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Heil Furnace from hell

Chuckster Member Posts: 4
I need some help with this one...

It's a Heil (model NULK) gas condensing furnace and what happens is the burner fires (HSI ignition) with no problem but the blower won't come on until the high limit for the burner opens. Then the blower runs for a very short while 5-10 seconds typically) until the high limit closes again and the burner circuit is energized. The blower shuts off and the burner starts up again. The burner and the blower will not run simultaneously.

I have isolated the thermostat (completely disconnected) jumpered Rand W

Believed that the trouble was in the fan relay/timer control. Replaced with same results.

I've also tried a new pilot module, system relay and have jumped out all my roll switches and safety switches. This one has got me baffled.

Has anybody got any ideas?


  • martin
    martin Member Posts: 144
    Heil From Hell

    Check the wire thats hooked to the heat terminal on the blower board. It might be possible you have a speed out on the fan. When this furnace goes into high limit it is supposed to turn the fan on but on high thru the a/c relay on the board. You need to somehow power up the the heating speed manually to see if its good. I've seen this happen only once. 
  • Chuckster
    Chuckster Member Posts: 4
    I powered

    each speed on the fan board separately with an outside power source. They all worked but I have encountered where the high and med. high terminals where both energized. I assumed the high was for A/C

    Also when the burner is running and I disconnect one wire off the high limit the blower will turn on and run with the burner. Is it possible I am shorting out in the blower board and running on two speeds at once?
  • Chuckster
    Chuckster Member Posts: 4

    I will recheck the med high heat terminal. That explains (the blower turning on through the a/c speed) why it does that. For tonight I have the furnace set on electric operation so that the burner and blower operate together.

  • martin
    martin Member Posts: 144
    Heil from hell

    Something is definitely screwy, when you remove 1 wire off the high limit the burner should drop out immediately and the blower kick in. The high limit and the rollouts are usually one big series circuit
  • World Plumber
    World Plumber Member Posts: 389
    High limit

    The blower should be coming on with time after the gas valve opens. Check that the dip switches are set right for the size duct work you have and the jumpers are in the right place.

         How old is this unit? If it is new check for a blocked secondary heat exchanger. They are having problems with the grease from manufacturing blocking them. The last one I installed was tripping the flame roll from the excess heat. 

          Check the operating sequence and time the interval to see if this is happening in less than the time the fan is supposed to start. If the blower is coming on late check the jumpers and dip switches. If everything else is right I would suspect the board. Look at the condensate if it is milky that an indication of grease in the secondary heat exchanger. I also noted that I had a low delta T of only 24 degrees after cleaning the heat exchanger it jumped up to 55 degree delta T.

  • Chuckster
    Chuckster Member Posts: 4

    The problem ended up being reverse polarity on the 24v circuit. Thanks to Martin for pointing out that the fan was supposed to turn on high A/C speed when the limit opened. From there I simply retraced all the circuits again and found the reversed wires at the module. They must have been that way for a while because the spade connectors were different sizes and could not be interchanged without putting on a new one.

    It's always an adventure out there!

    Thanks again to all who posted.
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