Another flow rate in unvented system question

Apologies for bringing up unvented systems - but I have searched and can't find an answer to my question.

My problem - in a nutshell the flow rate at showers etc isn't as good as I would have hoped.

My plumber (in retrospect I shouldn't have paid all the cash) blamed the flow on the mains, but didn't have a gauge to show me what it was and we left it at that as my builder would then replace the mains. (yes i know)

I subsequently had the mains replaced all the way back to the street assuming that rusted/crushed incoming pipework was the prob (replacement = 25mm? blue plastic all the way to the airing cupboard except for a tee to the garden/kitchen).

Static pressure is approx 5bar

A couple of questions.
1) Measuring flow at the (old) garden tap gives approx 22L/min. The tap is old and hisses / comes out as a jet akin to putting your finger over the end of a hosepipe. Am I correct in thinking this could be giving me an artificially low reading as it seems to be restricting flow (slightly) even when fully open?

2) I had a water softener installed after (chronologically- it is upstream!) the unvented system, but before the mains were updated. So as well as the 3.5 bar PRV combination valve with the system they installed a PRV before the softener. Is there a possibility of causing a problem if the first PRV is 3bar and the 2nd is 3.5 (ie the water pressure at the inlet to the combination valve is lower than the outlet rating) or will that make no difference.

3) While searching for answers I found many people talking of PRV's letting by. What is this? is it a failure of the PRV leading to higher pressures at the outlet or is it some other problem that I might have?

The instructions for the system say that min spec is 1.5bar-20L/min but performance won't be great. If my reading from the garden tap is accurate this is probably the cause. If so why is my mains flowrate so low with new pipework?

I have checked the pressure in the expansion vessel, cleaned the strainer in the combination valve and operated both check valves to ensure they discharge into the tundish and then close again.

Any suggestions appreciated

Open all the cold taps that you can at the same time. Then whilst they are all running measure the flow out of each one at a time. Adding together all the readings will give you a better idea of the max flow you can get through your 25mm main and the cylinder PRV. It's no good measuring at one tap only as the tap itself is a big restriction.

Obvious now you say it!

Another flow issue searches came up with was to check that all stopcocks (including external) are fully open. Which I have done (in fact got the council to replace my external one which was actually buried - I thought that would do it, but no and that is now a plastic quarter turn full open job.)

Tried it from the bath only, and then the bath and the trickle that comes out of the sink if you run both at the same time and both come out at around 17.25L/min.

I would say this is pretty poor for an unvented system, but you can't get out more than you put in, so the inlet is the prob (but at what stage?) I would expect a slight drop after passing through all the gubbins, but still can't find what the flow rate into the property is. I don't have a water meter, but there is a plug to fit one by the stopcock, is there a way of measuring pressure/ flow there? Although if there is a restriction in my house then presumably that would still show my same low flow rate unless the meter just discharged the mains into the street!

If you are running a bath on the first floor (ie the one above the ground in case you are American!) then the toilet cistern on the second floor (third storey) won't even fill.

(The cylinder is in the bathroom on the first floor, by the way)

The council came to measure it a few years ago and it was about 6.5 bar (my 5 bar reading came from a £10 gauge on ebay!) and I can't remember what the flowrate was, but he used the same garden tap as me and my stopwatch/bucket.

I would like to clarify if the serial fitting of PRVs is a prob if it isn't stepping down just to stop me barking up the wrong tree! Do PRVs work (well allow unrestricted flow) if the inlet pressure is less than the outlet rated pressure or is there some sort of NRV that would mean this is a problem?

How many lpm do you get from your kitchen cold tap ?
Also some water softeners can be quite restrictive - bypass the softener and see if there's a difference.

The softener is rated to 55L/min and I asked them to use the hi-flo installation (essentially using 22mm rather than 15mm hoses), but I have also tried bypassing it with no change.

The main kitchen tap is softened (not having washer/dishwasher softened missed half the point of installing it) so comes from the airing cupboard and after the prv's, there is a drinking water tap installed pre-softener but is fed via a long 3mm( maybe 5mm) plastic pipe and goes through a filter so won't really give a reading which helps.

So if I understand your readings correctly the garden tap delivers 22l/min and the bathroom and kitchen together deliver 17.25l/min. So I assume the garden tap is fed directly from the mains. My guess is that for a single tap the 22l/min sounds OK and that the flow shouldn't give you any worries about your supply. The tap is the biggest restriction and so it's max flow is what you're actually measuring.

The 17.25l/min from the bathroom and kitchen is low and suggests that the problem lies with the two PRVs and softener. Can you bypass the softener to get some idea of its effect?

Unless your new mains supply has been installed incorrectly then it shouldn't be the cause of your problems. At 30l/min the presssure drop through 10m of 22mm mdpe will only be about 0.25 bar.

How much 15mm pipework is there after the unvented cylinder? This is another possible restriction.

Regarding the flow rates - almost. The garden tap (where mains enters the property) is indeed 22l/min. The 17.25L/min reading was for the bathroom only - A reading for the bath alone was 17.25 and for the bath and bathroom sink was also 17.25ish (the basin was 4L/min and the bath 13.25L/min, didn't seem much point in opening additional taps given the dribble from the second one).

As listed above (we seem to have crossed posts) have bypassed the softener with no improvement. (although I haven't actually measured it after bypass, will do though)

The bathroom was redone as well so very little 15mm pipe involved in this. The tank is in the bathroom so a few metres at most to bath and basin!

How can I tell if the PRVs are causing the problem? I'm guessing the way is to put a couple of pressure gauges in the line - after the first and after the second. Any suggestions on where to get cheap ones?

OK, I see you've tried bypassing the softener.

Are the strainers in the PRVs dirty?

See http://www.rwc.co.uk/Public/prv/PRV-001.pdf for a the way the PRVs operate. I can't see how two in series with the second rated higher than the first would be a problem, but it would be worth trying to contact their technical department to confirm. It's perhaps possible that the 3.5bar PRV is overly sensitive to the downstream back pressure because the pressure into it is only 3bar, ie. it doesn't open as much.

Actually, regarding the PRV in series question - it is redundant in my case as the 1st is 5bar (awkward to get to it but finally got to see the label) and the 2nd is 3.5bar; so in my case (assuming they are working) is is a natural step-down anyway.

The strainer in the combination valve PRV was a little dirty (yellowy sludge, poss dislodged limescale) but cleaning it made no difference.

Do all PRVs have a strainer? The 1st one (5bar) looks a lot simpler - but if simply unscrewing the 'cartridge' reveals another (further upstream) strainer then that could be it!

What shower do you have?
What flow and pressure do you get at the hot and cold bathroom tap near the shower, assuming that is where its plumbed in to?

'standard' gravity fed shower. (I assume that is the term) with thermostatic mixer.

Part of the reason for installing the unvented system was to negate the need for pumps. It backs onto the cupboard the system is in so prob about a metre of pipe! Flow in there is no more than adequate, well to be honest the basins are fine - but the bath could fill quicker! Better than the electric shower that was originally there - but not 'luxurious'.

All taps in the property on all 3 floors are no more than adequate. And if a bath/shower is going then forget about using anything else. The kitchen tap seems stronger, but prob due to the small aperture creating a nozzle effect.

Wel, I cleaned the strainer on the 1st PRV - nothing there. More of a slight yellow discolouration than anything, no debris- came off with a rub of the thumb.

The housing on that PRV says 3bar, but it is an adjustable type with the 5bar sticker over the adjustment screw; so I assume the softener people just preset them at the warehouse.

I suppose a lump of gravel or something could cause the problem - but with the stopcocks fully open and it not making it as far as the strainers in the prv is it likely?

so are you just wanting a better flow of water at your showers ? and no where else.

Well I'd like to have enough water coming out of a tap to clean my teeth while someone is running a bath!

There are bathrooms on the top 2 floors (one on each, shower only in the top one) and I would like for guests to be able to use one without having to explain that they can't if I am using the other if we are all getting ready to go out together. 2 showers at the same time in a 5 bed house can't be that unusual surely?

To be honest I'm predominantly trying to work out why a system with new piping from external stopcock (also new) to my bath with 5 bar of pressure coming in provides a flow rate of less than 18L/min to my taps. That sounds to me like there is a fault with what is there - I'm not too keen to start adding to that to work round problems that could be cured. Presumably my next step is to add a pressure gauge after all the prvs but before the cylinder. Presumably I should just cut in a 22mm tee with the gauge attached to the protruding leg. Where is the best place to look. Saw some 1/4" BSP gauges, but couldn't find any 22mm/22mm/ 1/4" reducing tees anywhere. Or am I barking up the wrong tree entirely?