Need help with a SF 30 Suburban Furnace

[mikestock]

My original problem was the usual story. The fan would come on and the furnace would fail to light and the fan would then shut down. I, standing outside could hear the switch and smell gas but the spark, I couldn't hear due to fan noise and it was failing to light the gas. I brought the furnace home and checked the over-temperature thermostat and sail switch. They were good. I hooked up a 12 volt battery and opened up the combustion chamber to check to see if I was getting a good spark, which I was.

Took it back and reinstalled it and the fan no longer came on when I called for gas heat. I could, however, use the external 12v battery to supply power through the green and yellow wires while jumping the two blue wires together. With the gas hooked up, the furnace came on and burned, just like it should. I had the internal thermostat out of the circuit and the only hook up to the rv was the gas source. I hooked internal wires back, just like they were originally but nothing would happen.

I'm guessing that I have a thermostat problem. I may have caused it when I back fed power by accident, blowing the 2 amp glass fuse in the thermostat.

Lacking any expert suggestions I guess I will go spend $80 for a replacement thermostat. This one is a 3 way, AC, heat pump or gas furnace. The ac and heat pump functions still work.

[mikestock]

No gas.

It seemed that most of the many posts about Suburban furnaces claimed the same problem as I had. The fan came on and I couldn't hear the igniter sparking above the fan noise. I could smell no gas on the outside. Most every bit of information I read said and the circuit diagram showed that the over temperature thermostat and the sail switch were usually the culprits. I removed the furnace from the housing and set it up on a bench, running it with a 12v battery. The spark looked good but, of course, there was no gas. Everything looked good so I reinstalled it and it was just like before. No gas. I let two wires touch and then nothing worked, no fan, no nothing. I found the glass fuse in the thermostat and I was back to the original situation; fan ran, no heat. I took a chance that it must be the thermostat so I bought a new one identical to the OEM and everything works. End of story. I don't really know what was wrong with the thermostat and don't care. It works.

I had a previous problem with this furnace about 3 years ago. Found, what felt like, a 1/4 lb dirt dabber nest in the duct. Put screens on the outside to prevent this from happening again.

[Flyfisherman128]

So you are saying you got gas and spark but no fire.. I just put a new motor (it was going bad)I thought I want getting the correct air flow. I replaced the sail switch. Finally I replaced the igniter. I set the igniter where it was set at the factory. I still got it light intermittently. Not a good scenario for winter. I got out the owners manual and set the igniter as specified which was not even close to the way it came from the factory..yet it worked for 20 years.. To my surprise, problem solved.

[mikestock]

Yes

[dougrainer]

mikestock wrote:

emblethen wrote:
I think the OP is in over his head!

So clairvoyant. You remind me of my wife.

I received the new Coleman thermostat today and installed the it. The circuit board was identical to the one I replaced and everything works perfectly. I still don't know the exact problem with the old thermostat but the new replacement solved it.

I had already eliminated the most common causes, the sail switch, the over temperature switch or dirt dabbers. The furnace would work when I applied 12v. using an external battery source, so it left only the thermostat or some unlikely cause, like rats.

I think you stated you may have caused a backfeed short when you were diagnosing the Furnace system. That may have blown something on the tstat circuit board. Have you looked at the Tstat circuit board for any burn spots? Doug

[enblethen]

Glad to hear that you have resolved your issue.

[mikestock]

emblethen wrote:
I think the OP is in over his head!

So clairvoyant. You remind me of my wife.

I received the new Coleman thermostat today and installed the it. The circuit board was identical to the one I replaced and everything works perfectly. I still don't know the exact problem with the old thermostat but the new replacement solved it.

I had already eliminated the most common causes, the sail switch, the over temperature switch or dirt dabbers. The furnace would work when I applied 12v. using an external battery source, so it left only the thermostat or some unlikely cause, like rats.

[enblethen]

Good explanation, Doug
I think the OP is in over his head!

[dougrainer]

How does the Heat Pump Thermostat Work
The RvComfort.HP, the RvComfort.PHP, the Coleman True-Air, and the RvComfort.ZC
thermostats by RvProducts Inc. are all capable of running not only an Air Conditioning unit, but
also an Electric Heat Pump. Frequently we receive calls from customers who do not
understand the functions of the Heat Pump Thermostats. This guide is a quick run through of
the information already provided in the Thermostat Operation Manual, included with each
thermostat.
The Heat Pump is an electric source for heat. It will supply and maintain heat assuming the
outside (ambient) temperature is above 40 degrees. This number of course can be slightly
higher or lower depending on the humidity. Higher humidity can cause a heat pump to lose
efficiency at a slightly higher ambient temperature, while lower humidity can cause a heat
pump to lose efficiency at a lower ambient temperature.
Since no one wants to wake up to find that the outside temperature dropped below forty
degrees and it is now 50 degrees inside the coach, the Heat Pump thermostats are programmed
internally to recognize when the temperature drops five degrees or more from the set
temperature to the actual inside room temperature. When the temperature exceeds five
degrees or more between the two, the thermostat will default to the next available heat
source.
The thermostat, upon sensing a temperature split of five degrees or more in the electric heat
mode will bring the gas heat on to assist the electric heat. This is the first strike. A strike is
created by the thermostat having to change modes (or run dual modes to sustain a
temperature split). The electric heat and the gas heat will continue to run together until the
thermostat reaches the set temperature and satisfies. When the electric heat comes back on, it
will be in electric heat only at that point. If the temperature again drops five degrees or more
from the set point, the thermostat will again bring the gas heat on to assist. This is strike two.
The system will then go through the above stated procedures. If the temp should drop five
degrees from set point for a third time, the thermostat will give up on the electric heat, lock the
electric heat out for two hours (showing either DIFF on the display or FLASHING GAS HEAT on
the display) and default to Gas heat only. You WILL NOT be able to run any Electric heat
during this two hour lockout.

[dougrainer]

In case you did not know, this is the RVP parameters for a HP system. Electric heat refers to HP. Doug
Why does my furnace come on when I am calling for heat pump?
When your thermostat is in the “Electric Heat” mode, if the set temperature is five or more degrees above what it shows as the room temperature; the thermostat will consider this too much of a load for an electric heat source and bring in the furnace to assist. The furnace will continue to run in this case, until the thermostat is satisfied. When the room temperature starts to drop again, the thermostat will try to proceed as normal, with just the electric heat (unless the difference between set point and room temperature reaches five degrees again).
PLEASE NOTE: Some of our thermostats will not run your electric heat and gas heat simultaneously. They will shut down the electric heat in order to bring in the gas heat.
Multiple consecutive calls for the furnace in the “Electric Heat” mode, can cause the thermostat to lock out the electric heat function for approximately two hours, triggering the furnace only when it needs heat. For more information on the Lockout function of the thermostat,

[dougrainer]

https://www.airxcel.com/rv/coleman-mach/thermostats/digital-thermostats

This should be your Tstat. 6536A3351
2-Stage Heat Pump & Gas Furnace - White

It controls both Furnace and HP function. Plus AC. The color control at the Tstat will NOT be the same at the furnace to those 2 BLUE Furnace wires. AT the furnace, ONE of the 2 blue wires should be 12 volt positive. This goes back up to the wall tstat. When you select FURNACE, the second blue wire is then 12 volt positive from the wall tstat. This runs the furnace like your bench test. It makes NO difference which blue wire is used. IF one of the blue wires is 12 volt positive and when you select furnace the 2nd blue wire is NOT 12 volt pos, then you have a problem in the circuit from the tstat to the furnace and replacing a different brand/model tstat will not fix the problem. You should have 2 wall tstats, unless you have the single ZC wall tstat. If 2 just look at the wall tstat wiring of the other tstat to verify color code and how the other furnace is connected. I assume you have 2 furnaces. Doug

[enblethen]

Your model is for an A/C and heat pump. That is what HP on model number means. It does not have internal mechanism to run a standard propane or electric furnace.
You might what to get some professional assistance!

[mikestock]

emblethen wrote:
Your existing thermostat is for heat pump only.

Wrong. It has ac, off, gas heat, heat pump.

[enblethen]

If you are talking about the Lux, Switch is a mechanical one on the back of thermostat body. Your existing thermostat is for heat pump only.

[mikestock]

I'm looking at the touch pad menu and don't see anything that allows selecting gas/heat pump. The nature of the beast says that a heat pump will only work down to about 40 degrees. If I tell my wife she has to take the cover off the thermostat to do anything, she would look at me like I'm crazy. She has enough trouble with the settings right in front of her.