we call them combi boilers. I agree there is a certain amount of waste water, but where I live I'm not on a water meter. But the radiators get hot in seconds instead of waiting for the tank boiler to heat up.
Dafydd.
I just replaced my Tankless with a real water heater. I wasted more water waiting for the water to get hot than savings on my utility bill. Once the water was hot at the heater it had to travel through the pipes and push out all of the clod water. I was wasting 6 to 8 gallons of water per day. Also you have to have them flushed yearly, they may say every two years but that is not enough and it cost about $250 for a plumber to come out and do that. I did not and do not see the savings, actually my gas bill has gone down. It is just HYPE.

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