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srt 10-18-2012 11:51 AM

Heil DC90 Furnace Vaccuum switch problem?
I bought a house in Feb 2012 with a Heil DC90 furnace that worked great during the home inspection. After moving in, I realized that the furnace would kick on just fine and heat the house up ONCE but wouldn't light again until it sat OFF for at least 20-30 minutes. Through Feb and March, if I let the thermostat call for heat when it wanted, there was not enough time between cycles and on the second cycle the blower would come on but the furnace wouldn't ignite, so my furnace would blow cold air through the house until I switched it OFF for a half hour.

I noticed that the exhaust blower motor got hot while it was running and I suspected it wasn't pulling enough vacuum once it got hot, so I assumed that it was the problem. It seemed that when the motor was hot I'd get no pilot light and after it cooled everything fired right up. So I replaced that fan/motor assembly... but continued to have the same problems.

So I did a little more troubleshooting. There is a rubber air hose that runs from the exhaust fan up to a Honeywell vacuum switch if I remove this line and stick the end inside the hose of my vacuum cleaner the furnace fires up every single time. I can then slip the hose back on the exhaust fan and the furnace continues to run. I figured that meant it was a bad vacuum switch so I replaced that. But I am STILL experiencing exactly the same problem. (I also replaced the rubber air hose between the fan and the Honeywell switch.)

My furnace repair guy charged me $85 to tell me he thinks it's the pilot light assembly and wants to change it out for another $85 service call plus $150 for the parts. I just keep thinking that his theory doesn't explain why putting more suction to the Honeywell switch allows the thing to light... but on the other hand, I'm pretty much out of ideas. Anyone have some insight?

I can take pictures or video after I get home tonight if it'll help.

JJboy 10-18-2012 12:20 PM

if the induce draft motor and pressure switch with hoses and clean exhast are ok the controller board will send a signal to light the igniter. i think your problem is not these part.

do you have flame on all burners? clean the flame sensor. any micro amps under 2 needs to be clean

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srt 10-18-2012 12:28 PM

Once I get the pilot light to come on (which requires the extra vacuum on that hose I mentioned) all burners light every time. I never see the pilot light come on and go back off without having lit the burners. Do you think it could still be the flame sensor?

I should mention that the furnace service tech cleaned the flame sensor during his visit. I didn't notice any change after he was there.

JJboy 10-18-2012 12:49 PM

you need to check the gas valve output pressure. Should be around 3.5 WC

when the burners are on the igniter turn off

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techpappy 10-18-2012 01:30 PM

The OP already stated that with vacuum cleaner connected to pressure switch the unit runs fine AND that he has replaced hose and fan unit...

I would suggest you check and clean the orifices inside where the hose connects at both ends to make sure no debris clogging them then retry..if still does not work then something wrong with vent from ID fan (plugged) so, check vent line inside and outside for any obstructions such as birds nest, kid's ball dead bird etc. RE check fan install to make sure right size and gasket installed correctly.

beenthere 10-18-2012 02:42 PM

80 or 90% furnace. Nat gas or LP.

That switch is a safety switch. meaning a condition that is UNSAFE is present. So stop using a vacuum to run your furnace. Check the things pappy said.

ben's plumbing 10-18-2012 05:35 PM


Originally Posted by beenthere (Post 1033437)
80 or 90% furnace. Nat gas or LP.

That switch is a safety switch. meaning a condition that is UNSAFE is present. So stop using a vacuum to run your furnace. Check the things pappy said.

b t its a dc 90 ..90%....

ben's plumbing 10-18-2012 05:38 PM

did you try disconnecting the exhaust pipe and trying that....seem to me you may have a obstruction in exhaust pipe....may as well check intake while your at it...ben sr

beenthere 10-18-2012 07:00 PM


Originally Posted by ben's plumbing (Post 1033541)
b t its a dc 90 ..90%....

Duh, missed that.

srt 11-05-2012 03:28 PM

I have checked the exhaust pipe all the way to the outlet and it's clear. I confirmed this by exhausting the fan into the open room, with no change in the behavior of the furnace.
I confirmed that the correct exhaust fan assembly is installed (which was replaced during this troubleshooting) and the correct vacuum switch (also just replaced).
Orifices are clear. Hose has been replaced with new just for good measure.

Furnace runs on natural gas.

Also note, the vacuum is only needed to START the furnace... as soon as the pilot light kicks on I can quickly connect the hose back to the exhaust fan and everything will run perfectly until the thermostat kicks it off.

beenthere 11-05-2012 03:34 PM

Could have a cracked heat exchanger.

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