> I still have the electric heaters installed in my living room - they haven't been used a single time since 2010. As they're permanent installations I haven't got them removed, or I would have. That sounds great. And sorry if I go off topic but I'm curious how you "distribute" the heat throughout the house. Air vents? Underfloor heating? If you kept the old radiators I assume you picked an option that caused the least disturbance in the home yet is still efficient enough that you didn't need extra heating in a decade.

I haven't needed extra heating in a decade in the area where the heat pump is - I should have been more accurate. In other posts I added that I do have (ordinary) electric heating elsewhere, the main place these days is my home office (without that there wouldn't be much extra heating). Most of the other rooms are unheated or nearly so (100W-130W w/thermostat is way more than enough), they stay warm enough due to good insulation and some air flow in the house. Bedrooms don't need to be particularly warm, unheated is ok, and the basement (hall, storage rooms, etc) is fine at around 11-13C which it normally stays at even at the coldest time of winter (and fortunately not much more in the summer either - that part of the house is partly below ground and is an excellent stable cool environment).

Heat pumps come in two main flavors. They can be like a forced air system, where they heat/cool air in an air handler and blow warm/cool air all over the house. Or they can be ductless heat pumps, where there is a head unit inside the house that uses coolant lines to move heat back and forth between the exterior unit. The head unit blows warm/cool air, but they can only blow so far. You can add multiple head units and get extra zones, but that can get more expensive

As an aside, there are some things that you can do with having the air vents in the rooms and a forced air system with some control so that instead of "control baffles in the basement that lead to ductwork for one zone" it is instead "control baffles in rooms".

One such example: https://flair.co/pages/central-systems-smart-vents

You may be covering it with your second definition, but I'm not sure - there's also the option available in Ireland (and UK and elsewhere) of a wet heat pump. By that, I mean an air-source (or ground heat extracting) heat pump that connects to the existing wet radiator infrastructure of the house. Replaces a gas boiler, which is normal here.

I had to think a bit about the "wet radiator" and then realized we're talking about potentially older houses that circulate hot water to metal radiators in the house.

In that case, yea, it would be a {something} to water heat pump to heat up the water, and then circulate that water through the house.

It would be curious if there were installations that did the other way too. In theory, you could also circulate cold water in the summer. The problem would have been "how do you make the water cold?" and in many older cases, that would be impractical. Though... that could lead to condensation on the radiators in the summer which could have downstream effects (condensation drips water on the floor).

Not even older, it's a standard heating technique in the UK and Ireland, even in modern homes built last year. You can use an air-to-water heat pump to retrofit a condensing gas boiler. Some even come integrated in to the hot water storage tank - https://rointe.ie/electric-water-heaters/heat-pumps/

Theoretically you could indeed drive cool water to the radiators to cool the house, but you don't get the convection effect that a radiator produces when hot, nor thermal radiation. So you need to rely on transfer of heat from the air to the metal of the radiator, potentially with no airflow. Easier to open the windows. In the last 12 years, I've never wished I had a system to chill the air in my dwelling; Ireland is generally on the cool side (though this may change).

It's a curious thought of "could you close a room and point a fan at it and noticeably cool off a room?"

This would be kind of running a mini-split AC to the rooms using water as the refrigerant instead of what hvac normally runs.

It's not a "this is the best choice" or even "this is a better choice than opening the windows" but rather a "if you did this, would it work? kind of?"

> In theory, you could also circulate cold water in the summer

Yes, but it isn't very common at all for one simple reason: condensation. You have to keep the temperature of the coolant above the dew point, which requires knowledge of ambient temperature and humidity. That means your coolant isn't actually that cold, so it can absorb less heat before needing to be re-chilled. It's overall pretty inefficient.