Shower from combi not really hot enough

We have two of these showers fitted in the house:

SIREN SL EXPOSED MIXER SHOWER

One has been there for 3 years and the other one just installed (by me)

The original one has been great, except that I find it necessary to turn the heat control knob more and more to the hot end to get a good hot shower. The newer one is already like that, and in fact a little worse as you don't really get a HOT shower at all...just hot-ish, if you know what I mean.

The basins in the two bathrooms, which share the same feeds as the showers in either case, are plenty hot enough - in fact, you'd burn yourself if you just used hot water in the basins.

Might there be an issue with the thermostats in the two showers themselves? If so, are they adjustable in any way?

no

it's winter.

combis do that.

let's suppose your combi has the power to raise the temperature of 11 litres of water delivered by the boiler to the hot water taps with a temp rise of 35 degree C. So if the temperature of the incoming water main to your house is 10 degree C then the boiler will raise that temperature by 35C to 45C

let's suppose you shower delivers 11LPM.

Let's suppose that in summer the incoming water supply from the mains was at 15C.
The water at your shower would be (15+35=) 50C which is fairly hot.

lets suppose it is now winter and the incoming water supply from the mains is at 5C.
The water at your shower would be (5+35=) 40C which is not very hot.

You can turn down the mixer tap to deliver less water, and it will get a bit hotter.

Run your shower into a bucket, time it to full, calculate how many litres per minute of hot water it is delivering.

Thanks. I get what you're saying but I fitted the 2nd shower in the early summer (only just got round to using/fixing it!) so not sure the issue is one at the boiler. Plus, the basins are very hot..

after you've bucket-tested the shower, do the same with the basin tap.

I bet you 50p you will find it is delivering less water, and the temperature is therefore higher.

use a thermometer if you have one.

its all fed in 15mm...why would the basin be delivering less? Anyway, winter/summer doesn't seem to apply, given it was the same in the newer shower back in the summer.....

Appreciate the help.

ooh thanks....I'll take a look tomorrow in the light.

Thanks chaps - sorted.