So, I live in Maine, where heating is a big deal and availability of replacement parts and equipment can be a challenge. Also, it now regularly gets into the 90s with high humidity, so AC has become closer to a necessity. We're building two new, totally off grid buildings now, and one of my best friends is an HVAC wholesaler. With his advice and that of the best installer he can recommend, here's what we've decided to do:
- No to geothermal. Even in a mostly-heating environment where temps go below zero F, it just doesn't make sense given the recent advances in air source heat pumps. Geothermal was brilliant 10-15 years ago, but now it's just too capital intensive to justify the installation costs.
- Yes to air-to-water heat pump for heating, but only because of where I live (much need for efficient heat transfer) and a desire to use in-floor hydronic heat (I just really like it). But if I lived further south than about PA, I'd go strictly air-to-air, which is less costly and widely available. And I'm waiting on the new Mitsubishi units before installing the air-to-water unit.
- Separate air-sourced hot water heater for the domestic hot water, simply because they cost so little to buy after rebates and are easily replaced (even with a resistive tank, in a pinch) in an emergency. In other words, no combi-boiler. I had a friend forced to wait two months on parts for his during Covid, which taught me a lesson about redundancy. I also had to replace a water heater the afternoon before Thanksgiving once, which was a good reminder of Murphy's Law.
- Similarly, separate air-sourced heat pumps for cooling and shoulder season heating. I could use "chillers" with the air-water unit, but they're a bit less efficient, and having a separate, backup heat source gives me redundancy (see a theme here?). We'll also install multiple, separate heat pumps instead of a single multi-head unit. They're more efficient, and again, redundancy.
- Finally, a small propane instant hot water heater as an alternative way to heat water for the hydronic system if the weather is dismal for a long stretch of no sun. Given that we'll use a buffer tank in the hydronic system I'm not worried about short cycling or return water temps to the heater, and those units cost a fraction of what a dedicated boiler costs. If I wear one out after a few years I can buy several for the cost of a boiler. And if necessary I could even re-plumb it quickly for domestic water.
- Anyway, that's my two cents. Hope it's of some help to you, in your planning. Yes, I overbuild things. But I live by the motto of "two is one, one is none" and it has served me well. Good luck!
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