This depends on the heater. All heaters have varying degrees of efficiency in terms of BTU of heat output per watt of energy input.

Lots of electric heat sources like space heaters are just a fat resistor with an electric charge getting converted directly to heat and a fan to blow the heat around. This type is used as a reference efficiency of 1.

A modern heat pump uses electricity to move existing heat from one place to another. These are 3-4 efficiency last I checked based on memory.

All the hash chips in a miner generate heat because they are resisting the flow of electricity, the same as a dumb resistor in a typical space heater. All the electricity goes to the miner hash or to cooling fans. Remember space heaters need fans too though. That means that a miner has the exact same electricity in to BTU out efficiency of 1.

Main building heat is usually heat pump based now due to the higher efficiency.

So the efficiency question is, which type of heater do I have? If it is a resistor and you need less than 60c heated temp then the answer is use a miner and capex payoff from switching is your only issue. Your opex doesn't change.

If you are replacing a heat pump it is harder to calculate. You get roughly 4x the electric bill opex per BTU, so your costofmining = .75*electricityrate is that a profitable cost of electricity to mine with your miner? Don't forget to include your capex in your calculation.