Suburban SF30F - intermittent reliability

[We_ride___]

Hi - SF30F Propane heater with a manual Dometic thermostat.

We placed a thermometer next to wall mounted thermostat for comparison.

Symptoms are unique:
- comes on and off at desired temperature during the day.
- around 2 or 3am abouts' it will not come on as it gets extremely cold.
- we get up and adjust the tempurature slide on thermostat - stays off.
- while we're still up, we turn the thermostat to OFF then back to FURNACE and it works for remainder of the overnight period.
- come the chilly morning, the same thing will need to be done to sort of manually reset 'something'.

New motherboard has been replaced this year, already.

Help?

Cheers

[We_ride___]

yes, we have three that are good locally. I'll bring in the TDR and throw a part or two at it.

[enblethen]

Just thinking if there was a place near you to get time delay relay!
I see you have a few.

[We_ride___]

Lk Havasu, AZ - 73* during the day, 40* at night.

[enblethen]

Very possible!
Where are you located?

[We_ride___]

Sorry, yes - I had watched a couple of youtube videos on TDR's that suggested theres a small disc inside that is designed to heat, warp, expand a plastic piece of toggle - which completes a circuit, and allows the start up sequence to continue.
Was that not correct?

FYI - Based on that assumption - I had come up with the theory, that the likely reason that the furnace system works perfectly each day, just not between 3 and 4am (the coldest of the evening), was because the TDR disc inside was failing to properly warp (presumably due to coldness) and was making it inoperable for those few frigid hours - but was returning to normal again by morning light.

[enblethen]

OP had asked about the disc. Thermal cutout/ high temperature limit switch.
Old B: we are on same page!

[Old-Biscuit]

enblethen wrote:
The disc is a thermal cutout. It is a safety item. It is normally closed, when temperature gets too hot it opens shutting off the burner circuit.

Sounds like you are describing the High Temp Limit Switch NOT a TDR

Timed Delay Relay is OPEN until DC comes back from thermostat and heats the coil. Then DC goes to Fan Motor

When Thermostat opens (set point met) DC from thermostat is no longer on TDR heating the coil....fan continues to run until coil cools off stopping the DC to fan motor


No Fan operating when t-stat closes......TDR
DC MUST go thru TDR to get to Fan Motor
**And GROUND must be Clean/tight for DC to flow (Motor ground and ground thru TDR)


Differences in the TDRs.....
Design only.........function/wiring is same

[We_ride___]

WilCo

[enblethen]

Check any splices in the thermostat wires!

[We_ride___]

Will do, thanks

[enblethen]

There could be something contracting enough to cause failure. Since fan doesn't run, it must be in power to the unit.
Start with the basics. check the connections on the ground buss. Tug on each wire, to insure the crimp connections are good. Do the same thing to the positive lead. Check any splices in the circuit in same manner.
Take the time delay relay into the house. Locate the blue wire terminal from the thermostat and the green wire terminal. put it in the freezer for thirty minutes or so. Take it out and quickly use an Ohm meter to check for continuity. You should have 20-30 (?) ohms of resistance.

[We_ride___]

He runs it every night from Oct to April, basically.
Works correctly for every 24hr period (minus the mentioned 2/3 hr period in the coldest hours)

[enblethen]

No, it doesn't sound realistic.
How often is the furnace running over night? It could be something like condensation building up on the porcelain of the ignitor or on the hi-voltage wire causing the unit not to spark then going into failure.
Inspect the porcelain for black arcing traces. Inspect the hi-voltage wire for any nicks or black spots.

[We_ride___]

I did - but I am still uncertain as to what factor would cause the system to work 20 hrs a day; but not to work at all for three hours each day and only when its dark and chilliest.

To your mind is it possible for a time delay relay to fail only some hours of the day, for another reason?