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Community and Q&A

Heat pump and radiant floor heat in older house

bucksbear | Posted in Mechanicals on

I am doing a renovation and small second-story addition on our 1870s-era house in Western Massachusetts (Climate Zone 5). The house is expanding from 2050 to 2400 s.f. The house currently has an oil furnace and 50 gallon electric hot water tank (each unit is 10 years old). I pay a high rate for electricity (just went up to $0.26 / kWh for 2022!), but I am comfortable with the higher expense with the goal of moving away from fossil energy at home. Installing solar is not a great option for us because of large trees shading our roof. In the renovated and added spaces (kitchen, two bathrooms, and primary bedroom – 900 s.f. total, spread over two floors), I am also interested in radiant floor heat, mostly as a luxury feature rather than as a primary source of heat. We have 8 foot ceilings for most of the house, though want to vault the ceiling for the new addition (up to 14 feet at the peak for the 450 s.f. primary bedroom and bath).

 

My initial inclination for an HVAC upgrade has been to explore ground-source and air-source heat pumps, as the house already has ductwork distribution, with accessible unfinished basement and attic spaces (and no intent to finish these) and two chimneys that are getting removed (providing good opportunities to expand duct distribution). With a somewhat historic house, I’d prefer to not have mini split piping visible externally, and the thought was that I could also get whole-house humidification (winter) and air quality benefits from a centralized system. 

 

I thought a water-to-water ground-source heat pump system would meet all of our goals, along with providing cheap heat for radiant floors. However, I have had trouble finding a reasonable quote for such a system. I have gotten a few water-to-air ground-source heat pump quotes as well, and these would then call for a separate system to provide radiant floor heat. Our state has substantial rebates for ground-source heat pump systems and there are good federal incentives as well ($15k per system from the state, along with the 26% federal tax credit. If we instead went the air-source heat pump route, the credits and incentives are less (with no federal tax credit) but so are the upfront costs. However, at our high rate of electricity, the savings with a GSHP to an ASHP would be substantial. I’ve done a bit of cost modeling of the two options, and the net-present-value over 30 years of a GSHP system (including energy consumption) is about 15% less than an ASHP system (my assumption is that an ASHP system will consume approximately 50% more electricity over the year).

 

I have received heat load calculations of around 40k BTU for the whole house and 23k BTU for only the new area of the house (the full house includes the new area, and these two estimates were from two different providers). Both of these estimates were based on pictures and plans, not on a full Manual-J, which they will not undertake until I sign on the dotted line. I used an online heat load calculator and came up with 60k BTU for the full house (with the new space), but I’m no professional.

 

I have a few specific questions for the GBA community:

 

* Are my plans and assumptions reasonable?

 

* When working with a heat pump, I recognize that insulation is likely the best bang for the buck. Our structural engineer has recommended 2×6 walls and 2×12 rafters in the ceiling, all filled with closed cell. I don’t know a ton about insulation. Would you recommend anything different in my case?

 

* One potential GSHP contractor is hesitant to do the ductwork themselves, but finding a separate ductwork contractor is challenging for some reason (I’ve had a couple turn me down because they don’t want to get blamed for a bad GSHP installation, as an example). Should this be a red flag about the challenges of having multiple contractors work on an HVAC system?

 

* How should I think about sizing the radiant floor heating? My thinking is that at 900 s.f., it wants to be hydronic. Heat load calcs seem to normally use 1% design temps (0 degrees to a 70 degrees for heating in this area, for example) as if this is the only heat source, but in our case, we are expecting air distribution from the heat pump for the entire house, and the radiant floor heat only as a supplemental/luxury heat source, which I would think substantially reduces the load. Can this be served from a single water heater that provides both the radiant floor heat and the domestic hot water? The best case from my perspective would be to have a larger heat pump water heater (say, an 80 gallon unit) serve both purposes using a heat exchanger to isolate the domestic hot water from the radiant loops (we’d still be planning two floor heat zones). We are a family of four who bathe an average (or slightly-less-than-average) amount!

 

Thanks for any and all help!

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Replies

  1. Paul Wiedefeld | | #1

    https://www.greenbuildingadvisor.com/article/all-about-radiant-floors

    https://www.greenbuildingadvisor.com/article/are-affordable-ground-source-heat-pumps-on-the-horizon

    I’d read these two articles by Martin if you haven’t already. A new, well insulated addition will have a low heat load. This means the floor temperature will be very low, which is not exactly luxurious, especially when used to supplement forced air. Using radiant in the bathroom might be the best case scenario, which could be an electric mat.

    As to geothermal, it’s expensive. If it’s a luxury purchase, go for it! Consider it like a pool.
    I would not expect it to ever have a positive NPV, especially with those low heat loads for the old portion. I don’t think the COP will be 2x air source either. An air source system or two will likely be the easiest method since you already have the ductwork.

    1. Richard McGrath | | #2

      No disrespect meant to the author but " All about radiant floors " is filled with misconceptions and myths about radiant floors due to the paper being written based on early installs done by unqualified individuals .

      To the Op , if you do read all about radiant floors please do not get bored , it is a hell of a discussion and you should pay particular attention to what is said after the point where Robert Bean and Mark Eatherton get involved especially Bean's comments . It is long and loaded with radiant facts . ENJOY

  2. DCContrarian | | #3

    You're asking a lot of questions here, and to a certain extent they're not related -- for example, regardless of how your house is heated you want it to be well-insulated and well-sealed. So you might be better served and get more responses if you break it up into a series of separate questions about insulation, ground-source vs. air-source, radiant vs. forced air, etc.

    That said, one of the things that happens in construction with regard to mechanicals, insulation and energy efficiency is that there's no one involved with the project who sees the whole picture. You want the house designed as a system, where the architectural choices are made in concert with the mechanical and materials choices. Ideally while the architects are going through revisions of the blueprints there is a parallel process going on where a Manual J is done for each iteration. And just like there's a construction budge in dollars there should be an energy budget in BTU/hr, so that as changes are considered the impact in weighed against the energy budget.

    In most projects there's no one doing this work, so at every decision point there's no one advocating for energy efficiency. So the architects favor aesthetics over efficiency and everyone else involved favors expediency in the moment.

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