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 > Heater Thermostat Oddness

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dartmouth01

Massachusetts

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Posted: 11/03/22 01:58pm Link  |  Quote  |  Print  |  Notify Moderator

Hi all, I have a digital Dometic thermostat that controls heat and AC, and when I try to use the furnace now, it works until it needs to turn on again because min temp is reached.  I can hear some relay sounds in the overhead AC strangely enough, and the heater blower starts up, but there is no ignition.  I can trigger ignition by taking the desired temp up another degree.  Any thoughts on what might be the root cause of the issue?  If thermostat, I'll probably go for a microair replacement. Thanks!  

dougrainer

Carrolton, Texas

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Posted: 11/03/22 07:58pm Link  |  Quote  |  Print  |  Notify Moderator

You have a bad FURNCE ignition module. Once the furnace blower comes on, the AC and Wall tstat are out of the picture for ignition control. This type failure is common for a bad Furnace Ignition module. It is VERY hard to diagnose. This is where years of knowledge comes in. Doug

wnjj

Cornelius, Oregon

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Posted: 11/03/22 11:47pm Link  |  Quote  |  Print  |  Notify Moderator

dougrainer wrote:

You have a bad FURNCE ignition module. Once the furnace blower comes on, the AC and Wall tstat are out of the picture for ignition control. This type failure is common for a bad Furnace Ignition module. It is VERY hard to diagnose. This is where years of knowledge comes in. Doug

Except the OP said when they turn the temperature up one more degree on the thermostat, it lights off. I would think a bad ignition control would stay bad under that circumstance. Maybe the thermostat is bad and only pulses the heat wire briefly which causes the fan to start up but not light off?

dougrainer

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Posted: 11/04/22 08:45am Link  |  Quote  |  Print  |  Notify Moderator

wnjj wrote:

dougrainer wrote:

You have a bad FURNCE ignition module. Once the furnace blower comes on, the AC and Wall tstat are out of the picture for ignition control. This type failure is common for a bad Furnace Ignition module. It is VERY hard to diagnose. This is where years of knowledge comes in. Doug

Except the OP said when they turn the temperature up one more degree on the thermostat, it lights off. I would think a bad ignition control would stay bad under that circumstance. Maybe the thermostat is bad and only pulses the heat wire briefly which causes the fan to start up but not light off?


Thats why this type problem is very hard to diagnois. Logically, you are correct. But the fact remains the AC board and wall tstat have nothing to do with a furnace failure to ignite AFTER the furnace blower comes on. Doug

dougrainer

Carrolton, Texas

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Posted: 11/04/22 09:31am Link  |  Quote  |  Print  |  Notify Moderator

The furnace temp differential is factory set at 2 degrees. The OP can change that to 1 degree. Doug

3. To set temperature differential, system must be Off. Press button and simultaneously
press and hold / Mode button for three seconds. Press button to toggle between “d1”
and “d2”, “d1” for 1ºF differential and “d2” for 2ºF differential.

PS, this is for the 3 wire CT LCD square wall tstat.

I don't think this will fix the problem, but it does not hurt to try.

* This post was edited 11/04/22 09:39am by dougrainer *

dougrainer

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Posted: 11/04/22 09:38am Link  |  Quote  |  Print  |  Notify Moderator

This is for the larger CCC2 12 button tstat.

To set the 1 °F / °C differential, simultaneously press the PROGRAM button and the
?button (“dIF1” will appear in the display while the buttons are pressed). To set the
2 °F / °C differential, simultaneously press the PROGRAM button and the ? button
(“dIF2” will appear in the display while the buttons are pressed).

The 5 button CCC tstat does not have a programmable Differential capability

wnjj

Cornelius, Oregon

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Posted: 11/04/22 06:20pm Link  |  Quote  |  Print  |  Notify Moderator

If it were me, I'd disconnect the thermostat and directly connect the heat(W) and +12V(R) wires. If everything is good let the heater run for a few minutes then disconnect, wait a few minutes and repeat.

If that is all good, replace the thermostat. It's nothing more than a relay that shorts those two wires when heat is desired.

John Burke

North Dakota

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Posted: 11/05/22 06:53am Link  |  Quote  |  Print  |  Notify Moderator

Make sure your thermostat is set to auto, not heat. If not the heat strip in the A/C unit will be the first to come on. The heat stripe will heat, and the furnace fan will run.

dougrainer

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Posted: 11/05/22 08:47am Link  |  Quote  |  Print  |  Notify Moderator

wnjj wrote:

If it were me, I'd disconnect the thermostat and directly connect the heat(W) and +12V(R) wires. If everything is good let the heater run for a few minutes then disconnect, wait a few minutes and repeat.

If that is all good, replace the thermostat. It's nothing more than a relay that shorts those two wires when heat is desired.


This will not work as his Tstat has no W wires at all. Doug

dougrainer

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Posted: 11/05/22 08:48am Link  |  Quote  |  Print  |  Notify Moderator

John Burke wrote:

Make sure your thermostat is set to auto, not heat. If not the heat strip in the A/C unit will be the first to come on. The heat stripe will heat, and the furnace fan will run.


You are assuming he HAS a Heat Strip. Very few RV's have factory installed Heat strips. Doug

PS, IF the furnace fan comes ON, there is no way to stop furnace ignition. What type and brand Furnace and AC system do you have?

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