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  1. #1
    Join Date
    Aug 2005
    Posts
    40

    Question

    How long should a properly sized gas fired furnace run when the outside temperature is close to the design temperature of the system? This morning my system would run for about 7-8 minutes and then shut off and would stay off for about 40-45 minutes before refiring. The outside temperature this morning was 26 degrees and for this area the design temp is 23.
    Thanks in advance...

  2. #2
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    Sounds like it is working quite well, definetly not short cycling if it is off for that amount of time.
    "Go big or Go Home"

  3. #3
    Join Date
    Aug 2005
    Posts
    40
    Black Adder,
    Thanks for responding. My thought process was that the system was heating up the house too quickly. One starts getting a little too warm during the on time, and then a little cool before it comes back on.

  4. #4
    Join Date
    Jan 2004
    Posts
    108
    The furnace is working well, but it's oversized. It's running for 7-8 minutes out of about 50. That's only 15% of the time. A properly sized furnace will run near 100% when at design temperature. With a properly sized furnace, the temperature climbs slowly enough that you do not feel a blast of heat followed by chills when it cycles. Your furnace is about six times larger than needed to heat your house.

  5. #5
    Join Date
    Aug 2005
    Posts
    40
    engineerguy,
    That was my thinking too and I just needed some professional advice. I used the HVAC-Calc program and its coming up with approximately 36K BTUH heat loss which looks like thats probably more like what should have been installed. The problem I have is that this furnace was just installed in '97. I hate throwing out a perfectly capable furnace thats oversized for my house. Its an Amana 90+ condensing furnace. Have you had customer's requesting to downsize a furnace? What would I do with the existing furnace? ebay?? LOL...

  6. #6
    Join Date
    Jul 2005
    Posts
    717
    Shoot52; I would think you should be quite satisfied with a 6- 8 minute burner run every 50 minutes or so even at a "mild" 26 f temperature outside.
    If you have an cycle adjustment on your T stat, you could perhaps set it for a shorter burner run time, and it may add another cycle within the hour.

    It also is amazing how (engineer guy") someone can tell you your furnace is 6 times too big for your home without even knowing any thermal particulars about your home.
    May this is a "new type of rule of thumb" (guess blindly from afar).... lol...
    And with a statement like...." a proper sized furnace will run near 100 % when at design temperature" ...sure makes most of us glad we don't have 'engineer guys' furnace calculations... lol

  7. #7
    Join Date
    Aug 2005
    Posts
    40
    Deejoe,
    Does a 6-8 minute burner time hurt the heat exchanger or shorten its lifespan? I do have a VisionPro tsat and the cycle time is set to three. Are you saying I could try increasing that setting?

  8. #8
    Join Date
    Mar 2004
    Posts
    1,042
    Originally posted by deejoe

    It also is amazing how (engineer guy") someone can tell you your furnace is 6 times too big for your home without even knowing any thermal particulars about your home.
    May this is a "new type of rule of thumb" (guess blindly from afar).... lol...
    OK, so that may be a bit much. Still, he was three degrees from design temperature and running a 15% duty cycle. It may not be a factor of 6 oversized, but it's still way oversized. If you're at design temperature you at least want to be running, jeez, half the time, anyway.

    My vote is to build an addition to the house to double the square footage. Maybe then you'll have enough house to suit the furnace

  9. #9
    Join Date
    Aug 2005
    Posts
    40

    Talking

    Yes, I have to agree with wyonger. This is an 87K BTUH output furnace. One of those 2 stage variable speed 40K furnace's would probably be just right.

    wyonger, yea maybe I'll have to build a dog house addition for me cause I'll have to convince my wife that we need a new furnace.


  10. #10
    Join Date
    Jul 2005
    Posts
    717
    Shoot52; nowhere in your initial post do you say anything about being cold ot too warm in the home. So we take it that all is ok, except for the 7-8 minute burner run time. Thank your lucky stars that it is only running for that length of time with the high cost of fuel, etc.
    If you now think a 40 K btu will do the trick, how on earth did a 87k get in, or did 'engineer guy' or 'wyounger' do the heat sheet calculation.
    and impo, a 6-8 minute burner cycle will in no way harm the exchanger as long as the "heat rise" is within specs.

    Btw, manual J's are within reach .

    and try and set the T stat cycle to a slightly lower setting, it may just satisfy your tastes.

  11. #11
    Join Date
    Aug 2005
    Posts
    40
    deejoe; Sorry if I didn't mention whether it was too hot or too cold. The problem is that when the heat is on for that short time, I can really tell its Heat time and no wonder it only takes 7-8 minutes. To answer your question about how I got the 87K furnace, I probably don't need to tell you but the contractor never did a heat load calculation. I believe the contractor had this unit in his warehouse just waiting to be used. Not too long after my install, I find out the guy has retired or closed his business. The original unit was a 2.5 Ton heat pump.
    What did you mean by "Manual J's are within reach"?

  12. #12
    "And with a statement like...." a proper sized furnace will run near 100 % when at design temperature" ...sure makes most of us glad we don't have 'engineer guys' furnace calculations... lol"

    Sorry but I think Engineer Guy is exactly correct. If you did the heat loss calculation, and at the design temperature your furnace provided exactly enough btu to meet the heat loss, then it would operate near 100 pct and be at its most efficient. Of course this isn't possible, most of the time the design temp is colder than existing conditions, and the furnace will operate somewhat less time to provide the lesser required btus. And of course if it got colder than the design temp, the house would slowly cool due to deficient btus. We went through this in our recent furnace replacement.

    deejoe, he's going to burn x btu worth of fuel to make up the heat loss whether he does it in five minutes once an hour, or over 60 minutes worth of burn at a slower rate.

    Now deejoe, "how on earth did a 87k get in(?)" Well in my experience an HVAC pro came to the house, looked at the old 125k (50 pct efficiency) and sold us a new 125k 80 pct efficiency. We didn't know any better. We do now, thanks to this forum...but not all HVAC pros understand sizing to heat loss. Worse (and this is right from their mouths) they don't want callbacks for cold houses, and oversize from the start.

    We were lucky to have a super company work with us on our recent oil burner replacement, but this is also illustrative. Our present house had a 90k btu/hr 80 pct in place but only needs 45k btu/hr or so, and the smallest Thermopride available was 70k btu input 87 pct with a .5gph nozzle. You can't always get the exact right equipment for the requirement and need to compromise. And almost all wanted to sell me an 85k just to be safe.

    I don't think most HVAC companies have the expertise that seems to be present on this board, or (if the local guys here speak the truth) they find it difficult to find the employees to do the proper work they want to do. Just look at the Hall of Shame!

    Anyway, interesting discussion, and even though I'm done with our recent install I still learn every day from you all.

  13. #13
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,031
    Your home has a certain BTU loss per hour (BTUH). Those BTU must be replaced. If an 87K furnace runs for 7-8 minutes, then it would stand to reason that a 45K furnace would run about 14-16 minutes. During either of those times, the total BTUH output would be the same if the efficiencies were the same. (For those following, yes, the larger furnace would not be operating at peak efficiency with the short run time...let's keep it simple.)

    So, with the smaller furnace, we get longer run times and a more comfortable home. I don't care if gas prices are $200 per therm, the SAME amount of gas will be used to heat your home regardless of the size of the furnace. However, you won't have the roller coaster swings in temperature with the PROPERLY sized furnace.

    If you are close to design, then your run times should be much, much longer. At or below design temperature, run times would be nearly continuous if you were nuts on your sizing. However, furnaces come in 20K BTUH incremental sizes. So, you usually have to be 10 degrees or more below design to get close to continuous run times.

    EDIT: Gee...look what got typed while I was typing...
    Perhaps you should have read the instructions before calling.

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