[RegB]

This furnace is about 35 years old, original equipment, it came with the house.

The thermostat is a lot more recent, the gas valve thermocouple was replaced a couple of years ago.


SYMPTOMS:
The thermostat calls for heat, the main burner comes on, after a while the fan starts up, but after a few seconds it all shuts down and tries again in about half a minute. I have verified that the fan does actually spin.

IDEAS so far:
The gas valve, thermocouple, fan and main burner are probably OK.
I think there is "something" that is not acknowledging that the fan is spinning and pushing air. This might be an air flow sensor or a thermostatic switch, I can imagine designs that would use either.

[RegB]

I removed the filter temporarily to eliminate the possibility that it is blocking air flow. This made no difference.

[RegB]

On the idea that the thermostat might have been failing to hold the fan relay on, I disconnected the thermostat and shorted the red and white wires together. This simulated the original non-programmable round thermostat from 35 years ago, which was basically a mercury switch on a bi-metal coil.
Same symptoms.

[RegB]

Yesterday I replaced the (combination) fan and limit control.
No change.

Today I tore apart some flue sections to see if there were any blockages, dead squirrels, etc. Not much soot for 35 years, the chimney is also clear.
Still having the same problem.

BTW, are furnace fans usually 1 or 2 speeds ?
Does anyone from appliancepartspro ever read and respond to these questions ?
I was told by the (somewhat rude) live chat person to post here.
The person that answered the phone said basically the same thing, that I would receive a technical answer within 24 hours.

[#1 tech]

sounds like your motor is interaly bad (thermal overload). most any HVAC store will carry a replacement. be sure to take the old one with to get a correct replacement. if it is heat only then use the low speed. if it is heating and cooling then use low for heat and high for cooling. sorry i did not post sooner getting ready for summer at work

[RegB]

Originally Posted by #1 tech

sounds like your motor is interaly bad (thermal overload). most any HVAC store will carry a replacement. be sure to take the old one with to get a correct replacement. if it is heat only then use the low speed. if it is heating and cooling then use low for heat and high for cooling. sorry i did not post sooner getting ready for summer at work

Thanks for your reply.
It was indeed the fan motor, which I diagnosed without any help from APP.
BTW, are you an appliancepartspro employee ?

For anyone else who gets down this path;
At some point it occurred to me that, but for the new switch, the motor was hooked pretty much directly across 110 VAC. Since I had already replaced that switch ....hmmm, kinda/gotta be the motor, so I did in fact wire it directly across 110 VAC (with the shutdown switch set to OFF) and sure enough I got the same symptoms. The whole lower blower unit is mounted on rails that slide out after removing a couple of small retaining screws. The squirrel cage is mounted to the (1/2 inch diameter) shaft with a set screw to the milled flat, the motor is mounted to the opposite side of the housing with 3 bolts to arms that are held to the motor with a band. All of this is quite intuitive once you have it all out. The 3 arms and retaining band had to be transfered to the new motor, of course I took care to be sure that I got it on STRAIGHT, tight and as close as I could to the same distance from the motor face as it was on the old one. I took the old motor with me and picked what little brain I could find in the local sales counter staff. Well, it came with a schematic anyway. The new motor came with a longer shaft, which didn't matter because it just sticks farther into the fan cavity. I also turned around the 4 thin screws that run through the motor and hold the end plates on, to get their excess length away from the face of the motor where it MIGHT have mattered.

I now (for 2 days) have a very quiet and seemingly efficient heating system. It cycles more evenly, I suspect partly the new motor and partly the cleaning that I did are responsible for this.
About $200 total for switch and motor, the neighbor paid some $5500 to replace his about 5 years ago (same symptoms), but I only found that out yesterday (-:
What to do with the $5300 I saved ?
I'll think of SOMETHING I'm sure, some of it should probably go to converting it to pilotless ignition (hot surface), something I've thought about every few years as the co$t of natural gas has continued to rise.

[#1 tech]

no i am not ... just a HVAC tech who thought he could help out ... if i was you i would look into changeing your unit over to a newer more effecent unit .. if you look on the ID plate on the furnace it should tell you input and output of you furnace .. judgeing from the age i would say it is no more than 80% .. they now sell 90+ units which you can get to work with a heatpump ... by doing this you can cut your heating costs by a very large amount ... i have seen saveings of 2/3 of the yearly cost .. plus it would add a/c in the summer ... if you would care for ferther advice i would be happy to help ... i also have some plans for a quick and easy solar window box that a person could bulid and helps dramaticly with engery cost

[RegB]

If I could swap out the burner & ignition system for a higher efficiency unit that would be fine. If I had to replace the whole standing unit that would be impractical. Just getting a new unit in and the old unit out would require the moving of too much "stuff", walls, etc.

I could get 50 or 60 pound sub-assemblies about the size of the blower unit in there without too much trouble, but not a whole cabinet.

If there isn't a "guts replacement/upgrade kit", maybe there should be.

[#1 tech]

the technology has changed a lot in 35 years. while a deferent burner create more heat. The heat exchanger would not transfer it to the air. A good contractor should be able to disassemble the unit in place. Also you will more than likely find the new models to be smaller in size. Here is a link Alpine Home Air Products: Contractor-grade heating, air conditioning and indoor-air quality products at wholesale prices. ... you can match out the output of your furnace. They have dimensions and you can see if it will fit. Remember 20% of your heating bill is not being used. If you lowered that by 15% how much would that save you next year .. the year after ... in 5 years ... i saw gas at $3.39 yesterday .. sorry i will get off my soap box