Answer:
Air
Explanation
You turn on your furnace in the winter to heat up you house and it wont be cold So the other answer is WRONG a water heater is a boiler that heats up your water
The answer is water. The heat from the furnace heats up the water and hence the hot water move along the radiant coils and distributes/move this heat to the target site. The steam can be used to warm up a house or move a steam engine, etcetera
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