Green / Eco-Friendly

How many KWh do you use in a day?

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  • Apr 27th, 2021 2:23 pm
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Deal Guru
Jul 7, 2017
10217 posts
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SW corner of the cou…
mf1982 wrote: I have both a heat pump dryer and heat pump water heater. When the dryer is running, the "waste" heat is absorbed by the water heater.

In terms of the math on the water heater energy wise:

My Rheem unit has a COP of 3.8 (1kWh electricity = 3.8 kWh of heat moved)
My home is heated by a ground source heat pump at a COP of 4.0.

My effective water heating COP during the heating season is still about 2 because I need 1 kWh to bring 4 kWh of heat into the house, then another kWh to put those 4 kWh into the tank, so for every 4 kWh of heat into the tank I need to use 2 kWh of electricity.

At my rental property where I have a similar set up but the home heating is provided by cold climate air source heat pump
DHW COP 3.8
Heating COP 3.8 - 2.0 (drops below 2 at temperatures below -18°C I believe, without looking it up again)

So in that case I need to use 2 kWh of electricity to bring 4 kWh of heat into the house in the cold of winter, then 1 kWh of electricity to put those 4 kWh into the hot water, so the COP is only 1.33. When temperatures are 8°C or above it's 1 kWh ot bring the heat in, 1 kWh to put the heat in the tank, so COP of about 2 again

From mid-May until mid-September when the home is not being heated the COP of the water heater is the full 3.8. You could argue that during the cooling season (mid-June to mid-September) the COP is even greater as the cool air expelled from the unit, and the dehumidification from the unit are "free". I'd be paying for those functions separately anyways, but in this case the waste heat energy is being used for a function.
Good detailed analysis. The ground-source HP makes it work better (even better if the dryer and water heater are ground source too) but I can't imagine the HP dryer or water heater is great for a carbon-heated home (or my lower-COP(?) HP-heated one).

It would seem the HP dryer and water heater would be much more effective if you live in the southern/south-eastern U.S. as it serves to cool and/or dehumidify which is something pretty much required year-round.
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Sr. Member
Jun 24, 2013
550 posts
751 upvotes
The HP dryer generates its own heat, but then recycles the warm air. Some of it does get vented into the room. It's actually more beneficial in an area that has a greater heating load than cooling load.

The HP water heater would not be beneficial in a carbon-heated home or with a low COP heat pump unless you're all electric or it's a first step to electric, run it in all electric during the heating season then turn the heat pump for non-heating season.
Deal Guru
Jul 7, 2017
10217 posts
5530 upvotes
SW corner of the cou…
mf1982 wrote: The HP dryer generates its own heat, but then recycles the warm air. Some of it does get vented into the room. It's actually more beneficial in an area that has a greater heating load than cooling load.
I've never seen one but does the HP dryer ventless and condenses water vapour (so are the ventless condensing dryers the same as HP dryers)? In that case, it make more sense than dumping hot air outside.
The HP water heater would not be beneficial in a carbon-heated home or with a low COP heat pump unless you're all electric or it's a first step to electric, run it in all electric during the heating season then turn the heat pump for non-heating season.
Have never seen one either so a HP water heater can heat with heating element as well as through the evaporative/condensing cycle?
I smile when I see container ships sailing past my house laden with stuff made in China
Sr. Member
Jun 24, 2013
550 posts
751 upvotes
thriftshopper wrote: I've never seen one but does the HP dryer ventless and condenses water vapour (so are the ventless condensing dryers the same as HP dryers)? In that case, it make more sense than dumping hot air outside.
Ventless condensing and HP are the same thing - I think. It generates some heat, condenses water (it has an actual drain pipe), and recycles that heat through. It does heat up the room it's in because any waste heat is just kept in. From an energy efficiency perspective where heating load > cooling load it does make more sense than venting the heat out of the house, not to mention you can also close up the opening that is always dropping cold air into the house.

Have never seen one either so a HP water heater can heat with heating element as well as through the evaporative/condensing cycle?
Some models have both options, some have only the heat pump option. Mine has both. The electric elements are useful when there is high demand and you need more hot water quickly. There is also an Electric Only option that turns off the heat pump.

I usually keep mine on Heat Pump Only mode, but will occasionally turn on the High Demand Mode when we need more hot water than our 80 gallon tank can provide.
Deal Addict
Jul 13, 2007
1263 posts
803 upvotes
Toronto
mf1982 wrote: From an energy efficiency perspective where heating load > cooling load it does make more sense than venting the heat out of the house, not to mention you can also close up the opening that is always dropping cold air into the house.
I think another factor that's forgotten about vents is that you're not just pumping out exhaust air, but whatever air volume you pump out needs to get pulled back in from somewhere. Either as more pull through whatever cracks are in your home, every door opening, or an HRV if you have one, but they're not 100% efficient either.
Deal Guru
Jul 7, 2017
10217 posts
5530 upvotes
SW corner of the cou…
mf1982 wrote: Ventless condensing and HP are the same thing - I think.
Had to look it up due to the intermittently-malfunctioning Bosch condensing dryer thread in the H&G forum. Seems there are condenser dryers and HP dryers.

https://www.reviewed.com/laundry/featur ... ess-dryers

Condenser dryers don't have the refrigerant circuit that an AC, dehumidifi er or HP has.
I smile when I see container ships sailing past my house laden with stuff made in China

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