Why is my water temperature changing?

[TarqsTheHero]

So I've got this frustrating situation where the water temperature from my shower and taps sometimes changes temperature - it goes colder/cold - but for only about 10 seconds.

We moved into the house 6 years ago and had a new combi boiler fitted. We also had a shower with a thermostatic value(?) fitted that in theory keeps the temperature the same but lowers the pressure when someone turns the tap on in another room.

This worked fine for the past 5 years and I've rarely had a tap change temp but more recently the issues I've described has started happening.

We are on the mains with no tank and no water meter.

It feels like maybe an issue with the boiler but it works 99% of the time. Has anyone else had similar issues and how did you get it fixed. Thanks

Please fill a bucket at the kitchen cold tap (and the utility and garden taps if you have them). Time it. How many litres per minute do you get?

Now do it again at the shower, set to hot. How many lpm?

Now do it again at the shower set to hot, with the kitchen cold tap running. How many?

How old is the house?

How scaly is the shower head?

Oh, and a photo of your shower mixer, please.

What pigletjohn said. It's likely a water pressure thing. If you're directly on the mains, enough neighbors running baths and watering their gardens could reduce the water pressure enough for your boiler to not heat as well. Either the valve is scaled up or the pressure to your property is more precarious than it used to be. Any newbuilds, nearby?

Amazing. Thanks @PigletJohn & @PickAChew. Over the weekend I'll do this and get back to you.

We've got guests this weekend so I hope it isn't much of an issue in the meantime.

Could be scale in the heat exchanger if the boiler has been heating hard water for a long time.
Try turning down the hot water temperature on the boiler, should be about 60, turn down to 45 and see if it helps with the fluctuations.

Hi there.
Cold kitchen tap on its own - 8.4 litres
Both outside and kitchen tap - 15.5 litres. Outside 9 litres + 6.5 litres from kitchen
Hot shower and kitchen tap - 13 litres. Shower 6.8 litres + 6.2 litres from kitchen

Boiler, shower and head photos attached.

Thanks and sorry for the delay, its seems its virtually impossible to find a time when some person or appliance isn't using water.
@pigletjohn @PickAChew @johnd2

Hi there.
Cold kitchen tap on its own - 8.4 litres
Both outside and kitchen tap - 15.5 litres. Outside 9 litres + 6.5 litres from kitchen
Hot shower and kitchen tap - 13 litres. Shower 6.8 litres + 6.2 litres from kitchen

Boiler, shower and head photos attached.

Thanks and sorry for the delay, its seems its virtually impossible to find a time when some person or appliance isn't using water.
@pigletjohn @PickAChew @johnd2

Shower head

So I managed to fix it - I got a guy in!

Basically, the flow sensor in the boiler was getting stuck. Got that replaced and it's good as new, for now anyway.

By the way I fixed this in the end. The boiler needed a good service