acv smartline pressurised hot water cylinder

[oldengineer]

When the boiler is on and supplying hot water to the cylinder turning on a hot water tap it is noticed that for a few seconds the water pressure is higher than normal. The drop to normal is noticeable. A plumber told me this is ok but my old Tribune cylinder never did this. Is the expansion tank pressure set too high?

 

[ChrisR]

Can think up a few things it could be
Most likely perhaps, P reducing valve is letting by, pumping the cyl to a higher presure. WHen thet's gone, you get the set pressure again. That would take time tro accumulate the nigher pressure, and if left overnight the expansion relief valve would probably open. DOes it?

 

[simond]

This is normal and is due to the expansion vessel accumulated charge. After the first few seconds the flow and (working) pressure revert to the dynamics supplied by the incoming water main.

Because it is only a small volume of compressed air in the expansion vessel the boost only lasts a few seconds. NB: A mains water accumulator works on this principle but can be 100 times bigger.

 

[oldengineer]

Many thanks indeed to Chrisr and Simond, the expansion relief valve is not opening. Two things ,I don't remember my old Tribune cylinder doing the same and I would have thought acv would say in their blurb to expect this. They also don't say what the pressure should be in the expansion vessel and whether this is related to the mains pressure.

 

[simond]

The pressure in the exp vessel is preset by ACV to work with the design pressure of the combi valve, which is part pressure reducing valve, part non return valve.

The Range Tribune may have had a smaller expansion vessel and therefore the burst would be less detectable. As the combi valves get older the non return valve can let by due to poor seating, this stabilises the water pressure in the cylinder, and thus removes the burst effect (and any backflow contamination protection).

I see no reason why it should be mentioned in the instructions, very few people notice it. The ACV cylinder is sold as an open vented cylinder with an optional unvented kit, it also can be used with 85C water and a mixdown valve; so the instructions have to be more complex than most.

 

[oldengineer]

Many thanks again,Simon, I feel much happier now, my line was performance testing gas turbines ; the Tribune failed because my water here is very acidic so I hope the acv will prove more reliable. I discovered the expansion tank is set to 3.5 bar. The old engineer.