And heat pumps are more expensive to buy and impossible for a tenant to install themselves in an apartment they rent from, which is why people use small portable and cheap resistive heaters.

And in that case, a miner is just as efficent if not more efficient since it's making the user some money (maybe).

First thing I found on Amazon US

https://www.amazon.com/Honeywell-MM14CHCS-Portable-Condition...

Portable heat pump + air con. I've had something like this before, typically comes with something to slot/seal up into a gap from leaving the window open. Can be moved between rentals.

Efficient in what units? Heat pumps are more efficient in BTUs/kWh, but I doubt you've done the pricing to say they're more efficient in BTUs/dollar, because that's fairly complicated.

Sure, lets say $500 for a heat pump, $50 for a heater. Then what 'yield' are you getting on the $450. Let's say heat pump is 2x as efficient (should be understimate). Let's say that you run 2kw normal heater for 1 hr / day winter only for 3 months. That's 180kwh. @12c that's $21.6. So you save $10 a year. A 2.2% yield tax fee. So admittedly not great.

OTOH a mining rig, well who knows that IS complicated. You might lose money, or spend a lot of time tweaking but break even.