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Discuss Unvented Question (flow/pressure related) in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Hoping someone can sanity check me!

Bit of a project house - had an unvented cylinder fitted in the loft around a year ago. Gledhill horizontal, Worcester system boiler as well as wet underfloor throughout.

As part of this we had a new 32mm MDPE mains fitted. This enters the house and reduces to 22mm. Also have a minimax M2 softener.

Cylinder is supplied via the 22mm after the softener. We have cold kitchen tap and one 15mm feed (Currently isolated) pre softener. Everything else is on the balanced cold and softened.

The issue I’m having;

Recently fitted a bath. At first the flow is excellent and as I would expect. However after about 3 minutes of filling there is some awful water hammer (I suspect from the cold supply to the cylinder) and flow is massively reduced.

I have a calibrated flow and pressure gauge - connected to the 15mm supply pre softener (so as close to incoming mains as I can get) I’m seeing just over 4bar and 35lpm.

Using another 15mm supply but this time a softened hot one - initially I see 3 bar and around 30lpm. However this reduces after a few minutes to 20lpm. After immediately isolating pressure sits at 2bar and slowly creeps back up to 3 after a minute or so.

Bypassing the softener makes no difference.

Before I call my G3 guy - would I be right in suspecting the combination valve may be faulty or slightly blocked etc? My thinking is the cylinder isn’t being replenished quickly enough so it’s relying on the pressure from the expansion vessel for that initial burst which is then unsustainable. Checked all accessible valves and all are fully open.

Or am I just expecting too much of the unvented and this is usual behaviour? We have only had shower run via 15mm so this hasn’t really been noticeable until now.
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
can you test directly after the softener ?
 
I can with a bit of fiddling yes. Pretty sure it’s not it though - there’s a full bore 22mm lever valve to bypass it and it makes no difference wether the softener is bypassed or not
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Ok - easier than expected. Managed to test literally either side of the softener. Me being a fussy git asked for an upstands ready for outside taps - I wanted a soft cold to clean the car! So there is a nice test point either side!

Both of these are as close to the incoming main as possible, just the softener between them.

hard cold = 3.9 bar resting. Jumps to 4.5/5 bar when shutting off valve before falling to 3.9 in around 10-15 seconds. 29/30lpm

soft cold = 3.8bar consistently 25/26lpm
Post automatically merged:

There is also a hot take off right here.

static pressure is 3.6/3.7 bar initially I thought this was higher than expected as the cylinder should only be at 3. However around 7 meters of head would make sense putting the cylinder at bang on 3.

flow initially is 25lpm before dropping to 18-20lpm after 30 seconds.

shutting it off pressure sits at 2bar initially before climbing back to the 3.6over a minute or so.

pretty sure this isn’t right as I’m testing from a 15mm pipe. The 22mm mains should be able to more than keep the initial pressure and flow going the whole time - or have a misunderstood something and this is actually ok/to be expected?
 
Last edited:

Vee

Plumber
Gas Engineer
Hoping someone can sanity check me!

Bit of a project house - had an unvented cylinder fitted in the loft around a year ago. Gledhill horizontal, Worcester system boiler as well as wet underfloor throughout.

As part of this we had a new 32mm MDPE mains fitted. This enters the house and reduces to 22mm. Also have a minimax M2 softener.

Cylinder is supplied via the 22mm after the softener. We have cold kitchen tap and one 15mm feed (Currently isolated) pre softener. Everything else is on the balanced cold and softened.

The issue I’m having;

Recently fitted a bath. At first the flow is excellent and as I would expect. However after about 3 minutes of filling there is some awful water hammer (I suspect from the cold supply to the cylinder) and flow is massively reduced.

I have a calibrated flow and pressure gauge - connected to the 15mm supply pre softener (so as close to incoming mains as I can get) I’m seeing just over 4bar and 35lpm.

Using another 15mm supply but this time a softened hot one - initially I see 3 bar and around 30lpm. However this reduces after a few minutes to 20lpm. After immediately isolating pressure sits at 2bar and slowly creeps back up to 3 after a minute or so.

Bypassing the softener makes no difference.

Before I call my G3 guy - would I be right in suspecting the combination valve may be faulty or slightly blocked etc? My thinking is the cylinder isn’t being replenished quickly enough so it’s relying on the pressure from the expansion vessel for that initial burst which is then unsustainable. Checked all accessible valves and all are fully open.

Or am I just expecting too much of the unvented and this is usual behaviour? We have only had shower run via 15mm so this hasn’t really been noticeable until now.
Are the isolation valves around the water softener 22mm and the hoses too? Are there any inline strainers, including washers with strainers that may need cleaning? Are the pipes including final connections to bath tap 22mm, i.e. no restricting flexible hoses with reduced bore. Are there any isolation valves fitted inline to the bath tap which have reduced bore?
 
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Mains comes in and converts from 32mm MDPE to 32/28/22 copper via gate valve.

there are full bore lever valves either side of the softener. Softener is connected by the largest flexible hoses I’ve ever seen! Supposed to be good for 8bar and 55lpm. 22mm main is one continuous run of 22mm speedfit to the loft, it then converts to copper via a gate valve and connects to the combo valve.

no strainers on any of that that I’m aware of. Although I expect the combo valve has one somewhere. I’m not-going to be fiddling with that though and it was only serviced last week.

bath is 22mm copper from the balanced cold and hot. Connected by crappy flexi hoses that could well be causing a restriction.

my concern is that it seems to be losing flow even when it’s tested from just a 15mm take off. It is less noticeable for sure than the bath - but it’s still a noticeable drop
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
I would say combo valve likely
 

Vee

Plumber
Gas Engineer
Mains comes in and converts from 32mm MDPE to 32/28/22 copper via gate valve.

there are full bore lever valves either side of the softener. Softener is connected by the largest flexible hoses I’ve ever seen! Supposed to be good for 8bar and 55lpm. 22mm main is one continuous run of 22mm speedfit to the loft, it then converts to copper via a gate valve and connects to the combo valve.

no strainers on any of that that I’m aware of. Although I expect the combo valve has one somewhere. I’m not-going to be fiddling with that though and it was only serviced last week.

bath is 22mm copper from the balanced cold and hot. Connected by crappy flexi hoses that could well be causing a restriction.

my concern is that it seems to be losing flow even when it’s tested from just a 15mm take off. It is less noticeable for sure than the bath - but it’s still a noticeable drop
I would change the crappy flexi hose prior to the combination valve.
 
I would change the crappy flexi hose prior to the combination valve.
I will change them - but I don’t think that’s the root cause. They wouldn’t be affecting what I’m seeing on every other outlet too. Plus they are capable of handling the higher flow, as they do initially which leads me to believe that while they may not help, they aren’t nt what’s causing the problem.
 

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