Water pressure/flow problem with Vaillant Unvented

[Modokon]

Hi
1st post, looking for a little advice from the professionals
...this rambles a bit, sorry.

30 year old gravity fed system died last month. Of the various options (fully pumped & keep old tank, big combi, or unvented) I went for what was advised as the "Rolls-Royce" solution: Vaillant branded unvented total system.

Problem
When I open 2 hot taps from the DHW unvented tank, I get a huge flow drop on both DHW outlets. 3 at one time reduces it to a trickle on 1 tap.
If I open 1 hot and 1 cold at the same time the flow drops through the floor.
It is currently not possible to use 1 shower and even 1 small mixer concurrently without a significant drop in flow. Opening 1 hot and 1 cold tap causes a similar problem.


1month ago I had quotes and surveys done by British Gas and another local firm. They found we had just under 4.0 bar static pressure at the garden tap dropping to 1.5~1.7 bar when the kitchen tap was fully open and a combined flow rate of 24-25l/min when garden, kitchen and cloakroom tap were opened concurrently. So my mains supply looks good???

So where do I stand? I reckon as installer's the competent person, he should have checked all of this out to his satisfaction prior to installation and he's liable for sorting this out.
I have paid 60% of the job already, but not willing to pay another penny until the job is complete.
Atmosphere is now a little tense after I emailed him my concerns.

If you were in this situation, what would you do as a pro?

 

[bookahandyman]

Is your hot water supply pipe run in 22mm pipe as far as possible?
is the cold water supply to all outlets except the kitchen sink coming from the inlet valve of the unvented cylinder , which should be just after a pressure reducing valve, this gives balanced hot/cold water pressures throughout house? did 'competent person put new stop cock on supply pipe or is it and 30 yr old one?

 

[neiln]

check the strainer on the inlet side of the unvented cylinder

 

[Modokon]

Is your hot water supply pipe run in 22mm pipe as far as possible?
is the cold water supply to all outlets except the kitchen sink coming from the inlet valve of the unvented cylinder , which should be just after a pressure reducing valve, this gives balanced hot/cold water pressures throughout house? did 'competent person put new stop cock on supply pipe or is it and 30 yr old one?

Yes 22mm when possible (used about 30m in total), but 15mm on old existing runs to far bathroom.
All cold water outlets (including kitchen) coming from the "3.5 bar" PRV.
Stopcock is old one.
Today the Water Board measured inflow at water meter point: 3.5bar 30l/min. Measured at garden tap. 3.5bar at 18l/min.

 

[bookahandyman]

if you have 3.5bar at the garden tap then pressure seems adequate , it could be a blockage in the PRV or strainer if there is one. another is that the old stopcock is not opening fully. another option is to contact valliant tech helpline and see what they say. if you open just the hot tap on the kitchen is it the same pressure as the cold on its own, and likewise in each bathroom? if so then it coulkd be down to pipesizing.

 

[Modokon]

Just to update:
New 32mm (oh yes!) blue poly from meter to stopcock, reducing to 28mm to 22mm to PRV.

Flow at the stopcock was approx 60l/min (10 litre bucket filled in just under 10 seconds.
Upstairs now 30l/min from a bath tap after balanced PRV.

Combined from the 2 hot big bath taps now about 45-50l/min.

Result? Home win.