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I have a heil furnace / air conditioner unit. The heil is model # N9MP2075812A3 serial# L024970895. It has an intl comfort evap coil Model EPD24B15C2 serial# X120393352. It was installed in 2003. I turned on my air conditioning and the furnace keeps cycling on and off. I checked the filter and it is new. It is a gas furnace and the gas valve is blinking stadily which means it is ok. It humms....tries to turn on.....blows for about 10-20 seconds....turns off...tries to turn back on...again and again....then turns on again....blows for 10-20 seconds then turns!!!

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Not sure I understand the description.
You said the "furnace" keeps cycling on and off when you turn on the AC.
But you must have meant the AC cycles, because if the actual furnace went on, that would an extremely seroius problem.

And I don't think it is necessarily bad that the AC would have a short cycle. If the AC were to stay on all the time, it would cost a fortune. So I believe they are supposed to have varying cycle length, depending on how much temperature difference there is between the room with the thermostat and the desired temp.

But also consider if the thermostat is too close to an output duct or something.
Try turning the AC down to a colder temp, and see if the cycles get longer.

Tell us more about it.
Unlike a furnace, where there is a lot of heat stored and the fan can run long after the flame is off, AC units generally should have the fan stop as soon as the pump stops.

Posted on May 13, 2010

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  • keith_busch May 13, 2010

    I guess I should have said its the blower that cycles on and off. the actual furnace is not kicking on its just the blower or fan that cycles on and off. They are very very short cycles for only about 10-20 seconds. The evap coil is very cold and the blower/fan kicks on to blow the cold air but only does so for a 10-20 second cycle. The furnace is in the basement and the thermostat is on the 1st floor. I cranked the thermostat down to 69 to get the furnace to blow cold constantly and it still does the same thing. Kicks on for about 10-20 seconds.....kicks off......stays off trying to restart again for about 30-60 seconds....kicks on for 10-20 seconds.....then the same thing

  • Kirk Augustin
    Kirk Augustin May 13, 2010

    Well I guess I can assume you don't have a duct outlet blowing right on the thermostat, or else the heat would also have been shutting itself off.

    And if you set it way down and it still rapidly cycles, I think there is a problem.

    But the next step is to isolate if it is the thermostat or AC control unit.

    If you have the instructions for the thermostat, determine which 2 wires you would need to shunt together in order to keep the AC on steady.

    If you don't have the instructions, go to a place that sells them, like Home Depot, and look up the instructions for a new one of the model you have.

    I think the thermostat is most likely.

    If not, then I am afraid it is the control unit for the AC itself, or the pressure is building too high and there is a cutoff.

    That can be because of moisture in the system that causes an ice plug in the evaporator.

    If the evaporator orifice can be accessed, one way to tell is to hold a candle under the orifice tube.

    That will heat it enough to melt an ice plug.

    If that works, then you need to replace the receiver/dryer, because the dessicant is saturated with water and is not keeping it out of the system. Then it would need to be re-evacuated and recharged. A pro can save and recycle all the freon, but you can do it yourself if you don't mind either renting the equipment or purging the freon to the atmosphere and buying new.

    Just melting it once won't be a permanent fix because it will go back through the system and plug again eventually.

  • keith_busch May 13, 2010

    There is a small box with a plastic tube running into it from the furnace that was filled with water. I have emptied the box and the tube that was running out of it and into my sewer pipe. I was hping that that was the problem that it was not pulling the moisture from the system. Could it be that this box is defective and where do I check the evaporator coil for ice would it be in an inlet pipe or outlet pipe or the entire coil?? I can hear the blower trying to turn on twice before it turns on for about 10 seconds then shuts off again. Could it be a faulty switch on the blower and what would that look like??

  • Kirk Augustin
    Kirk Augustin May 13, 2010

    The water collecting in the plastic box is condensation from the moisture in the air, collecting on the cold evaporator.

    But that water is not much of a worry, although it is good for that to be run to a drain.

    The moisture I am worried about it inside the freon tubing.

    Any moisture in that can easily form an ice plug, because the freon has to be sprayed through a tiny orifice, that gets to something like a -70 degrees.

    Unfortunately you can't see any ice or moisture there.

    But that is why there is a receiver/dryer, with silicon dessicant.

    It is the cylinder right after the condenser outside (which is the radiator that gets hot).

    It is what the liquid freon under pressure is collected in.

    This is also what has the sight glass.

    The need to remove moisture is also why a vacuum pump is run before a new system is charged with freon. A vacuum will cause all the water to evaporate and get sucked out.

    The orifice that can plug up with ice, is always after the valve that electrically turns the freon on and off, and before the evaporator (the radiator that gets cold). It usually is a separate item connected to the tubing. But it can be part of the evaporator itself.

    It is best separate because it often gets clogged and needs replacement, by corrosion or dust, if not ice.

    One way to tell, is with a gauge set. If there is too much pressure, then the orifice is probably clogged somehow. You can buy a gauge set at Harbor Freight for $39.

  • Kirk Augustin
    Kirk Augustin May 13, 2010

    Forgot to mention that freon can be under a lot of pressure, and a fine spray can quickly freeze and harm skin, especially eyes. Always wear eye protector and gloves if risking any possible contact with freon.

  • keith_busch May 14, 2010

    Thank you for all your help!!!!!



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