Mains stopcock reducing flow to unvented cylinder?

Hi there,

This is related to another problem with my unvented cylinder boiler. A new mains has been fitted (a year back in fairness) and I'm not noticing any better flow...its been a nagging problem that I've had enough of and need to solve. 9LPM at best I get - and that's in the taps next to the boiler close to the mains inlet.
Anyway, I've replaced the PRV to no avail and I'm now looking elsewhere... I have a 25mm plastic mains pipe. The boiler is fed by a 22mm pipe and pipes leading off the boiler are all 22mm. However, the stop cock is what seems to be 15mm instantly where the blue pipe enters the house. This continues at 15mm, hits the PRV and then continues under the ceiling and turns into 22mm which arrives at the boiler.
Would this reduction to 15mm have an impact on flow rates? Pressure at the taps seem great but the flow rate not good. Its an Ideal Istor all in one unit - bad reputation but its serviced every year and hasn't caused any major problems. It advertises up to 35LPM and 11LPM minimum I think on an empty tank.

Thanks in advance for any suggestions.

That would indeed change the flow rate, in the same way that turning the stop tap half off would. Static pressure would remain the same, however.

Yup, stop tap would normally be 25mm to 22mm. That being said if you have suitable pressure then flow shouldn't be so low at the entry point, 15mm pipe can flow a lot more than 9L/min. Has the static and dynamic pressures before and after the PRV been checked?

In your previous thread you said that you turned the prv up a bit and the flow rate increased a lot. (Albeit causing the pressure relief valve on the istore to drip)

Still sounds like the prv is limiting the flow too much when set to the 2 bar required by the istore.

Some prv's are a lot worse than others for this.

Why not contact ideal and explain your problem and ask them to reccommend a suitable prv

You really need to measure pressure before and after the PRV. Is there a plug on the PRV where a gauge can be screwed in? You'll need to be ceeative with the mains inlet side.

Then take readings at different flow rates. A shortish length of 15mm shouldn't affect the flow rate that drastically.

Hi All, thanks for the replies. I purchased a weir gauge - think its called that - and measured the flow rate on my mains entering the house. The bit I measured from had a small outlet that I had to fit a hose pipe too. I have a plastic blue/black tap but not the usual spout off it and that tap I assumed stays open. I noticed the bit I was measuring from had had 15mm piping and added to that the restriction of a hose added to it so I could test - I had to turn the bit where the handle would be with my pliers. (I'm assuming its some kind of valve to drain off the system?) Anyway, off the mains with this size reduction limitation im getting far in excess of 22LPM - it was hurtling out of the gauge which has a maximum read of 22LPM. This was far above that - and I actually don't think I'd opened the tap fully with being in the basement and not wanting to soak everywhere!
So, I get good flow even with a 15mm pipe sizing, but once it hits the PRV and makes its way down the 15mm piping to the 22mm piping to connected to the boiler all the flow rate is lost. The new PRV is on the correct way but it does seem to be 15mm in and 15mm out. Ideal I think would like this to be 22mm.
My issue now is a plumber recommending a 22mm pipe setup from the mains but if I'm getting a major drop in flow with 15mm, that is excellent before the PRV, whats to say I wont get the same drop with 22mm piping. In the process I'd have to rip out a decent section of ceiling. I have 22LPM+ coming in with 15mm pipe restriction yet its easily less than halved when it come out of the taps/shower.
Is it worth going to these measures? The PRV - which is a JET one does say it has good flow rate. Ideal use a Caleffi PRV - maybe I should just get one of those? Thanks a lot

Oh, so you have a 15mm PRV? That may explain something if the characteristics of the PRV are reflected in a smaller unit. Can you look up the characteristics online?

Check that the strainers on the PRV are clear. Screw in a gauge to the PRV if there's a cap for one. Otherwise try a 22mm PRV with a reducing set.

Yeah the PRV purchased was a 15mm PRV and the strainer is new so should be clear. Had a bit more time to explore today and it gets worse. I have the 15mm pipe coming out of one end of the PRV and the other 15mm bit on the other side attached to the 25mm external pipe via a tap/valve. The bit behind the ceiling void (I cut a section of the ceiling void open today) is a grey plastic pipe running a good 10ft or so. It looked again to be 15mm, but I recognised the pipe as I had some in a spare room that must have been left over from the installation. It is polyplumb pipe and when I looked at the internal diameter its around 11/12mm at best. So yet again another flow restriction.
I also noticed alll of the cold water pipes coming out of the boiler and around the house are 15mm. Yet the hot water pipes are all 22mm. Will this cause things to be unbalanced? I'm looking at getting a plumber to see if he can replace the pipes leading to the boiler with 22mm and add a 22mm PRV in the process. This should hopefully make a massive difference. I just wonder now if i will get an imbalance between hot and cold at showers and mixers as although they're at 2 both at 2 bar one will be 22mm? Thanks again all

I used to have a 3lb lead mains with an ID of about 11mm and it used to flow 19L/Min @ 2.5bar, it's not the 15mm pipe that's the issue, you need to confirm the supply's static and dynamic pressure before and after the PRV.
The 15mm should easily be able to sustain a suitable supply. If the static/dynamic pressure doesn't ever peak above 3Bar, then you don't really need a PRV. What is this PRV protecting?

Your un-vented cylinder should have it's own combination group and that would normally include a PRV.You say the boiler's fed with 22mm pipe, is this the cold mains feed and it's a combi boiler working with an un-vented cylinder and that PRV is protecting the combi?

Cold water pipes coming out of the boiler? Depending on the type of boiler you would normally have 22mm primary pipes (flow/return) for the radiator circuit (nothing at all to do with the hot water pressure) and only a 15/22mm cold inlet and a 15/22mm hot outlet if it's a combi but you shouldn't have a cold outlet??

Hi Madrab, its an ideal istor all in one unit: https://www.mytub.co.uk/ideal-istor-he-260-cylinder-module-product-107518

The boiler itself comes with a PRV that sits seperate to the unit - I think earlier models it was integral. I seem to have 22mm piping attached to the boiler but the entire hot/cold to the house is limited by this PRV which is directly after the mains supply. there isnt a seperate mains cold water - its all downstream from the PRV which is why I think flow is particularly reduced from the 22+LPM I was getting when testing before the PRV gets involved. In terms of measuring the pressure - I can turn the PRV up and register more than 3 bar but the tundish on the boiler begins to leak so I know there is more than 2 bar available. However, the boiler requires 2bar to operate safely. I'm going to get rid of the small piping leading to thee boiler and see if we have any joy.

Ok, so that PRV is to protect the un-vented module , though 15mm pipe @ 2 bar should easily feed more than 9 L/min but never say never...

I'd disconnect the pipe after the PRV and add a test pipe in there and check the flow, that should be definitive if the PRV is the cause. That before replacing all the pipework up to the boiler with 22mm.

Thanks Rob, I'll aim to give that a try some time this week before I rip the place apart!

Check against the PRV flow figures and see if it's flowing correcly, if it's a jet 15/25 then it should flow up to around 40L/min so it certainly should have no issue at flowing 22+L/Min.
Time to Test

Well, I took the ceiling out last weekend and asked a plumber to fit me some 22mm copper piping. He has run a 22mm all the way to the boiler and a section of 22mm all the way to the cold (which is 15mm). It seems to have increased the flow rate now to around 15lpm which looks and feels a whole lot better. The only issue now is the cold taps are off 15mm all over the house (no a seperate feed off the mains) so the flow rate from the taps is around 11LPM - noticeably different. The plumber recommended fitting a seperate PRV on the cold (which can be done the he's branched off) and increasing the pressure on the cold to match the flow rate of the hot taps. I know this would work but wouldn't there still be a pressure inbalance even if the flow rates are equalled? I debated with the plumber and he seemed to think if the flow rates were the same pressure would be similar? So therefore not causing an issue with my thermostatic showers (which do say equal pressures ideally)?

If he's right I may just give him the green light and go ahead with a PRV on the cold as well? Does this make sense?