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Geothermal System for Heating and DHW

BartlettNH | Posted in Green Products and Materials on

Hello, I’ve just purchased a home in Northern NH and would like to get off of propane if possible. The home is using an older Series DE boiler (PeerlessBoilers) from 2005 to heat the DHW and hydronic baseboard. From what I can tell, the hydronic system was also installed in 2005. All rooms have hydronic baseboard and the pipes appear to be in good condition. There are two LP tanks outside used to fuel the boiler. My question to the group is if I can replace this old propane-fueled boiler with a new geothermal heating system: Can a new geothermal heating system create enough hot water for both DHW and hydronic baseboards (forced hot water heating system)? If so, what make/model do you recommend? Note that I don’t have any ductwork and am not looking to use geothermal for AC/cooling, just the ability to heat the DHW and hydronic. The house is 4-bed/3-bth and ~3800 sf, and I believe we have enough land to install either a horizontal field or vertical field for the geothermal. We are pretty far north (parallel to Northern Michigan and Wisconsin) and I’m not sure if there are geothermal systems powerful enough to heat water up to the 87+ MBH needed for both of our homes systems. Thanks for your help.

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Replies

  1. DC_Contrarian | | #1

    First step is to read this article to figure out the size of your system using historical fuel usage:
    https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

    Then go around your house and measure the feet of baseboard. With a heat pump you're probably going to use a reduced water temperature, the question is going to be whether you have enough baseboard to heat the house at a low water temperature.

    An air-to-water heat pump is probably going to be more practical than geothermal. Do you know your design temperature?

    1. BartlettNH | | #3

      Thank you for your reply. I hadn't thought of an air-to-water heat pump but have been reading more about it, and think it makes sense given lower costs. As you mentioned, the issue seems to be the water temperature. My hydronic baseboard system now is at ~180F and it looks like most air-to-water heat pumps get the water up to ~120F (sufficient for DHW but not hydronic baseboard). So my thinking is that I'd install a separate commercial water heater (also electric) to bring the 120F water up to 180F for the hydronic. Here is one from Rheem, for example, https://www.rheem.ca/product/rheem-commercial-electric-water-heaters-marathon-180 What do you think about this concept?

      1. DC_Contrarian | | #4

        If you read the article, it says that older boiler systems are typically oversized by a factor of at least two. If that's the case, your radiators will produce adequate heat at 120F. That's why it's important to do the sizing calculations first.

        If the existing radiators are inadequate for 120F water, I think you'd be better off adding more radiation than trying to get the water temperature up.

      2. charlie_sullivan | | #5

        I'm in central NH and have a system that's about 15 years old that very similar to what you are describing. It was expensive to install but works great. I did very much what DC_Contrarian suggests: started with a load calculation, and then added radiators to get enough heat output. The heat pumps available now can output higher temperatures than they could back then, so you might not need to add as many radiators as I did, but figuring out the load is definitely the first step. You can also improve the envelope to reduce the load, which was part of how I made it work.

      3. paul_wiedefeld | | #6

        You can use a water to water heat pump to do the same thing - resistance will be pretty expensive. An electric boiler is probably the better choice for the resistance route. Or just keep the boiler for the coldest nights. Make sure you actually need 180 on design day - sizing is a bit of a guessing game, would be surprised if you were correctly sized.

  2. paul_wiedefeld | | #2

    Yes, geothermal or air source heat pumps can work - the exact configuration depends on your heat loss compared to baseboard length, and you may keep the boiler as a seldom used backup heat source. Geothermal isn’t really a brand thing - sure there are different brands of “liquid-to-water” units but they all are around the same efficiency and don’t make up all that much of the equipment cost. Geothermal, especially on the hydronic side, is more of a collection of brands, unlike a furnace or air source heat pump. The installer is much more important.

  3. greenright | | #7

    You will get 120-130 water out of an anything-to-water heat pump. With that temp and regular baseboard you will get nowhere in your zone and house size. Your design day is what- minus 10? Last time I looked at regular baseboard and low water temps I found out the hard way that any water below 140 pretty much cannot get meaningful heat transfer to the room. If you look at the heat transfer graph for your baseboard (slant fin has it for example) you will see that heat transfer is negligible at 120-130 water.

    If you want heat pumps you are looking an air delivery setup- ducts or a multi split setup. Keep the baseboard as a backup if you wish.

    If I was you I would first invest in sealing and insulation and then see what is what calculation- wise…

    One experiment you can do is lower your boiler temp to 130 and see if you can maintain your desired set point in the house in relation to outdoor temp. This way you will get a rough idea of 130 water will work for you in Jan and Feb. my gut feel is that it would not and you will need hotter water for most of those two months.

    Good luck.

    1. DC_Contrarian | | #8

      This isn't true at all. Heat output from baseboards is a linear relationship with the temperature difference between water and the room. If the water is at 170Fand the room is at 70F, that's 100 degrees. If the water is at 120F that's 50 degrees, you get half the heat output. Now, half is a big hit, but it's not nothing. Considering that scientific sizing of heating systems only really started in the last 20 years or so, there's a good chance existing baseboards are twice the size they need to be, in which case half the output works just fine.

      I do agree that empiric testing is the best way to go. That's why I recommended fuel bill analysis above. If you get a day at your design temperature you can try reducing the water temperature, if your boiler will allow it. I wouldn't recommend doing that for long periods because some boilers will have issues with the water temperature that low.

      1. greenright | | #9

        https://www.slantfin.com/images/stories/Technical-Literature/ratings_fineline30_r.pdf

        At 130 degrees at 1 gpm he gets pretty much nothing given any meaningful, practical length of baseboard. And remember- he has a big house in Northern NH where it gets cold.

        But yes- he needs to experiment and see where he is at.

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