Originally Posted by zenarmy
our 70' Overlander orig heater works great except that sometime in the middle of the night it turns to cold air....
Pretty common problem on the older furnaces. Two or three things may be causing it.
1: low voltage. When the battery gets down, the voltage drops below the necessary range to keep the gas valve open. That is generally 10.5 volts. Cause is a low battery, poor connections, poor grounds etc. You will need as high a voltage AT THE FURNACE as possible. Voltage drop through the wiring makes it important to measure it at the furnace.
2: Slow blower speed due to low voltage (see above) which causes the sail switch to drop out, shutting the power off to the gas valve. Also, the blower speed might be slow, just due to old age.
3: This is the hardest one to find, and is generally solved by replacement rather than testing.... the high limit over temp klickson switch has become interminent. On your original no circuit board furnace, if the temp of the interior of the furnace goes too high (and this is common, due to small ducts and low blower speeds) there is a little switch called a Klickson (brand name) which will shut the gas valve off until the heat is dissipated by the blower. The real name is the "high temperature limit switch" These switches get old and tired and when they open, they shut the gas off, but when they close, the contact does not make good and the gas valve will not come back on. Often, when completely cool, and run through another cycle from scratch, they will start to work again, but then fail again. In other words they become intermintent, which can drive you crazy finding the problem.
So, on your old pilot light furnace, I am pretty sure one of those three things is causing your problem. There is a slight possibility that the gas valve itself is defective and is temp sensitive, but that is not too common.