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Replacement/upgrade for thermostat


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I have 83 Sunrader with the original Robertshaw CM60 24 volt mechanical thermostat controlling the furnace. No AC. The thermostat works (and therefore the furnace) when I am plugged into shore power. Furnace triggers and runs fine. 

The thermostat does not function when I am running on 12 volts so I currently do not have a functioning furnace when I camp without shore power. Would like that option. 

 

Should the thermostat still work on 12 volts? Operation and install Manuals make no specific statement but it looks like only 120 (stepped down to 24). 

If not, is there a good replacement that would allow both 12 volt and 120 volt?

Furnace is Suburban NT12s. Power Center is a B-W 6325. 

Thanks

Andy

Edited by akwcanoe
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Most are no more that a switch they have no voltage dependent parts other than the switch and generally low voltage. I have an off the shelf computer thermostat that uses AA batteries running my furnace in the camper but because most are no more than a switch operating mechanically pretty much any will work.

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Thanks Maineah . I'm getting 12 volts into the thermostat so it looks like almost any simple Tstat will do. 

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38 minutes ago, akwcanoe said:

Thanks Maineah . I'm getting 12 volts into the thermostat so it looks like almost any simple Tstat will do. 

Get one that has a definite “off” setting. Some go down to low numbers but not “off”.

I found this out the hard way while getting into my truck on a very cold morning. It was too warm for the outside air temperature, then I heard my furnace fire up. Switched to the correct one and no more problems.

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Thanks Fred. Will do that. 

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This maybe totally unrelated to your thermostat. 

The RV heaters are designed to run the fan blower to purge the chamber of propane and it opens the valve and makes a spark to get things going.

A safety switch is involved. The switch has a sail on it and the fan has to blow a certain amount of air to trip the switch. A fan motor may not have enough umph to trip the switch if the house battery is weak. Things work if your plugged into shore power, because the converter makes a higher voltage than a weak battery.

It's voltmeter time

Edited by WME
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Thanks WME. The fan does cycle before, during and after the burner runs - when I am plugged in. This suggests to me that everything is working fine on the furnace itself. It should do the same while on 12 volt but I get nothing at all. No fan. No burner ignition. 

When not plugged in, a volt meter across the thermostat shows 12+ volts until the contacts engage. At that point the voltage drops to zero. Coach battery voltage is over 13 volts while this occurs. Thought maybe my coach battery was toast and the load from the furnace was causing a voltage drop. That doesn't seem to be the case. Will try again while running the engine (and therefore the alternator). Will also test the voltage at the Tstat while plugged in to see what happens. 

It seems to me that the furnace is not getting what it needs from the Tstat. Either it's busted or it was never designed to work on 12 volts. The wiring diagram that comes with it only shows a 120 transformer stepping down to 24 volts and rates it as 24 VAC - alternating current. No mention of 12 VDC at all. Which really doesn't make much sense. Will continue to troubleshoot it and figure out what's happening before I order a new control. 

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Yes, running the truck is an excellent idea. It will show if the extra voltage fixes the problem.

The problem can also be caused by a funked up fan motor.

FWIW the thermostat is used as a milli volt  switch, it never sees 120v. Your furance is 12v only, again it never sees 120v.  

Possible MAJOR weirdness Surburban does make a "Park" model that is 120v. A replacement by P/O? But it is not an NT model

Edited by WME
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If it does any thing blows air what ever the "T" stat is working demanding heat. It should go thru a "pre purge" then ignite with a "clunk" when the gas valve opens plugged in or not. If you have doubts just twist the wires together and see what happens it should go through the song an dance and light that basically is all the T stat does just completes the circuit it is a super simple gadget. It only sets the rest of the furnace. stuff in motion. Propane has a lot of "what if" safety procedures but again the T stat only puts all that in motion.

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Thanks everyone for the help. I disconnected the Tstat and twisted the wires together as Maineah suggested and it fired right up. Cleaned the contacts on the Tstat, reconnected the wires and it worked. Just 38 year old dirty contacts. Coincidentally this is exactly what it took to get my 3-way fridge working on 12 volts. When in doubt, disconnect, clean and reconnect all electrical contacts and grounds. Why does it always take me so long to remember this!

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1 hour ago, akwcanoe said:

Thanks everyone for the help. I disconnected the Tstat and twisted the wires together as Maineah suggested and it fired right up. Cleaned the contacts on the Tstat, reconnected the wires and it worked. Just 38 year old dirty contacts. Coincidentally this is exactly what it took to get my 3-way fridge working on 12 volts. When in doubt, disconnect, clean and reconnect all electrical contacts and grounds. Why does it always take me so long to remember this!

Well age does play a part in may respects! Glad you got it going. My camper has two heating systems the propane one is controlled by a computer T stat so if I'm out exploring it comes on before I return to a nice warm camper. The second is a line voltage home made system made from a bath room over head heater that's used when I'm plug in paying for power any way. By the way the 120 volt T stat is the same concept it's just a switch but it carries the heavy load of a electric heater.

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Maineah.   The your second system makes good sense and would be relatively easy to configure with a radiant heater like you mention. Food for thought. Thanks again for your suggestion. 

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23 minutes ago, akwcanoe said:

Maineah.   The your second system makes good sense and would be relatively easy to configure with a radiant heater like you mention. Food for thought. Thanks again for your suggestion. 

Yes that would work just as well mine is fan forced electric heat.

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