Hmm, I don't think heating is a strong enough use case. What about as cloth dryer? Those things uses a lot of energy to produce heat and you need them even in hot summer. Especially in dense city apartments.
A water heater may be better - warming incoming water to save hot water cylinder bills or heat a pool if there is enough heat. Something that needs short bursts of heat would be harder to implement. I know someone who uses a waste water to raise the incoming water temperature 1-2 degrees C and it saves a measurable amount of money on power bills.
