750mv vs 12v Thermostat Question UPDATE 2

My furnace is an Hydro Flame Excalibur 8500 II, but it does work like that as above. I had some fun getting the furnace to work after getting this 1991 RV (wasps etc) but still had trouble due to suspect thermostat. Now that is fixed, so another victory getting this 1991 RV all going right.

So far so good--the important things are all fixed. Good thing I have been on this forum and picked up a few clues about RV systems so I can be my own tech--hate to have to pay for all the work/time it takes to trouble-shoot everything and fix stuff--yipes.

If it's a suburban furnace, one blue wire is connected to +12VDC, the other blue wire is connected to the sail switch, limit switch time delay and module control. so when the thermostat closes you are connecting +12VDC to the sail switch and limit switch time delay circuit. once it sees the 12VDC the fan motor comes on and the after a time delay 12VDC is applied to the control module to signal opening the gas valve and firing the ignitor. the gas valve won't open if the limit switch is open or if the sail switch is open. (e.g. if it is either to hot in the heat exchanger or if the fan motor doesn't start running to give enough airflow to close the sail switch)

I'm sure other furnaces are similar since virtually all of them can use a common aftermarket ignitor board.


see link for schematic.
http://gooddy.org/suburban-rv-furnace-wiring-diagram.html

pianotuna wrote:
Glad it worked out!

I can't remember having such a good time! :(

BFL13 wrote:
Some mentioned about the adjustable swing feature on the better ones. I got the lower cost digital without that. I can see how that could be a useful feature in some scenarios.

I have no clue why those two blue wires have a voltage across them. I started out thinking it was just one of them that was being switched and the other was a ground. Didn't look closely enough at the old one before starting all this. Anyway it turns the voltage on and off between the two wires.

The blue wire shows up in the schematic here and there to the various furnace switches, so it must all have something to do with that instead of just switching one of them. Whatever. "Not my problem!" :)

If there were no voltage across the wires (when not connected together), the furnace would have no way of detecting when the thermostat switch closed. The wires would basically be virtually shorted together already--which basically means that their voltage is identical--and without any voltage difference, there would be no current that flows when the switch closes and no voltage change to boot.

These are not power supply wires. They are signaling wires for the furnace to turn on and off. It's pointless in this case to try to determine if one is positive and one is negative; they just need to be connected to turn the furnace on and disconnected to turn it off. Similarly, it doesn't matter for a light switch which wire to the switch is positive and which is negative, and also there those descriptors are not entirely accurate.

(There's never any voltage between wires that are shorted together by a switch or anything else. That's basically what shorting them means; they are, from a circuit analysis standpoint, part of the same node.)

Glad it worked out!

Some mentioned about the adjustable swing feature on the better ones. I got the lower cost digital without that. I can see how that could be a useful feature in some scenarios.

I have no clue why those two blue wires have a voltage across them. I started out thinking it was just one of them that was being switched and the other was a ground. Didn't look closely enough at the old one before starting all this. Anyway it turns the voltage on and off between the two wires.

The blue wire shows up in the schematic here and there to the various furnace switches, so it must all have something to do with that instead of just switching one of them. Whatever. "Not my problem!" :)

BFL13 wrote:
I decided to put the blue wire that was on the old Thermo R Common onto R on the new one, and put the other blue wire from old W to new W.

I still think that must be backwards (isn't "common" negative?)

Anyway, it worked. With it set to "heat" I set target temp at 17C and the display (yes you can see it without any back lighting) showed ambient at 50C so the furnace fired up blower first then heat--all good.

When the ambient got to 17 it kept heating away but then there was a click and the "heat on" went out and the heater stopped and then a bit later the blower. By this time ambient was showing at 18C--confirmed by another thermometer nearby as a cross-check. Good!

So I waited to see if it would stay like that for a while, it did, so I opened the RV doors and a window to cool things down and waited to see if it would come back on and at what temp.

It came back on with the ambient reading 17C but this took a while because it stays warm inside the RV (which is good) So it cycled.

I don't know if that one degree C drop counts as a "short cycle". It would be ok by me if it dropped to 16 or even 15, which is how the old slider temp Duotherm in the 5er acted. But it is ok as is where it would have stayed off longer if I had not opened the doors and a window to cool it down inside. Should be able to live with that action as is.

So another day of adventure completed! Can't say all that was much fun, but it was "interesting" for sure. Thanks again for all the advice, some of which I actually took! :)

in reality, for a RV setup which wire goes to W and R really doesn't make any difference.

and how much temp swing you get between on and off is determined by the thermostat. some are adjustable some are fixed. a typical fixed system has about a 2-3F or 1-2 C swing between on and off.

and often they have an "anticipator" circuit that determines when to shut off knowing temp will keep rising for a while. often those are set by what type of heating you have. on the mechanical systems that often is a heating element on the bimetallic coil. But those are designed to run from the 24VAC, so 12V DC won't give the same result.

the ones that are electronic often let you set the swing anywhere from 1F to 5F or so.

shastagary wrote:

take the two wires going to the furnace. touch them together. If the furnace starts to run, which i suspect it will, connect one wire to the W and one wire to the R on the thermostat.

if he connects his 2 thermostat wires tougher the furnace will run that's what a thermostat does. it is a switch controlled by a temperature controlled relay circuit in the thermostat polarity does not matter home furnaces have ac control power to the thermostat. its not that complicated buy the thermostat and hook it up thousands already have.

that was my point. it verifies he has the two correct wires to connect to the thermostat. once he verifies that, then it's easy peasy to get the thermostat connected up and working.

I decided to put the blue wire that was on the old Thermo R Common onto R on the new one, and put the other blue wire from old W to new W.

I still think that must be backwards (isn't "common" negative?)

Anyway, it worked. With it set to "heat" I set target temp at 17C and the display (yes you can see it without any back lighting) showed ambient at 10C so the furnace fired up blower first then heat--all good.

When the ambient got to 17 it kept heating away but then there was a click and the "heat on" went out and the heater stopped and then a bit later the blower. By this time ambient was showing at 18C--confirmed by another thermometer nearby as a cross-check. Good!

So I waited to see if it would stay like that for a while, it did, so I opened the RV doors and a window to cool things down and waited to see if it would come back on and at what temp.

It came back on with the ambient reading 17C but this took a while because it stays warm inside the RV (which is good) So it cycled.

I don't know if that one degree C drop counts as a "short cycle". It would be ok by me if it dropped to 16 or even 15, which is how the old slider temp Duotherm in the 5er acted. But it is ok as is where it would have stayed off longer if I had not opened the doors and a window to cool it down inside. Should be able to live with that action as is.

So another day of adventure completed! Can't say all that was much fun, but it was "interesting" for sure. Thanks again for all the advice, some of which I actually took! :)

take the two wires going to the furnace. touch them together. If the furnace starts to run, which i suspect it will, connect one wire to the W and one wire to the R on the thermostat.

if he connects his 2 thermostat wires tougher the furnace will run that's what a thermostat does. it is a switch controlled by a temperature controlled relay circuit in the thermostat polarity does not matter home furnaces have ac control power to the thermostat. its not that complicated buy the thermostat and hook it up thousands already have.