It's not that simple. If you have a gas boiler, you're most likely exchanging heat using radiators which are _designed_ for certain amount of heat exchange. Wit

It's not that simple. If you have a gas boiler, you're most likely exchanging heat using radiators which are _designed_ for certain amount of heat exchange. With high water temperature, you can get away with smaller radiator and deliver the same amount of heat. The story gets worse when you put heat pump to the same system - now you have to run it to its limits to have just enough heat in the water to make the radiators release enough heat to barely heat up the room. Of course, it's only that simple to replace one heat source with another, why wouldn't anyone do it already?