Extend water main via loft for unvented cylinder

[twillmo]

I'm looking to upgrade my vented system to an unvented system, and as part of that I'm looking to upgrade the 15mm pipe from the stopcock to 22mm.

We've got 25mm in to the house and the stopcock is in the downstairs loo, it's in the boxing in of the soil stack. The pipe run to the airing cupboard is complicated, and pretty long.

I'm thinking of running 22mm plastic pipe up the soil stack boxing, across the loft about 4m, and back down to the airing cupboard on the 1st floor. Is this a sensible idea?

Whilst at it, I'm thinking of running a second 22mm supply next to it for the cold water back down to the downstairs loo/utility/outside tap. I believe all cold water should be fed from after the pressure regulating valve next to the unvented cylinder?

I'm also toying with only running plastic on the vertical soil stack part, and copper across the loft just in case any pesky squirrels or mice get in there.

 

[twillmo]

Am I being a bit stupid about the second pipe run coming back down? Can I just fit the PRV next to the stop cock and tee off the loo/utility cold supply before it runs up to the unvented cylinder?

 

[Baths'N'Gas]

Do whatever you got told when you did your unvented course, I often use my Btec book if I ever get stuck

 

[twillmo]

I won't be doing the cylinder myself. All of the plumbers I've had round so far have looked at me like I'm speaking a foreign language when I've talked about running a new feed to the cylinder, which has left me pretty worried.

I'm struggling to find someone who fills me with confidence, it seems like they just want to do the bare minimum and not touch anything more complex

 

[Madrab]

As long as the cold water supply is adequate to supply an unvented then yes the supply pipe could be run up, over and down.

There is also a balanced cold water supply that can be taken off of the combination valve to supply the bathroom.

 

[oldbutnotdead]

There's another thread on here about unvented- any mixer taps need to be fed from the balanced cold (so shower for sure, bath etc depends on yr taps), single taps should be fed from mains.

 

[twillmo]

There's another thread on here about unvented- any mixer taps need to be fed from the balanced cold (so shower for sure, bath etc depends on yr taps), single taps should be fed from mains.

So even though there is a pressure reducing valve just after the stopcock, the balanced cold isn't until after the pressure reducing valve just before the cylinder?

 

[oldbutnotdead]

I'm no expert, there's a lot of informed discussion here https://www.diynot.com/diy/threads/unvented-g3-ongoing-install-sanity-check.564521/
Part of the installation should include a balancing valve at the cylinder- that's where you'd get your balanced cold water feed from (not sure why it is needed, I can't see there being much pressure drop caused by a cylinder but as I said I ain't no expert.)

 

[Madrab]

An unvented will usually be set to 3bar. Therefore anything that will use that hot water, especially mono/mixers wants the cold pressure to be the same. The balanced supply off of the combination valve (PRV) will be at the same pressure as the cold feeding the cylinder and therefore the hot water.

It really comes into play when the mains pressure is higher than the unvented HW.

 

[twillmo]

An unvented will usually be set to 3bar. Therefore anything that will use that hot water, especially mono/mixers wants the cold pressure to be the same. The balanced supply off of the combination valve (PRV) will be at the same pressure as the cold feeding the cylinder and therefore the hot water.

It really comes into play when the mains pressure is higher than the unvented HW.

That makes sense thanks. So really it's for an instance where your incoming is 6 bar, so that you don't have a pressure difference of 3 bar at the mixer tap which would cause the cold to take massive presidence.

Theoretically if your incoming pressure is only 2 bar then the whole system could be considered balanced straight from the stop cock

 

[muggles]

Yes, correct

 

[Madrab]

As suggested yes.

That's why it's always important to know your dynamic mains pressure and flow before deciding whether an unvented is actually suitable to a given situation.

E.g. A system @ 2 bar static but then drops to 1bar and 10L/Min when 2 or 3 outlets are open (dynamic) isn't really suitable for an unvented, but a system running @ 2.5 bar and drops to 2bar dynamic and the flow stays up 15-20L/min mark could be ok (planned use dependent) but wouldn't really need to use a balanced supply.

A system @ 5 bar that drops to 4.5 bar dynamic should use a balanced supply.