Discussion Starter · #1 ·
Normal operation: Thermostat @ 70F, furnace runs until it hits 70-71F, then you can hear the gas valve/flame shut off while the fan continues to run for another minute or two, then turns off.

Problem: Thermostat @ 70F, furnace runs until it hits 70-71F, then you can hear the gas valve/flame shut off while the fan continues to run. HOWEVER, about 10 seconds after the flame is off and while the fan is still running its shutdown cycle, the flame reignites, runs for another minute or so, and keeps repeating this process. This repeats 3-4 times per heating cycle before it cuts the flame and allows the fan to run and turn off normally.

This happens with a Nest smart thermostat and a non-programmable battery operated thermostat. Also tried hardwiring heat power wire (Rh) to heating call (W). Furnace runs normally from 68F to 75F with no interruptions or odd behavior.

So thermostat and any thermal protection on furnace is ruled out. It's acting almost as if the temperature it's reading falls down to it's turn on temp of 68F immediately after hitting its target temp of 70-71F and switching into the fan cooldown step. Ideas?
 

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Discussion Starter · #7 ·
The hole where the thermostat wire penetrates the wall.
This makes more sense haha. I'll check after work today.

The hole for the thermostat wire is centrally located in the house, runs down through an interior partition wall, through a return branch duct, and then into the general basement space where it runs over to the furnace. The return branch ducts are just a few basement joists with sheet metal nailed to close off the bottom.

Are you thinking that a cool draft is making its way to the back of the thermostat or being pulled past it, into the wall, and back to the return duct and causing a quick drop in temperature reading after the flame first shuts off?

Also check for a drop in the heating call with your meter at the furnace.
If I follow correctly, 24 VAC should be seen between the heating call wire (W) and the common wire (C) at the furnace control panel when the heat is running. When the flame cuts off when first reaching target temp, this should show no voltage while the fan continues to run. But then when it reignites, 24 VAC should be seen again. If this is the case and the heating call properly follows the furnace flame, this is a strong indicator of something funky happening at the thermostat, not at the furnace?
 

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Discussion Starter · #10 ·
Drafts affecting thermostats are fairly common.
As far as everything else, only further diagnostics will show what’s going on.
So it’s definitely cool air in the tstat hole, but not necessarily drafty. Although this interior wall cavity runs down into a return duct, so it’s possible that when the fan is running, it is pulling air through there.

Anyway, tried the easiest option first of plugging the hole with some insulation. And success! It must have been cold air at back of tstat causing the strange behavior. I’ve also noticed the cycle to cycle time is a bit longer under normal operation and that the house temp seems slightly cooler than before. As in we used to keep it at 69F and now need 70-71F to be as comfortable as before.

Anyway, thanks for help!

EDIT: oh, and nest does NOT have any minimum time off setting as far as I can see
 

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You’d be much better off without the Nest.
Although labeled as “smart”, it’s about as dumb as a brick and equally as reliable.
A lot of companies don’t want to warranty the equipment when a Nest is used, due to the issues they cause.
 

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Discussion Starter · #12 ·
You’d be much better off without the Nest.
Although labeled as “smart”, it’s about as dumb as a brick and equally as reliable.
A lot of companies don’t want to warranty the equipment when a Nest is used, due to the issues they cause.
I’d have to agree. After first getting it, the Smart learning, home/away detection, and some other features sounded cool. Then started to realize how unreliable they were. At this point I only use the WiFi temp control and normal weekly scheduling.
 

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Discussion Starter · #13 ·
So the cool draft behind the thermostat wasn't the problem, just a coincidence that it stopped happening for some time after plugging that hole.

Proved out all of the limit switches and traced the problem back to the draft inducer motor pressure switch. You can see 0 Vac between the two pressure switch terminals when running normally, but when the flame cuts out during its error state, you can see ~28 Vac between the two pressure switch terminals.

Now the question is determining if the problem is a failing pressure switch, leaky tube, clogged tube, or something causing the inducer motor to not pull enough vacuum. First thought is to try cleaning out the pressure switch tube and tube ports out to see if a clog is the problem. If that doesn't do it, then buy a cheap manometer and monitor the actual vacuum when running normally and then after it enters the error state. Then this would point to either the pressure switch if vacuum remains steady or towards the inducer motor if vacuum does drop right before it enters error state.

Open to other ideas to try as well.
 

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Discussion Starter · #14 ·
Quick update, ended up being condensation build up in the pressure tube. The loop dipped down in the middle between the pressure switch and DI motor and allowed this to pool. No problems since emptying.
 
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