pressure problems - unvented system

[Docb]

Hello All
First post so please be gentle

I have a peculiar problem with my unvented system.
My mains pressure is good, aprox 4 bar and around 24 lpm flow.

The hot water pressure is much lower than the cold. all taps are the same, bathroom, kitchen (right next to the cylinder) and shower.

flow from cold is as above 24 lpm, hot on its own is down around 12 lpm.

The peculiarity is when the boiler is on and heating the water, i have what can only be descibed as a pressure surge. for maybe a second after turning on the tap or shower the pressure will be extremely good (at what i would expect it to be) and then drops back to normal.
This only happens when the boiler is on.

Are the two linked?

I have repeatedly asked the plumbers who installed it if they can sort it and they have no real interest and shrug their shoulders.


cylinder is an OSO.

Thanks in advance

Mark

 

[Nige F]

shrug.......draws breath thru` teeth........maybe the pressure reducing valve is "dynamic" not "static" ie. it reduces the pressure of moving water only ....yawn, what do you think Charley

 

[Docb]

thanks for the quick reply Nige F

Not being up to speed with the forum terms are you yawning because the question has been asked before?

So are you saying if the PRV is dynamic is should be static?
Would simply changing the valve solve all my problems (of a pressure nature of course )

Are you talking about the PRV that sits on the supply to the cylinder?

sorry for all the questions

Mark

 

[simond]

The OSO is fitted with a 2.1bar pressure reducing valve if the dip tube is used. Most OSOs are supplied in this way, but on request the cylinder can be supplied with an external expansion vessel and a different combi valve.

I expect that your installer hasn't balanced the cold supply through the combi valve so the hot water pressure through the cylinder is set at 2.1 and your incoming cold is 4. You would notice the difference. The answer is to run the cold feed through the OSO combi valve. If you like higher pressure then get the upgrade kit fitted from OSO to use an external vessel, this can take the full 4bar standing pressure you have at your disposal...... You will achieve a larger useful volume of stored hot water this way, too.

The surge you describe is the compressed air in the bubble top of the cylinder pushing the water out faster. Due to the small volume of air needed to maintain equilibrium, the boost is very shortlived. This is normal and is noticeable with all unventeds.

 

[Docb]

forgot to mention that I have quite a bit of water escaping out to the tundish, will this also be related?

 

[simond]

Yes, you might have told us!

The air gap will need recharging. A constant stream of water through the tundish will affect performance to a degree, depending on the actual loss.

 

[Docb]

Thanks for the advice

So to summarise

1. it could be the valve that is dynamic and needs changing to a static.

2. to boost pressure seperately from the issue use an external expansion vessel

3. the cold and hot need to be balanced. I presume the effect of this is to drop the cold pressure down to the level of the hot? so both would be at 2.1 bar as limited by the PRV?

4. Air gap needs recharging.

I feel confortable at DIY but would assume only point 4 is a DIY job.

Any tips on how to do it?

Thanks again

Mark

 

[ChrisR]

1.
No. I don't recognise those terms in connection with Pressure Reducing Valves! The valve should not let water through at high pressure even at very low flow.

2.
An external EV enables you to remove the dip tube so you get more water stored, and take up the expansion on heating, externally. You can then also change the prv to a fixed or adjustable one set to a higher pressure. This needs a qualified engineer.

3.
Yes, which is why you could do with a higher pressure PRV.
2.1 bar is often found to be perfectly acceptable though. Many shower pumps only produce about 1.5 bar. It depends on your expectations and aspirations!


4 Yes.
Turn off cw input to the cylinder
Turn on your lowest hot tap(s). If your air bubble is deplete, not much water will come out.

Hold open the relief valve which is on the side of the cylinder. This is boring, * you might have to be there 15 minutes - a pair of "mole" grips saves some effort.
When it stops glugging, release the valve, turn off your hot tap and restore the supply.

*Some people prefer to put a hose on the cylinder drain cock to get water out, which can be more effective and quicker. You still need a simple tap open to let air in, ie one without a non return valve - or again you can use the relief valve.

 

[Docb]

excellent thanks ChrisR

as to point 3. I am now thinking that something is not right, as the shower performance is well below any pumped electric shower i have been in before.

how could I measure the pressure at the shower/bathroom tap?
I guess it will require some kind of wonderful gadget that i can go and buy?
or is there a rough guide for non plumbers?

Thanks again

Mark

 

[ChrisR]

AH well, pressure and flow....
They're related, but different. Much like Voltage and Current. What feels good is Power, which is the product of the two.

I always stick a pressure gauge on an unvented cylinder. They cost about a quid to us! But lets assume you have 2bar at the cylinder. You should have some difficulty holding your thumb over a hot tap, but be able to do it. With zero flow, you have the same pressure at the tap as at the cylinder.

Measure the flow you're getting from the shower head, on hot only, full open. Same for cold only. Use a 9litre bucket, say, to get flow in litres per minute.
Then unscrew the handset from the hose, and try again out of the hose itself.
Then try a simple hot tap, perhaps the basin or kitchen sink one. Some taps and mixers, are surprisingly resistive
With the numbers you can get an idea of the effect of the resistance of the pipes and tap. Eg maybe the inner of the shower hose is all screwed up.

 

[Docb]

Thanks ChrisR

I will try all those and report back.

Thanks for the help, much appreciated.

 

[Docb]

OK

so I have done the recommissioning thing, and ther isnt a great deal of diference in the flow rates, before and after the recom.

Kitchen cold 19
Hot 6

Bathroom shower hose on Cold 12
Hot 7
hose off cold 15
Hot 10

Bathroom sink Cold 18
Hot 12

I am at a loss now as to what the problem could be if there is a problem.
very confused
I have noticed that there is a seperate PRV on the supply, about 2 feet from the cylinder. I have just spoke to the plumbers and they put it on for safety?

Could that be a problem?

 

[chrishutt]

I have just spoke to the plumbers and they put it on for safety?

Do the plumbers have the unvented ticket - the identity card that shows that they are competent to work on unvented cylinders? If not they shouldn't be installing the things at all. If they do, how come they don't understand what the pressure and flow problem is?

 

[simond]

I've told you the most likely cause of your flow imbalance, have you checked to see if the cold water supply is branched off after the 2.1bar combi valve or not?

I have seen two OSO installs this year alone with precisely the same installation defect. Get in a G3 fitter if you aren't sure what is going on.

Do you know the difference between a Preessure reducing valve and a pressure release valve? Stop blabbering on about the valves being dynamic or otherwise and check the obvious thing.

 

[ChrisR]

I have noticed that there is a seperate PRV on the supply

Only just realised?
Right, ok you don't understand.
The cylinder has to havea (one only) P reducing valve, they all do.
That leaves you with the HW at 2.1 bar.
Your CW is unregulated (probably) at 4 bar or whatever, which accounts very well for the differences in flow you measured.

Yes certainly you could take the CW off the system after the PRV then it would match the HW pressure. Whoopy hay - still pretty poor pressure and flow in your shower. It would make absolutley no difference to your shower - as you or your mixer has to throttle the cold back to make the mix temperature correct.

There's no great benefit in having the plumbing changed to give you "balanced" hw and cw in your (and many) cases. If they aren't very different pressure (such as with 9 bar mains, which does happen), thermostatic mixers cope perfectly well.

So get the mods we said made to the cylinder, then your HW can be say 4 bar. If the mains does go up to something much higher, which is pretty unlikely, you might notice it, but you're still safe.

The air gap recommissioning thing was to stop the discharge to outside, you wouldn't expect the pressure to change apart from the pulse you had.
Your numbers don't indicate any inconsistency - though your showerhead is rather restrictive.