Balanced pressure: Definitions

This is part 1 of a four-part article which have to be read together for the full presentation.

• • Balanced pressure continues to be misunderstood in terms of what the requirements are surrounding it
  • The reason why we experience things like unbalanced pressure is because the municipal mains coming into the property has throttled the incoming water supply
  • Older houses are more difficult to balance pressure than the newer houses which are plumbed differently

“Balanced pressure continues to be misunderstood in terms of what the requirements are surrounding it. ‘Unbalanced pressure’ is a term used to describe a state of unequal pressure between the hot water supply and the cold water supply to a mixing point. If you’ve got a cold supply, on the right-hand side there to a sink point, or a mixing point of 800 and the hot is 400, then the mix is unbalanced. If one isn’t equal to the other, then it’s unbalanced – that’s quite clear,” says Richard Bailie, member of PIRB’s technical advisory committee and IOPSA compliance auditor.

“The reason why we experience things like unbalanced pressure is because of the fact that the municipal mains coming into the property – and especially in the Cape where there’s a serious water crisis and a lot of municipal areas have throttled the incoming water supply – the pressure is lowered somewhat. It’s not always the case but is mostly the case where the municipal mains pressure incoming is much higher than that of the rating of the hot water cylinder.

“The hot water cylinder is a pressure vessel that needs to be tested and approved, and it must comply with certain product standards, in this case SANS 151 (Fixed electric storage water heaters). Within those tests it would be rated to withstand a continuous pressure of 400 kPa, for example, and can’t go beyond it. The pressure coming into the cylinder must be reduced down to 400 kPa.

“The hot water outlet from that is going to be at a lesser pressure than the municipal mains. Because the standards require that hot and cold water pressure to mixing points must be balanced, at some point we have to reduce the pressure of the cold water as well. Depending on how it’s been plumbed, we know that older houses are more difficult to balance pressure than the newer houses which are plumbed differently from the geyser outwards, while in an older house they tend to have been plumbed from the outwards inwards. This makes balancing pressure difficult. The reason for that is that 30 years ago, you hardly ever saw a mixer, you would always see two separate taps at a basin, sink or bath. The only mixer would be a shower.

“With an unbalanced system, you would usually have a situation where you’ve plumbed from the outside inwards with a main supply coming from the meter, a supply to the geyser through a pressure reducing valve (PRV), then a supply that continues on to the points etc, so you’ll see that these branches to the points have not been reduced in any manner. In terms of pressure, in order to balance this would be a challenge because you need to balance it prior to these points on the main supply, and that brings up other issues because there are shutoff valves between the geyser, there’s reduced pressure to high flow points such as garden taps, which should always be full pressure. Consequently, to balance an old home is quite tricky.

“The more modern way of planning an installation would be to have a main incoming supply to the geyser, then reduce it down and from there balance the pressure on the cold and plumb the points. There’s still the main outside line feeding the garden high-flow taps, and high-flow points such as a flush master, because that you can’t reduce and so on. That’s the fundamental difference between balanced and unbalanced pressure: one would plumb the bathroom and kitchen after the pressure reducing valve.

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