How to fix a broken thermostat

by John Stapel

I am increasingly appreciating the concept of appropriate technology. The story begins with us being unable to switch the rear furnace (our RV has two 12V/LPG driven furnaces. The front furnace heats front half + bathroom and the rear furnace heats the bedroom.

From my perspective, the furnace is a black box. Once it is triggered by the thermostat, a 12V fan switches on. After the furnace verifies that the fan is running it automatically ignites the gas. Your LPG heater in your stick house probably works much the same (except it runs on 110V).

In general I can fix many things once I understand how they operate. Luckily we have an old coach, so it is equipped with fixable $25 mechanical thermostats rather than unfixable $125 electronic thermostats. If an electronic thermostat stops working, there is little you can do to fix it. Simply throw it away and pay $125 (they are pretty easy to install).

Conversely, with a manual thermostat it is easy to peel off the cover (after switching off the current at the circuit breaker panel) and see what is going on.

The idea is very simple. There is a coil of bimetal (two metals fused together) that bends according to the temperature. The reason is that different metals expand to different lengths depending on the temperature. Those of you living in countries where utility cables still run overhead might have noticed how they are strung tighter in the winter than in the summer.

When the temperature increases, the tip of the coil extends towards a magnet. Once it gets close enough the magnet pulls it tight making the connection.

The first problem is if the contact is not good. Use a soft brush or my favorite, a can of compressed air (in retrospect I am realizing what a boring life I led before discovering canned air), and blow the dust away. If that does not fix the problem, it is time to clean the contact. Use some coarse paper from e.g. a brown paper back or an envelope.

The second problem is the on/off switch. You will note how it is very simple (I love this!). This can also be cleaned with the coarse paper method.

Luckily I did not have to go any further. The furnace switches on and off again.

The next step would be to take out the thermostat and hot wire the furnace to an on/off switch. This would probably save money in our case given that it takes about two minutes to heat a room. Try beating that stick house dwellers! πŸ˜›

Maybe it’s time to take my old electronics stuff out of storage. I have grand plans, such as wiring an actuator to the black water (human waste) tank valve and connecting it to the right turn signal for easy dumping. Or maybe not πŸ˜€

This post was part of Everything Home DIY – 11th ed

Originally posted 2008-11-11 07:22:46.