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RV Maintenance Furnace Thermostat

Furnace Thermostat

A common thermostat in RVs and Homes is this basic mechanical Bimetallic coil, two wire, device. It is reliable and cheap. But it has much to be desired for an RV.

The on-off indent position is not reliable.
It is just a piece of metal that slides over another piece of metal. Normally to turn off, the temperature setting is lowered to the extreme left to an indent position, necissarily invalidating (loosing) any termperature setting.

The device is not designed for an RV:
A temperature mode for unattended storage is not available.

There is no night light.

There is no indicator of when the thermostat is on or off.
One can place an ear close to the thermostat and listen for a "click", but I find the indirect method quite annoying and quite contrived. So I installed an LED. ...A supper bright LED, and there is no dought about it.


The translucent plastic that these things are made of lends itself well to being lit. Also, now I can see to reach out of bed at night and make a correction to the temperature. ...No first finding a flashlight.


Now this switch is definite! It is either on or totally off. RV furnaces are different than home furnaces in that the fan will Always come on and run down your batteries even with no gas. Also, you can leave your temperature settings alone - the way that you like them - and simply turn off the unit with an independent switch.


The way that I began was to do all my work behind a board. I did not have to mess up the thermostat. I installed a DTDP switch on the board and three LEDs. I installed a Blue LED inside the thermostat housing in an already established hole. Behind the board I of course had to drill holes in the wall.


I made a mistake on my first attempt at this, and I wanted the mistake to remain visible to you. I don't want you to make the same mistake. The mistake involved wanting TWO indicators to display the state of the furnace: a bright pink LED to indicate ON and a cool blue LED to indicate OFF. And with neither one on if the unit is off. The "greyed-out" LED is the blue LED. I never took a single voltage measurement before I started this project. That is because people say that I think that I know everything. And I assumed the thermostat was a TWO state device. It is not! There are three states. The control line shown in orange can be high at +12v, or low at 0v (that is two...), but also "floating" at a high impedance state (that is three). In the high impedance state you will see that both LEDs are on! Half brightness, but they are both on. The more serious problem is that the blue LED affects the operation of the furnace during the cool down phase. Do not include it!

Here is the main reason for the modifications:
When we are on the coast of Washington, Oregon, or California, we want to leave the RV and travel ahead for a couple of days. If inland a little, or especially in back in Chico, the temperature may be below freezing. I never worry because of this next modification.

The thermostat can be calibrated to any temperature by taking a small screwdriver and carefully moving the post inside the center of the coil. It is a friction fit, and will move. I use the indent position, which used to function as the off switch, as a well calibrated 33F degrees. The object is to have the furnace come on at 33F degrees and off at about 36F degrees.

There are a couple of ways that the calibration can be done:
One is to remove the thermostat and place in a bucket of ice water. Place the temperature adjusting peg to the indent position, which is extreme left. Then slip the inside center portion of the coil to have the contacts close at 33F degrees. You can play with the hysteresis, if you want, by varing the magnet distance.

I did not use this method. I simply waited for freezing weather in Chico, and went out to the RV and adjusted it for 33F degrees in the indent position. It is there every time.

If you do not make this modification, the coldest that the thermostat will go down to is about 50F degrees. And the exact value is "iffy" because you are somewhere above the indent position.

Now, for a question...
What is now the warmest setting possible? After all, we have mucked with this thing. The warmest temperature will come down exactly by the same amount. My warmest setting, with the peg shoved all the way to the right, is now 76F degrees. I don't know about you, but that works for me.



Automatic, unattended operation.
Location is Chico, Ca

Temperature outside is well below freezing, at 19.3 F degrees. Inside temperature is just a hair above freezing. And that is perfect! Slider is in indent position, a precise set point.




This is my nieghbors, but one more thing to note:
Check for obstructions
In the input, the flame will not be a good "blue". And performance will suffer.
In Output, could catch fire.

Luckily, this one is near the surface and can be seen. In fact, dramaticaly seen! Kinda gets your attention.





Made a modification...
Added a Bipolar PNP to always give a lighted status of either onn or off.
Of course the "Power Onn" switch must be Onn.
Found one unexpected event:
If the Batteries are growing week, and sence the PNP Emitter draws 12v power from Utilities while the Base draws 12v power from Furnace, the LED for "Off" starts to just barely light while the furnace is Onn. The 12v Furnace drops lower than the 12v Utils. But that is OK, a rather cool consequence too.