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Thread: Furnace question

  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.

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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?

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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.

  7. #7
    DIY Senior Member Hube's Avatar
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    NMNUNNA32.
    Just curious as to the size of your home that would require an 175000 btu input furnace.(WOW!!) Most average sized homes(2000 sq ft or so only require approx 100,000 btu's) ,so you either have a very,very large home of around 4000 sq ft or more, and if not you must have a terrific lot of heat loss due to lack of insulation and poor fitting windows,etc. Your best to obtain an heat loss/gain consenus of your home before getting an new unit.
    Most reliable heating/cooling firms will provide a heat loss/gain sheet in order to give you a properly sized unit.
    Kindly post back as to the size of your home and we can provide you with more info on newer heating/cooling units
    Thank you.

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    It was and is common for a builder to oversize the furnace. A good one will do a heat loss analysis, and only provide you with one sized properly. Unless you have real plans to build on in the near term, it doesn't make sense to buy a bigger furnace than you currently need. Extra capacity eats up energy since part of what you use just warms up the furnace, and you don't recover all of that into the living quarters, so it is lost each time it fires. Extra capacity does potentially allow the house to warm up quicker if you set back and keep the house warm at colder days than designed for. If you design it for -10F, and it is a perfect match to your house (i.e., can just provide enough heat to maintain say 72), and for some reason it drops to -11, it may only be able to keep the house at 71 - it doesn't fall off a cliff and suddenly you're frigid.

    If the air flow is proper through the heat exchanger, a furnace doesn't usually reach the overtemp limit. Yes, it can, causing it to cycle, but if the fan speed, heat output, and ducts are properly sized, you could run the burner nearly all day, if required, to keep the heat on.

    A properly sized furnace on the cold design day (the coldest day it is designed to maintain the preferred temperature) would run constantly, and less and less as the days got milder. This actually is more efficient and comfortable than a bigger one that turns on and off periodically to maintain the temp set.

    A heat loss analysis looks at things like the amount of insulation, the number of windows and doors, the orientation of the house, the size of the house, and the lowest anticipated outside temperature you want to maintain the house warm. To do it right, you need to measure sizes and look or know what insulation you have in both the walls and the attic. the quality of the windows and doors makes a difference, too, since outside air inflitration (leaks) can play a big part about how much you need to heat.

    If on a really cold day in the winter, the furnace doesn't run nearly all the time, it is too big.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9

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    Hube,

    My house is 3600 sft (excluding basement). I have a finished basement but we don't have any bedrooms in the basement. Basement is another 1800 sft. I do have supply vents in the basement but I closed them all. There are no returns in the basement.

    First Floor 8 supply registers 6 returns
    Second Floor 8 supply registers 5 returns
    Basement 4 supply registers no returns

    The main return trunk that runs across the basement is 18 x 8 and it is connected to a 20 X 10 trunk which drops down (about 7 feet) to the side of the furnace where it is connected to the blower compartment.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    While the size of the house plays a part in deteriming how big a furnace you need, the bigger things are issues like how many windows, what type, and the type of insulation. Whether you have 8' ceilings or 10 or 12 or higher affects things as well (more surface area and volume). So you can't go from what you have to determine the size of the furnace needed...you must do a heat loss analysis. These aren't all that hard to do, and you can find some calculators (spreadsheets) on-line that aren't bad. A heating professional may have access to more accurate ones, but you should be able to get close doing it yourself. I'd use this as a sanity check against what the contractor says. If they look at the old one and just quote replacing it with similar sized unit, IMHO, they aren't doing their job. Same thing with a/c sizing. The worst thing you can do for comfort is to oversize an a/c. You'll get cold, clammy air. It's much more comfortable with it cool and dry, and that takes a properly sized unit - an oversized one can't do it.

    If you current one is sized to heat the basement, and you never heat it, that should be a consideration, too.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    In the trades Dana's Avatar
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    Quote Originally Posted by Hube View Post
    NMNUNNA32.
    Just curious as to the size of your home that would require an 175000 btu input furnace.(WOW!!) Most average sized homes(2000 sq ft or so only require approx 100,000 btu's) ,so you either have a very,very large home of around 4000 sq ft or more, and if not you must have a terrific lot of heat loss due to lack of insulation and poor fitting windows,etc. Your best to obtain an heat loss/gain consenus of your home before getting an new unit.
    Most reliable heating/cooling firms will provide a heat loss/gain sheet in order to give you a properly sized unit.
    Kindly post back as to the size of your home and we can provide you with more info on newer heating/cooling units
    Thank you.

    100KBTU/H in a ~80% AFUE furnace would be about 3x oversized for my ~2000' home in ~7000 heating-degree-day central MA. (It would be about right if I vacuumed out all of the insulation in the place though. :-) )

    If the place has any insulation at all and isn't so leaky that it's drafting 10+ air exchanges/hour 100K is gonna be oversized by a factor of at least 2 for most homes in the northern US (maybe not for a barely insulated super-leaker of a house in northern ND or MN though.)

    ACCA Manual-J heat loss analysis (or similar) will calculate the true design-day heat load to within 25% or so (it overestimates a bit). As long as you keep it to less than 25% over Manual-J you'll likely meet the AFUE number for efficiency. If you're 3x oversized it'll be sliding down an efficiency cliff, where the only time it's running reasonably efficiently is on the coldest hours of the year.

    If you're going for 2-stage condensing high-efficiency version oversizing it won't matter too much for efficiency, but oversizing will wear out the furnace faster. (And you'll be paying for too much equipment up front.)

    If they aren't already, sealing the joints of the ducts with mastic and insulating at least the supply ducts to a minimum of R6 will improve the operating efficiency too, by delivering the heat where it was intended, and not running the semi-conditioned basement at a warmer, more lossy temperature.

  12. #12
    DIY Senior Member Runs with bison's Avatar
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    In looking at the Heil 95% furnaces recently the input ratings were roughly 20,000 Btu/hr per burner if memory serves. The original quote was for a 5 burner unit at roughly 100,000 Btu/hr input, which was clearly more than I needed. I'm considering dropping from the existing 110,000 Btu/hr input single stage 80% AFUE to about 80,000 Btu/hr input two stage 95% with variable speed driver.

    If the unit can spend most of its operation at the low fire rate and only need the higher fire on cold, clear windy winter nights then I figure that is the most efficient way to run. I've got a pretty good idea of what my gas consumption per delta (set point - OAT) is already. I could probably drop down to a 3 burner based on that and still easily handle the coldest nights, but I'm not sure if that would result in a mismatch with air handler for the AC which will probably remain a 4 ton unit. This is for ~2800 sq. ft. in a 5200 HDD/yr windy climate, high summer humidity.

  13. #13
    DIY Senior Member Hube's Avatar
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    NMNUNNA;
    your problem of having the burners shut off on high limit is because your return duct of 18x8 is way TOO SMALLfor the amount of supplys you have that are open. You say you have a total of 16 supplys that are open and if these are 4" in size that amounts to approx 200 sq inches of are while your return is only 144 sq inches in area. If by chance some of the supplys are 5" then the total supply amount is even greater than 200 sq inches.
    Return air area should ALWAYS be slightly more than what the supply area is.
    This may be the cause of your furnace shutting the burners down prematurely from time to time because it is overheating due to lack of return air. As you say you have a total of 20 supplys (8 on one floor. 8 on another floor, and 4 that are shut off in the basement ,this supply total would require an return area of AT LEAST 260 sq inches.

    You would be best have a heat loss/gain calculation done to your home(manual J )and also have duct sized (manual D )

  14. #14
    In the trades Dana's Avatar
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    Quote Originally Posted by Runs with bison View Post
    In looking at the Heil 95% furnaces recently the input ratings were roughly 20,000 Btu/hr per burner if memory serves. The original quote was for a 5 burner unit at roughly 100,000 Btu/hr input, which was clearly more than I needed. I'm considering dropping from the existing 110,000 Btu/hr input single stage 80% AFUE to about 80,000 Btu/hr input two stage 95% with variable speed driver.

    If the unit can spend most of its operation at the low fire rate and only need the higher fire on cold, clear windy winter nights then I figure that is the most efficient way to run. I've got a pretty good idea of what my gas consumption per delta (set point - OAT) is already. I could probably drop down to a 3 burner based on that and still easily handle the coldest nights, but I'm not sure if that would result in a mismatch with air handler for the AC which will probably remain a 4 ton unit. This is for ~2800 sq. ft. in a 5200 HDD/yr windy climate, high summer humidity.
    True dat, within limits. If very oversized and short-cycling, you'd still be kinda scrawed on efficiency. With multi-stage the lower the lowest stage the fewer the total cycles, but if it's fixed timer type algorithm for stepping it up after X minutes of low fire it depends- higher output at low fire might prove more effiecent since it kicks into high-fire less often. Either way fuel consumption is still going to be better than with a comparably-oversized bang-bang controlled furnace.

    Smart controls with outdoor air temp sensing or PID & variable speed ECM blowers can make a huge difference in comfort, and will save measurably on the heating season electricity use as well. (Old-skool air handlers are real power hogs!) Downsizing to the 2-stage variable speed is going to be a win all the way around.

  15. #15

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    Hube:

    I think it is even worse. I measured one of the supply ducts in the basement. I think they are 6 inches dia.

    My 18 X 8 return duct is under solid ceiling. To replace the duct we have to break the ceiling. There is some portion of the duct that is in the proximity of the furnace in the utility room that is accessible. What other options do I have in order to make up the loss?

    Can I add a couple of 6 inch ducts to the 18 by 8 and draw return air from basement?

    Or is it possible to build a box under the furnace and draw the return from the bottom of the furnace? I don't know how this is going to help because we are still constrained by the 18 by 8 duct????

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