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  1. #1
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    214

    ran my own calcs, need advice

    I've posted here a lot lately, but I will summarize what I am doing. Installing a new propane furnace, current one is 90k input 90%, and I know it is too big. I have had several HVAC guys in to quote, some did calcs, others did not. Left me confused because even the calc guys were all over the place. I plan on having a Goodman modulating furnace put in. I purchased the software from above, and carefully worked it through. Very carefully. On "average infiltration" the calc came back 63K. On "tight" it came back in the 50's. I used a design temp of -5, since on a normal winter we get that at least at night fairly often (the software suggested +5)
    I am not sure what to use for the infiltration.. The house was built in 1990, average construction grade. We have lived there since 1999 and I have sealed and caulked, added storm doors, and I also put plastic on most of the windows in the winter (huge difference) I know it has a Tyvex wrap, but who knows how well? I am in the heart of the NE Ohio snowbelt, and we get pretty good winds coming across the huge cornfield during the winter.
    What do you all suggest at this point? Thanks, Mike.

  2. #2
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,148
    Troy

    I would not size a new furnace to a rare extreme low temp.

    Will home have any other type heating source?

    The Goodman mod's smallest size is 60 KBTU. Based on your load calc that suggests a borderline situation of being undersized.

    You may want to exhaust all available and cost effective opportunities to reduce the heating load.

    IMO

  3. #3
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    214
    Doing the calc at +5 degrees lowers my BTU needs to something like 62K, if I recall correctly. One HVAC guy, who did the calcs, suggested the 80K, saying it would not run in full high fire very often, but the 60k would be a little too small. the house is a 2100 sq ft colonial, not counting the basement. The basement is heated, however (not finished and not heated to 70, stays about low - mid 60's during the winter)
    Looked into dual fuel (heatpump) but at this time was planning on staying with full propane, and then in a few years change out my standard single stage AC with a heatpump. The Goodman can work with both. Right now, the much higher cost of the replacing everything at once just doesn't work.

  4. #4
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,829
    Since you're taking on the task of the contractor to size the furnace, you've come up against one of the limitations of load calculations, which is the actual equipment sizes with which you can work. Btu needs and furnace outputs are part of that puzzle. The additional piece when sizing a furnace is the accurately size the blower as well. Some furnaces are available with more than one blower and knowing the proper size for that reduces the likelihood of a mis-matched heating/cooling system.

    So if your home needs >60mBtu but <72mBtu, I'd say you need an 80mBtu gross input gas furnace. Now size the blower and you'll know what model number should appear on the quote. Staging and when 1st stage or 2nd stage will be needed is easy to calculate. Just do a load analysis at two different temperatures, 5-degrees apart. You'll find that the Btu increase and decrease is linear. So calculate the Btu output of the furnace on 1st stage, find the nearest house loss value that most closely matches that number and you've found the 'design limit' for 1st stage. Any OAT less than that will require 2nd stage heat.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  5. #5
    Join Date
    Jun 2007
    Location
    Greenwood Indiana (Indianapolis)
    Posts
    420

    Thumbs up

    the goodman comes in a 80,000 btu. size with a 5 ton drive blower motor (gmvm96). since its a modulating furnace it will always try to give you the right amount of heat , btu's you need. sounds like the right size to me.

  6. #6
    Join Date
    Sep 2004
    Location
    Jersey
    Posts
    389
    just to add my 2 cents, some contractors add 20% to the heat load calc to ensure proper heating, so the guy telling you the 60 is just under may or may not be correct but he may be adding that 20% to cover his tail. also the blower size may be included in the factor like skip touched on.
    Thats why many contractor have different suggestions. there is a right and wrong way. the right way may have different solutions
    Ride hard on a Harley!!

  7. #7
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    214
    I've been charting my actual run-time for my current furnace, and based on the BTU output of that furnace for a 24-hr period then doing the math, it comes out to something like 22k btu output per hour. (can't remember the exact, have to look at my notes) for an average temperature over that timespan of something like 35 degrees.. I figure I can change the design temp in the software to 35 degrees, and see what it says I need for btu's per hour. Should be a good way of double-checking my load calculations, correct?
    I am pretty much set on going with the modulating furnace, so I can figure at what temp the furnace should run constantly on the lowest output... I think I read somewhere that constant run on the lowest output should happen at about 45 degrees? Is this correct? This could be another way to check my numbers..

  8. #8
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,829
    Quote Originally Posted by troyport View Post
    I've been charting my actual run-time for my current furnace, and based on the BTU output of that furnace for a 24-hr period then doing the math, it comes out to something like 22k btu output per hour. (can't remember the exact, have to look at my notes) for an average temperature over that timespan of something like 35 degrees.. I figure I can change the design temp in the software to 35 degrees, and see what it says I need for btu's per hour. Should be a good way of double-checking my load calculations, correct?
    I am pretty much set on going with the modulating furnace, so I can figure at what temp the furnace should run constantly on the lowest output... I think I read somewhere that constant run on the lowest output should happen at about 45 degrees? Is this correct? This could be another way to check my numbers..
    Not correct. The size of the furnace needed is based on outdoor temperature, insulation between indoor and outdoor temperature, indoor temperature and the rate of exfiltration and infiltration air. The only reasonable method of using historic home information for sizing would be calculate the area of the heated space and then using actual high and low daily temperatures, calculate the "degree day" value. Then using the historical fuel data for the home, you could calculate the heat loss in Btu's per square foot per degree day. The national average is 4 but your results would likely vary.

    Load analysis is based on "Bin Data", which is years of weather temperature and humidity history that's crunched into averages over a very long time. The average high temp is used for cooling and the average low temp is used for heating. The actual calculation does include a 'fudge factor', though Manual 'J-8' has less of a factor than did Manual 'J-7'. However, load analysis is the best 'educated analysis' for both past and future needs of the building. Nobody can predict whether the unit will be properly sized, over sized or under sized for any weather that substantially departs from the Bin Data.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  9. #9
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,635
    just to add to the confusion.............

    is the furnace you intend to install actually rated for LP gas or will it be converted, (15% derate) from stated mfg info?
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  10. #10
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    214
    the de-rating does not add to the confusion, it actually puts me more solidly toward the 80k.. de-rating make the 60k more far-fetched. But I thought in reality the output with propane was not that different? It had something to do with needing more combustion air?

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,763
    When you did your calc. Did you enter that you keep the basement at 70, or at 65 like it stays?
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    How many times must one fix something before it is fixed?

  12. #12
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,941
    Quote Originally Posted by troyport View Post
    I've posted here a lot lately, but I will summarize what I am doing. Installing a new propane furnace, current one is 90k input 90%, and I know it is too big. I have had several HVAC guys in to quote, some did calcs, others did not. Left me confused because even the calc guys were all over the place. I plan on having a Goodman modulating furnace put in. I purchased the software from above, and carefully worked it through. Very carefully. On "average infiltration" the calc came back 63K. On "tight" it came back in the 50's. I used a design temp of -5, since on a normal winter we get that at least at night fairly often (the software suggested +5)
    I am not sure what to use for the infiltration.. The house was built in 1990, average construction grade. We have lived there since 1999 and I have sealed and caulked, added storm doors, and I also put plastic on most of the windows in the winter (huge difference) I know it has a Tyvex wrap, but who knows how well? I am in the heart of the NE Ohio snowbelt, and we get pretty good winds coming across the huge cornfield during the winter.
    What do you all suggest at this point? Thanks, Mike.
    Nobody does without a blower door test. It's hard enough trying to find a contractor that does load calcs; finding one that doesn't guess at infiltration is even harder. What R-value to you assign to your walls? This pic is one we took on a collonial:
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    Do you have returns that use the wall joist? Better make sure that wall is air sealed or you are pulling attic air into your system and losing efficiency:
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    Collonial with a second floor & knee walls? Check the air connection between the 1st floor ceiling & the upstairs floor and you will likely see that it is open to the unconditioned attic. This pic is one I took of a similar upstairs floor as seen from looking from the unconditioned attic into that section of floor/ceiling in the conditioned area. Notice the recessed canned light from the kitchen below that is not insulated or air sealed. If you have this stuff, either fix it, or factor it into your load calc:
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    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  13. #13
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    214
    I didn't see on the software where I could put in a different temp for the basement and the main part of the house. I will certainly look into that. There are heat registers in the basement, and the temp of the basement is more determined by how much the furnace actually runs. I expect that with a modulating furnace it will be closer in temp to the rest of the house.
    Also, I do not have knee walls, and the heat registers do NOT travel through the exterior walls.. They run up the center to the second floor, then travel horizontally between the first floor ceiling and second floor floor, then surface at the edge of the floor. I suppose the insulation is at least "squished" at the exterior wall, but that is it. The returns are all metal where they run down from the second floor. I know where they run down, two main trunk lines, right where my wife wanted to hang pictures, go through the drywall and hit sheet metal. Makes it tough for a hollow wall anchor. First floor returns are fine, of course. This house was built in 1990, so many things have been done correctly. Just would love to know how it was wrapped.
    Someday, I want to have a house built, then I would know these things.

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