The biggest heat pumps

But especially with old houses, with any "not insignificant change" to your electrical setup, you have to bring everything up to modern standards.

Also, and connected with that, metering is weird in Germany. If your consumption rises above a certain amount (I think 10MWh for a single-family home) you are required to get a "smart meter", meaning a digital meter which includes the possibility (just the possibility, not actually the real thing) of online reporting and minute-by-minute pricing. In the rest of the world, that would just be swapping the meter or slapping some Wifi-to-IR-interface on it, but not so in Germany. You need to install a new metering cabinet that provides space for at least one digital meter (but better provide more than one...), one remote control receiver (for old-style night/day tariff switches, obsoleted by smart meters but still required nonetheless) and a smart meter gateway. That new metering cabinet needs to conform to standards set by your local electricity supplier (which can be as small as a city), so there is no nation-wide standards, more like 50 of them. And the metering cabinet is huge, not someting like the 30x40cm thing you see on sidewalks in spain or something, no. More like 200x140cm in the smallest(!) version. So those are really expensive just because the market is tiny and the requirements are completely crazy: Even though most smart meter gateways are wireless nowadays (UMTS or GSM) and usually such a gateway won't be installed anyways (because just the possibility of installing one is required), you also need to provide for cabled internet uplink to the metering cabinet. That uplink is a Cat6 cable to the gateway, but it crosses through the electrical uplink part of the cabinet. So the insulation of that cable has to be certified for at least 10kV insulation voltage, at least 80A current on the shielding and more stuff like that. So e.g. just that one stupid half-meter Cat6 cable will set you back 50Eur. Installation isn't any less crazy. You definitely cannot do anything yourself. Even your licensed electrician can only do the menial preparations. When your licensed electrician is done with the prep, you request an appointment with the local supplier's meter installer, who, after a typical wait time of 2 months, will install and seal the meter in the presence of your licensed electrician (who is there to receive complaints about incorrect prep, followed by another 2 months wait time for another appointment...).

That's why it's 4kEur...

And if you want your power to be a few cents cheaper (28ct/kWh instead of the usual 35ct/kWh), you need an extra meter for your heat pump. Have solar? Another extra meter. Want to charge your electrical car? You guessed right. And you cannot bring your own. Each of those "smart" meters has to be rented from your local supplier at around 20 to 120Eur/year.

There have been a few cold snaps here where the weather has been down to -2 some days, but it’s been fine. I had a couple of minor installation issues (eg 3 way valve set incorrectly) but once those were fixed my house hasn’t dropped below 19C.

And a dirty tank of water in the attic to act as a "in-house water tower" because only one tap may be connected directly to the mains. Really archaic.

and that house still has the sewage stacks on the outside of the house, as do almost all homes in Bath and environs.

You don't see them on new builds, I think, probably because the pipe going from inside to outside would reduce insulation effectiveness.

But not everybody has those opportunities. Not every home has an existing conditioned space within which to put a water heater. Not every person is equipped (mentally or physically) to engineer and use tempering loops and/or water-cooling compressor motors.

As a practical matter: In a warm-climate home that already has a water heater in the garage (which is very common in the American south, from my limited direct observation), replacing a traditional water heater with one that uses a heat pump can make a lot of sense.

This replacement is something that any person and a friend with minimal plumbing and electrical experience can accomplish on their own in one afternoon, without incurring the expense and inconvenience of relocating their water heater somewhere else. There will be no drywall dust, and no paint.

It's a natural fit.

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And don't take any of this the wrong way. You've got some great ideas there.

But not all environments are the same. During the warmer months in my own city, I've measured incoming water at 76F/24C -- warmer than the house, and also warmer than the basement where the plumbing lives. A tempering loop may make sense for you in your environment, but it would be the opposite of useful in my environment: "Oh neat! A thing that makes my home harder to cool in the summer!" (Unusual? Perhaps. But it's my reality anyway. I've never run out of hot water in this city during the summer. Not even close. But things do change in the winter -- maybe I'll measure the input temperature again when I get home tonight.)

It's fun to think about niche concepts that don't have broad-scale adoption. And sometimes, it makes sense to set forth and make them a reality.

But it's always important to remember that there's often very real reasons for them to remain niche concepts that aren't broadly utilized.