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Just lift the top off and you should see something like mine, downstairs as well with the Flomasta type filler, if the filling part is offset like mine then should be OK to install the ballcock type, take a snapshot of the inside of the cistern and show it to the plumpers supplier. I installed a...

Assuming a max of 120L rads+system, the max expansion vol will be 2.5L when fully hot, my 30L F&E cistern, LxBxH, 381mmx254mmx305mm will rise by 10.4mm/liter so 2.5L will cause a rise of 26mm, say just over a inch, not a lot, depends obviously on the cistern dimensions, measure your own.

Under really no circumstances should you require that UPS3, a giant of a pump, on its max setting of CC3, which, in effect is a constant head of 6.4M up to over 19.0LPM, I'm surprised you're not getting even more carry over and air ingress, CC1, at over 4M should be adequate, one green, if not...

And the pump details ?

Gas or oil boiler/ Internal or external circ pump? If external, post make/model/mode/setting or/and a post of any LEDs when running.

Next time you vent air out of it until you get a dribble of water which should be done with the Boiler off, start up the system&boiler and then open the vent, if no water then comes out, shut down the boiler and see if water then re appears, if so, its pulling a slight vacuum at the towel rail.

Maybe replace it (yourself) with something like this, shouldn't get any water hammer (ballcock type) as it gradually reduces the flowrate as the cistern fills.

What type of filling, is it the traditional ball cock type or something like this Flomasta type, the latter fill at full throttle and then shut off pretty abruptly which may cause water hammer.

Its serious from a safety point of view if the PRV is not lifting at 3.5bar, is it??. If it is, then each time the boiler/system is cold the pressure should be a little less.

3.5kW so ~ 1.43LPM at 45C & 0.91LPM from water at its very coldest winter temp of 5C, wonder what happens if you throttle the flow right down to a trickle, does it just run hot/cold then like a electric shower when its TCO (thermal cut out) operates?.

I suppose when one thinks of a instantaneous water heater one means that the water only starts heating when there is a flow through the heater like a electric shower, there will obviouly be a small delay while the heating element itself heats up. The "boiling" HW taps are around 3.0 kW which...

A external blending valve is just a thermostatic mixing valve (TMV) to reduce the maximum water temperature after leaving the boiler so presumably set to 60C, the shower's TMV will then further reduce this to whatever showering temperature required, say 36C to 42C ish. I don't know where...

This was Vaillant's response to someone's query...... "You are correct. The ecoTEC plus 938 is able to give a blended flow rate of 20 L/min for 10 minutes after that the actoSTOR will be depleted so the boiler flow rate will drop to 15 L/min, all Vaillant combi boilers come fitted with a...

I think the 938 has a 15L store so if heated to say 60C will have a effective store of 24L with mains at its present 7C and a 40C showering temp. 38kW will give a flowrate of 16.51LPM, from 7C to 40C. If the max flowrate is limited to say 22LPM (2 showers) because of mains pressure constraints...

Ensure there is enough of a air gap when cold to ensure it doesn't overflow each time it heats up especially if the bottom, off peak element has been used, look into the F&E cistern just after reheating, very little, or practically no make up should be required normally via the ballcock.

OK, so, is it heated only by the electric Immersions or is it also directly heated by a gas/oil fired boiler?

That looks like a Vented thermal store?. Is it only heated by the electric immersion(s)?. Is the DHW just forced through the coils from the mains to heat it?, if so, there may be a pin hole in the coil causing contamination of the store water but you then should see the F&E cistern overflowing...

I've seen this mentioned numerous times but its not my many many years experience of using TRVs, mine last ~ 6/8 years and some do fail closed but I just renew them then. I don't touch mine year after year and at the start of the heating season, they just operate away normally. I'd like to know...

Here's a bit of info from a Worcester Greenstar Compact..........

Is the bottom of the CWS cistern higher or lower than the bottom of the Feed&Expansion cistern and is the CWS cistern water level higher or lower than the water level in the F&E cistern??.