Replies

1. 
   paul_wiedefeld | Jul 19, 2022 12:14am | #1

   For a house with that heat loss, consider skipping the geothermal entirely. Doesn’t make much sense to install a ASHP today, spend a bunch of money on ground loops, wait 20 years, then maybe save a trivial electricity amount (if any). Instead, there are upgrades that make more sense today, both enjoyment and energy wise.

1. 
   lance_p | Jul 19, 2022 09:43am | #2

   Hi Paul, thanks for your points. This will be a very efficient new build and about 3,500 sqft. Not much left on the upgrade front.

   The idea of using an ASHP is great, but the unit that’s available with this furnace is not capable of two tons heating in our rather cold climate. Our design temperature is -26C (-15F) and we get plenty of nighttime lows beyond that.

   Stabilizing our heat pump output with a ground loop would be more about enabling the hp to do its thing without relying on auxiliary heat sources, and less about saving electricity/money.

   I’m thinking I can get the loops installed for the cost of an excavator for a day, some clean fill above the loops, and the cost of the pipe and my time. Roughly $1000, which is pocket change compared to doing it afterwards.

1. 
   paul_wiedefeld | Jul 19, 2022 09:52am | #3

   Electric resistance heat for the handful of hours under the capacity of a cold climate heat pump (like a Mitsubishi hyper heat or equivalent) will be much cheaper than thousands spent on ground loops, especially if you won't be using them for a few decades.

1. 
   lance_p | Jul 19, 2022 10:14am | #4

   Understood. There are two points to make here.

   One, the available heat pump is far from being a capable cold-climate unit. I don’t remember the specs off hand, but it’s not going to work in our climate. I don’t know if a suitable cold climate unit can be substituted or retrofitted.

   Two, I’m planning to use the ground loops immediately to pre-condition my incoming ventilation air, and within 5-10 yrs for a GSHP. These loops will obviously be overkill for ventilation, and if that was my only planned use I would go much smaller.

1. 
   paul_wiedefeld | Jul 19, 2022 10:30am | #6

   Is there time to just install a GSHP to start?

2. 
   charlie_sullivan | Jul 19, 2022 10:15am | #5

   One of the challenges with ground source heat pumps is that sites vary significantly, and so sizing is somewhat guesswork. Generally, in your climate, you would want to have the tubing a lot deeper than several feet, to be well below the frost line. However, the special situation of being able to lay the tubing right against bedrock could make the performance better even with less-than-ideal depth. There are ways to do detailed thermal modeling of a scenario like that but that's way to complex to do on a routine basis. Another thing that is sometimes done is to make a single test loop and measure its performance before deciding the total system needed. But approaches like that are more suited to commercial scale systems, because the study might cost more than the system.

   In addition to your approach of taking a guess but then oversizing, another approach is to mitigate under sizing by combining with an air source heat pump, and using the air source heat pump whenever the outside temperatures are mild and only using the ground source during the coldest hours. But then you have the cost of installing two systems, and the complexity of figuring out how to combine them into one air handler. Some ways to do that include making both systems hydronic so you can easily use valves to direct the water as needed, using minisplits for the air source part, or just stacking heat exchangers and making sure that your fan has enough capacity.

3. Expert Member
   Akos | Jul 19, 2022 11:04am | #7

   Lance,

   I would talk your contractor and see if they are flexible on the equipment they will install. You don't want a sub par heat pump in cold climate. The COP on lot of the units drops significantly bellow freezing, they might provide heat but it is useless if COP is around 1.

   For a new build, you want a proper hyper heat cold climate unit. If they can't supply one, you can look at something like MrCool Universal DIY. This will provide 100% of nameplate capacity down to -5F, you won't even need resistance backup at all. If you are on the handy side, the equipment install should not be too hard and the HVAC folks can than connect the ducting to it.

   I recently bought 3 tons of cold climate ducted heat pump for less than what you will spend on the pipes, fittings, manifold and pumps for your geo loop. Never mind the cost of trenching or the actual GSHP. As far as I'm concerned in zone 6 or less, GSHP is dead.

4. 
   walta100 | Jul 19, 2022 10:46pm | #8

   Sounds like you have your heart set on a geo system and we are unlikely to change your mind.

   You will find like mined people on the geo exchange forum.

   https://www.geoexchange.org/forum/

   Walta