At least some of the Sanden water heaters are not freeze-protected to NH type mid-winter cold. Most are good down to at least -10C/+14F, others down to -20C/-4F or lower.

The efficiency and capacity of any hyrdonic air source heat pump is highly dependent upon the heating systems peak water temperature requirements. A low-temp radiant floor solution might get you there, but you have to really calculate both the heat load numbers and the peak water temp to know if it's going to cut it or not. I'm not sure if it has been characterized for capacity & efficiency with the glycol/antifreeze that would clearly be necessary in your climate.

You may have an easier time designing the heating system around a 2-ton Chiltrix modulating reversible hydronic chiller than hacking on a Sanden water heater:

http://www.chiltrix.com/small-chiller-home.html

thanks for the reply, the chiltrix unit looks interesting, any preference on it vs. mitsubishi or fujitsu mini-split systems, we don't have any load calculations yet just exploring possibilities

Fujitsu's mini-duct series have better air handler drive than the Mitsubishi mini-duct unit, and more flexible mounting possiblities, with more capacity and efficiency, fully rated down to -5F, which is fine for a southern NH /US climate zone 5 location.. If you have a full basement or a conditioned attic to install it in, it's likely that you'd be able to heat & cool the whole house with a single 1.5 ton -18RLFCD.

The VERY high modulation range of the 3/4 ton Mitsubishi FH09NA make it a real winner for very low load homes. At +47F the Fujitsu 9RLS3H can only drop back to 3100BTU/hr, whereas the FH09NA can drop back to 1600BTU/hr. Why is this important?

Say the calculated heat load of your house comes in at 14,000 BTU/hr @ -3F (Manchester's 99% outside design temp) at a 68F indoor temp. That about 200 BTU/hr per degree below 70F. So the Fujitsu starts cycling when it's 3100/200= ~16F below 70F, or ~54F. If heated with a single Fujitsu that may be OK, but if it needs more than one, they'll start cycling on/off at about 40F. In southern NH it's above 40 F for most of the heating season, and on/off cycling takes a toll on both efficiency and comfort. Ideally you'd still be modulating rather than cycling at least in to the 50s F outdoors, something that you can do with a pair of FH09s, but you'd have to turn one of the 9RLS3s off.

The _ _ RLFCD mini-ducted series all modulate down to the 3100 BTU/hr range, so if it can be done as a single zone (a one-story rancher) you can get good heat distribution and pretty good modulation with a single unit. But if you have to go to wall-coil types, the FH09NA is really in a class of it's own.

With the kind of whole-wall R values you're talking you should be able to get the design load even lower than 14,000 BTU/hr, even with an insulated 1400' full basement below. If it's 700' upstairs and 700' wall-out it might be 10-12,000 BTU/hr or even less, at which point it really calls for two mini-splits minimum for zoning purposes. Between the differences in heat gain/loss between the walk-out and the upstairs, and the natural temperature stratification it really doesn't work well as a single zone.

The Chiltrix may be overkill for your shoulder season heat loads too, and may need thermal buffering to keep the efficiency up. The minimum modulated output isn't readily discernable in their online data, nor is their extended temperature capacity totally clear, but it looks like you can get the water temp requirements for your radiant floor down to 100F or so, it'll be able to deliver 14,000 BTU/hr @ -4F:

http://www.chiltrix.com/chiller-technology.html

http://www.chiltrix.com/heat-table.jpg

To design a system around it would require more information, and I'd be looking for information on whether/how it auto-defrosts, etc. Between the custom design and the more expensive hardware it's going to be at least 1.5-2x more expensive than a pair of 3/4 ton mini-splits or a 1.5 mini-ducted solution (either of which should be well under $10K.)