Vitodens 222-F combi storage boiler and loft shower pressure.

[Max Hill]

I have a viessman vitodens 222F combi storage boiler on the ground floor. It has 100l capacity inbuilt cylinder which is delivered on mains pressure.

I bought it hoping I could have two showers across floors running with hot water even if it’s a short while, while avoiding an unvented cylinder. The marketing said this should have worked. I’m having problems on the loft shower though when there is any demand on the lower floors.

So this is my setup.

Combo storage Boiler is on the ground floor.

I have a 1st floor standalone walk in shower and separate bath each with thermostatic valve

I have a 2nd floor shower bath combo with thermostatic valve

I would have hoped I could have a warm shower on the second floor and first floor at the same time.

However when I use the shower in 1st floor the second floor shower pressure drops completely to a trickle almost.

Some observations that might be relevant :

I can run the bath’s shower head and walk in shower with hot water on the first floor at the same time with really reasonable pressure. I conclude from this that it’s not a flow rate. Correct?

I can also run cold showers on both floors with decent pressure , so it’s not mains pressure issue. Correct?

So if it’s not hot water flow rate and not mains pressure, what could it be?
Could there be some balancing issue ?
Are the thermostatic valves in the loft somehow involved in this?
Anyone near Bromley that would want to help me out diagnosing this in person perhaps

Thank you