Friday, January 21, 2011


Along with this FAQ, you may also be interested in the Furnace FAQ.

How a mechanical thermostat works

The thermostat senses temperature -- via a bimetallic strip -- to "decide" when to turn on the furnace. But a clever, and simple, timing device decides when the furnace shuts down. There is a small resistive strip in the thermostat that acts like a little electric heater. When the furnace starts running, the strip starts heating up. Eventually, it will warm up the bimetallic strip enough that the furnace shuts off. So the thermostat doesn't directly sense room air to shut down. This minimizes the effect of drafts on the furnace operation. The heating of the resistor strip is supposed to be an analog of the heating of the rig and the resistor is adjustable. As I'm sure that you understand, the thermostat simply allows a signal to go to the card that controls the furnace. Upon seeing that 12VDC signal the blower should come on for about 17 seconds to purge the combustion chamber, then the ignition sequence (ignitor and gas valve energized) begins. If after 7 secs the card does not sense the flame, then the card will de-activate the ignition sequence and run for 3 minutes and lock out. If the flame is sensed (normal mode) the ignitor will stop sparking and the gas valve and blower are left in an "on" state until the thermostat sees enough heat and then it disconnects the signal to the card. At that time the card will shut down the gas valve and the blower will run for 90 seconds to cool down.


Some folks choose to replace the simple mechanical thermostat with an electronic model that can be programmed.

- What you need is a millivolt heating setback thermostat that is battery operated. Most models are used for both heating and A/C, but it will still work fine for your application. There may be heating-only models still available somewhere. Most thermostats nowadays are 24 volt powered. To see the various available thermostats, check out Air & Water, Inc.

- Kent replaced his with the Hunter Digital "Just Right" thermostat. They have two models 42999B and 42996; the latter comes with a remote. This is a basic digital thermostat but way more accurate over stock analog thermostats. It has off position and Heat and AC selections, but it will only control the furnace just like the one you are replacing. He installed the 42996 with remote because he likes to have the temperature cold while sleeping but wants it warm when he gets up. He can raise the temperature by couple of degrees with the remote while in bed and then get up to nice warm space. He reports that the furnace does not run as often and he doesn't have the big temperature swings. It is easy to install, just remove the old thermostat marking the wires for position and transfer to same position on new thermostat- I replaced mine with the Honeywell RTH111 programmable about 3 years ago and have had no problems. Whatever you get be sure to check measurements to be sure it will fit. Some of us with MB's had to move the magazine rack to make room.

- I just finished putting a Honeywell FocusPRO 5000 thermostat in my LD, just wanted to share how it turned out. It runs on batteries and is totally compatible in either heat only or heat/cool application. The thermostat is maybe an inch or so larger than the old Robertshaw thermostat in my 91 FL; I mounted the base plate to cover the old screw holes for the old thermostat and it fits perfectly! It has a nice green back-light and big buttons for the temp control which are easy to stab in the dark. It's not programmable but I don't need it. Only 2 wires to hook up and a few minutes to input the settings for the thermostat and you're off and running. It took me 20 minutes to install, including answering a 4 year old's questions while I was doing this work.

Clever Suggestion

Larry Wade suggests adding a switch at or near where you sleep, reachable from when you are still in bed. Wire the switch in series with the existing thermostat. At bedtime, set the thermostat to the desired morning temperature and turn the switch off. In the morning, turn the switch on and then wait for the heater to do its job. It was easy to wire. The thermostat is just a temperature controlled switch. It has two wires running to it. I cut one wire and spliced two new wires onto the two ends and ran them to the bunk location, next to the LD provided night light switch. The wires are hidden inside the overhead storage bin. The floor of the storage bins is a false bottom that can be removed, providing access. Inside you will find other wiring that LD installed for lighting.


Furnace won't start

1. It's fairly common for the thermostat's contacts to oxidize, especially between heating seasons when it's not in use. It's easy to diagnose this.

If the furnace won't start even with the thermostat turned all the way up, take off the plastic cover. You'll see a red wire and a white wire going to two screws on the left side of the case.

Using a straightened-out paperclip or a short piece of wire, bridge those two screw terminals together. The furnace should start immediately. If it does, you know you have a thermostat problem. On the other hand, if the furnace *doesn't* start when you bridge the thermostat's screw terminals, then you have a furnace problem.

Usually, moving the lever back and forth a dozen times or so will clean off the contacts and cure the problem. Another method for cleaning the contacts is put a piece of paper (not sandpaper) between the contacts and move it through a couple of times. That should clean the contacts and renew the connection.

Furnace re-cycles improperly

- When you remove the cover by pulling straight out, you will see a vial of mercury above a spring coil. To the right of the spring is a 2" long metal strip with numbered markings on it; this is the anticipator adjustment. The factory default setting is "1.0". Try setting it on "0.4" and the heater will cycle on more often, but run for a shorter time period, thus reducing temperature extremes. I just reset mine to give it a try. Just slide the moveable brass pointer up until it's on the .4 mark. You can experiment with this setting without hurting anything.

- For those of you with a 2006 model that have the furnace cycling problem, this is what East Bay RV did to mostly fix the problem:

They reset the thermostat, which you can do by turning the black on/off switch to off, then pressing the Mode and Zone buttons at the same time. When you turn it back on you will see that it resets the furnace temperature to 68 and the air conditioner temperature to 72. This has seemed to help my problem. The furnace still turns off and on, but not as frequently as it used to, and it may be because of the location of the thermostat, right next to two heat vents and the air intake.

Furnace Won't Shut Off

Remove thermostat wires and check for continuity.

- If no continuity, check condition of wires. One LD'er recently found that during the installation of a new thermostat, a cover plate he had mounted where the old thermostat was located has compressed the wires slightly, enough for vibration to wear through the insulation and create a short. Taping the wires solved the problem.

Remove circuit board and have it checked with a Fenwal tester. See the LD Companion article on furnaces for more trouble-shooting tips.

Contributors: noel, Larry Wade, Heidi, Kent, Joe Hamm, Ken, Barry Barnes

Revised: 26 Mar 11