Affordable Air-to-Water Heat Pump for climate zone 6B, Colorado
I’m looking for an air-to-water heat pump that I could use with a low temperature hydronic distribution system for space heating. I would also like to use the heat pump as the preheat for domestic hot water.
The mini-split air-to-air heat pumps are pretty ideal in many climate zones in the USA. On the western slope of the Rockies in Colorado, air conditioning isn’t usually necessary. So I’d prefer one heat pump to provide space heating and much of DHW.
I’ve seen that some manufacturers provide air-to-water heat pumps for European markets. There are also some rather expensive units available in the USA. Any news on affordable units coming to the USA market?
For the cost of some of the more expensive units currently available, I may be better off going with a larger PV array and an electric boiler. I could use surplus summer production to offset heavier winter electric consumption. (My utility only pays avoided cost for any annual net meter surplus.)
Solar thermal is pretty effective out here, and there are some rebates available, but the heat pump option seems like the lower maintenance route.
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Replies
Pat,
If you haven't seen it yet, I suggest that you read this article: Air-to-Water Heat Pumps.
Air to water heat pumps and reversible chillers exist that might work in your location, but "affordable" is a function of both your wallet pain tolerance and design skills.
Unlike mini-splits, which are a "system in a can", with the input & output heat exchangers already designed and sized to work well together, hydronic systems require designing the radiation and heating distribution systems to operate with adequate capacity & efficiency. To hire a competent hydronic design engineer to do the job right may cost as you as much in just the design services as the cost of a mini-split.
Cold climate mini-splits will operate with specified capacity output at zone 6 Colorado type 99% outside design temps, and heat pump water heaters work too.
Before you can even guess how much a hydronic solution (or a ductless air-air solution) would cost, you need to start with the room-by-room heat load calculations at your 99% outside design temp. A 2 ton reversible chiller like the Chilltrix CX30 might be an "affordable" solution for a small high-R house operated as a single zone with low-temp radiant floor slab radiation. It'll be more expensive than a mini-split solution of equal capacity though. At -10F you can't expect more than about 12,500-13,000 BTU/hr out of it, even at radiant-slab water temps, so that's going to be a fairly modestly sized & high performance house.
http://www.chiltrix.com/chiller-technology.html
From a design risk and total up-front cost point of view cold climate mini-splits are a much safer/cheaper bet.
Martin,
I did read that excellent article and the comments when it first came out. I just read it again to refresh my memory. I'm hoping for an air to water solution that is as elegant and reliable as the better mini-splits. Clearly, we aren't quite there yet.
Dana,
Thanks for your thoughts.
I was imagining an air to water heat pump unit, a buffer tank, and low temp hydronic baseboards. I'm designing a co-household (shared home) that offers a balance of communal and private areas. So separate zones that offer individual controls are highly desirable.
Ideally, with the buffer tank I could do some preheat during the day when the COP is better due to higher ambient temperatures. (The low temperatures would limit my heat storage capacity, though.) During extreme cold I could supplement with some electric heating elements in the tank. I could also supplement with solar thermal. ("I'm not dead yet." -- Monty Python) If the buffer tank and the heat pump were reasonably well matched for efficient heat transfer and COP, the distribution should be manageable.
The Ecodan is available overseas;
http://www.innovations.mitsubishi-les.com/en/ecodan/waermepumpensets
The Nyle Cold Climate Heat Pump is still under development:
http://water.nyle.com/cchp/
I'm trying to avoid the radiant slab approach because of the cost, time lag, and performance qualities. (A friend of mine did a radiant slab several years ago in a super-insulated home and says he won't do it again. It never really feels warm because the heat load is so small.)
Air conditioning isn't necessary or desirable at this point.
The cold climate mini-splits are a cheaper/safer bet, and a compromise in terms of zone control and contribution to DHW.