A heat pump water heater is a complicated product. Understanding the physics is essential to having the installation not be a complete fiasco.
You already have two heat pumps in your house. The refrigerator and the air conditioner. Both of them do the same thing: Remove heat from one place, and put the heat another place. The refrigerator dumps the heat into your kitchen. The A/C dumps the heat outside.
Coldness is simply less heat.
Heat pumps are pumps. They need a rotating shaft. And if you're not inside the hood of a car, the best place to get that is an electric motor. Those make no exhaust. As you know, your refrigerator does not need an exhaust stack.
But where does the heat come from / go to?
Your refrigerator removes heat from inside the fridge and dumps it into your kitchen. Fortunately it's not very much, or you would need extra air conditioning in the kitchen. Wait. What does a heat pump water heater do?
Well the heat pump water heater takes heat from in the utility room and puts it in the water. That means it makes the utility room colder. But unlike the fridge, it takes a ton of heat to heat water, so we're dealing with a lot of "cold" in the utility room. This is a problem.
This is the pinch-point issue you need to understand.
Now if your ventilation can spread that "cold" around the house and it's air conditioning season, awesome. Free A/C. However the rest of the year you will need supplemental heating to replace the heat shoved into the water. Wait. Doesn't that - sort of - defeat the purpose?
Yes, it does. And this is the weak spot of heat pump water heaters, **and it's why people shouldn't be putting them in simply because they're "trendy" or "good/better/BEST".
On the other hand, if you could have the heat pump water heater's evaporator (cold side) sitting outside, that would be great if the heat pump can function at that temperature. It may not be tuned to do so.
What about just ejecting its cold air stream outside? No, because that has the same problem as portable air conditioners: their intake air is being stolen from the room, and you paid good money to get that to the right temperature and humidity, and it's thus pulling in more unconditioned air from outside, making your HVAC work harder. If the heat pump could also get intake air from outside, like a 2-hose "portable A/C" does, that would be OK if the water heater can function at that temperature.
Dancing around government requirements
But my guess is you're not buying a heat pump unit because it's trendy. You're buying it because it's mandatory for 55 gallon or larger water heaters. I rarely say a government requirement is stupid, because if you understand the issue, they're not. But this one is super stupid. Because this government mandate has "chilling effects" on your house. And each house needs engineering review to see whether those chilling effects can be used productively, or just make matters worse. Getting it wrong costs you money, messes up your house, and actually defeats the government's purpose.
So my strongest advice is to do your best to slip the collar of this government regulation. How does one do that?
Well, let me talk about another maddening pet peeve with water heaters. Ever notice the very long wait to get hot water from the far side of the house? You may have worked around that with a hot water circulation system, which is extremely wasteful and not energy saving at all. Why on earth is the water heater on the far side of the house??? Well, because that's where the chimney is, and the fuel heater has to vent exhaust up a chimney. But electrics don't have stacks, and can go anywhere.
So let's kill 2 birds with 1 stone. Let's get under the 55 gallon limit so we can avoid government crazy... and also move the water heaterS plural as close as possible to the points of use, so we get rid of that looooong delay without needing circulation pumps. Most houses have a cluster of hot faucets in the kitchen/laundry area, and a cluster in the bedroom/bathroom area. So put 2 or 3 total 20-45 gallon Plain Old Electric Water Heaters as needed to cover all loads.
Now if you're having trouble squeezing 3 water heaters onto the electrical Load Calculation, here are some tricks. First, you can get water heaters with 3800W elements instead of 4500W elements. These only require 20A circuit instead of 30A. They take 25% longer to recover. (who cares?) Second, a "30A" 4500W heater can work on 120V/15A, at which point it actually draws 1100 watts. However they will take 4 times as long to recover, which is a pretty big deal.
