I was asked recently to get furnace going for a friend, it's a 7900 series Coleman in a 1980 trailer house,, I said "Replace it" but money is the issue so I would like to ask the community some questions about this unit or a resource which could help me troubleshoot this things woes. Using the digital thermostat , I set it for "fan on" but the motor only hummed, Burned out I think.The squirrel cage rotates but not under its own power,, this is problem one,, can a replacement motor be found or can I find a new downflow furnace for $$$ and just yank the old one out... thanks for reading and have a great week... olderguy

There is really no good way to answer this question without the catch 22 showing up.

First you might try a new run capacitor for the motor in question. They are cheap and may solve the problem if not then see the next line.

Blower motors are available for that unit since it does not take a special motor. Remove the blower assembly then remove motor and take it with you for a match up. When you get the new motor get a new run capacitor exactly as specified for use on the new motor. Then follow the directions on the new motor for wiring and you should be in business. While the unit is apart you might also clean the blower since it is probably very dirty in the air scoop areas. NOTE: Make sure to mark the areas where the motor wiring goes on the unit so you can reconnect to the correct terminals with the new motor.

Now for the catch 22 problem.

A 1980 mobile home furnace might be close to its end of life cycle or it might already have cracks in the heat exchanger assembly. Before spending money on a motor you might test the heat exchanger or give it a complete visual inspection to make sure it is OK. As you are well aware a bad heat exchanger can KILL YOU by releasing CO and in most cases the people in the mobile home do not wake up in the morning if such a leak from the heat exchanger is present. By helping your friend (which is a very honorable thing to do) you have placed yourself in a liability situation if anything happens. Most HVAC companies carry a very large errors or omissions insurance policy (mine was capped at 10 million for my business) but you probably do not have that coverage. It is always possible that the survivors or relatives could contact a lawyer after the tragedy and this could cost you big bucks just for your defense let alone a court award for damages etc.

As far as used units are concerned most of them have there own set of problems and that is why they were replaced to start with.

Good luck and post back if you have further questions.