Off-grid and gshp brainstorming

Discussion in 'General Discussions' started by elkjunkie, Nov 11, 2021.

  1. elkjunkie

    elkjunkie New Member

    Hey all
    Over the next couple years my wife and I are planning a move to off grid. We found a 40 acre parcel and she suggested geothermal as part of the build. We heat 99% with wood and use a couple window a/c units intermittently in the summer. We have an electric furnace that we just use for circulation.

    As we have 2 plug-in cars, the solar required will be 20+ kw (70ish panels), propane backup generator and appliances.

    Primary heat will still be wood. Building permit will likely require an alternative heat source. This is where a GSHP comes in. It will likely only be used to condition air in the summer.

    My current home is 2,000sqft and set up for a 3T pump. Our build looks like 3000 sq ft. As the load will be primarily summer only, calcs for a 6 ton seem excessive. I am leaning toward 4 ton with possible propane backup.

    I am just doing a site plan now so ground loops are the initial consideration. Does 4 or 5 600 ft loops sound adequate?
     
  2. Timothy Fossa

    Timothy Fossa New Member

    I’d ditch the propane all together. Get a couple mini-splits and a battery backup system. Go all electric with the appliances. Home insulation is very important too.
     
  3. elkjunkie

    elkjunkie New Member

    I would ditch propane if I could connect to the grid. Bringing grid power to the property could push six figures. Solar with a 54kw battery bank is around 80 thousand. That's if I go with some propane appliances.
     
  4. xSpecBx

    xSpecBx Member

    There are people here much smart than me, but I’ll throw my 2 cents in. I’m struggling with following what you have and plan to build so, just to make sure I have my facts straight I laid out the below. Please correct anything that is wrong.

    current home:
    - 2,000 sq ft (total sq ft or just what is above grade?)
    - heat source is predominantly wood, but with a propane furnace
    - window units for a/c in the summer

    planned build:
    - 3,000 sq ft (total sq ft or just what is above grade?)
    - heat source is predominantly wood
    - undersized GSHP to cover a/c and partial heating load
    - lots of solar to cover power usage along with battery storage and propane backup generator

    Assuming the above is correct, first, where are you located? Second, have you done a manual J on your proposed build? If not, coolcalc dot com is a free site where you can perform one. Third, use the information from the manual J and go to looplinkrlc dot com to figure out you loop size.

    I live in SECT. My house is 2,400 sq ft above grade with a walkout basement (so partially below grade and partially above) which makes up another 600 sq ft. My house uses 2 300’ closed vertical loops and a 4T unit. Average low design temp where I live is 8*F. The advantage I do have is we are straight into rock (and my wells start on a slab of ledge), so good thermal transfer. Obviously a horizontal loop would need to be larger for the same size unit.

    I have only had my system since May, but it was very efficient during the summer, which is probably why you are considering it. I think the idea of ductless mini splits is a good one to use for backup heat and a/c, but the above links will help you properly size a system to make an educated design choice. Frankly, GSHP is an expensive system for essentially just an a/c unit.
     
    Last edited: Nov 21, 2021
  5. elkjunkie

    elkjunkie New Member

    current home:
    - 2,000 sq ft all above grade
    - heat source is predominantly wood, but with a electric furnace (3T ready)
    - window units for a/c in the summer

    planned build:
    - 3,000 sq ft single story, insulated crawlspace
    - heat source is predominantly wood
    - undersized GSHP to cover a/c and partial heating load (circulate wood heat)
    - lots of solar to cover power usage along with battery storage and propane backup generator

    I am in southwest Washington. I only included info about my current house to illustrate heating and cooling needs. The calc on the new house came in over 60k btu. That is more than the solar can handle and more than I would need heating with wood. The building permit will require another heat source. I like mini-splits but would lose the benefit of whole home circulation. I could go with a propane furnace but the cost increase to a gshp is negligible as I would do the install myself with the build. My wife and I built our current home 12 years ago for 80k only hiring subs for sheetrock and insulation.

    My main question was in regard to the predominantly cooling nature of the somewhat undersized 4T unit and the demand that places on the loops. My apologies for being horrible at explaining things.
     
  6. xSpecBx

    xSpecBx Member


    My house has a 38k btuh for heating loss and 19k btuh heat gain for cooling. For areas like mine, where heating is the primary driver for loop design, the loop winds up being oversized for cooling and barely works in the summer.

    I would think that, as long as your loop is sized appropriately for the expected cooling load, you wouldn’t have an issue. That 4T is probably more than you need for cooling You need to size the unit to the heat loss or gain and than size your loop to the corresponding unit size. An oversized loop doesn’t necessarily do much for you. It will keep your EWT higher which will improve system efficiency slightly, but will increase your pumping costs and the installation costs.

    Loop link lets you pick which load condition, heating or cooling, is predominant and than sizes the loop accordingly.
     

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