Questions/Issues with new UVC System

[HughF]

I use mlcp extensively, I’ll measure fittings bore for you this weekend.

 

p.s. Unless you’ve got naff all water pressure, the reduction from the fittings won’t make any noticeable difference.

 

your issue is 100% the incoming supply. Your plumber should have advised a new 25mm mlcp into the house.

[Spinny]

Any feedback on the last post and photos please ?

 

Is this tundish plumbing compliant to regs and acceptable ?

 

ta

[ProDave]

  On 06/04/2024 at 12:29, Spinny said:

Any feedback on the last post and photos please ?

 

Is this tundish plumbing compliant to regs and acceptable ?

 

ta

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I would say no

 

 

Picture from here (first google link that found the picture I wanted)  https://www.e-lindsey.gov.uk/media/4257/050-Unvented-hot-water-storage-systems/pdf/050Unventedhotwaterstoragesystems.pdf

 

You are supposed to have 500mm of straight pipe below the tundish.  Yours appears to go straight into what I think is a waterless trap.  Whether that will cause a problem I could not say.

 

You could try manually operating the over pressure or over temperature relief valve and see how the water flows?

 

[Jenki]

  On 06/04/2024 at 12:29, Spinny said:

Any feedback on the last post and photos please ?

 

Is this tundish plumbing compliant to regs and acceptable ?

 

ta

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you need a tundish, this has to have an air gap, and to summarise G3  need to be in a visible position or the outlet to the drain need to be prominent and visible.

 

so a waterless trap is ok, but you need a tundish before this. then as @ProDave mentions you need a straight length of pipe after the tundish & before any bend. looks very tricky to achieve with your install, which would make me question the plumber, this is the basics. G3 is common sense and discharge pipes are straight forward, but they need planning / calculating before you install.

[Spinny]

He has fitted one tundish which is the black one directly above the trap in the photos. Here it is in close up in another pic. 
 

Tundish not easy to keep an eye on tucked behind the cylinder when it could have been further forward.

 

Also wondering if there should be 2 separate tundishes, one below each valve so you can see which one has opened ?

 

He has plastic waste pipe below the trap which runs across under the floor into a drain pipe - as per final photo.

 

Is there a way I can tell if the pipe is OK for 95C water ? As I understand some plastic pipe is OK for high temp and some not. It is going to be big problem to put a new pipe under the floor now as it is glued & screwed down with u/floor heating pipe in it !

[JohnMo]

  On 06/04/2024 at 14:37, Spinny said:

Also wondering if there should be 2 separate tundish

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No need, one is fine

[Spinny]

So one year on and I now today have a new 32mm MDPE barrier pipe (23m) connected into the water main replacing the old 1930s lead pipe.

 

Via outside tap/u-stairs toilet cistern which share 15mm pipework directly off the mains, I have gone from 50PSI static and 22.3 l/m before, to 52PSI static(3.6bar) and 37 l/m.

 

Via utility tap fed by the UVC with a 3 bar PRV, I have gone from 40PSI static (which is actually only 2.75 bar !) and 17 l/m before, to 40PSI static and 19l/m afterwards.

 

However my shower has gone from 11.6 l/m (shower head removed) to 11.6 l/m (shower head removed) - no change. This is still very significantly less than from the old gravity system with monsoon pump (3 bar static & 2.5bar @ 9l/m) even though through the same mixer valve. CONCLUSION: the plumbers pipework to the shower is crap and will need to be replaced ?

 

I am very puzzled by the following result though... I can put a static pressure guage onto the utility tap. With no outlets open it reads 40PSI (2.75bar) which is roughly similar to the system pressure from the PRV of 3 bar. If I then run the shower (head off) at 11.6 l/m the pressure at the utility tap falls to just 20PSI (1.4bar). It I then also open en ensuite tap, the pressure at the utility tap falls again to just 15 PSI or about 1 bar.

 

So why is this system fed from a main at 3.5bar and supposed to run at 3bar, losing half its pressure when you draw off just 11.6l/m ? Especially as the main is capable of delivering 37l/m !? Something would seem to be choking this pressurised system off...

The restricted bore on the MLC pipe connectors  which feed the system ?

The water softener, not currently fitted but connection is there in bypass mode ?

The PRV or some other location having a blockage/restriction ?

A pipework problem ?

A faulty cylinder ?

 

 

 

 

[Nickfromwales]

  On 31/03/2025 at 20:17, Spinny said:

However my shower has gone from 11.6 l/m (shower head removed) to 11.6 l/m (shower head removed) - no change. This is still very significantly less than from the old gravity system with monsoon pump (3 bar static & 2.5bar @ 9l/m) even though through the same mixer valve. CONCLUSION: the plumbers pipework to the shower is crap and will need to be replaced ?

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You cannot compare an open pumped system to one running off a cold mains, as the pump is a huge performance booster and usually way OTT for what you actually need to be able to shower under, adequately.

 

The static and dynamic flow rates will obvs be affected when more than one outlet is opened simultaneously, so no shock there, and obvs the more outlets you open the more dynamic performance you will see.  Simple maths methinks! 
 

I’ve spec’d a full house plumbing arrangement with a 400L UVC and the cold mains is hovering a bit above 2.5 bar static. I’ve suggested fitting a 300L cold mains accumulator to maintain flow rates as I know without it the showers will suffer.

 

Cold mains supplies aren’t infinite, so this is always down to diligence of the installer; they need to put on their thinking cap before just going ahead with a knee-jerk spec and installation of kit that won’t meet expectations.

 

The only way you’ll better the performance here is with an accumulator. 

[Spinny]

  On 05/04/2024 at 08:13, HughF said:

I use mlcp extensively, I’ll measure fittings bore for you this weekend.

p.s. Unless you’ve got naff all water pressure, the reduction from the fittings won’t make any noticeable difference.

 

your issue is 100% the incoming supply. Your plumber should have advised a new 25mm mlcp into the house.

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  On 31/03/2025 at 20:38, Nickfromwales said:

.....

Cold mains supplies aren’t infinite, so this is always down to diligence of the installer; they need to put on their thinking cap before just going ahead with a knee-jerk spec and installation of kit that won’t meet expectations.

 

The only way you’ll better the performance here is with an accumulator. 

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I appreciate these are different comments from different people, but why is this apparently some kind of trial and error black art ? First I need a new main (£2000), now I need a giant pressurised replacement for the old water tank in the loft (££££) ? This should be about the physics of fluid dynamics, not ask a different plumber get a different answer. (not trying to offend or insult anyone, I appreciate all comments/responses - just wanting to get to the root of the matter,)

 

At present 'the glorious system' appears to be turning a 37l/m torrent from the mains supply, into a pathetic 11.6l/m (shower head off) (and just 7.3l/m with showerhead on). 

 

It seems to me 'the glorious system' £££££££££ is just a giant flow inhibitor ?

 

It was 'advertised as' capable of 2 simultaneous showers, while the mrs is running the kitchen tap, and the washing machine is running. The wonderful new system that avoids people shouting around the house to 'turn that outlet off'.

 

It still seems something is amiss here to me !?

[John Carroll]

Are you saying that you are only getting 1.4bar (static) pressure where indicated at a shower flowrate of 11.6LPM??

Can you take the pressures where indicated with  the shower only in service on its hottest setting, the dP (pressure drop)  through the 23m of 32mm MDPE should only be ~ 0.04bar at 19LPM and practically nil, 0.02bar,  at the shower flowrate of 11.6LPM,  trying to see if there is a problem with the UVC PRV (pressure reducing valve), maybe the strainer is full of crap after commissioning the system or something simple like that.

I don't think you said if a balanced cold is taken off just after the UVC PRV, if so, open and measure the flow from a (balanced) cold only and take pressures.

 

What is the height of the shower (head) above the top of the UVC roughly.

Edited April 1 by John Carroll

[Spinny]

Just done another measurement to measure the static and dynamic pressure for the 15mm copper pipe outside tap outlet which is upstream of 'the glorious system'.

 

Static pressure from the mains is 50PSI (3.45 Bar), and the dynamic pressure whilst discharging at 37 l/m is 38PSI (2.6 Bar).

 

Therefore 'the glorious system' has a raw input capable of delivering 37 l/m at 2.6Bar.

Yet when the output from 'the glorious system' is just 11.6 l/m from the shower outlet pipe, the pressure in 'the glorious system' falls to just 1.4bar.

 

So to me that seems like conclusive proof that there is a problem in 'the glorious system'.

 

Any suggestions on how to find the fault ? Change the PRV ? Take the water softener bypass out of the flow line ? Replace the MLC pipe feed to the PRV/Cylinder ?

 

(Move the outside tap up the wall and shower naked outside under 37 l/m of cold water ?)

[John Carroll]

Where is the "1.4bar" measurement been taken?, is it  directly off the UVC?.

If a balanced cold is taken off where shown, try and install a PG on any tap that its supplying and note the pressure with the shower flowing 11.6LPM, that should show the PRV downstream pressure. If the balanced cold is unused and capped off then shutting the mains supply to the UVC should allow the removal of the cap nut without draining down the UVC as there is/should be a check valve,  take care when slackening off the cap nut to ensure pressure falls off after shutting the mains. 

[Spinny]

  On 01/04/2025 at 17:55, John Carroll said:

Are you saying that you are only getting 1.4bar (static) pressure where indicated at a shower flowrate of 11.6LPM??   YES, exactly that. Which would seem to indicate that the pressure of the system is falling from ~3 bar with all outlets closed, to 1.4bar when the shower pipe is open (yet the mains supply is now capable of providing 37 l/m at 2.6bar).

 

Can you take the pressures where indicated with  the shower only in service on its hottest setting, the dP (pressure drop)  through the 23m of 32mm MDPE should only be ~ 0.04bar at 19LPM and practically nil, 0.02bar,  at the shower flowrate of 11.6LPM,  trying to see if there is a problem with the UVC PRV (pressure reducing valve), maybe the strainer is full of crap after commissioning the system or something simple like that. The original PRV was supplied with the Gledhill cylinder. When I first raised the flow issues with the builder's plumber he replaced the Gledhill PRV with a new one from Caleffi, saying the original one might be faulty/blocked, but it had no effect on performance then (which was before I replaced the lead supply pipe).

When I opened up the new MDPE supply pipe I opened the outside tap to try to bleed the air out from the new pipe, but I was only getting a dribble from it. Today I replaced that tap and found the old one had gummed up with some muck/grit from the connection work. I suppose it is possible that some muck might have got down the system pipe to the PRV.

 

I don't think you said if a balanced cold is taken off just after the UVC PRV, if so, open and measure the flow from a (balanced) cold only and take pressures.

OK. There is a more detailed sketch of the system posted on page 1 of this topic/thread and yes there is a balanced cold. I will take some more measurements tomorrow.

 

What is the height of the shower (head) above the top of the UVC roughly.

Shower head is about 2.5m-3m above the top of the UVC.

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[John Carroll]

OK getting there slowly I think.

 

IMO, if you take a pressure reading off any tap fed from the balanced cold then that will tell a lot as its downstream of the PRV, that pressure should be showing ~ > 2.8bar ish if the PRV is set to 3.0bar and working properly and with no debris in its strainer while supplying only 11.6LPM to the shower.

 

Can you (again?) say where exactly that "1.4bar" pressure is taken from?

[Spinny]

[Spinny]

  On 01/04/2025 at 20:58, John Carroll said:

OK getting there slowly I think.

 

IMO, if you take a pressure reading off any tap fed from the balanced cold then that will tell a lot as its downstream of the PRV, that pressure should be showing ~ > 2.8bar ish if the PRV is set to 3.0bar and working properly and with no debris in its strainer while supplying only 11.6LPM to the shower.

 

Can you (again?) say where exactly that "1.4bar" pressure is taken from?

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Yes the 1.4bar is the pressure on the utility cold tap when the shower is on (set to a warm mix of hot & cold) and outputting 11.6 l/m. And the utility cold tap is fed from the balanced cold on the downstream side of the PRV.

With no flow on any outlet, the static pressure at that utility cold tap is around 40-42PSI or 2.75Bar ish. When the shower outlet is turned on that pressure falls to just 1.4bar.

[John Carroll]

Well then that PG should show (with the shower off) the dynamic pressure when the outside tap is flowing 37LPM or whatever, is that the 2.6bar you mentioned above?.

[Spinny]

[Spinny]

  On 01/04/2025 at 21:20, John Carroll said:

Well then that PG should show (with the shower off) the dynamic pressure when the outside tap is flowing 37LPM or whatever, is that the 2.6bar you mentioned above?.

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No.

You can see the utility sink cold outlet on the bottom right of the sketch.

The 2.6bar is the pressure in the outside tap 15mm copper feed pipe (centre left of the sketch) while it is open and discharging 37l/m through the outside tap. I can measure it because the same copper pipe to the outside tap also feeds the cistern in the understairs toilet (not shown on diagram), and I can attach the PG to the cistern outlet and measure the pressure while the outside tap is running.

So the mains supply is about 3.5bar static, and with 37l/m flowing through it the dynamic pressure in the mains supply is still 2.6bar. So even if the outlets downstream of the PRV are discharging a combined 37l/m, the PRV would still be able to receive an input pressure of 2.6bar. So there seems no reason for the pressure in the system downstream of the PRV to fall below 2.6bar. Yet the downstream pressure measured at the utility cold tap is only 1.4bar when the shower is running.

It seems like something is choking off the supply through the PRV.

So do you think it is going to be the PRV where the problem is ?

There is about 6m of 25mm (internal bore 20mm) MLC pipe connecting the mains supply to the PRV, and this MLC pipe has a crimped connector at each end where it connects to copper. The internal diameter at the connectors reduces to 14mm. (but the outside tap still discharges 37l/m via 15mm copper pipe).

[John Carroll]

Agree,  but IMO, the "1.4" bar PG should read exactly the same as the cistern PG which reads 2.6bar with a flowrate of 37LPM as long as there is no demand from the PRV, and allowing for any elevation difference between the two PGs. IF it does and it would have to IMO (static measurement) then the problem might appear to be somewhere between the tee off to the water softner (including its now bypass valve) and the 25mm MLCP etc + and including the PRV.

 

Also if the shower is flowing 11.6LPM @ 1.4bar then assuming a "normal" minimum of 2.7bar should give a shower flowrate of 11.6*sqroot(2.7/1.4), 15.8LPM.

Edited April 1 by John Carroll

[Spinny]

Photo showing the water softener bypass valve...

[Spinny]

OK. Definitely a huge problem somewhere between the mains connection and the downstream side of the PRV.

 

The system pressure of around 40PSI as measured on the utility tap (cold or hot) drops dramatically to 20 PSI if any single outlet is turned full on. That shouldn't happen because the system should be refilling and pressurised from the mains to a much higher pressure than 20 PSI. With two outlets turned on the system pressure falls to just 16 PSI. I took some readings with the shower outlet partially opened with the following results...

Utility Tap Pressure reading:

All outlets off                         40PSI

Shower running at 2l/m        29PSI  !! (A dramatic drop in system pressure as soon as

Shower running at 4.4l/m     25PSI

Shower running at 8.5l/m      22PSI

Shower running at 11.6l/m     20 PSI

Even flushing the loo sends the pressure down to 24 PSI

There is also a sound from the area of the PRV valve a bit like a cistern refilling noise when opening and closing any outlet. Makes me think maybe the PRV is jammed and not responding properly when water is drawn from the system ?

If it isn't that either the water softener bypass (seems unlikely), or the MLC pipe run have a problem - maybe some crap stuck in the pipe ? Is it possible to kink MLC pipe ? Or over compress the end fittings ?

I guess if the PRV is removed the water flow and pressure at the input to the PRV could be measured which would show whether the water softener bypass/MLC pipe was choking off pressure/flow.

 

It frustrates me that the plumber fitted no gauges to show the pressure or temperature of the cylinder.

 

If I get a new PRV fitted (again) which one is best ?

 

 

Edited April 2 by Spinny

[John Carroll]

What does the Utility tap PG read when drawing 37LPM ONLY from the outside tap?

[Spinny]

  On 02/04/2025 at 19:12, John Carroll said:

What does the Utility tap PG read when drawing 37LPM ONLY from the outside tap?

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It continues to read 40 PSI with no change.

I think this is to be expected I believe ?

[John Carroll]

 

Exactly, I'm happy now.

There is one "foolproof" method of checking if the problem is with the PRV and that, as you suggested, is to remove the PRV and install a spool piece but this ia a bit messy as the balanced and EV connections must be removed and the spool piece made up,  easier IMO to remove the balanced cold at the PRV, make up a connection and install the PG here, then check this pressure with shower on (HW) only and/or other HW users, this will also show, if the PG reads say 2.6/2.8bar,  that the problem is downstream of the PRV.

Edited April 2 by John Carroll