OSO versus Megaflow and low pressure issue

[chrishutt]

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.

Eh? Can't quite follow that. Look, if the mains dynamic pressure is say 2 Bar with a flow of 20 l/m, that will be the pressure, more or less, at both hot and cold outlets - therefore balanced. Why should hot pressure be more or less than cold pressure in such circumstances?

 

[Dan Robinson]

Seeing as the balanced draw off could be anywhere AFTER the regulator I always assumed it was irrelevant what the pressure itself was (ignoring performance for a while).

That is certainly what Heatrea and Ariston proclaim anyway...

 

[Agile]

Even if its only the shower, at least that should be connected to the balanced output after the regulator.

However, varying flows before the regulator will have more effect on the HW than if its after the regulator where any reduction will be balanced equally between the H & C .

Tony

 

[gas4you]

Chris, I can sort of see where you're coming from, but the way I look at it the hot and cold after the balancing valve can only remain balanced if the pressure before the valve is greater than what it is set for.

If you only have 2 bar incoming and then hot and cold after the valve will share this 2 bar, = balanced, if another balanced hot or cold is turned on the pressure will then be divided between 3 outlets, = pressure drop.

If the incoming pressure is say 4 bar then the valve reduces this to 3 bar so a balanced hot and cold will then share this 3 bar. If another balanced outlet is then used at the same time, the pressure drops, but as there is 1 bar 'spare' the valve opens instantly maintaining the pressure at 3 bar, thereby balancing and maintaining the 3 outlets pressure instantly.

Obviously any cold draw offs before the valve will not be included in this action.

This is my understanding of the balancing valve or am I completely off track??

 

[chrishutt]

Dave, the original point of the "balanced branch" connection for cold supplies taken off the combination valve (PRV) was to avoid having cold water supplies taken off upstream of the PRV at a much higher pressure than the hot supplies downstream of the PRV. For example a static mains pressure of 5 Bar and a PRV setting of 2.5 Bar would result in a shower valve having the cold inlet at twice the pressure of the hot.

However that situation only arises if the upstream pressure is significantly higher than the downstream pressure, which is of course limited by the PRV. If the upstream pressure is no higher than the downstream pressure (i.e. the mains pressure is no higher than the PRV setting) then it makes no significant difference whether you take off the cold supplies upstream or downstream of the PRV.

Of course there are other factors affecting dynamic pressures which will vary with flow rates and pipe resistances, but these apply more or less irrespective of whether mains pressure is higher or lower than PRV setting. Of course the exact location of the cold supply branch will have some bearing on this - for example a branch upstream of the PRV will suffer slightly less pressure loss than one downstream, but this is not really significant.

 

[Agile]

If the upstream pressure is no higher than the downstream pressure (i.e. the mains pressure is no higher than the PRV setting) then it makes no significant difference whether you take off the cold supplies upstream or downstream of the PRV.

There is always some resistance across the PRV so connecting the shower to the outlet will always be better.

In any case the mains inlet pressure can vary widely across the day.

Tony

 

[chrishutt]

There is always some resistance across the PRV so connecting the shower to the outlet will always be better.

Tony, I covered that point in my third paragraph above. The resistance across the open PRV isn't likely to be significant in relation to other variables.

 

[gas4you]

Cheers Chris, always willing to listen and learn

 

[plumbermitch]

New boy---my first (and maybe only!!) thread!!!
I read this thread with interest because my plumber is outside putting in a new OSO 210 indirect cylinder to replace a traditional cylinder which had burst.
My old system also had a a Monsoon twin negative duty 1.5 bar pump to get H and C water to an attic shower and was also used for a bidet (old-fashioned type with a spray in the bottom of the the bowl) with mains cold elsewhere as normal.
My question is this--will the new "all mains" system be ok to feed this bidet or do we run the risk of burnt nether regions if someone runs another tap somewhere while the bidet is in use?
Plumber seems only "fairly confident " that it will be alright as" if a tap is opened elsewhere the resulting pressure drop will affect both the H and C the same!
Any comment would be much appreciated.

 

[chrishutt]

-will the new "all mains" system be ok to feed this bidet

No. The Water Regulations do not allow that type of bidet to be connected direct to the mains. There is an obvious risk that soiled water could be drawn back into the mains if mains pressure failed. If you want a bidet you must get an over-rim type. Your plumber is probably not competent if he doesn't know this, which is worrying if he's installing an unvented cylinder. Does he have the unvented ticket? If he get's it wrong this could happen.

my first (and maybe only!!) thread!!!

Could well be.

 

[Agile]

There are a lot of nieve homeowners who dont check the credentials of people they get to do plumbing jobs.

He clearly does not have any Water Regulations qualification and may not have the necessary qualification for unvented which is essential if he is installing one so that he can notify it to the Building Regulations department.

Tony

 

[Dan Robinson]

Scary isn't it...