I was thinking the same thing. You don't get something for nothing. Assuming the water heater is in the conditioned space, it will just make the central heate

I was thinking the same thing. You don't get something for nothing. Assuming the water heater is in the conditioned space, it will just make the central heater work harder (and consume more fuel) during the winter months. Maybe this is offset during summer months when it can help the AC cool the space. None of this applies if the water heater is not in a conditioned space, but that's not feasible in (common) environments with cold winters.