Heat-pump water heater in conditioned space
I am building a net-zero energy home to Passive House standards in the Seattle area. The best scenario for a heat pump water heater is in an attached garage, unheated basement or in the conditioned space of a house in a humid heating climate (why doesn’t everyone in Florida own one?). Unfortunately, my only option is within my conditioned space in a heating climate. My heating system will be a ductless minisplit air source heat pump. I would prefer to not remove the heat from my utility room only to make my heating system work harder. Exhaust venting only doesn’t make sense, because the air would be replaced with outside air at an even lower temperature and my house is as airtight as I could make it.
My vented crawlspace is too low to place the HPWH there, but what about getting my supply air from the crawlspace and venting the exhaust air to the outside? Each duct run would be about 8 ft long. I could disconnect the ducts in the summer and use the water heater to cool and dehumidify my house. By how much would ducting decrease the COP? I believe that AirGenerate, Geospring, AO Smith and State water heaters all have direct vent options. Which are the best options?
I’ve read that it’s best to get the largest possible tank to minimize the use of the electric resistance backup heater. My family of four is frugal with hot water, so I’m not sure if we’d really need the larger tank. As long as the heat pump is able to heat the water at the rate it is being used, isn’t a smaller tank more efficient (less standby loss)?
Even if I don’t direct vent the supply or exhaust, I could just use it on the electric resistance backup mode during the heating months and as a heat pump water heater for the rest of the year. The total efficiency over the year would decrease, but it would still be more efficient than an electric water heater. Of course, it would significantly increase the payback period. I wonder if it would still pay for itself within its lifetime if used only half the time as a heat pump water heater.
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Replies
Gerald,
Some houses have a room where you can install a heat-pump water heater. Others don't.
Here's my advice: don't try to cut corners on the size of the room where the appliance is located. Strictly adhere to the manufacturer's instructions; a bigger room is better than one that barely meets the requirements, and a smaller room is unacceptable.
Don't try to duct air to a heat-pump water heater. It's a Rube Goldberg device; the fans waste energy; and eventually something breaks.
For more information on this topic, see these two articles:
The Water Heater Payoff
Heat-Pump Water Heaters Come of Age
My take on the duct idea: It would work, but might be a hassle. If your hobby is fussing with stuff like that, go for it. If you want to build something and be done with it or to sell the house and have it continue to work, heed Martin's advice.
You may have an easier time hitting Net Zero energy with a drainwater heat exchanger, if you have at least 4-5' of vertical drain between the shower and where the drain exits the house, and would effectively double your showering time/capacity. It won't help batch draws, like bathtub / washer fills, but in most houses bathing would be something like half the total hot water use, and the only really big draws, and may eliminated the argument for a larger tank. A smaller tank would have faster recovery in heat pump mode than a large tank, and with drainwater heat recovery the water entering the tank in winter would be 70F instead of 40F.
EFI is a distributor for Reneability's PowerPipe series (and you can get the wholesale price if you have an account set up with them: http://test.efi.org/sites/default/files/power_pipe.pdf ), but they can also be ordered direct from the manufacturer, or through the big orange box store. Natural Resources Canada keeps third party testing data on the vertical gravity film types using a standardized test conditions to keep apples-to-apples comparisons:
http://www.nrcan.gc.ca/energy/efficiency/housing/home-improvements/12356
There are horizontal versions, but there isn't any third-party performance data on them. Marc Rosenbaum's blog just covered a meeting with the manufacturer of one of those:
http://blog.energysmiths.com/2014/11/a-visit-with-david-velan-of-ecodrain-drainwater-heat-recovery-comes-of-age.html
Rather than a Rube Goldberg contraption, the AirTap Ati-xxx series heat pump water heaters were DESIGNED to be ducted early in the product design cycle. (I haven't seen ducted versions of the others, if they exist they're "me too" designs of very recent vintage.) But keeping the duct runs short & straight would still be important. Parking the thing tightly into an under-stairwell closet but ducted to an adjacent larger space works fine, running a duct 10' with three elbow turns doesn't.
Thanks to the pointer to Marc's blog about the new horizontal drainwater heat exchanger. It sounds like powerpipe is the way to go if you can do vertical, and ecodrain is a good option if you have to do horizontal.
Gerald,
I have a very similar situation in a home I'm just now completing. In about 2 months I'll know if there are any major issues with this strategy:
1. I'm using an 80" long vertical Power Pipe, which should cut water heating requirements in about half.
2. Although the small utility room is in the middle of the living space, it is close to the heat pump outlet, in hopes that comfort issues will be minimal.
3. I think there is a timer on the HPWH that can force it into resistance mode during times of occupancy if there are comfort issues. If the water storage capacity of the HPWH is large enough, the timer can ensure that the heat pump only runs during unoccupied time periods.
4. The utility room is well vented to the heated space. With a high vent and a low vent, natural convection will cause air circulation through the room and out into the heated space.
5. The utility room also contains the back of the refrigerator, so whenever the refrigerator is operating, the cold air in the utility room passes under the fridge and gets preheated.
6. If the space heating heat pump has a COP of 3.0, and the HPWH has a COP of 2.2, I calculated the overall water heating system COP to be roughly 1.5, which is significantly higher than resistance heat at 1.0.
7. In summer, the cooling work of the HPWH is delivered directly to the living space. This may be enough to cool the entire first floor of a low energy house.
8. No ductwork or blowers.
9. In summer and winter, three fourths of the refrigerator's surface area is exposed to the lower temperatures of the utility room. This may save a little energy because the fridge doesn't have to run as much.
So my biggest fears are comfort problems (cold air finding the occupants), and noise. Not that the space heating heat pump will be working too hard. It's small, but it's twice as big as the HPWH in terms of btus per hour.