ASHP vs Geothermal in coastal MA?
TomServo
| Posted in Mechanicals on
I just bought a house near the south coast of MA that’s been more or less abandoned for five years. Among other major renovations, I want to replace the old gas-fired furnace and DHW with a heat pump system. With three air handlers and roughly 5000 sqft to condition, I’m wondering if there’s a significant advantage to either air source of ground source (provided I tighten up the building envelope significantly). First time homeowner, generally feeling very excited and also pretty overwhelmed 🙃
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Start with a plan to get the building to your end performance goals, Net Zero Onsite Energy and electrification without a service upgrade are good goals. Separate the plan into stages based on when the existing building materials reach end of life and have to be replaced anyway. In a Coastal environment an Air Source Heat Pump (ASHP) should not need any supplementary electric resistance heat and stay at a higher COP for the entire year.
The main advantage of a ground source heat pump (GSHP) is a consistent COP over the entire year, but at a higher upfront cost to install the ground loops. The GSHP should be able to provide or at least preheat the DHW using the same unit. Having a skilled local designer and installer is often more difficult to find for GSHP.
If you find a good designer and installer, get quotes for both a ASHP and a GSHP and compare upfront cost vs operating cost vs warranty.
this legislation supposedly applies a 30% incentive to gshp installs, but i couldn't find the detail yet on how that incentive actually works and what exactly it applies to (eg does it apply to wells or distribution components like radiant-floors, towel-warmers etc:
https://bipartisanpolicy.org/blog/inflation-reduction-act-summary-energy-climate-provisions/#:~:text=30%25%20credit%20for%20geothermal%20heat,2033%20and%2022%25%20in%202034.
I have a house nearby, on Buzzard's Bay. As you probably know, the price of electricity has skyrocketed in the region lately, we've gotten a few posts recently from people who switched to ASHP and were shocked at how much more expensive it was than the fossil fuels it replaced. I wouldn't recommend switching to all-electric without also aiming to be net-zero and all-solar.
Right now, solar is a great deal with net metering. But it's not sustainable for the power companies, they can't stay in business if their customers don't pay. So when planning for the long run assume that net metering will be less generous in the future. I wouldn't be surprised if in the future you can only net out the cost of generation, not the transmission. Or if there's just a monthly service fee for being attached to the grid.
Geothermal is theoretically more efficient, but it's a niche technology and ASHP's are produced in such quantity that they are much more advanced. If geothermal was as refined as ASHP's it would be more efficient, but it's not. The most efficient ASHP now exceed the Energy Star standards for geothermal. Geothermal is a lot more expensive to install, and there's a lot more that can go wrong.
What is your current heating system? One of the questions with switching is whether you keep your existing distribution -- ducts or radiators -- or go with ductless minisplits. Are you far enough inland that you're going to want air conditioning in the summer?
Sizing with heat pumps is much more critical than with fossil-fuel furnaces and boilers. Probably the biggest complaint we see here is that a system is oversized.
I'll add that where I am there's about 12' of soil before you hit bedrock, drilling wells is expensive.
Thanks for your response. I'm close by in South Dartmouth, just a few blocks from the water, so we expect to need some AC in the summer, but hopefully not too much. The house currently has a ducted system with three air handlers that we'd like to keep, provided we can get Servpro out to inspect and clean the ducts.
I definitely intend to get an audit done including a Manual J calc to make sure a new heat pump system is sized appropriately. We have a lot of roof space, but it's somewhat shady, so I'll need a solar quote to inform how much of our usage we could offset.
There’s usually a premium involved with ground source. If that premium divided by the discounted kWh saved vs ASHP is less than your rate, it’s a good deal. It’s usually not, but a big house in a cold climate with high electricity rates is about as good a scenario for GSHPs as you’ll find.
For me it is a numbers game and if you have city gas generally the cost bet BTU is much lower with gas than any heat pump. If you gas is propane then get a heat pump. The much lower cost of installation for an air source heat pump will make the payback on the ground source heat pump longer than the unit is likely to live.
The numbers are ever changing with the cost of the fuels and you guess at inflation combined with the very high ground loop cost. The only way to really make this decision is to build a computer model of you home enter the costs from the bids in the model with you local weather and fuel costs and your guess at inflation and interests rates and run the numbers.
Free modeling software
https://www.nrel.gov/buildings/beopt.html
Training videos to use the software
https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
We see this question regularly and I invite anyone to post a BEopt model where the ground source is the winner. I have yet to see a model with a ground source winner.
Note the ground source proponents are always talking about the 55° temp of the ground but when I see posted loop water temps from late in the heating season the numbers seem to be very close to 32°
It seems for many they are unpersuadable by facts and numbers. If you find yourself in that camp the Geo exchange forum is a place where you will find like minded advice.
https://www.geoexchange.org/forum/
Walta