(Really off topic, here) I'm curious about the overall efficiency of
heat pump HW heaters. If it's in your garage (or basement), and it's
drawing heat from the garage, that's going to create a very cold garage,
right ? Further, if adjoining spaces or spaces above aren't fully
insulated from the garage, you'll also be increasing the heat load in
those spaces. All this would seem to reduce the efficiency of the HW
heater dramatically.
So, shouldn't the heat pump be vented to the outdoors ? If so, then its
efficiency would be determined by the climate where you live. In winter
it may rely solely on resistance heating. Though it should do quite well
in summer in most parts of the US.
Peri
------ Original Message ------
From: "Robert Bruninga via EV" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Cc: "Robert Bruninga" <[email protected]>
Sent: 18-Jul-19 1:23:02 PM
Subject: Re: [EVDL] Solar off grid with an EV? (DC AC/heatpumps and
waterheating)
*-Because a heat pump system is using energy to "move" existing heat,
it gives you 400% (or more!) efficiencies. [i.e. you use 800 watts to
drive a compressor and fans, but get 3,200 watts of heat into the
tank, while cooling the area around the water heater.]
I think it is more like 3 to one. The higher temp you want the water, the
lower the efficiency and can range between 2 to 1 or 4 to 1.
I haven't shopped for heat pump water heaters. It looks like I should.
I have recently resolved to stop using my propane water heater...
Here is my summary. They are often called HYBRID water heaters with the
heatpump heating the incoming water at the bottom of the tank and then (if
enabled) a resistance element at the top to provide rapid response and
higher temperatures. Remember the heatpump is the most efficient when it
is throwing energy at the incoming 60F water with a greater delta-T
The efficiency goes down as the temperature rises. SO I have mine set to
heat the bottom of the tank to only 105F (where it is still pretty
efficient) and then the top coil heats the rest to 115F.
IN fact, I really have the Heatpump one in series with the old pure
electric one and both have an added 4" insulation around them. So I let
the heatpump one heat its entire tank to 105F, from there it goes into the
old heater which has the bottom element turned off and the top element set
to 115F. SO the bulk of the heating (55 to 105F (50 degrees) is done at
3:1 electric effdiciency, and the final 105-115 (10 degrees) is straight
electric at 1:1 (but saves wear and tear on the heatpump, working much
harder j ust to get the final 10 degrees).
Remember you can drastically change the "capacity" of a water heater
simply by the temperature setting. If you set it to 140F, then when you
take a shower, you only use a little bit of hot water mixed with more cold
water to get to final temp. This gives you a lot of "hot" water capacity.
If you don't use that much hot water, then set the temperature to 110F,
but now then your shower will be using mostly hot water from the tank and
only using a little bit of cold water. Now your capacity is much less but
you save energy by not throwing a higher temperature away mixing it all
with cold water.
Your wife may vary.
Bob
http://aprs.org/Energy-Choices.html
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