Gas Heater

The interesting part of this was convincing the fellows at the parts place to sell me a new motor. They only wanted to sell to a "real A/C technician" or something they had words for. I should have told them that I work as an engineer at the observatory (which I do) and routinely install things costing many orders of magnitude more than a cooler motor. I didn't. I agreed to waive the warranty (their concern apparently was that I would incorrectly install the motor, burn it out, then try to return it under warranty). All of this provides annoyance balanced almost equally with amusement and by noon the motor was installed and working just fine.

Now the heater won't blow hot air

The thermostat works, the flame lights, but the fan never comes on. I suspected (wrongly) at first that this might be related to my replacement of the blower motor. It was not. Jumping ahead, I have now learned that there is a bypass lever on the fan/limit controller that will turn the motor on, any time, no questions asked, but I did not know about this when I got started. This would have been a quick and simple way to check the motor itself.

My heater has the usual (but very handy) wiring diagram glued on the inside of the panel covering the electrical stuff. Here is what I learned after a couple of hours of studying it and fooling around.

The "G" wire from the thermostat controls the fan relay. This will serve to turn on the blower when things get hotter than the thermostat likes, and this is how things work in the summer with the A/C.

In the winter, the "G" wire is inactive (apparently), this leaves the normally closed side of the relay ready to control the heater blower. It is all the same blower incidentally, but a lower speed winding on the motor is used in conjunction with the heater. What actually controls the blower when heating is going on is the fan/limit module. The thermostat asks for heat, the flame comes on, the furnace box heats up, and this triggers the fan/limit module to turn the fan on (or off again when the flame cools down). In theory the house could get hot enough to make the unit want to turn on the high speed fan that goes with cooling, but this somehow never seems to happen.

My problem was with the fan/limit module. It has an 8 inch long tube with a bimetallic strip that extends into the furnace chamber. This yields rotation that actuates switches via some adjustable cams. I never noticed any rotation watching the "dial" on the end of the shaft, so my guess is that some part holding the strip has broken or rusted out. So I am ordering a new module. Good old Ebay will get me one (A White Rodgers E12 type 5D51-90). This has an 8 inch long tube and will cost me $90.

While I was at it, I bought a "high temp limit safety switch" rated at 250F. The existing one is/was adjustable and is/was set to 235. The existing one has been bypassed for several years. A new one cost me $25. Another quick and easy Ebay purchase.

Another note on the fan control for heating. The diagram shows a time actuated switch wired in parallel with the fan/limit. I guess this would be a failsafe on some units (it is marked on the diagram as present in downdraft units). At any rate, there is no such thing in my heater. It is the fan/limit switch or go home.