Emergency Thermostat Repairs - Don't Be Cold!

The Aero Cruiser

    I don't like to be cold. Most other people don't either. One of the greatest things about an RV is the shelter and comfort it provides when the weather outside turns nasty. Waking up on a cold morning when the furnace is working and the coffee is perking is one of life's small (but wonderful!) pleasures. That is, until something doesn't work and you have NO heat! Then waking up and getting out from underneath the covers isn't all that fun. That's just what happened to me a couple of weeks ago when I was at a swap meet/flea market. It got very cold at night (rainy too!). I thought, "No problem, I'll just set the thermostat and get some more sleep." Nope. Slid the switch to "on," set the temperature slider and waited (it takes a few seconds to start up the furnace) and waited...Nothing! Now What?

Pay No Attention To The Red Jumper Wire, YET!

The typical RV mechanical thermostat is a pretty simple device. It allows 12 Volts to flow to the furnace at anything below the temperature you've set. You don't really NEED a separate on/off switch, but I like having one. It absolutely makes sure you cannot get 12 Volts to the furnace and have it turn on. Unless there is a short someplace else.... For the temperature sensing, inside is a small coil of flat bi-metal wire. As the coil warms up and cools down it expands and contracts. When it drops below the temperature set on the thermostat, it completes the 12 Volt circuit and the furnace turns on. It will then run until the temperature reaches your set value and the coil has contracted enough to break the contact. Simple...not super accurate, but it works and doesn't require any electrical power.

Thermostat "Guts"

What happened to mine? I pulled the cover off the thermostat and looked. Nothing. Everything looked fine...no burn marks, no melted plastic, no bent metal....weird. I gently pushed the contact at the end of the coil manually up against the contact, hoping it was just a faulty coil. It made contact easily, but no furnace startup. Hmmmm..... I then connected a jumper wire (I keep several lengths and gauges of wire around with alligator clips on each end just for this kind of testing) to each side of the circuit on the front of the thermostat. STILL nothing????? I went outside (in the cold) and opened the access door to my furnace. On the door is a simple diagram of the wiring. I located the terminals that fed the thermostat and jumped them....SUCCESS! The furnace turned on and ran fine. HMMMMmmm. Connecting the wires out here would run the furnace, but it wouldn't turn off...it would get VERY hot inside!! I removed the jumper wire and closed up the furnace. Back inside.....

I figured that I must have a break in the wire between the furnace control box and the thermostat. OK, I can run new wire. It wouldn't be fun, but it's do-able. Next, I removed the thermostat from the wall (it's only two screws) and disconnected the wires from the two screw terminals. When I did this, they touched. And lo and behold, the furnace turned on!! Obviously there was no break in the wire in between. I thought, "Loose connection?" So I reconnected everything and turned on the thermostat switch, re-set the temperature to max and waited....and waited. STILL no heat.  WHAT?!? After a lot of head scratching I tried connecting the jumper wire to the BACK of the thermostat (at one screw terminal) and on the FRONT to the coil contact. It WORKED!!! Huh?

I pulled out my tester and used its continuity function (it beeps when you have a connection between two points) to see where the break was. To make a very long testing process short, turns out, where the metal strips attach from the front to the back, on one side..there was no actual contact. The rivet they used must have gotten loose or oxidized enough to NOT conduct electricity. So the thermostat never connects the 12 Volt wire and ignites the furnace. Never seen this before. Ever. Temporarily, I just left the jumper wire in place...since it all worked. Once home, I filled in the rivet with solder and everything was back to normal.

This is a perfect example of something simple to fix that could ruin your trip. It pays to figure these things out and have the right bits to do the figuring. I could have also gone to a big box store and purchased a house-style thermostat for "heat only." That MAY work..depends on your particular installation. Of course, you have to GET to the store. Now that the heater works...I am stuck winterizing the RV. It's getting too cold for my water system to survive....there's always next season!

Be Seeing You...Down The Road,

Rich "The Wanderman"
www.thewanderman.com