Balanced pressure: Masterbox

This is part 2 of a four-part article which have to be read together for the full presentation.

• • Two problems immediately become highlighted when using a Masterbox installation
  • Every plumbing system will have some sort of friction loss within it
  • The location of the hot water cylinder needs to be serviceable and the need to be easily replaceable and generally worked on

“In another example of how one would balance an older property that’s plumbed from the outside inwards:  You can have a sort of Masterbox arrangement on the outside, though this does cause certain problems with restricting the flow to high-flow points, which is actually in the standards. The standards say ‘you shall not reduce pressure before or upstream of any high draw off water points such as taps’. Therefore, these, as far as possible, should be supplied with full municipal mains pressure,” says Richard Bailie, member of PIRB’s technical advisory committee and IOPSA compliance auditor.

“But further to that there is a situation whereby, if one is supplying a geyser, two problems immediately become highlighted when using a Masterbox installation – depending on the distances. The first is the distance itself: when you have higher pressure systems such as 400 kPa. The manufacturers – though it is not in the standards – will say they don’t recommend that you install a PRV further than 10 linear meters away from the geyser, because of friction loss. By the time the water has travelled 10 meters it’s already lost some head – they call it friction loss. And if it’s 15 or 20 meters, or sometimes even further than that, of the section of pipe between the Masterbox and the geyser, you’re losing significant pressure. It’s not recommended.

“The second problem is the location of the hot water cylinder, which needs to be serviceable and the need to be easily replaceable and generally worked on. In the instance of a double storey house, or a property designed in such a way that the geyser is difficult to get to, one can’t treat that as a geyser shut off, because it first of all shuts off the whole house, including outside taps and toilets. Therefore, you need to put a shutoff valve at the geyser whereby you put a valve between the pressure relief component at the top [between] the tank – which is not allowed. You have to include an additional pressure relief valve at the geyser on a T with an overflow pipe. This is fine as long as you can get access to the shut-off valve to maintain the geyser and it leaves nothing between the relief component of the geyser and the geyser itself.

“It’s critical to include this relief component. And they’re available as a half inch male thread screwed into a T with a female on the anti siphon loop and take an overflow out.

Static pressure

‘But the trick is that balanced pressure doesn’t refer just to static pressure. If you look at the standard SANS 10252-1 (Water supply and drainage for buildings Part 1: Water supply) it says the following: ‘that water to mixing components must be balanced so that the residual dynamic pressure does not vary by more than 20%’. This requires that the layout and the pipe sizes shall be correctly calculated in terms of 10252-1. If you haven’t looked at that standard, there’s a section in there that is dedicated to pipe sizing. It comes with various calculations of friction loss, and head loss through not only pipes but various types of valves and fittings. The net result of having a network of pipes – in its simplest terms – with water or fluid running through them, is that the more length you have the more friction loss will occur. And the more changes of direction you have, the more friction loss will occur. And the more valves and so on, it needs to flow through, the same will apply.

“Every plumbing system will have some sort of friction loss within it. Consequently, if you don’t size your pipes correctly, and you have for instance a three-bedroom property with two and a half bathrooms supplied with a half inch copper pipe – or even a 22mm multilayer pipe – if you do the maths from the geyser you will find that it is not a balanced system, but rather has a residual dynamic pressure. This means when someone is having a shower it affects the pressure at which the hot and cold comes out and when somebody flushes a toilet or opens a sink somewhere else in the property, the drop or change in flow rate at that point may not vary by more than 20% of what it was.

“But typically there’s a situation of someone having a shower, and somebody opens a sink, and it varies by 40%, and that shower gets cold. As a result of getting cold, they shut off the cold water in a panic and when the sink or tap is later closed, then they get hit by hot water only and may get scalded. That’s the only reason behind the requirement for having balanced pressure, which refers most importantly to residual dynamic pressure.

“As to how one figures out the balanced pressure, one’s got to actually measure it. If one follows the recommendations and guidelines, in terms of pipe sizing, and size the pipes correctly within the property, and route them nicely this situation won’t occur and there will be a balanced system. It just requires putting a little effort into it, perhaps be willing to spend a little bit of money and time on it. It is something that should be done at design phase, then costed into quotations.

“However, construction companies and developers are the ones that hold the purse strings and they’re the ones that usually pinch on costs. But ultimately if that installation is audited, you the plumber are going to be found out if it’s not a balanced pressure.

Written by Eamonn Ryan based on an IOPSA Tech Talk by Richard Bailie on Balanced pressure