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Where is the vr65 ? your boiler is being controlled by the vaillant ebus and needs an interface between the ebus and 240v 2 port valves.

Your welcome.

The plastic handles tend to strip their splines as they get older, unscrew them and use a small spanner on the brass valve.

Yes

Your boiler should not be running continuously, 24v link taken out?

Not sure exactly what your asking but if you put 230vac on the LS connection in the uh 8 you will have 230vac on the LR which is the boiler enable , this would connect to the boiler switched live, you dont say how the hot water is controlled.

Sounds like the pump is on its way out.

Something isn't right, more than one engineer has quoted a 35kw boiler and your calculations are less than 9kws , your heat load is probably nearer the 9 than the 35 but need more info.

A lot depends on level of insulation, windows (double glazed, single glazed, window area ect) did you try the link provided my muggles ?

Your boiler can do either heating or hot water. it cannot do both, the diverter valve sends primary water round the rads or through a plate heat exchanger, your diverter is stuck half way so needs replacing.

Turn the cold to the new tap off and see if it makes a difference.

The expansion vessel needs recharging, it should have been done on the annual service, with the boiler cold run the hot tap and see if the far left 22mm pipe starts to get hot, if it does the diverter valve needs replacing.

PCB or air pressure switch.

More likely to be the heating 2 port valve stuck, it probably freed its self but will stick again at some point.

Your supplies are unbalanced, all the hot and cold outlets should be taken from the pressure reducing valve.

As in all things mechanical there is a compromise between size, cost, resistance to flow , efficiency ect, if you read the link I posted you will see the average plate has a head loss of 20 to 40 kpa, that is 2 to 4 metres in old money.

Have a read of this...Plate heat exchanger principle - SWEP Heat exchangers (heat-exchangers.uk).

The manufacturers of the plate heat exchangers design for turbulent flow to increase the heat exchange between the two mediums, turbulent flow increases the resistance to flow, so what you see as a short circuit across the plate is more like half the pump output to drive the primary water...

google reynolds number.

The heating will modulate the gas rate as the temp rises , the hot water thermistor sends a value that the hot water is luke warm so the gas rate ramps up, if the pump is running slow it cant push enough water through the plate to clear the heat the main heat exchanger is making, a plate cant be...