Is it feasible to do this at smaller scales? Would be cool to use my compurers to heat water at home. Put all that useless heat to good use.

Air conditioners could do it too, right? Pump heat into a water reservoir instead of just throwing it away?

I have a pool heater and an air conditioner, and I'm running both at the same time. They're fifty feet apart, but this thought crosses my mind constantly.

Linus Tech Talk (LTT) did a whole series on doing this on the pool at the channel hosts’ house. Extravagant home upgrades are a frequent topic on that YouTube channel… business expense write off yada yada. My general takeaway was, yikes, all that piping and infrastructure would be a nightmare to maintain and will likely just be closed off whenever an issue comes up (or he sells). I’m no expert, but I am a home owner, and have come to form a deep appreciation for maintaining simplicity when it comes to the operation of your house.

> Is it feasible to do this at smaller scales?

You need a lot of heat to do anything useful. I would need to run something like 14 kW of servers to heat my home through winter - that's a couple of hundred thousand in hardware at current prices.

>Air conditioners could do it too, right?

Some heat pumps do this. E.g. Panasonic Aquarea EcoFleX. When cooling the house, the domestic hot water tank is used to dump heat into (up to a certain temperature).

I have connected the radiator of my homeserver liquid cooling setup to the heat exchanger of my hot water heat pump. Not sure how efficient it is, but I get a measurable drop in CPU temperatures while the heat pump runs.

Some people use cryptocurrency miners to heat their homes. It's certainly better than dumb resistive heating, but depending on various conditions it can cost more than installing a heat pump.

heata.co do precisely this with hot water tanks

(I work there)