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Heat pump for a remodel – Questions for a remodel in a Marine (3C) zone

Dirtfox | Posted in Mechanicals on

Thanks for your thoughts in advance!

I’ve been reading everyone’s comments and so many articles to come up with efficient ideas for an impending remodel, and I’m curious what others think at this point.

I’m in zone 3C (Marine), and the ocean moderates the climate in my neighborhood so heavily that it’s always slightly chilly, but never freezing. No one installs air conditioning.

For those of you who like added data, my current rolling average is 150 only cooling degree days annually, compared to 3300 heating degree days. A little bit of heating almost constantly, and we’re usually too excited to feel temporarily warm to mind there is no A/C.

I’m planning a gut and remodel to convert what’s effectively a garage on a concrete slab to cover a small bathroom and open concept kitchen and living room. Probably about 450 sq feet with vaulted ceilings.

There’s no way the heating system in what remains of the rest of the house will do anything useful for this remodeled area.

With mild year-round temperatures, I was looking at air heat pumps as an efficient energy source. I currently have natural gas heating, but solar covers basically all of my electricity needs so I prefer electric over gas heating if I can keep it efficient.

Here are the main areas I’d like feedback:

  • The ground is sand. Literal sand. Was I informed correctly that’s too unstable to make ground heat pumps a smart choice long term?
    Any experience with an air heat pump system in marine zones? Do all the metal fins rust really fast like I’m picturing they will?
    Is radiant flooring worth it? It sounds so nice compared to just a warm air vent from a ductless split-pump system and I’ll barely use the air cooling half.

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  1. Expert Member
    Dana Dorsett | | #1

    Got a ZIP code?

    Most zone 3C locations have a 99% outside design temperature of about 35-40F.

    A rectangular 450' space with a common wall with other conditioned space, insulated to IRC 2015 code min will have a 99% heat load less than 5000 BTU/hr, but a careful load calculation should be performed to confirm that. Most ductless mini-splits would be sub-optimally oversized for loads that small, but Mitsubishi makes a highly efficient half tonner, the MSZ/MUZ FH09NA, that would probably work out OK, since it can modulate down to 1600 BTU/hr @ +47F, according to the submittal sheet:

    A less expensive but less efficient solution would be a half-ton point terminal (PTHP), which is similar to a through-the-wall air conditioner but a heat pump. LG's LP073HDUC series is capable of 6400 BTU/hr in heat-pump mode, and has three different options for resistance heat backup for when temps drop into the low 20sF. Amana and Gree have similar models. Some have compatible wall thermostats (recommended), others don't. But run the heat load numbers and we can refine the options a bit more.

  2. Dirtfox | | #2

    Thanks Dana, great details. The zip code is 93955.

    I'll play with some more calculations and take a closer look at your small-sized suggestions.

    Your reminder that most of what's on the market is oversized for a space that small makes me wonder if I ought consider placing any heat for the remodel where it can warm the other 900 sq feet and take a load off the ancient fossil fuel furnace that does the job currently.

    The monkey wrench is this is old construction with almost all walls made of foot thick uninsulated masonry - and that includes the wall between the currently conditioned space and the remodel. Great thermal mass, but both good and bad.

  3. GBA Editor
    Martin Holladay | | #3

    Dirt Fox,
    You should consider your existing house as you make this decision. Your "ancient furnace" will eventually need to be replaced, and you may be interested in improving the air sealing and insulation details in the old section of the house.

    Run the heat loss numbers on the old house as well as the new house. If you can get a supply duct and a return air duct to the new room, you may be able to use one furnace for the entire house. It's even possible that your "ancient furnace" has plenty of extra capacity, even if it doesn't need replacement.

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