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  1. #1
    Join Date
    Aug 2009
    Location
    Juneau, Alaska
    Posts
    2

    Sizing GSHP: Should I go up or go down?

    I'm planning to retrofit my Juneau, Alaska house with a ground source heat pump. Heat loss calc. for the house is 32,300 BTU. Cooling isn't an issue here.

    My contractor is recommending a 4 ton Econar Ultra unit (~40,000 BTU at 28F EWT; 45,700 BTU at 32F EWT).

    I'm wondering whether a 3 ton unit might make more sense (~28,000 BTU at 28F EWT; 32,800 at 32F EWT)

    Will be installing a new duct system, and the contractor seems familiar with designing for a GSHP system.

    We've always heated with electric resistance and wood, closed off portions of house during infrequent coldest spells, usually keep the house cool (typically 65F), and intend to keep using wood as a backup and for power outages.
    Additionally, while I want a desuperheater, I would expect to turn it off in coldest weather.

    Also, I have no reason to think that the 28F EWT winter minimum is realistic -- it doesn't seem based on local data, and I know from 20 years of gardening that the area the ground loop will be located in (buried 6-8 feet) generally does not freeze more than a few inches and never more than a foot deep. A water seep/spring often melts snow in the area.

    Cost is partially the issue of course, but so are issues of equipment wear from starting/stopping more frequently. Should this be of concern?

    Should I go up to the 4 ton, or down to the 3 ton?

    What other factors should I consider?

    Thanks for any advice or information.

  2. #2
    Join Date
    Apr 2002
    Posts
    11,808
    I am used to well insulated houses in cold climates but the only time I have seen heat loads that low are for places under 2000 total square feet and I am including a basement in this square footage.

    I am wondering if perhaps you did your own heat loss calculation and the contractor thinks it is higher and is therefore suggesting a larger unit.

    Concerning the 28F and 32F EWT. With snow cover the frost will not go down all that deep, Juneau seems counter intuitively warm compared to the likes of central Canada and the dakotas etc, must be due to the pacific.

    However when you are extracting heat from the ground, you are going to make the ground colder than normal, and I will also say that the majority of the heat you get from the ground will be from making frost so your EWT is going to be down at or a little below freezing by January. Late in the heating season you could approach that 28F or lower but it does depend on how wet the soil is and how much pipe you have in the ground.

    As the specifications tell you the lower the EWT, the less heat the unit can extract from the ground. Lower loop temps in late March will not be the end of the world as the heat load in March is not like January.

    Do not shut off heat to areas of the house unless you have proper zoning, a heat pump is going to move more air than an electric furnace.

    I would not be concerned about short cycling

    Concerning the desuperheater, if you have a preheat tank and then a water heater, shutting the desuperheater off in cold weather is six of one and a half dozen of another.

    If anyone here starts going on about you being oversized when it comes to air conditioning, just laugh or ignore them the rules and threat of elevated summer RH will not apply to you even if you did find the need to run the AC
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  3. #3
    Join Date
    Mar 2007
    Location
    Cedar Rapids, Iowa
    Posts
    121

    A 3 or 4 ton unit...

    That depends on the costs to operate the unit vs the cost to install the unit. Your contractor should be able to show you how much you can expect the 3 ton unit running some Aux electric strip heat and the 4 ton unit's operating cost to be. Remember, an added ton of Geo adds more cost to the unit and more loop field costs. This could add between 2~4K to the install cost. If you do go with the 4 ton unit, I would still recommend Aux back-up strip heat. If anything goes wrong with the compressor the strip heaters can help heat the home.

    Where A/C is not a concern, a single stage unit can be used. Again, your contractor can show you the operating costs of a single stage vs a two stage unit. Whatever unit you choose make sure it is energy star rated for the 30% tax credit. This information is available at energystar.gov. Your contractor should also give you a print out of the units compliance with the energy star regulations.

    To get the full benefit of a desuperheater a buffer, or pre-heat, tank is needed. The desuperheater will take away about 10% of the units heating capabilities so turning it off may not actually save you money. The Geo is operating at 3, or better, COP while your electric water heater is working at 1 COP.

    Bergy

  4. #4
    Join Date
    May 2005
    Posts
    160
    My vote is for the 3 ton provided that heat loss calc is accurate. Even if you were slightly undersized you could always supplement with some wood burning. That's what I do here in Michigan in the coldest months. I tried to size mine for A/C, which made me a little undersized for heating. So now I'm a pyro.

  5. #5
    Join Date
    Aug 2009
    Location
    Juneau, Alaska
    Posts
    2
    Thanks for the helpful responses.

    The heat loss is the contractor's calculation -- I believe he followed industry standards and is consistent with a quick and dirty one that I did.

    He argues that the heat pump should supply 100% of heating requirements, and that the 4 ton is required to do that.

    I've always supplemented my electric baseboard heat with wood heat, and intend to keep doing that, partly because I want a reliable backup for electric outages.
    My contractor does not seem able to show me how many days per winter a 3 ton system would be undersized -- should industry standard heat loss programs be able to do that? I'm not sure he completely understands what the program can and can't do, or certainly couldn't explain it to me. He is the only heat pump outfit in town and his ground source projects seem mostly to be big ones designed by others, although he is doing increasing number of smaller residential ones this year.

    The house is about 1850 sq. ft., 2 story (1250 upper floor, 600 lower, with unheated crawl space). It's an old log home that I've done quite a bit of tightening and insulating to (including a new highly insulated 'back porch' that covers the entire north wall). We have a moderate,cool maritime climate in Juneau -- average annual temperature is 41F and it rarely goes below 10-15F. The house is located on the ocean and has even warmer winter temperatures than the Juneau averages.

    Based on contractor's information, and on my reading, it seems that the 3 ton will do what I want -- which is provide most of my heating most of the time, considerably cheaper than my existing electric resistance/wood combination. But of course I don't know all of the considerations.

    What is a "preheat tank" for water heating? Is it between water supply and my electric water heater? or between supply and the desuperheater? How big?

    Carnak: "Do not shut off heat to areas of the house unless you have proper zoning, a heat pump is going to move more air than an electric furnace."

    What is the best way to zone? I have 3 major use areas -- the back porch, which has storage and workshop and could basically be closed off completely in coldest weather; the downstairs -- mostly a large master bedroom area which we keep cool; and the main floor, which we normally keep cool at night and warmer during day. We're designing the duct system to provide for returns from each area.

    thanks again.

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    get an electric water heater to use as a preheat. Do not wire the power to it.

    run your cold water supply to it

    use your desuperheater to heat this tank

    the hot water outlet of this tank gets piped to the cold water inlet of your real water heater.

    these systems move a lot of air.

    These units move a lot of air, a badly zoned duct system could be a disaster. Shutting off vents in rooms you want to close down could be a disaster as well.

    So you need to bypass supply air back to the return side when zones are shut off. You now have warmer air entering the coil, and you make the geo a little less efficent.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  7. #7
    Join Date
    Mar 2007
    Location
    Cedar Rapids, Iowa
    Posts
    121

    Cost of operation

    Make the contractor show you the cost of operation for both units. If a four ton is cheaper to operate than a 3 ton w/ aux. heat, go for it. If not, stick with the 3 ton and a 10kW strip heater.

    Here is how to pipe a desuperheater...

    Bergy
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