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  1. #1

    Default Furnace question

    I have a gas furnace that is 17 years old. It is made by HEIL. It is rated at 175,000 BTU (Input).
    The furnace is equipped with FAN/LIMIT CONTROL device. I am just a novice so I am describing what I was told. This device senses temperature in heat exchanger and if the temprature crosses a certain preset limit then the device will turn off all burners but let the fan run for few minutes until the temprature cools down to 90 to 100 degrees. This is a safety device.

    In my furnace ever since I know (last 7 years), my burners cycle on and off but the fan keeps running during this burner on/off process. I have timed this. Typically when the room thrmostat calls for heat, the burners turn on first followed by the fan. The burners stay on for about 150 seconds and turn off while blower fan continues to run. After about 95 seconds the burners start again. This process continues until the room thermostat is satisfied.

    I was told by a technician yesterday that the funrnace is not supposed to do this. It is doing this because the fan/control safety device is coming in to action. The root cause is my return duct is undersized and so the blower motor is not getting sufficient air so the temperature in heat exchange is frequently reaching the safety threshold. He wants to install couple of return ducts in the basement (furnace is located in the basement in a utility room) and replace the return duct on the side of the furnace with a bigger size.

    But another technician, couple of years ago, said that the on/off process is normal and good. When the burners are off I am obviously saving gas while the blower is still working to remove already hot air in the heat exchanger. He described this as a good thing.

    Today I did a little experiment. I removed the front panel that covers the blower compartment and put a duct tape on the limit switch. The burner ON/OFF behaviour is totally gone. The burners were on continously until the thermostat temparature setting is reached. I help a toilet paper in front of the blower and I noticed the blower is drawing lot of air from the front. (The return duct is on the side of the furnace.)

    So, what is correct here? I appreaciate if you could tell me if this is the way it is supposed to work or if I indeed have a return duct sizing issue.

    Thanks in advance for reading this long story...

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    On/off cycles of the burner are inefficient. Normally, the filter is on the inlet side. Have you checked the filter? They should be cleaned (if it is reusable) or replaced fairly frequently. If it is clogged, it won't be able to draw enough air and you could get this effect.

    The installation manual normally describes the size of the duct for return. Do you have it? Check it out.

    Normally, return ducts don't have dampers in them. But, it's possible...if you do have any, open them. Do you have furniture or something in front of the return duct inlet(s)? Move it.

    Check the fan speed, it may be set too high for your system. The manual should say how many cuft/min you need, and relate that to your ducts and the size of the furnace. If it is electronically controlled, it may be set too high. If it has a separate motor and pulleys, it may be on the wrong pair (if stepped). It is not uncommon to adjust the fan faster for a/c, and slower for heat. It is also common to use one setting all year, but you can often improve comfort if it does change.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Plumber jimbo's Avatar
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    It is dangerous to operate the furnace with the blower door removed, it that is what you are describing.

  4. #4
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    A perfectly designed and balanced system will allow the burners and the blower to run for a very long period. However, no matter how big your returns are, eventually the burners will overwhelm the blowers ability to maintain a set temperature and the high limit will shut the blower off. It's normal, natural and what is supposed to happen. As soon s the high limit comes to it's set differential, the burners fire again. When the thermostat is happy, the burners shut down and the blower continues to run until the heat exchanger has cooled below the low limit setting. Undersized returns or supplies will cause some degree of short cycling and a reduction in efficiency, but if you have been using this thing for 17 years now I think I would leave it alone untill it's time to replace it.

  5. #5

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    Peter,

    Thank you!

    I am already thinking about getting a new one. My current furnace is HEIL 175,000 BTU/135,000 BTU input/output (17 years old). The manual says it is 70.7% efficient (ICS AFUE).

    What BTU rating should I be looking for in the current models? Can I get some thing in the energy efficient range for tax credit?

  6. #6
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    The initial reaction is to replace what is in there with something of equal btu output, but there is a good chance that you could be greatly oversizing the unit. the best thing would be to have a reputable company come out and do a heat loss for the structure and then size to the loss and the existing ductwork. High efficiency units are great but the more they cost, the longer the pay back period. I like some thing in the 90+ range.

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