grohe shower

hi, i have got a grohe shower which is run off my combi-boiler, the problem being when i am in the shower the boiler keeps knocking on & off. i have tried reducing the pressure on the cold feed going into the boiler & also reducing it, but still no joy, any ideas? cheers keith....

I am not a heating engineer.

Sounds like a lack of pressure diffrence producing a low flow rate so that the stat in the DHW system is cutting off the DHW when it gets too hot.???
Possibly wrong though
Try increasing the input pressure if you can and turn the hot tap fully on then balance the temp at the shower head after the hot temp has stabilized. My shower runs from my combi without any problem and its only a mixer type.

Need lots more info. New/old boiler or problem? What boiler? What flow of hot at shower if you put to hottest temp? Measure litres per minute. What flow at showeringtemp? Is the mixer thermostatic?

Try having a shower with a bathroom hot basin tap turned on a bit- not I'm not mad!

I HAVE THE SAME PROBLEM SHOWER GOES HOT AND AND SLIGHTLY COLDER-HAVE YOU SOLVED THE PROBLEM I WAS TOLD THE BOILER WAS AT FAULT "HUNTING"?

try it with the hot water temp on the boiler lower .if the mixer is thermostatic it could be reducing the flow of hot water to much for boiler to stay on. could also try a higher flowrate hose & head,

really baffles me this one tried varying the temp knob at different levels but it doesn't make much difference to the actual temp coming from say the sink tap. the boiler is 6 yrs old now and is a potterton puma 810e it has recently had the temp sensors changed by bg who said that as the problem with the heat being too hot. we have recently had the bathroom refitted and a grohe 3000 fitted. pressures seem fine and when the shower is turned on the demand for hot water is their from the shower and stays for a few seconds 10-15 perhaps and then the flame goes out when the water gets slightly cold in the shower the boiler restrikes from demand from the shower but in the time it takes to do this the shower has gone a few degrees cold in waiting for the new hot water to reach the shower head.if you have a long shower the cycle/demand for hot water then seems to go out of sync and the shower goes cold for 20 seconds. i really do think it is the boiler - has anyone else had any problems like this or can help?