Combi boiler question

Doesn't sound like it! There should be a knob on the boiler to set the water output temperature. Turn it down a little

Sorry Bob, I think you've misunderstood. I was complaining that the hot water wasn't as hot as the CH water.

There *is* a knob on the front of ther boiler for hot water, and it's turned to maximum. The CH knob is at "summer heating" right now.

Ah, the last bit would have helped! As it's a combi, the hot water temperature will depend on flow rate - you may have to shut off the inlet valve for the cold water feed a bit to drop the flow. A simple diagnostic check would be to run just the shower (which restricts flow a bit) just on hot - does that get any hotter than water out of a running tap? If it does it's flow related.

(smacks head) ding-a-ling-a-ling ... makes sense ... will investigate.

Simple reason is that the DHW is a once-only heating and the CH is a recirculation reheat situation. One only has one go at the burner and the other continuously passes through it.

For hot water, the boiler is heating cold water from the mains which will be at a near constant temperature of under 10C.

The heating circuit water arrives back at the boiler to be re-heated at probably 60C plus...

The water temp knob is a MAXIMUM temperature knob. In fact half way up should produce very hot water if you run the water at the right rate.

Fully up will allow the boiler to produce a trickle of extremely hot HW. Something akin to device found often seen in Cafes made by "Still".

If you are not getting a reasonable flow of HW then the cause may be scaling of the domestic heat exchanger.

HTH