Connecticut Residents, $400 Rebate on a Hot Water Heat Pump

jpete wrote:I think you are leaving out the heat of compression and heat of work. The electric motor is converting electricity to rotational force and heat. The compression of the refrigerant raises the temperature of it.

So while some heat is extracted from the room, it's a fraction of the total amount.

If that were truly the case, then it would be much more expensive than resistance heating of the water. Converting mechanical work to heat is less efficient. Since it is not more expensive, then the major portion of the heat is coming from the room.

franco b wrote: If that were truly the case, then it would be much more expensive than resistance heating of the water. Converting mechanical work to heat is less efficient. Since it is not more expensive, then the major portion of the heat is coming from the room.

It's more the heat of compression from the refrigeration cycle. But the few BTU you take from the room at the evaporator, is being put back by the electric motor so it's something of a wash.

From what I know of freon vapor cycle units, I agree with Franco.

When you evaporate the freon it absorbs heat (the heat in the room, house, etc). The compressor does pump it, and of course when you compress a gas it gets hotter. But the heat from the house, etc is released into the atmosphere (in this case the DHWHT) via the condenser, where it is changes it's state back to a liquid.

The major portion of heat comes from the ambient air passing through the evaporator (low side).

If that were truly the case, then it would be much more expensive than resistance heating of the water. Converting mechanical work to heat is less efficient. Since it is not more expensive, then the major portion of the heat is coming from the room.

It's more the heat of compression from the refrigeration cycle. But the few BTU you take from the room at the evaporator, is being put back by the electric motor so it's something of a wash.[/quote]
If that were true then it only would be slightly less efficient if you got rid of the evaporator. Transforming mechanical work to heat using electricity is far less efficient than resistance heating. If compressing a gas were all that was required , why not just use air?

Because air isn't nearly as efficient. But if you'd like to test it, go down in you garage and turn your air compressor on. After it makes pressure, grab the copper pipe from the outlet side of the compressor. That sizzling you hear is your palm sticking to the hot copper.
And if you can, notice how much cooler your garage is.

jpete wrote:Because air isn't nearly as efficient. But if you'd like to test it, go down in you garage and turn your air compressor on. After it makes pressure, grab the copper pipe from the outlet side of the compressor. That sizzling you hear is your palm sticking to the hot copper.
And if you can, notice how much cooler your garage is.

You are assuming that the heat is coming from the air surrounding the compressor and its air intake, cooling the garage. In reality it is coming from the work done in compressing the gas being transformed into heat, the heat is generated internally, not absorbed from outside the machine.

If what you say were true there would be no need for the refrigeration cycle in heat pumps or air conditioners.

jpete wrote:
It's more the heat of compression from the refrigeration cycle.

You cannot magically create heat, the heat in the refrigerant is extracted from the room air. When you compress anything you are compressing the heat as well which drives the temperature up. Think of a simple air compressor. If it's empty and you fill it then the tank is going to be slightly warm, if you immediately blow some air from it on your skin it's going to be room temperature. If you let it sit for a few hours to let the compressed air reach room temperature when you blow it on your skin it's going to feel cold.

Physics 101.

jpete wrote:Because air isn't nearly as efficient. But if you'd like to test it, go down in you garage and turn your air compressor on. After it makes pressure, grab the copper pipe from the outlet side of the compressor. That sizzling you hear is your palm sticking to the hot copper.
And if you can, notice how much cooler your garage is.

True the compressor and it's piping gets hot, especially the tank feed line from the high side of a two stage commercial duty compressor.

But, your last line goes against one of the rules of physics - compressing a gas causes it to heat up. An air compressor will not cool the space (your garage) it's operating in. Just the opposite, it will heat it. You may not have a big enough air compressor for the space, or run it long enough to notice how much heat the compressor dumps there. The motor makes and sheds heat as part of the work of driving the compressor. There's a lot of friction from the drive v-belts and pulleys. And then there's even more heat made by the compressor head. Especially the high side of a two stage.

As the motor, belts/pulleys, and the compressor head shed heat that heat is added to the surrounding area. Nothing about an air compressor is lowering the room temp since it's starting at room temp and only getting hotter. It may seem cooler if your down wind of the compressor head fan, but that's just your skin's wind-chill affect. A thermometer held in the same breeze would show an increase over garage temp.

If you were to have the compressor in a smaller space, say like I have my shop compressor in a cabinet in a corner of my spray booth, you'd easily feel the temp rise in there. In fact, shortly after I built it, I had to install ducts and a fan to circulate cooler shop air through the compressor cabinet because it would get so hot in there during long sessions of sand blasting a car body, or chassis, that it used to cook the rubber off the face of the air outlet check valve. And, the shop room that duct air exhausts into gets noticeably warmer than the rest of the shop building.

Paul

jpete wrote:... the few BTU you take from the room at the evaporator, is being put back by the electric motor so it's something of a wash.

It is SOMETHING of a wash, yes, but only a partial wash. If the heat pump is electrically more than 100% efficient – and why have one, otherwise? – then the room is cooled by the evaporator more than it is heated by the waste heat of the compressor machinery. The difference between the water heater and the air compressor is that, with the air compressor the air tank (or fins on the compressor/piping/tank) IS the evaporator. So the air compressor will add net heat to the room, and the water heater will remove net heat from the room and transfer it into the water. (Of course if you don’t USE the hot water, the insulated water tank gradually cools and the overall system becomes just like the air compressor).

rberq wrote: It is SOMETHING of a wash, yes, but only a partial wash. If the heat pump is electrically more than 100% efficient – and why have one, otherwise? – then the room is cooled by the evaporator more than it is heated by the waste heat of the compressor machinery. The difference between the water heater and the air compressor is that, with the air compressor the air tank (or fins on the compressor/piping/tank) IS the evaporator. So the air compressor will add net heat to the room, and the water heater will remove net heat from the room and transfer it into the water. (Of course if you don’t USE the hot water, the insulated water tank gradually cools and the overall system becomes just like the air compressor).

But that's the whole thing. The idea that a heat pump water heater is "stealing" BTU and somehow not warming your house as much is pointless because the heat pump motor itself is "adding" heat to the room.

While I agree that it would probably be "better" to just run a hot water coil off the stove, but even that "steals" heat from the system. But doing DHW off the stove isn't an option for everyone.

And the fact that actual people, actually using these devices report an ACTUAL reduction in their electric bill and as yet, none have complained their heating bill increased, I'd say the amount of "stolen" BTU's are negligible and therefore not even worth considering.

Sunny Boy wrote:But, your last line goes against one of the rules of physics - compressing a gas causes it to heat up. An air compressor will not cool the space (your garage) it's operating in. Just the opposite, it will heat it. You may not have a big enough air compressor for the space, or run it long enough to notice how much heat the compressor dumps there. The motor makes and sheds heat as part of the work of driving the compressor. There's a lot of friction from the drive v-belts and pulleys. And then there's even more heat made by the compressor head. Especially the high side of a two stage.

Paul

I know that. And you know that. But some people claim that in order for one medium to heat up, it must "steal" heat from the surrounding environment which would necessarily cool it down.

That doesn't happen in practice so something else must be going on. And that something else is heat of compression and heat of work.

jpete wrote:But that's the whole thing. The idea that a heat pump water heater is "stealing" BTU and somehow not warming your house as much is pointless because the heat pump motor itself is "adding" heat to the room.
While I agree that it would probably be "better" to just run a hot water coil off the stove, but even that "steals" heat from the system.

Ah, yes, I see what you are saying: one way or another heat from the house is being transferred into the water, and except for a thermosiphon coil all the methods probably use some electricity. The heat pump water heater uses more electricity than a circulator pump from a boiler coil, and definitely more than a thermosiphon, but its electric waste heat remains in the house so it is not lost and partially mitigates the cost of the extra electricity. In THEORY the cost of any system using electricity will be more than the thermosiphon method, but theory doesn't always work. There are so many ways heat may be lost before getting to the water that any one of the methods may be best in a given situation.