hot water when tap turn right down?

When the tap is marginally open, the flow of water is low enough that the boiler has time to heat the water. When the tap is fully open, the flow of water is too high, thus the boiler hasn't enough time to heat the water. Modern boilers overcome this by limiting the flow of water until a preset temperature, usually set by the user, has been reached.

Regards.

Rich UK
January 2010