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  1. #1

    Question Load calculation sizing. Chart reading

    I had a load calculation already done:
    Heat loss: 30,050
    Heat gain: 12,570

    A 2.5 ton was suggested.
    I looked up Goodman’s 3 TON 13 SEER heating capacity here:

    Am I interpreting this correctly ?
    If I need a load of 30,000 btus hr., the chart says the HP cant provide any more than that when the temp gets below 40 degrees outside? And the kWHours are 2.8 at that temperature.
    I pay .10 cents a kWH.

    So without even calculating any backup heat strips or natural gas at that temperature of 40 degrees or below (balance point), the MINIMUM electric consumption for just the HP would be: 24 hours a day at 2.8 kWH (24 hours x .28 cents/hour = $6.72 day/ $200 dollars month) to maintain a load of 30,000 btu’s . Plus any cost for natural gas or resistance heat on top of that when the temp outside gets lower?

    Is that correct?

  2. #2
    Join Date
    Apr 2007
    Where are you at?

  3. #3

  4. #4
    Join Date
    May 2007
    DC Metro Area (MD)
    Your heat gain would be your cooling load. You generally size for cooling for heat pumps and size the heat strips to meet the heat loss. If the load is accurate, you need just a little over a ton of cooling.

  5. #5
    Ok now I’m confused.

    Isn’t the whole reason for a HP to heat more efficiently than electric strips?
    At 20 degrees outside from last example, If I did get a suggested 1.5 ton HP (smallest Goodman sells), it has a output of 11,300 Btu’s at 20 degrees/ .14cents an hour.
    So I would still need 19,000 btus of electric heat? .55 cents hour
    That’s about .70 cents an hour x 24hrs = 16.80 day / 500 dollars a month?!

    Doesn’t it make more sense to over size the HP if your main purpose is heating because a HP has a higher COP than electric?

    Or is a HP then just not practical in Michigan?

  6. #6
    Join Date
    Apr 2007
    You want the HP to do the majority of the work look for COP. Are you stuck on heat strips (COP of 1)? Why not a furnace?

    Do you really even use A/C? You just wouldn't want a 2.5 ton HP system trying to cool off a 12,570 btu heat gain.

  7. #7
    Join Date
    Jan 2004
    Its ok to over size a HP by 25%.

    Your not doing the calc completely right for operating cost.

    It wouldn't go into 24 hour run (atleast the aux heat )except at OD temps that are colder then the systems total design load.
    The unit would cycle off the same as a gas furnace would.

    Dual fuel heat pumps are to save money in the milder part of the heating season.
    Heat pumps with electrci aux heat, still provide more heat then they use electric at lower outdoor temps.

    What is your gas rate.
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  8. #8
    BigJohn, a HP WITH furnace seems like the way to go here in MICH. Initially I oversized the HP to make up for the heat loss, but that would be way too much for the cooling load as beenthere said. I realize t would cycle endlessly wasting energy.

    Beenthere, The gas rate, that is one thing I'm not sure. I will try to look it up online.
    I have a wood stove, and when you have access to wood it is great. But the stove needs replacing and I'm looking at options. I heard one can even use the stove and a heat exchange coil with a HP but thats another thing entirely at this point.

    You have me wondering what is the average HP/Furnace cycle on/time 50percent? I realize that would change with temp but is there a industry average used in rough energy calculations?

    Either way I'm leaning towards a HP/Furnace combination.

  9. #9
    Join Date
    Apr 2007
    In cooling mode at design conditions it should practically run the load decrease the cycles increase...very general.

    Wood stove.
    Imagine a giant hole with heat being pushed out of it and it's fueled by the air in your home....So your furnace/HP will be working to warm the home and the fire place will be pushing all of that warm air out the chimney.

  10. #10
    The stove is fed any incoming air from outside air piped in, that issue was already looked at when I installed it. None of the air in comes from inside the home.

    On that note, I have heard of ppl heating a very large container of water in their unfinished basement with their wood stove (steam explosion proof of course) and using a small pump with an exchanger coil to their HP when it gets really cold. Yes the warranty is kaboshed !

    Beenthere: Michigan is $1.26 per 100cubic feet

  11. #11
    Join Date
    Apr 2007
    • 1 cubic foot = 1000 Btu
    • 100 cubic feet = 100,000 Btu = 1 Therm

    Good news about the fresh air for the stove.

  12. #12
    Join Date
    Jan 2002
    East Grand Forks, MN
    I did some calculation for your area: based on your rates and Heat Loads; it would cost $100.00 more a year with a conventional system than an HP/fossil system (Dual-Fuel) set-up. You decide which one you like.

    BTW, I used a very good program to figure this out!

  13. #13
    Thanks for the links BigJohn and the calculation arc8, you guys are great

    Because I will only be in the home for 1 maybe 2 more heating seasons, I will continue to heat with the stove. It just doesn't make sense to put a lot of $ in a new system if I will leave soon. Not to mention it will be harder to recoup that expense come selling time with home prices continuing to drop here in MICH.

    When I do move I will throw everything I can at a Geothermal HP for a long term residence.

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