Replacing old HW cylinder

[John Rumm]

so far so good...

Yup although unvented cylinders usually have a pressure reduction valve on their input from the mains, that will usually limit to around 3 bar (assuming your mains pressure is that or more). That will be significantly more than available from a gravity fed system in the vast majority of cases.

The actual process is fairly trivial it has to be said.

Unless you have menopausal toilets with hot flushes, they are unlikely to be fed from the hot water cylinder :-)

If you replace gravity fed cold feeds with mains one, then yes you might get a problem of that nature - but it is pretty unlikely IME. (you can also get far more noise from old ballcock style float valves when fed with mains pressure)

It will work, but the flow rate will be more limited than it otherwise might be. It will depend a bit on the incoming pressure. (higher supply pressure will compensate a bit for smaller diameter supply pipes)

20+ lpm is good and will probably run a couple of adequate showers at once.

Most unvented cylinders also have a pressure balanced cold mains output as well as the hot one, so you can feed both sides of shower mixers from the cylinders PRV if you want.

[AlanC]

Sorry, I didnt explicitly say but we were also going to remove the cold water storage tank in the loft so that everything would be in mains pressure. The change from vented to unvented HW is preparation work for having the bathroom and cloakroom refitted so any noise should only be short lived.

Good to have an expert confirm that the 15mm feed should be OK.

I had assumed that the "normal" thing to do is feed everything from the unbalanced feed and feed the showers from the balanced feed. The showers (2) will be thermostatically controlled and I assume it will be better to have a balanced hot and cold feed to them.

[John Rumm]

It will depend a bit on what the mains cold pressure actually is. If it is lower than the PRV set pressure on the cylinder it makes no difference at all. If it is a bit higher also probably not much to worry about. (I expect the only problem would be if you have a true mixer tap or shower, and the cold main is able to force water back into the cylinder via its hot output)

Here the mains pressure is over 6 bar, so I used the PRV output on the cylinder for one shower that was in the room adjacent, and then installed a second standalone PRV in the cold feed to the other bathrooms. So pressures are broadly equal at most locations apart from the kitchen tap which has a full pressure cold feed. You can see the difference in flow rate, (which is actually good for filling pans / kettle etc), but it does not cause any problems. (the tap is a concentric spout mixer rather than a true mixer)