Low flow rate in mains pressure system

[Randomiser]

A relative has just had an update to their plumbing (existing house, not a self build).  They have had an unvented cylinder installed but now have problems which they are told are being caused by a low flow rate, rather than water pressure.  Apparently the water pressure is about 4 bar and the flow rate 12 (litres per minute?).  They have been told that with a mains pressure system they cannot have a pump added but could add an "accumulator" to the system.  They asked me what I thought and I had to confess I had never heard of an accumulator before.  I did a quick bit of online searching and think I understand the basic principle of one works, but what I have read seems to suggest it is installed to help with low water pressure as it returns the water pressure to static pressure from working pressure even when water is being drawn.  What I can't find is if this will do anything for the flow rate in the system.

 

Any thoughts / experience / info I can pass on would be much appreciated.

[Temp]

Their installer should have checked tge flow rate before recommending and installing a mains pressure system. The problem could be the size of their supply pipe from the road to their house. However check that their stopcock(s) are fully open and no other restrictions or undersize pipes somewhere in the house.

 

An accumulator helps when you have good static pressure but low flow rate. It works by filling slowly with water at whatever rate the main can deliver, then releasing it suddenly/more rapidly when you open a tap/shower. It needs to be correctly sized (bigger is better) or it may run out and revert to low flow, perhaps when a second person wants a shower immediately after the first. 

 

Check if it would be cheaper to have the main supply upgraded to a larger diameter pipe than fitting an accumulator - although I suspect not.

Edited October 11, 2019 by Temp

[Randomiser]

42 minutes ago, Temp said:

Their installer should have checked tge flow rate before recommending and installing a mains pressure system. The problem could be the size of their supply pipe from the road to their house. However check that their stopcock(s) are fully open and no other restrictions or undersize pipes somewhere in the house.

 

An accumulator helps when you have good static pressure but low flow rate. It works by filling slowly with water at whatever rate the main can deliver, then releasing it suddenly/more rapidly when you open a tap/shower. It needs to be correctly sized (bigger is better) or it may run out and revert to low flow, perhaps when a second person wants a shower immediately after the first. 

 

Check if it would be cheaper to have the main supply upgraded to a larger diameter pipe than fitting an accumulator - although I suspect not.

Thanks for that Temp.  Yes, uncle not happy that it has got to this point before problem spotted.  I imagine changing pipes to house would be a big issue as he is a long way away from the road and if I recall correctly his water metre is at the end of the drive, so a lot of new pipe he would have to run.  So from what you say an accumulator should help as the static pressure is around 4 bar, which I think is pretty good.  What you say about sizing is interesting, should the accumulator be the same size as the unvented tank he has had installed, 200 litre I think?  That would presumably mean he would run out of hot water at the same time the accumulator stopped adding additional flow to the system.  Or does it need to be even bigger to work effectively?

 

Thanks again for your help.

[scottishjohn]

totally  agree

as has already been said if you got 4 bar then its pipe sizing  or partially  obstructed somewhere 

they only have to give you 1bar ,usually they aim to give you 2 bar --so if you got 4 bar  then its pipe sizing somewhere 

 really old house ?

 

Edited October 11, 2019 by scottishjohn

[scottishjohn]

the way the water board checks flow is to put a gauge on somewhere handy  then open up a cold tap full and see what it drops to --if it drops then there is a restriction  somwhere in the piping .

 

[Randomiser]

26 minutes ago, scottishjohn said:

totally  agree

as has already been said if you got 4 bar then its pipe sizing  or partially  obstructed somewhere 

they only have to give you 1bar ,usually they aim to give you 2 bar --so if you got 4 bar  then its pipe sizing somewhere 

 really old house ?

 

Old house that was then extended many times before my uncle bought it, last one was in the 80s.  Suspicion is that the plumbing system has just been added to every time it was extended without ever having been fully replumbed.  He tells me there are "pipes everywhere under the floors and in the loft".

[scottishjohn]

7 minutes ago, Randomiser said:

Old house that was then extended many times before my uncle bought it, last one was in the 80s.  Suspicion is that the plumbing system has just been added to every time it was extended without ever having been fully replumbed.  He tells me there are "pipes everywhere under the floors and in the loft".

theres your answer then possibly

some  plumber or some DIY has fitted small piping somewhere no doubt

I,m guessing it was  a gravity system with water tank in loft when house was built 

was not as critical with gravity system ,as the ballcock  would just keep open longer to fill the large tank --but now  it will matter

 

Edited October 11, 2019 by scottishjohn

[joe90]

I re plumbed an en-suite because of lack of hot water flow, customer wanted a pump to make it better but I noticed lots of 90’ bends and tees etc, I replaced it with 15mm plastic pipe straight from the tank and only one 90’ bend, (able to wind the plastic pipe around obstructions etc) customer very happy with the result, then I told him I didn’t use the pump and he could get his money back ?. It’s amazing how restrictive 90’ bends are to water flow especially in 15mm pipework. In my new build we only have a 25mm incoming main and the water flow is excellent.

[Randomiser]

2 hours ago, scottishjohn said:

theres your answer then possibly

some  plumber or some DIY has fitted small piping somewhere no doubt

I,m guessing it was  a gravity system with water tank in loft when house was built 

was not as critical with gravity system ,as the ballcock  would just keep open longer to fill the large tank --but now  it will matter

 

OK, thanks again.  It sounds like the accumulator is the answer as otherwise if he is unlucky and it is found quickly he will need pretty much the whole house ripped apart to find the restricting pipework.  Do you have any thoughts on the size of the accumulator relative to the size of the cylinder?

[scottishjohn]

a big accumulator and it would need to be good size will be hundreds of pounds --i would go looking for the problem before spending on  that

[joe90]

1 minute ago, scottishjohn said:

i would go looking for the problem before spending on  that

 

I agree, it depends where the pipes are, loft, underfloor but worth finding or re routing.

[scottishjohn]

2 hours ago, joe90 said:

I re plumbed an en-suite because of lack of hot water flow, customer wanted a pump to make it better but I noticed lots of 90’ bends and tees etc, I replaced it with 15mm plastic pipe straight from the tank and only one 90’ bend, (able to wind the plastic pipe around obstructions etc) customer very happy with the result, then I told him I didn’t use the pump and he could get his money back ?. It’s amazing how restrictive 90’ bends are to water flow especially in 15mm pipework. In my new build we only have a 25mm incoming main and the water flow is excellent.

that is exactly the reason why I like the uponor shink-fit  pex system --16m pipe and fittings are same internal size as  pex pipe is

I intend to run separate pipes to everything so all joints are somewhere where you can get at them 

no its not as cheap as hp20  -not something i want to revisit in my life time

 

[joe90]

12 minutes ago, scottishjohn said:

I intend to run separate pipes to everything so all joints are somewhere where you can get at them

 

Exactly what I did in my new build (but in hep20).

[PeterW]

2 hours ago, joe90 said:

It’s amazing how restrictive 90’ bends are to water flow especially in 15mm pipework.

 

Welcome to fluid dynamics ...

 

16 minutes ago, scottishjohn said:

uponor shink-fit  pex system --16m pipe and fittings are same internal size as  pex pipe is

 

So in Hep2O and all copper fittings, the internal size of the bore is equal or larger than the pipe bore. Only valves have smaller bores unless they are marked as full bore. 

 

The issue is one of fluid dynamics, and not just about the size of the pipe. Most 90 degree elbows and tees are the same internal size but very tight when it comes to the direction change. This causes significant slowing of the flow as the pressure is lost in the pipework as a secondary reverse flow is set up as the water “bounces” off the end of the pipe. You can test this by using pressure gauges before and after bends. 

 

These are also incredibly damaging pressure waves as they can in time make joints weaker - the trick is to smooth the flow, and using street elbows in copper, or long radius elbows as they are sometimes called, reduces this issue. The slower the bend, the less the flow reduction due to pressure reduction from back pressure. 

 

 

 

[Temp]

Before you install an accumulator you need to check that the problem really is the supply pipe to the house and not the pipework in the house. The pipework between the accumulator and the showers needs to be good or the accumulator wont help. 

 

If possible measure the pressure near where the main comes into the house while running the shower/bath. If that drops then the supply pipe is the problem. If the pressure stays reasonably high at that point then the problem is in the house.

 

 

[Nickfromwales]

22 hours ago, Randomiser said:

Old house that was then extended many times before my uncle bought it, last one was in the 80s.  Suspicion is that the plumbing system has just been added to every time it was extended without ever having been fully replumbed.  He tells me there are "pipes everywhere under the floors and in the loft".

I'd say you have far bigger problems than slow showers I fear.

If this was a retro-fit then a few things that would have needed doing, so lets see where you are with these before adding anything....

• All mixer taps ( every single last one of them ) should be installed ( or retrospectively connected to ) in a manner that prevents back-flow to the UVC. That means, the cold feed to EVERY mixer tap should originate from the multi-block ( MB ) balanced cold outlet. One of these should already have been fitted within 500mm of the UVC and the cold feeds should be connected to that, and NOT to the generic cold pipework that traverses the property. If that has not been done, then a G3 installation requires that a non return valve ( NRV ) is fitted in the supply line to / at EACH and EVERY hot outlet. The idea is, if a mixer tap is open, 4 bar pressure cannot creep back up the hot pipe and back-pressurise the UVC ( sat at 1 bar lower so cannot 'fight back' ) beyond its 3 / 3.5 bar stated working pressure capacity ( and it goes pop, which I've seen happen and isn't pretty ).

Or, to save all that grief ......

• A whole of house pressure reducing valve ( PRedV ) like this should have been fitted to the one single stopcock that supplies cold mains water to the house. Hopefully the house is still served by the one stopcock?
• A 22mm NRV can be fitted to the hot outlet of the UVC, IMMEDIATELY off the UVC, which will negate all the satellite NRV's needing fitting. Quick and cheap / simple, plus G3 compliant  

If those have not been done ( some pics would help ) then you do not have a G3 compliant install and it's on borrowed time. If they have been done, you found a top installer and I'll shut me mouth ( doesn't happen very often  ).

On 11/10/2019 at 08:28, Randomiser said:

They have had an unvented cylinder installed but now have problems which they are told are being caused by a low flow rate, rather than water pressure.

Is it only the hot that is suffering?