Suburban furnace works only when thermostat is bypassed

[danjoe91]

I have a 2 year old Suburban furnace NT30SP that will not come on. When I move the thermostat, nothing at all happens. I checked for debris, blockages, and power, and all was good. I then took the two wires from the thermostat and touched them together ( and then separated them), the fan came on, but no gas. When I left the wires together, the furnace worked great. Even though I did not think this was the problem, I bought a new thermostat, but it did not fix the problem. I cannot find this problem in any troubleshooting info.

[bradleyoder]

Similar problem here...

[Lumatic]

Really sounds like the thermostat. Are you able to adjust the new thermostat for on/off temperature range. Put a meter on the thermostat and check for continuity.

[lewster]

If connecting the furnace wires together at the thermostat ignites and runs the furnace, the problem is not with the furnace. A basic t/stat is simply a temperature controlled SPST switch.

When you set the t/stat above your ambient temperature, the switch is essentially open as the furnace will not operate. By lowering the t/stat, you create an internal connection in the device (basic ones use a bi-metal spring which expands or contracts with temperature) to connect those 2 wires and activate the furnace.

It's entirely possible that you have a bad t/stat and that the new one was also bad. As stated above, find the terminals where the wires attach and using an ohm meter or DVM, look for continuity across those terminals by activating the slide bar that sets the temperature. If you get nothing across the entire range of motion, the t/stat is bad.

[n2916s]

When you hear hoof beats, think horses, not zebras. The fact that physically twisting the wires together fires the furnace up normally pretty much screams "faulty thermostat". Or does it? My thermostat has an "0n/off" switch in addition to the slide switch for setting the temperature. That "on/off" switch, on my rig, has always needed to be firmly moved to the"on" position.

Look closely, reset the switch firmly, hope for the best...

Mike

[lewster]

Originally Posted by n2916s

When you hear hoof beats, think horses, not zebras. The fact that physically twisting the wires together fires the furnace up normally pretty much screams "faulty thermostat". Or does it? My thermostat has an "0n/off" switch in addition to the slide switch for setting the temperature. That "on/off" switch, on my rig, has always needed to be firmly moved to the"on" position.

Look closely, reset the switch firmly, hope for the best...

Mike

Good point!

It always helps to be sure that a device in 'ON' before use.

[danjoe91]

The reason I could not figure this out, is I bought two new different thermostats ( heat only) with no luck. I then realized that maybe there was a difference between rv and home furnace thermostats. I ordered a new Suburban thermostat through Amazon, and that did the trick! I knew the whole time it had to be the thermostat, but had not figured (right away) that one available from a hardware store would not work. I have no shame to tell you that I am glad I did not let that ignorance stop me from pursuing the thermostat solution.

[Lumatic]

Originally Posted by danjoe91

there was a difference between rv and home furnace thermostats.

A simple home thermostat should work. It may say 24 volts but that does not matter. A thermostat in which you can adjust the on/ off temperature setting will avoid the furnace short cycling. This is what I have in one of my Airstreams.

[lewster]

Most analog t/stats made for RV furnace use generally have an 'anticipator' control that is adjustable. This governs the temperature differential between the furnace re-starts. Most are set to a degree or two, but you can widen the spread if you like.


Lew Farber
RVIA/RVDA Nationally Certified Master Tech
ABYC Certified Master Marine Electrician
Master Tech Energy Systems, Inc.
AM Solar Certified Installation Center*AMS Lithium Batteries
Lifeline AGM Batteries**Magnum Inverters
541-490-6357

[idroba]

The best conversion thermostats for RV furnace work that I have found are ones which have two AA batteries to power them. They will work with virtually any system, 12 volt, 24 volt, millivolt, and have various anticipation rates that are switchable internally. Use the red and white wire terminals to connect them up to the furnace wiring. Generally under $50. I have no specific brand which I think is best, all that I have used worked equally well. Honeywell, however, is my favorite brand, but White Rogers, Hunter and others are fine too.

Batteries seem to last several years (only to fail on a cold night when you have no extras...lol) When in doubt replace them every two years is about right.