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  1. #1
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    221

    60k or 80k.. I'm sorry, but can't decide

    After very careful manual j, and finding out that with the burner and gas valve change that goodman does on their modulation furnace when converting to LP which makes for no derate, I can't decide which furnace to get. With different scenarios of "tight" or "average" infiltration and indoor temp at 70 or 67,(so 4 different scenarios) 60K makes the grade. If I add a 20% BTU need as a safety factor, then the 60K does not make the grade on 3 of them by between 2K and 9K.
    Also, the house is cooled by a 3-ton unit, the 60K has a 3-ton blower, the 80K has a 5 ton blower.
    Current furnace is a 81K output, and runs not even 6 hours during a 30 degree stretch over 24 hours.
    what would you install in my house?

  2. #2
    Join Date
    Feb 2011
    Location
    North East Ohio
    Posts
    785
    Why not 80K with a 3 ton blower?

  3. #3
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    221
    Just going by what the HVAC guy is offering to install..

  4. #4
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,861
    Im not sure you need 30% safety factor ,the reason for doing a load calculation is to get the correct size unit no safety factor needed

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,586
    There's usually a little fudge in the calcs anyway. So what is the final requirement? If more than 56K or so, the 60K may be too close although being a couple degrees cool on a rare bitter morning isn't a big deal.

    I'd be less likely to oversize with Goodman's mod. Like Lennox, from what I've read, it does not throttle back as the temp gets closer to setpoint. At least on single and 2 stage stats. May be different with the communicating control.

  6. #6
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    221
    The software, for my location, calculates for 5 degrees. Currently I keep the house at 67 during the day. (A little cool, kids complain sometimes, but they would like to run around in shorts and t-shirt, like kids would)

    looks like all are good, except maybe the last one... how much safety factor?

    Here's the breakdown:

    tight infiltration, 5 deg, 67 deg 43.9K
    tight infiltration 5 deg 70 deg 46.0K
    average infiltra 5 deg 67 deg 51.8K
    average infiltra 5 deg 70 deg 54.2K
    tight infiltration 0 deg 67 deg 47.4K
    tight infiltration 0 deg 70 deg 49.5K
    average infiltra 0 deg 67 deg 55.9K
    average infiltra 0 deg 70 deg 58.4K
    Last edited by troyport; 02-27-2012 at 10:44 PM. Reason: clarify

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,586
    My house would get the 60K with those numbers.

  8. #8
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    221
    Yes, the numbers certainly support that. I can't imagine I have made a mistake in doing the calcs, only one of the techs I had in here said a 60K, most were 80k with or without a calc. I'd hate to have trouble keeping up at 20 degrees, although that would be a huge mistake in the calcs. Current furnace outputs 81K, I'd be going down to 57K, assuming the current furnace is exactly right (which it isn't, I am quite sure it is too big) how bad could it be?

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by troyport View Post
    Current furnace is a 81K output, and runs not even 6 hours during a 30 degree stretch over 24 hours.
    what would you install in my house?
    I think that answers the question. At 30F you're roughly at 50% of design conditions and your system is running only 25% of the time. So it sounds like your actual heat loss is closer to 45k BTU's... so the 60k should be plenty big enough, even allow you to use setbacks.

    You can also work backwards using your gas bill. Subtract out the estimated protion used for water heating and/or cooking (subtract the average summer bill), then look at the units of gas used and the average daily temperature compared to design temeprature. 1 unit of gas (therm) is 100k BTU's input.

    You'll find that even in the coldest month, a properly sized system won't run more than about 60% of the time total unless it's an unusually old month.

    From there, at least a safety factor of 10% should be plenty... which should have you rounding up to the next size unit anyway.

  10. #10
    Join Date
    Dec 2011
    Location
    Hiram, Ohio
    Posts
    221
    I thought the whole idea is that the system should run all the time on high fire at the design temp? (5 deg for me) Maybe add a little safety factor to make sure you don't lose ground if it stays below 0 for a few days straight. But if I want it to run 60% of the time on the coldest month, then I would want the 80K??? What am I missing with your last post?
    I do agree with your assessment of my current furnace and its run time, but are you contradicting yourself?

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