Mains-pressure cylinder hot water system - 15 or 22 mm piping?

[John Rumm]

3 - 3.5 on my Unistor

[John Rumm]

That is why the PRV on unvented cylinders typically provide a pressure balanced cold output as well - so you can feed a shower with balanced hot and cold.

[John Rumm]

Not always easy to do in a retrofit situation.

On my setup I do use the PRV provided cold for one shower (literally the other side of the wall from the cylinder), but for both the first floor showers, it was much easier[1] to just use the full mains pressure cold (~6.5 bar here), and then have a second independent PRV there for the cold feeds there.

[1] Historically the system was plumbed for a vented system so the cold main went upstairs to fill the cistern, rather than down to the cylinder. So when I converted it to unvented I looped back the rising cold main from upstairs to downstairs to provide the mains feed to the cylinder. Hence there was already mains cold upstairs, and no spare pipes to being it back again from the cylinder PRV.

[The Natural Philosopher]

That is why there is a post pressure regulator cold feed takeoff on all tanks .

wrongly installed then

Easily remedied by feeding the showers from after the tank pressure regulator

[Chris B]

Fine for new build but not in our case "easy" for the retrofit to a bath/shower room some distance from the tank.

[The Natural Philosopher]

Actually ALL you have to do is run a new cold feed to the tank, and switch the old incoming cold feed to after the regulator. You have then moved your whole house cold supply to be after the regulator.

[Roger Hayter]

It would be just as effective to fit a separate pressure regulator to the cold feed to the shower/bath, set to the same pressure.

[John Rumm]

See the thing on the bottom right with the green cap:

One is also included as a part of the multifunction valve feeding an unvented cylinder. See diagram and picture here:

The copper could be a secondary low pressure cold feed for a shower (typically tapped off the cistern a little bit lower than the main feed to the hot cylinder in case the cistern runs out of water - that means the hot feed to the shower would be interrupted first. The plastic pipe might be the overflow from the cistern.

[bert]

We have this type of system on 15mm having converted from traditional header tank. It's all 15mm from the mains inlet. The shower(s) is/are the critical device(s). We have decent mains pressure - though haven't actually measured it. Showers are no more than OK. Modern shower heads seem to want much greater pressure/flow. Two showers together are bit poor. Oh, and you aren't allowed to put a pump on this type of system. Wouldn't recommend it.

[Fredxx]

This might be more DIYable:

This has the combination and safety of a low pressure tank but hot water at mains pressure.

[John Rumm]

That is why its important to measure your flow rate as well as the pressure (they are not the same thing). If you have not got at least 3 bar or better, and at leat 25 lpm or better, the system may not perform that well.

There will be limitations with 15mm pipe everywhere (especially for more than one shower at a time).

With a appropriate pressure and mains flow, you will get excellent performance from an unvented system. However if you install one in circumstances where its sub optimal, then that is the performance you will get. (it will still beat any combi, but the "burst" performance of a vented stored water system with pumps etc will beat it, simply because your mains supply can't deliver water fast enough.

Would it be fairer to say you would not recommend the system you have?

[John Rumm]

Or for really DIY:

However in realty they are all DIYable.

There was an argument that unvented systems were "more complicated" because they were included in section G3 of the building regs, while traditional vented systems were not. Hence there was all the stuff about notifiable building jobs etc to contend with. Now that all hot water systems are included in the building regs (by virtue of the conservation of heat and power clauses if nothing else), then it becomes a moot point.

Yup there are pros and cons, and initially I was considering implementing something similar. In the final analysis however the unvented system was simpler, cheaper and quicker to implement for my circumstances and the performance equally good. (it also works in a power cut since it does not need a pump!)