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A2W Heat Pump sizing question

skipt Member Posts: 3

I'm posting for the first time here, but I have been reading for quite some time, and I appreciate the expertise you people dispense. Thank you!

Building a new house in the mountains of southern Colorado. 2500 sq ft, single floor, slab on grade with radiant heat, using the betterbuiltnw calculation I get just under 37,000 Btu/hr. Lots of window area.

The builtbetternw calc is using 6 degF as the design temperature. It is quite common for overnight lows in Dec and Jan to be 12-17 degF, so I think is is sensible to try to NOT count on supplementing the heat pump with resistance heat for those relatively common nights.

Looking at air-to-water heat pumps, the one that strikes me as a good choice is the 4-ton SpacePak ILAHP, which seems geared nicely toward lower temperature performance. From their charts looks like it has a capacity of 42,000 Btu/hr at 95 degF LWT and 17 degF ambient.

Just for information, the common daily highs that go along with those cold Dec-Jan nights are in the 40-45 degF range. The days are nice and sunny. Big variation between day and night temperatures year round.

Should I worry about oversizing and short cycling with this choice? It seems to me that the 3-ton choices might not have enough capacity for my situation.

Thank you very much!


  • fentonc
    fentonc Member Posts: 157
    What temperature water would your radiant system need to meet your heat loss (37K BTU/hr) at the design temperature (6F)? The ILAHP48 is inverter-driven and it looks like it might support something like a 4:1 turndown ratio, so that at least helps reduce short cycling and improves efficiency (especially as I think these are usually paired with buffer tanks). The manual shows it can also be paired with a backup heat source like an electric boiler.
  • hot_rod
    hot_rod Member Posts: 17,468
    A buffer tank is often recommended for addressing the short cycling. As @fentonc mentioned, the latest version cold climate A2WHPs have inverter drives and can modulate, reducing the size, or possibly eliminating the need for a buffer.

    A good look at the technology and application concerns here.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • skipt
    skipt Member Posts: 3
    Thank you very much for your comments, it is a great chance for me to learn.

    Definitely we will have a buffer tank. I think the schematics in Idronics 27 (which I discovered when you ref'd it in a post some time ago, Mr. Hot Rod sir) will be a guide. What great info that is!

    Thanks for explaining the inverter drive aspect and the ability to turn down to lower levels. This, along with the use of a buffer, alleviates my concern I think.

    Fentonc, re your question about target water temperature, when I use the 2 Btu/hr per sqft per degF rule of thumb, my radiator area, which is the entire floor, does not need a big delta T. So I think using 95 degF is a safe number.

    Thank you for your help. I am now taking a look at the Arctic 050ZA/BE 5-ton unit, which also seems to be in the same ballpark winter-capacity-wise (at 19 degF, capacity 39,400 Btu/hr at 115 degF LWT). It also seems to be much less expensive, which I'm still trying to understand.

    Stay warm!