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Old 10-22-2009, 09:34 AM   #1
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Heil fan runs continuously


Help ! I have a Heil DC90 furnace, installed in 1997. Whenever the power is on, both the draft inducer blower and the main blower come on immediately, and contunue running, until I shut the power to the furnace off. The burner never comes on, and it makes no difference what the room temperature is, or what the thermostat is set to.

I suspect that either the limit control or the control board is bad.

Can anyone tell me how to test those parts, or have any other ideas?

TIA

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Old 10-22-2009, 04:49 PM   #2
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Heil fan runs continuously


The high limit is a normally closed limit and you can test it by ohms or continuity.

You should have a low ohm resistance or you should have continuity through it

How dirty is the filter?

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Old 10-26-2009, 07:16 PM   #3
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Heil fan runs continuously


The filter is not very dirty, it's about 8 months old, and the furnace is only used to heat a garage, so it really only has about 3 months of use. Which one is the limit switch? The one with the two red wires, behind the gas pipe?

Thanks.
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Old 10-26-2009, 10:36 PM   #4
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Heil fan runs continuously


TIA - In my past experience it use to be that a simple thing as a blown transformer would cause the furnace blower to run continuously. I don't know if you tested that or not but you need to make sure you have your 24v.
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Old 10-27-2009, 05:56 PM   #5
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Heil fan runs continuously


Thanks for the tips !

I have 28 volts (no load) coming out of the transformer. When I dosconnected the wires from the transformer to test the voltage, and turned the power on, the fans did not run.

There is no continuity between the two terminals of the switch behind the gas pipe (in the picture), where the two red wires go. Is that the limit switch, and is it supposed to have continuity at room temperature?
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Old 10-27-2009, 06:02 PM   #6
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Heil fan runs continuously


That is a limit but a roll out limit. It should have contuity. In between the two wires there should be a nipple sticking out. Pust it in to reset.

If that limit tripped then you need to consider why it tripped. I have never seen a roll out go bad always a problem. Some of the possibilities are cracked heat exchanger and high gas pressure. If you reset the switch you need to have it checked out.

Heil is an ICP product and I have seen several heat exchangers where the rings inside the collector box have failed. Just like a Comfortmaker RPJ
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Last edited by Plumber101; 10-27-2009 at 06:04 PM.
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Old 10-27-2009, 06:28 PM   #7
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Heil fan runs continuously


I pressed the reset button, and the furnace works normally now. Thanks!!

My friend is a plumber/HVAC guy, but he has been out of town for a couple of weeks, on a big job. I'll tell him what I found, and hopefully, he can come out and take a look at it soon.

The furnace is in a detached garage, and I only heat it to 55 degrees most of the time. I'll be careful, and open the garage doors, when I'm going to be in there, in case of CO.

The funny thing is, that rollout tripped in the summer, not in the winter, when I heat the garage. I went out to the garage one day in late June, and noticed that the fan was running. So I just turned the furnace off, and figured that I'd look at it later. Could it have gotten hot enough in the attic of the garage, to trip that rollout switch?
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Old 10-27-2009, 06:36 PM   #8
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Heil fan runs continuously


I had one of those with a cracked heat exchanger the other day. After 5 minutes of running the crack opened up and the centre burner started flame rollout and tripped the rollout switch. That switch is probably set at around 300 degF so it is hard to believe it gets that hot inside the burner box in the summer (even in Texas). If someone accidentally flipped the heat on, maybe. Probably the heat exchanger is cracked, the "metal" ICP used is "suspicous" IMO.
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Old 10-27-2009, 06:37 PM   #9
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Heil fan runs continuously


It could have tripped from the heat.

I think the temp setting on that limit is 127 deg. Not 100% sure. Somewhere on the round part of the limit there is the temp setting. Very small print
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Old 10-27-2009, 06:43 PM   #10
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Heil fan runs continuously


On a mid it is 300-350 degF. On a high they would expect the air to be no warmer than the entering combustion air.
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Old 10-27-2009, 06:45 PM   #11
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Heil fan runs continuously


Thanks guys. At any rate, I'll have my HVAC buddy take a look at it. I already had a call in to him, and I'll give him an update. If it trips again, I won't reset it, until he checks the heat exchanger.
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Old 10-27-2009, 06:51 PM   #12
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Heil fan runs continuously


The one I had was 2 yrs older than yours. Have him and you watch the burners with the front cover off the burner box carefully. After the fan comes on and it has heated up good you will see flame distortion or it coming back at you. Mine did.

Good Luck

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