Considering a HWHP unit but not sure of feasibility and/or if a little room addition and ducting could make it work, and if it is worth it.  Live in Boise ID, currently hot water is gas and in the garage, and the HWHP must go in that garage which can get quite cold at times.

My idea is to build a well insulated mechanical closet in the garage.  This closet would not be near the size suggested for a in house location, but would be adjacent (shared exterior wall) to the conditioned laundry room in the house.  I would use a ducting kit and ducting w/dampers set up to supply conditioned air from the laundry room in the winter (via pass through duct and grill) and then adjust damper(s) to get supply from the garage during the summer.  The exhaust ducting would be setup to exhaust to the garage in the winter and then exhaust to the laundry room in the summer.

Worth the bother?  Feasible?  Closet would be of sufficient size for unit clearances and have the room to run all the 8" duct.  Would a closet even be necessary if all supply and return is via insulated duct work?  Can't seem to find any lit on how well the WH jacket is insulated and if you can use an insulation wrap like the gas heater has for added thermal isolation.

Reason I am thinking about going this route is that my 6kW solar array produces more power than I currently consume.  I get 1 to 1 kw/h credits but who knows how long that will last.  Idaho Power is already trying to change the rules so would like to use my excess power generation to reduce my current gas consumption.  Any thoughts appreciated, thanks.

By "HWHP" are you referring to a heat pump water heater (HPWH)? If yes...

Your scheme can work, provided the laundry room has sufficient volume, but it may be better to just use the garage air, and spend the money you would have spent on the closet & ducts to insulate & air seal the exterior walls of the garage, maybe even improve or replace the garage door with a better insulated better weather-stripped version. In your location an insulated reasonably air tight (garages can never be perfectly air tight) garage won't drop much below 50F even during a cold snap if you keep the insulated garage doors closed. The binned hourly coldest-weeks mean temperature in Boise is still north 25F, and the deep subsoil temps are a few degrees north of 50F, so it's really all about controlling outdoor air infiltration rates and wall losses.

https://weatherspark.com/y/2142/Average-Weather-in-Boise-Idaho-United-States-Year-Round

http://mb-soft.com/solar/soilmap.gif

If you go the closet route, any duct runs will work better if very short, straight, and hard-piped (no flex).

HPWH tank insulation is typically as-good or better than those on standard electric water heaters, which is better than the insulation on gas-fired water heaters.

Thanks Dana,

Yes, I did mean a Heat Pump Water Heater. Based on your comments I think my plan now is to skip the enclosure (garage is insulated but unheated) and just concentrate on the ductwork. The runs will be less than 30' equivalents and I will concentrate on the sealing and insulation of those short runs.

Do you think it makes sense to route the exhaust back in to the house during the summer? Manufacturer literature seems to suggest it could reduce AC load, but I can't find any info on what the dehumidified temperature of the exhaust would be. Winter supply temps would be ~70F and summer supply around ~80F.

Anyone have any suggestions as to best brand? I'm leaning toward the Rheem models right now. It is already set-up for ducting and seems to be as reliable as any other. Thanks.

30' is quite a bit of duct impedance unless it's pretty fat stuff. Check the manufacturers' specs.

Is the laundry room a large enough volume to support an HPWH on it's own?

The supply and return air have to both be garage-only or conditioned space for a couple of reasons:

1: The garage air needs to be isolated from the house air for fire code & indoor air quality purposes. (Can't have the water heater pumping air or gasoline fumes into the house.)

2: Supplying the air from one side of the garage/house pressure boundary and returning it to the other pressurizes or depressurizes the house relative to the outdoors, since the garage is usually leaky enough to be always closer to the outdoor air pressure than the indoor air. The pressure difference to the outdoors will drive outdoor air infiltration (or worse, garage air infiltration) whenever the water heater is running.

It's fine to source the air from one part of the building's pressure envelope and return it to a different part, as long as there is adequate return-air paths between the supply & return locations within the house, such as makeup air grilles on any doors that might be closed, or jump ducts, etc.

In a Boise clikmate with an insulated garage I'd be inclined to skip the ducts, let it use garage air only, and only have easier to air-seal plumbing penetrations between the garage & house rather than duct penetrations too. If the ducts ever leaked it would create infiltration inducing pressure differences too (though not as badly as sourcing from one side, exhausting to the other.)