Insufficient flow Hot Water from Unvented Cylinder

JanSE · 25 Jan 2018

Hi - I have a situation where the hot water pressure in the shower reduces significantly when I turn on a hot basin tap, but not when I turn on a cold basin tap.

This is strange because there is a 28mm pipe from mains (~3bar) up to the attic where unvented is. It splits in 2 22mm pipes right next to cylinder: 1 for cold water and 1 going to the unvented cylinder. So this should not be a flow issue from the mains since opening the cold tap makes no difference to hot water pressure. Rather it seems to be a flow or pressure loss in the unvented cylinder - so for hot water only - when I turn on 2nd hot water tap.

The 3bar pressure reducing valve has already been replaced and this has helped raise static pressure to 2.5bar inside cylinder (old one was rusty and broken). The pressure reduces a little when I open shower tap and then more when I open 2nd hot water tap (but not with cold water tap). The hot water pressure loss in shower is actually already a bit better now, but there is still a drop.

What else could cause this pressure drop over the cylinder?

Hot&Cold · 25 Jan 2018

JanSE said:
Hi - I have a situation where the hot water pressure in the shower reduces significantly when I turn on a hot basin tap, but not when I turn on a cold basin tap.

This is strange because there is a 28mm pipe from mains (~3bar) up to the attic where unvented is. It splits in 2 22mm pipes right next to cylinder: 1 for cold water and 1 going to the unvented cylinder. So this should not be a flow issue from the mains since opening the cold tap makes no difference to hot water pressure. Rather it seems to be a flow or pressure loss in the unvented cylinder - so for hot water only - when I turn on 2nd hot water tap.

The 3bar pressure reducing valve has already been replaced and this has helped raise static pressure to 2.5bar inside cylinder (old one was rusty and broken). The pressure reduces a little when I open shower tap and then more when I open 2nd hot water tap (but not with cold water tap). The hot water pressure loss in shower is actually already a bit better now, but there is still a drop.

What else could cause this pressure drop over the cylinder?

When was the 3bar prv exchanged ?

JanSE · 25 Jan 2018

Hi! Only yesterday!

Hot&Cold · 25 Jan 2018

JanSE said:
Hi! Only yesterday!

Great stuff,so the prv installer has seen your system and you would have raised the flow issue with him/her.

What did the prv installer recommend to overcome your low flow problems ?

JanSE · 25 Jan 2018

Said the mains pressure was probably too low - but I don't get it. Hence asking for a second opinion. ok - only 3bar at mains and 5m up to attic where unvented is.

Hot&Cold · 25 Jan 2018

JanSE said:
Said the mains pressure was probably too low - but I don't get it.

its the flow rate which is important and the flow rate could be to low for what it has to feed,even unvented systems have their limitations.

Has your local water company been to test and report on your incoming flow ? some are better than others

JanSE · 25 Jan 2018

Ok - no they haven't. Will look into that.

I just thought that flow rate would be ok given I can turn on a cold tap and the shower pressure does not change. Only when hot tap is turned on. Both cold pipe and cylinder cold input split from same 28mm pipe. So hence flow should be ok?

Thx for the tip on local water company though.

Hot&Cold · 25 Jan 2018

There will be some water flow resistance through the cylinder,your cold will have hardly any flow resistance.

The hot water tap that causes the reduction in flow could have a flow regulator of around 4 lpm fitted but not much help on a bath tap also the shower would need checking for flow regulators and clean filters.

Any isolation valves on the hot water system would need to be full bore to avoid any local flow restrictions,

good luck

JanSE · 25 Jan 2018

Hi Old&Bold - yep - I think that your suggestions make good sense. More resistance somewhere on hot water flow would result in this behaviour. Had not thought about shower taps themselves. Thanks very much for the tips.

fezster · 25 Jan 2018

Does your cold pipe going to taps split before or after the pressure reducing valve? If before, it may have far more pressure available from the main than the hot has available to it.

Also, what is the pipe run for the hot and cold taps? For example, if the hot has more 22mm leading up to the tap, but the cold is only 15mm, the hot tap will steal far more flow than the cold will.

JanSE · 26 Jan 2018

Thx. I am now thinking:
- The 3 bar pressure reducing valve is not doing anything since the mains is 3 bar and the feed goes up to 5m so I am getting ~2.5 static in cylinder.
- Therefore the valve is not stabilising the pressure in the cylinder: it goes up and down as more water is being drawn from it.....and yes it is strange that turning on the cold does not have the same result but a lot of the resistance must be added in the cylinder (and valves) itself. It looks like the isolation and prv add quite some resistance.
- So maybe it would make sense to run the cylinder at 2bars instead......then at least the mains pressure would stabilise the pressure in the cylinder and we would not get annoying changes in flow in shower
OR
- Put a prv of 2 bars on the shower hot water only to stabilise the pressure there.

Fezster: Cold splits before prv - but as per above the prv does not do anything (except add resistance

) and the pipe runs for my basin taps are both the same: first 22mm then going into 15mm at tap. Thx!!

fezster · 26 Jan 2018

How do you know the mains is 3 bar throughout the day and does not fluctuate? When did you measure it? You'd need to measure it at the times you are experiencing these issues.

You are correct that you lose half a bar going up 0.5m, but this would not explain why the cold tap does not impact the hot.

Next thing to do is to measure the flow rate from the hot tap and the cold tap. And also measure your incoming flow rate by using the kitchen tap and garden tap (and possibly others) and seeing what the total incoming flow is, whilst also measuring the pressure whilst in use (the running pressure). You cant rule out a restriction in your hot supply taps, which could also be causing this.

John D v2.0 · 26 Jan 2018

If some taps are affecting each other, it has to be something those taps have in common.
So the fitter is wrong to suggest any supply problem or blame individual taps.
There must be a restriction in the hot system affecting flow rate.
Reducing the prv will make things worse not better, unless you have independent ones for each tap after whatever restriction there is.

Do you have any balanced cold feed used eg for the shower? Teed off after the prv. If so is that affected?
I think if it's that bad the only solution is to start taking things off in the water run and checking if they are clean. Unfortunately not something you can do yourself

JanSE · 26 Jan 2018

Hi John - yep - a restriction in hot system affecting flow rate makes sense.

Given I have a pressure meter on the prv where I can see a pressure drop, and the only items between the meter and the T where the 28mm mains feed split in cylinder feed and cold, are the prv itself and a isolation valve, then maybe those 2 components are causing the pressure drop. The prv cartridge is brand new - so that leaves the isolation valve? It is a gate type.

John D v2.0 · 26 Jan 2018

Fair point, gate type isn't normally used on mains pressure, you can change it yourself as it's before the cylinder etc
It's possible there's just muckch there

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Insufficient flow Hot Water from Unvented Cylinder

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