New study on heat pump water heaters’ reliance on the home’s heating system and viability of ducting the HPWH
mackstann
| Posted in Mechanicals on
https://www.aceee.org/files/proceedings/2014/data/papers/1-1203.pdf
This came out sometime in 2014 and I haven’t seen anything about it before.
The points that I took from it are:
- The heat “stolen” by the HPWH is less than expected, at least when located in a room that is partially on the exterior surface of the home.
- Exhausting the HPWH to the outside without also ducting its intake can have a harmful effect on the home’s heating energy use, since it needs makeup air, which comes from outside and can be even colder than the output air of the HPWH.
- While they found somewhat positive results in fully ducting the HPWH, this seems dubious to me given that they drew intake air from the crawlspace where they admitted that ground coupling added about 4 degrees to the outdoor air before it was sucked into the water heater. Then again, crawlspaces are pretty common in the PNW so maybe it is an appropriate scheme to test.
- Another thought… energy aside, HPWHs can still cause comfort issues when their placement causes part of the house to feel cold.
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Replies
Nick,
Thanks for sharing this link. Here is a quote worth sharing:
"In addition, the experimental data indicate that the penalty of installing a HPWH within the thermal envelope of the home may not always be as large as simplified modeling studies suggest. The interactions between the HPWH and the space conditioning system may be more complex, potentially due to the buffering of interior walls resulting in localized cooling in the water heater closet, with very little impact on surrounding interior temperatures. In this testing, only approximately 43.4 ± 12.2% to 37.2 ± 4.7% of the heat removed from the closet by the HPWH was made up by the HVAC system, in the heating and cooling seasons, respectively. The study also verified the benefit of HPWHs installed in conditioned space in providing supplemental cooling, decreasing HVAC energy use by 9.3% compared to an exhaust only or fully ducted HPWH."
Nice info.
The use of Mini's for space heating in heating dominated climates, provides with the possibility of using air heat that already has a lower cost than 1:1 . Doesn't that negate partially the impact of using a HWHP in cold climate during heating season ? If i remember correctly, Dana pointed that out in a recent discussion.
So in fact, a HPWH should be well within the conditionned envelope to benefit from the minis high COP during all but the coldest days.
I will need to review my POV on using HPWH in cold climate and their impact, this study points out that the impact on the space heating might be less severe than previously thought.
The same water heater could also be supplied with "hot air" coming from and adjacent room, heated up by a wood stove by only using a small wall circulation fan raising its COP significantly.
Jin, Dana posted a useful calculation in an older thread:
If we follow the study's numbers, roughly, and say that the HPWH only gets about half its heat from the home's mini split heating system, then the cumulative COP is more like 2.0 in winter.
Another interesting thought is that if the HPWH is in a basement or garage space that has uninsulated concrete in contact with the earth, then there is probably some heat being taken from the earth... a super cheap ground source heat pump.
My thought remains the same for homes like mine with unheated basements. Today I just checked the temperature in my basement that has an electric hot water tank and all exposed plumbing. 35 degrees.
So for me, I would run a HPWH only after my heating period had ended and too warm summer started. July 15th. So now I could run it till September 15th which is just before heating season because I would not want to cool my cellar. At that point I would have to switch to external air to rob heat from via a fully ducted scenario.
I hate the idea for my particular home. Just plain nuts.
I have great southern exposure. Best to home build a solar thermal system or go the PV way but then I have to somehow talk to the tree Gods and get them to kill off a few of my neighbors 100' PV blocking trees.
Martin, Dana, How much PV is collected via the best 6 months? My neighbors trees only block the winter 6 months.
As to HPWH, quote me from here on in, I will never own one at my present location. I will own one from day one if I move to below the Mason Dixon line.
I'm in zone 4 and the HPWH is not nearly so bad here. My unheated/uninsulated (also with no insulation in the joists) basement gets down to about 50F in the depths of winter. The HPWH does not lower this by more than a degree or two, and since it's above the HPWH's cutoff of 45F, it never has to fall back to resistance heat. Any heat it "steals" from the mini split that heats the floor above is still working out to a cumulative COP of well over 1.0. Life is good.
Nick: i don't get Dana's maths there... but i am a n00b to this level of comprehension ..
How can using an energy that costed you less than 2:1, on another 2:1 machine, bring it down to 1.5:1?? I would like to understand! :)
When you say cumulative , do you mean HPHW itself or in conjunction of the MINI ?
Think of it this way. The mini split has a COP of 4, so it spends 1 BTU to get 4 BTU, or, to make things easier, let's instead say it spends 0.25 BTU to get 1 BTU (1/4=0.25).
The water heater has a COP of 2.5 so it spends 0.4 BTU to get 1 BTU (1/2.5 = 0.4).
So, cumulatively, if a BTU is brought inside from outdoors through the mini split, and then is put into the water by the HPWH, it incurs both the 0.25 BTU expenditure from the mini split and the 0.4 from the HPWH. We have now spent a total of 0.65 BTU to get 1 BTU from the outdoors into our hot water. 1/0.65=1.5384615384615383, so our "cumulative" COP for this chain of two heat pumps is about 1.5.
Good! makes more sense now than on a single line :p
Very counterintuitive to think that 2 systems start with higher than the resulting cop of their sum of work.
Here is how i thought it worked :
The mini uses 1btu to gather and move 4 btu from the outside to the interior.
Then the HPWH uses 1btu to store 2.5btu in the water tank.
So we've spent 2btu of energy to accumulate 6.5 btu in the tank .
So 6.5 / 2 = 3.25cop. And then you need to add the energy loss due to the exchange of the HPWH??
In my house the HPWH gets a COP of 1 and when it finally gets up to 2 it keeps my basement running too cold for even more months. My basement is only at the temperature for 3 months.
I should build a room with water wall and glass and a HPWH along with an external motorized thermal shutter.... Need to invite the Zen one for a building vacation.
Wow reading my post back relax at home now, it doesn't even make any sense. lol
Why the heck would i add the two in the water tank.. ok my bad
I'll go use the shower with my 1+:1 hot water now ( electric water tank + standby losses ) :p
and get some relaxin' time!
Another try , don't laugh and please explain what i am doing wrong.
Mini uses 1btu to produce 4btu to the interior space
then HPWH uses 1 btu to produce 2.5btu of hot water and removes 2.5btu from conditionned space
hot water = 2.5btu
air space = 4 - 2.5btu = 1.5btu
energy cost = 1+1 = 2btu
total btu generated = 4btu
total cost energy = 2btu
4/2 = cop of 2 ??
Try this, Jin:
HPWH moves 2.5Btu of heat energy into the tank...it uses 1.0Btu of electrical energy and removes 1.5Btu of space heat to do so.
Mini-split replaces the 1.5Btu of space heat...it uses 0.375Btu of electrical energy to do so (1.5 / COP4 = 0.375).
Total electrical energy used is 1.0Btu + 0.375Btu = 1.375Btu. Total heat moved into the tank is still 2.5Btu. COP in this scenario is 2.5 / 1.375 = 1.82
Thanks John for helping me train my apprentice brain cycles !
Could you please explain why the HPWH would remove only 1.5btu from space?
Did you remove the 1btu of electrical usage from the 2.5btu output from the COP value because COP includes both the electrical and the resulting output ?
Just for kicks i used John math to calculate how much yearly savings a HPWH would yield in my situation. I guesstimated our current water usage @ 500$/year .
Using COP of 3.0 for the HWH during cooling season, and COP of 2.0for the minis during heating season, i ended up with savings of approx 220$yearly, to which i would add a drain water heat recovery @ 25% to be safe... Yearly savings of approx 170$CAD @ 0.088$/KW/H .
Not to shabby, more than a northern guy like me would've guess thinking that the HP would hurt more during heating season.
Still a bit tight on value VS initial cost if you factor in an expected life of 10 years.
But i guess that it would start making much sense just a tad south of us where climate is easier and energy costs are higher.
Jin,
Yes. Of the 2.5Btu of energy moved into the tank in our example, 1.0Btu of electrical energy is moved into the tank as thermal energy. Therefore, the other 1.5Btu has to come from the space.
The refrigerator in our new home (Northern Minnesota) will back up against the adjoining wall of the 6ft 6" x 6 ft 6" mechanical room where our GE GeoSpring HPWH will be located (with louvered door). Makes sense to me that I could vent the refrigerator's warm air into the mechanical room. The two condenser's could keep each other happy.
Our mechanical room will also house a ceiling mounted UltmateAir energy recovery ventilator. Does anyone see any problems with the HPWH and ERV both being located in the same small space?
Cabin will be heated by air source heat pump.
https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/designing-hvac-system-cold-climate
Randy: GE says you can install it in a smaller than 700 cubic foot space if you use a louvered door, but your room is only a third of that. I'd make sure that your warranty won't be voided.