Combi boiler and shower not triggering hot water

It's certainly an option. But once you get the boiler going (e.g. turn on the tap as well) it works... for now. 

At first I thought it was definitely going to be a problem with the boiler. But turn on other hot taps on it turns on. But not with the shower (with head connected). I'm off the next few days so will have another go at cleaning the head. A replacement isn't going to cost much but if it's not the fix then a waste of money.

I'm now wondering if water pressure is a little lower than it usually is.... I could measure/time something filling but I have no reference point.

We aren't in a hard water area. But maybe it's slightly blocked. Will try with pins like suggested. I'm off for a few days now.

It is indeed a thermostatic mixer - you correctly guess, but then it's off a combi.

I'll have to look up how to take it apart. I'll no doubt find it online as it's a Grohe one.

Thanks ON. Just been busy and living with it so far. I could just get a plumber in but I'd like to narrow it down.

All other hot taps or mixer taps do trigger the water heating but not the shower mixer. But once the boiler has started to heat water then turning on the shower tap keeps it going. That then makes me wonder if it's the shower head, mixer tap filter or the shower hose. I've checked the hose for blockages.

The thing is I think the water pressure from the shower was lower for a few weeks but I might have been imagining it.

According to the installation page for the boiler hot water flow was measured at 12 litres per minute and I'm sure it is that from a single hot water tap on full flow. Not measured it for more than 20 seconds.

So it still might be the shower head, hose or mixer tap itself. Or maybe the diaphragm in the boiler not moving enough when the shower is used.

I was off work last week so perhaps I'll find more time to investigate this now I'm back in work!

Thanks again. Something to try tomorrow. But I know I need to measure water flow too. I'm not sure it's stuck because you can change the temperature and it does keep going once it's on.

I know I could just go and buy a new shower hose and head and it won't cost much... but pointless if it's something else. I suppose removing the filter from the Grohe mixer/thermostat tap is not beyond me but if I did mess up we're without water until I can get the plumber in.

I realise it's not sensing water requirement and that's independent of which tap is in use. But what if the shower was getting a lower water pressure because the filter in the combo-boiler is blocked. Or what if the sensor is not as sensitive as it once was and the shower just has a slightly lower pressure.