Gas and Oil Home Heating Furnaces - Heil Gas FA max's out the fan control,cuts out?

My Heil 7000 vintage 1990 gas FA will only operate with the thermostat set to the "fan on" setting. At this setting the burner ignites and continues until the fan control dial reaches the 170 peg promptly shutting down everything including the circulating blower. The fan control dial spins right up to the max temp and nothing happens until the heat exchanger cools down sufficiently to allow the whole sequence to repeat itself. This can't be good for the heat exchanger?
Please help as I can't get the house above 60-63 in this mannner!

Has this just started doing this?

How's the air fliter?? What type do you use?

Any vents closed off?

Return vents blocked off?

Any changes on the duct work?

You done anything to the furance itself latley?

Don't operate your furnace like this. Unless you've got a extreemly dirty filter, closed off returns you need a pro to look at this unit.

Has it ever been cleaned?

Thanks for the reply! Actually I didn't want the post to drag on but, yes, there's more.... My problems began when the exhaust blower became noisy as the plastic turbine developed an ever-widening crack and subsequent wobble. I have 2 identical units (HEIL 7000's) so I robbed the "upstairs" unit so that I could at least get by. This was a year ago and , as warm weather came it dropped in priority until...oops, it's winter again. However, the problem of only operating in the "fan on" mode started at about the same time. At first the blower would just endlessly run as you would expect in this mode, but at least I could keep the house at my desired temp. Now, however, the sequence of instant on, constant burn until the upper limit setting cuts off the burner as well as the blower effectively turning it into a very inefficient "gravity Furnace"! I fear this may be hard on the heat exchanger, but also I can't get a decent level of warmth in the house.
Specifically, I have changed the ultra high efficient filters I thought within the past year but my wife thinks longer. I will try that, thanks!

If that filter hasn't been changed in a year. take it out! Get a new one in soon as you can.

Jay11J, Mattison,
Thanks, I can't believe I could be so stupid about this but I tried removing the filter and bingo, it's now working as one would expect! My first stop will be Lowes to get a filter,or two. Still, I am now left with the problem that it will only operate in th "fan on" thermostat setting?

Those air filters should be checked every 30 days, when they are dirty change them! hehe

Even those 3M filters says good up to 90 days, doesn't mean ignor them for that long..

As for the fan not working, the low "heating" speed of the blower speed maybe burnt out? check to make sure you are getting power to the lower speed.

When you turn the fan ON at the stat, it feeds anoter set of wires for cooling and fan ON.

If you don't have a tester, turn off the power and look at the power door, it should tell you what wire is used for heating, and color wire for blower speed. Most of the time Red is used.

Jay,
Thanks, but I'm losing you here. The AC side of the thermostat works ok in the summer with the fan switch set to "auto". However, in the heat mode, with the fan switch set to "auto" the exhaust fan comes on, the burner lights, but the blower never comes on. The burner continues until as with the dirty filter the max temp limit shuts everything down. I don't ever recall noticing a difference in blower speeds?

THere are two types of blower out there.

Belt driven and they are single speed. (has single belt from motor to fan pully)

Direct drive. (right inside of the blower housing, no belts) and they usely are multi speed motor.

The multi speed motor use one speed for cooling/fan on (Higher), and for heating, it use another speed (lower).

Chances are that one of the windings in the motor that uses for heating is burned out. Try another speed tap and see if that works.

Even tho if the fan does work on "ON" than "AUTO" your using another set of windings in the motor itself.