Mixer tap temperature fluctuates

[Lower]

You mentioned you have basic flow control on the incoming supplies. Are you sure they're not pressure reducing valves? They may be working against each other, hence the regular pressure fluctuation.

 

[Donsboig]

You mentioned you have basic flow control on the incoming supplies. Are you sure they're not pressure reducing valves? They may be working against each other, hence the regular pressure fluctuation.

I'm just learning about all of this so not entirely sure! But from some googling I thought they were probably basic isolation valves that were set by the previous owners to a part-closed position. If fully open the shower pressure is a bit forceful...

Photo shows one of the isolation (?) valves at the bottom, with the pipes heading up to the underside of the tap at the top

 

[Florine]

Those thing never work particularly well as they are so sensitive to water pressure fluctuations, it may be something as simple as your neighbours flushing their loos at the same time causing a small pressure drop, which in turn will affect your shower.
If it were me, I'd fit a thermostatic mixing valve under the bath.

I'm not sure whether you're in a house or a flat? We had a shared mains supply with the downstairs neighbours, and if they flushed the loo our (electric) shower would run cold immediately.

 

[Donsboig]

I'm not sure whether you're in a house or a flat? We had a shared mains supply with the downstairs neighbours, and if they flushed the loo our (electric) shower would run cold immediately.

In a flat (Victorian conversion), but the temperature fluctuations are constant and very quick (going hotter-colder-hotter over 5 seconds ish) so I don't think it's due to flushing loos or other taps being used

 

[Johntheo5]

Its a strange one allright, one of my showers has gravity hot HW and mains fed cold from two > 50 year old taps, I just open the gravity (0.25bar) fed hot fully and then just crack open the mains (2.5bar) fed cold to give the required showering temperature, seems to work fine even though the combined florate is only ~ 5LPM.
It might be worth setting the HW flow through the shower head with the hot tap (only) fully open to say 8LPM then do the same with the cold only even though I don't think it will solve the problem, you use those "penny" slotted head valves underneath the bath to set the flowrates.