Walking away from heat-pump water heater

I believe this is a reasonably common and efficient approach. But the double tankage doesn't work in some homes, that is why various other solutions try to fit into a single tank. I had thought about solar DHW since the turn of this century, but local installation costs are high and the layout of my home puts the storage tanks quite far from the good collector locations, making winter freeze protection more difficult.

I finally went the HPWH route a year ago, installed in the garage (moved away from the kitchen closet). Most of the units had ambient low temperature cutouts right in the middle of my typical winter garage temperature range, so I went with the ($$) Airgenerate both for its lower ambient cutout, and for its external venting, keeping the garage from chilling as much. It has worked quite well.

Too bad you must decide quickly, before a new generation of very high efficiency R-744 (CO2) heat pump water heaters reaches American shores. Here is a lab report on an R-744 mini-split system acquired in Australia and tested locally:
Lab Test Report on Sanden Split System CO2 Refrigerant Heat Pump Water Heater
https://conduitnw.org/Pages/File.aspx?RID=1901 (Sorry, I can't embed the link today.)

The Lab Test Report contains the performance test results for this CO2 refrigerant, split system heat pump water heater. The results include the tests required for all heat pump water heaters for ranking according to the Northern Climate Specification. The tested Northern Climate Specification Energy Factor is 3.2 and its Delivery Rating is 7.5. This report is posted in order to share it with the Heat Pump Water Heater Collaborative and the Advanced Heat Pump Research Advisory Task Force. The date of this report is 9/18/2013 and its author is Ben Larson. It was produced under contract with Washington State University Energy Program which is funded by BPA to do this research.


For reference, all the other HPWHs on the American market have Northern Climate Spec EFs of just 1.8 to 2.2. The 3.2 rating of this unit would be a game changer, if the price is affordable. Apparently the R-744 systems can hit COPs of 4 to 5 in warmer climates for water heating alone, or 8 when integrated into AC systems. See discussion at:
CO2 Heat Pump Water Heaters
https://conduitnw.org/Pages/Article.aspx?rid=425

PS. Here is a graph of our daily water heater energy use, as measured on a standard watt-hour meter installed in its branch circuit.
The first month, upper left corner, is the previous regular electric heater, not fully insulated after a temporary move. The rest is an AirGenerate ATI66. The first gap is due to the watt-hour meter being temporarily moved to the new solar PV system while awaiting the official production meter. Other apparent gaps represent trip shutdowns, not actual averages over long spans. Blue line is for the quasi- daily reading, red line is a moving average (N=5).

 

If the heater is going to be out side on a south east facing, you should look into a passive system.

Build a Solar Water Heater: An Intregal Passive Solar Water Heater

 

I was thinking along similar lines, particularly in the Southern US. If the heat pump was placed in a well insulated room with fixed South facing windows it could conceivably operate in a 90+ F ambient environment year round. The COP would be fantastic.

I would also think about a way to pump the cool, dehumidified exhaust air into the home for free cooling during the warm season (is that year round in AL ?)

Regarding 'classic' passive DHW: it seems (and can be) very attractive in cost and eco profile, but long runs of copper become expensive, and forget about it if you have to pay a plumber.

 

If planning this route, do check out the max ambient temperatures of the intended HPWH, compared to the conditions in the intended space. My Airtap/Airgenerate has a default max ambient temperature of 110F. It can be reprogrammed to up to 130F, but I don't know just what problems that could lead in to.

This is where a ducted unit will help. The Airtap was always available with outlet ducting, but I believe the GE Geospring, and the A.O. Smith-Voltex (and its many rebranded versions) can now be ducted too. Probably even retrofitted to existing installations.

I see some other units (Bosch, Nyle) with ducting available. But as they are not on the Northern Climate approval list, I am not familiar them.

 

Bob, can you unscrew the outlet pipe from the HWH with a pipe wrench and replace it with a nipple to stretch it's use while you decide what to go to for better efficiency?
I switched to an Ariston water heater from Lowe's, under $200. and the energy side of my power bill is now under a buck a day...convection oven, induction cooktop and LED lights are helping too, and I'm no longer dumping waste heat into my home for the AC to wrestle with. Good luck! R.

 

Hey Bob, a passive solar system hooked up to an instant flash water heater (ours is natural gas) might be a good way to go.

 

Bob you want a plastic insert for the hot outlet nipple? you may be in luck..it already exists...your cold side already has a "dip tube", a white plastic pipe with a small flange on the upper end that just slides inside the cold water inlet nipple and makes the incoming cold water go all the way to the bottom of the tank to avoid mixing with and warming the hot water that is in the top of the tank. I see it as a replacement part on Home Depot site, "water heater dip tube", $4.00 you would need to cut it short, about 3" long, and clean and dry the inside of the offending fitting, and glue your new liner-tube inside with your favorite snake oil. R.