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heat pump water heaters?

DoRight's picture

Anyone have thoughts about heatpump water heaters which take teh heat out of the air to heat the water?

A heating system using a heat pump has the unit outside the home, thus removing heat from outside the home and bring it inside.  Makes sense right?  So why would you wish to heat water by taking heat out of the air INSIDE your home, thus cooling your home and having your heting system have to expend energy to replace the heat.?  That makes no sense to me.  Of course in the summer, it would be ok and might save you energy (money?) over a traditional hot water heater.  But six months out the year, it would be a wash at best.  If this is not that case why would you not put your heating heat pump unit inside the house.  I think the answer is obvious.



I don't know a lot about (post #207150, reply #1 of 6)

I don't know a lot about them, but what I've read suggests that they are only feasible in months when you are cooling the house.

So if for many months of the year you are faced with high energy costs to cool a house, you might as well pump that heat into something that you want to be hot, namely domestic hot water, rather than simply dumping the heat outdoors.

There are some schemes that (post #207150, reply #2 of 6)

There are some schemes that effectively switch the air exchange from inside to outside by the season.  But the units are primarily intended for more industrial situations where continuous cooling of some area is needed.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

That is what I thought (post #207150, reply #3 of 6)

That is what I thought.  I just wanted to see if I was missing anything.

As for the other poster saying they are primarily for commercial situations, that is ot what the Big Box stores want you to know.  They sell them and I am sure poor people with energy consumption hesteria will be all over them.  Sad.


HPWH are usually more efficient than the alternative... (post #207150, reply #4 of 6)

There are several scenarios where a heat pump water heater (HPWH) inside the shell of the house would be just great during the heating season. 

First, if the HPWH is inside the house but in an unconditioned space (e.g. basement, utility room that is vented).  In this situation, the cooling in the proximity of the HPWH will have no appreciable impact on the house heating load.  This would be the case in 11 of 13 houses/apartments I have lived in the past 35 years.  I'm not sure if this is representative of the country but is certainly not uncommon.

Second, it depends on the rest of your HVAC equipment.  Below are a couple simple examples which all produce 5 units of heat and leave the house heat balance unaffected:

  • HPWH (COP = 2) and whole house heat pump (COP = 2.5).  HPWH pumps 5 units of heat into potable water system while expending 2.5 units of energy and creating 5 units of cold.  Whole house heat pump pumps 5 units of cold outside while expending two units of energy.  Total energy use is 4.5 units of energy.
  • HPWH (COP = 2) and high efficiency furnace (eff = 0.98).  HPWH pumps 5 units of heat into potable water system while expending 2.5 units of energy and creating 5 units of cold. Furnace offsets 5 units of cold by burning 5.1 units of energy.  Total energy use is 7.6 units of energy.
  • HPWH (COP = 2) and low efficiency furnace (eff = 0.8).    HPWH pumps 5 units of heat into potable water system while expending 2.5 units of energy and creating 5 units of cold. Furnace offsets 5 units of cold by burning 6.25 units of energy.  Total energy use if 8.75 units of energy.
  • electric hot water heater (eff = 0.9).  Total energy use is 5.6 units of energy (5 / 0.9)
  • gas hot water heater (eff = 0.6) Total energy use is 8.3 units of energy  (5 / 0.6)

So HPWH compares well on an efficiency basis at it's worst comparable point of the year.  The cost efficiency of these systems will all vary depending on what part of the country you are in but by any measure that I can think of, heat pumps of any type should always be part of the HVAC consideration.

We've had a heatpump water (post #207150, reply #5 of 6)

We've had a heatpump water heater for most of the 16 years we've been homeowners here in Japan. When we built the house, we had the Trio System by Daikin. The heat pump was connected to a large water heater and also to three air conditioners inside the house. The theory was that in summer, heat sucked out of the inside air by the air conditioners would be rechanneled to the water heater, thus heating the water. In winter, heat pumps aren't very effective below freezing, though, so independent heating coils would heat the water at night, when electricity prices were lowest. We also had a 300Liter solar water heater on the roof, but that actually spoiled the purpose of the Trio system. Namely, during the hot months of summer, we were able to rely about 98% on the solar water heater, so it wasn't necessary to have the electric Trio system working virtually at all. Or, at least, we needed the trio system much less for heating water, so we used it mostly on the hottest days just for cooling the indoors; in essense, during the summer it was more inexpensive to rely on the solar water heater and just split the difference with the electric air conditioners.



". . . and only the stump, or fishy part of him remained."

Heat pump (post #207150, reply #6 of 6)



I found this discussion very interesting and came to know that an air source heat pump extracts heat from the outside air just like a refrigerator extracts heat from its inside. It can generate heat from the air even when the temperature is as low as -15° C. Heat pumps have some impact on the environment as they need electricity to run, but the heat they extract from the ground, air, or water is constantly being renewed naturally.