I have now carried out some further checks.
The standing water pressure measured at an outside tap
adjacent to where the water main enters the house is 3.2 bar. However, this soon lowers with the use of water; kitchen tap
on full 1.6 bar; toilet
refilling 2.6 bar; bath tap
on full 1.0 bar. We are in a rural area that suffers from low pressure!
The problem with the unvented cylinder, as I see it,is that the following sequence of events is taking place:
1. After a period of heating and water expansion, the cylinder at a higher pessure than the incoming water main (> 3.2 bar). The pressure reducing valve is off.
2. Turning on the hot water allows water flow, the pressure will reduce. At a point below 3.2 bar the pressure reducing valve will open and allow water to enter the cylinder.
3. The flow of water causes the incoming water pressure to lower. When below the the cylinder pressure the valve closes again.
4. The incoming pressure returns to 3.2 bar, the valve opens again.
5. This all repeats, causing the juddering noise until all the extra pressure in the cylinder is released.
The way we can prevent the juddering is by running a hot tap
slowly first thing is the morning to release the extra pressure.
I have tried reducing the expansion vessel pressure as suggested by Ariston to 3.0 bar (from 3.5 bar) but it does not make any difference. In any case, Ariston suggested the EV pressure should be 1/4 bar higher than the mains pressure. This would put it back to 3.5 bar.
Are we stuck with this problem or are there solutions for this type of case?