OSO versus Megaflow and low pressure issue

[glynstab]

Hi anyone help or have an opinion please?

I have just had an OSO 20 RI (indirect) 250litre and a Greenstar 24i System Condensing Boiler fitted but not commissioned... the builder had said Megaflow...the plumber fitted OSO...it still isn't fully commisioned and is running off the immersion heater..ok so far except that the pressure to the shower in the newly converted loft is nowhere near what we had been led to believe the "MEgaflow" would produce!! So questions..

1. WOuld the pressure increase once all the other bath / shower facilities have been commisioned?
2. Would the pressure increase when the boiler is commissioned as well as the cylinder, people say it wont make any difference logic seems to tell me it should...but I'm no expert!

We have a large blue flexible more than an inch diameter connected to the water meter just outside the property boundary, this runs in directly to the main stop cock and from there on seems to run in mostly 22mm flexible hose, 15mm flexible to the rads off the 22mm feeds. As I say I think the 22mm runs most of the way or all the way up to the shower in the loft. We will have soon hopefully a shower and bath on the 1st floor and an ensuite shower in the loft.

Please can anyone shed any light on why the pressure seems so poor, the builder thinks its poor and the plumber doesn't seem to have a clue...OSO said the pressure is it is 2.1 bar and thats as good as it gets?!!

I've read various posts on here and it seems the Megaflow is superior and that OSO can only be tweaked with the addition of some valve or another available from the manufacturer?

Thanks in advance

 

[Dan Robinson]

Haven't I read this somewhere before?

 

[Dan Robinson]

Right oh - which thread are we posting on?


Your OSO is going to be fine. The problem will be an installation issue - undersized water main, restriciton on the pipe - knackered stop cock.


Pressure and flow are related but are not the same.

You can have high pressure-low flow and low presure-high flow. 2.1bar is fine which indicates that you either have a blockage, or undersized pipe.

 

[Buzzark]

Yes Dan, sounds quite familiar...

Had a similar question myself recently glynstab, but mine was prior to choice so nothing fitted yet.

You're not going to gain any flow or pressure when the rest of the system is commisioned.
Our stoptap was in 15mm but the rest of the incoming main was in 25mm blue pipe leading to 22mm copper. Swapping this to a 25mm stoptap helped us gain some flow and dynamic (running) pressure.

Here's the results from my thread:
14l/m at the bath tap (fastest flowing tap and close in height to the shower)
Static pressure 2.2 bar
Dynamic pressure 0.7 bar

After the work:
20l/m at the bath tap
Static pressure 2.2 bar
Dynamic pressure 1.3 bar

Check obvious things first like the stopcock being fully open etc.

Recently checked my parents house and ended up really jealous - 5 bar static pressure and well, pretty much 5 bar still with a couple of taps open.

 

[gas4you]

You really need at least 3 bar incoming mains for an unvented cylinder as the prv will probably be factory set at 3 bar, so less than this won't let it balance tha hot and cold supply's.

 

[Balenza]

gas4you wrote

so less than this won't let it balance tha hot and cold supply's.

If the plumber has installed the system correctly he will have used the balanced supply port provided on the inlet control group (one piece (multiblock)) which I believe is reliance gear on the OSO .

 

[Dan Robinson]

I believe is reliance gear on the OSO

It certianly is; and he certainly should have.

[/quote]

 

[gas4you]

Hopefully he has, but my point is that unless op's mains pressure is greater than that of the prv setting, usually 3bar, whether on block or seperate, there will be no excess pressure left for the valve to be able to perform the 'balancing act'.

 

[Dan Robinson]

I take your point.

If it's all been done is plastic then I worry that something has been crushed or kinked. Even with 2.1 bar you should get reasonable showers. You could always try a lower pressured regulator for testing purposes I suppose, but it's not really the point.

There could be lockage sin the filters, but it is all push-fit then that is a bit less likely.

Have pressure and flow readings been taken at the appliances yet? That should be done before any closing up is carried out by the builders.

Remember also that your expectations and the sales banter of the salesman are nothing without direct comparison. If they showed you your neighbour's and then said "thats what you'll get" then you have the right to expect it; however.... well I guess you understand.

 

[Agile]

The installer should be qualified to fit unventeds and should have filled in the benchmark certificate and notified Building Control.

I doubt that he is qualified or has done anything!

Dont pay for the fitting if he is not qualified!

Tony

 

[chrishutt]

You really need at least 3 bar incoming mains for an unvented cylinder as the prv will probably be factory set at 3 bar, so less than this won't let it balance tha hot and cold supply's.

If the mains pressure is lower than the setting of the PRV the hot and cold pressures will be "balanced" anyway - i.e. both at mains pressure. The point of the "balanced" cold outlet from the combination valve is to ensure balanced pressures when the mains pressure exceeds the PRV setting.

 

[simond]

If my memory serves me correctly the 2.1bar limitation on an OSO can be raised to 3bar if you do not use the internal expansion bubble top and add an OSO accessory external expansion kit. The OSO cylinder is well up to taking 3bar, it is the limited volume allowed for expansion that makes them fit the 2.1Bar combi valve as standard.

The Norwegian OSO is a much better built unit than the Megaflo, a fact that is obvious just from lifting it.

If you have insufficient FLOW then the addition of an accumulator will transform the performance. I suspect this is the problem as an unvented working on 2.1bar will give a lively performance if there is sufficient flow rate to keep the operating pressure up in use.

If you decide to fit an accumulator (find a specialist) the dip tube in the OSO can be removed and the actual water storage capacity thus increased.

 

[chrishutt]

The OSO cylinder is well up to taking 3bar, .

The last OSO I saw had an 8 Bar Expansion Valve and a 10 Bar Temperature and Pressure relief Valve, so it was clearly designed to cope with 10 Bar. Incidentally the dip tube should be removed (to increase storage capacity) if you fit any external expansion vessel, not just a very large one (an accumulator).

 

[Agile]

The air bubble will provide a considerable accumulator effect when using a hand basin although obviously it will quickly run out when using a shower.

Its often forgotten just how much accumulator volume is needed because the effective usable volume is hardly 50% of the total volume.

I would expect the minimum accumulator volume would need to be at least equal to the storage cylinder volume.

Tony

 

[gas4you]

You really need at least 3 bar incoming mains for an unvented cylinder as the prv will probably be factory set at 3 bar, so less than this won't let it balance tha hot and cold supply's.

If the mains pressure is lower than the setting of the PRV the hot and cold pressures will be "balanced" anyway - i.e. both at mains pressure. The point of the "balanced" cold outlet from the combination valve is to ensure balanced pressures when the mains pressure exceeds the PRV setting.

Sorry to disagree on above, but if you have less pressure than the prv is set for then there is no excess pressure left for the valve to allow more water through when another outlet is turn on on the balanced circuit to keep it balanced.