safety question - keep pushing the auxillary switch and turning off electricity

[blton]

we have a Heil DC90 ultra high efficiency furnace which would turn on and off OK for one time, the next time it would turn on OK, but then it will keep running for a while, then the heat will stop, but the fan will keep running. If we turn off the thermostat, the fan still won't stop, and I'll have to turn off the electricity switch to turn the fan off. So first question is, is it safe to keep doing this - turning off electricity to turn the fan off?

Then the heating won't come back up again, unless I push the auxillary switch on the gas chamber once. After pushing the auxillary switch manually, everything will come up fine when the thermostat is turned on. So second question is - is this method of pushing auxillary switch safe?

What could be causing this on and off problem?Any insight would be appreciated! Thanks.

[Ian Gills]

How long does the fan keep running after the burners go out and are the burners turning off when the desired thermostat temperature is reached or before that? Many furnaces will keep the blower going after the burner switches off to get every last drop of heat from the heat exchanger to boost efficiency. Newer furnaces will also keep the fan blowing much longer than that to circulate the heat around the house to avoid cold spots.

You have a problem though if the burners are turning off before the room temperature set on the theromstat is reached. Is this happening?

Also check that the fan setting on your thermostat is set to "auto" and not "on" if there is this function.

Be sure the filter is clean. Lack of air through the unit can cause these symptoms too.

[blton]

The fan keeps running hours/overnight after the burners go out, and it seems it won't stop unless the electricity is turned off. is it safe to turn off the fan by turning off electricity? and After that is it safe to push the auxillary switch on the burner?

the fan setting on thermostat is on 'auto'

the burner is turned off maybe when the room temperature set on the theromstat is reached, but not totally sure.

will check the air filter later. Thanks!

[jadnashua]

If the house isn't getting overly warm, it is likely the burner is turning off.

The thermostat could be defective or wired wrong, or you may have a short between leads that are causing the fan to stay on. I suppose it could be a sticky relay control. Many of the newer HVAC units are controlled via electronics...does yours have any flashing lamps? You may need to take the cover off to view them...again, look at the user manual. the lamp (probably an LED) may be on or flashing, and if flashing, may be flashing a code. This should help dianose what's going on.

Not sure what the auxilliary switch is supposed to do...what does the manual say?

Have you ever had a light bulb blow out after you turned it on after it has been running a long time? Almost never...the shock of turning it on is what causes it to fail. Few things like a furnace are designed to have primary power turned off and on frequently. Starting and stopping operation is a different matter.

[Runs with bison]

Sounds like a pretty new system (90% Heil). Why not have it examined and repaired?

[Ian Gills]

I now think it might be a limit being breached which might be due to lack of air flow, which might be due to a dirty filter.

Given what you have described it's not very safe doing what you are doing nor is it very good for the furnace. I'm less worried about the turning off part but more concerned with you shutting everything down and essentially resetting the thing for the problem to occur again.

If a new filter does not do the trick, keep doing what you are doing to stay warm but call a pro ASAP to service it.

[Bill Arden]

My furnace would do that if I set the firebox "fan off" temp too low, but My furnace is an old unit with a mechanical firebox temperature dial that has adjustable set-points.

I'm not sure how the newer ones are set.