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Balanced pressure: The Formula

This is part 3 of a four-part article which has to be read together for the full presentation.

    • “It’s a very simple formula”
    • SANS 10252-1 contains all the information
    • In an older house where they have separate taps for most points except the shower – in the event of an unbalanced system, how do you balance?

“It’s a very simple formula: it’s the litres of hot water, multiplied by the temperature of the hot water plus the litres of cold water multiplied by the temperature of cold water over how many cold litres plus how many hot litres equals the temperature mix. If one is having a shower at 38 degrees Celsius under a head that is delivering 10 litres a minute, let’s say it’s supplying five litres per minute, hot water at 63 degrees Celsius and five litres a minute, the cold water at 13 degrees Celsius. If the cold-water pressure and flow rate drops by 20% from five litres per minute to four litres per minute, the mixed temperature changes from 38 to 40.8 degrees Celsius, it’s warmer but not dangerous,” says Richard Bailie, a member of PIRB’s technical advisory committee and IOPSA compliance auditor.

“That is an example of dynamic pressure alters but it doesn’t alter by more than 20%, which is key to the residual dynamic pressure after a point, or any number of points has been opened. In other words, you’ve got to ensure that your pipes are big enough to supply all points that are likely to be used simultaneously. SANS 10252-1 contains all this information.

“There are factors which are included in calculations when we’re determining pipe size factors which take into account the likelihood of various points or different types of points being used simultaneously. It gives that factor for various installations. If you follow those steps – and it’s quite easy to do – you’ll find that you’ll never have a problem with scalding. It’s not as if you would all of a sudden be plumbing a normal two-bedroom house in 28mm copper pipe – it’s just traditional plumbing with one or two areas that need to be upsized.

“To take another example. In an older house where they have separate taps for most points except the shower – in the event of an unbalanced system, how do you balance? Nine times out of 10 in those old houses you also have a gravity fed geyser. If you’ve got a tank type system or a closed vented geyser supplied by a tank it’s still a tank fed situation without a mains incoming supply. This is going to be reduced by PRV which you can utilise to supply a balanced shower. If that’s not the case, you can just take it off whatever that main supply is, it’s the same head of water, which is supplying your geyser.

“You can just cut it into the cold-water supply to the geyser. In the case of a gravity fed geyser or an assistant type geyser, you can cut a little port into the storage tank of that geyser and in a normal combination geyser a little 20mm hole saw, a half inch brass nipple, and two brass back nuts with two fibre washers, and you put a stopcock on, and you’ve balanced the cold supply to shower.

The effects of unbalanced pressure

“That’s the case with older houses, and there’s still a lot of these geysers around, as well as the fact many are still being installed today because they will last 30 years and so many people actually prefer them.

“The effects of unbalanced pressure are quite numerous: we have a backflow situation where cold water overwhelms the hot water and starts to push backwards into the geyser. It starts to over pressurise valves which have reverse flow from the geyser into the valve and it usually picks up a lot of sediment at the bottom of the geyser. It goes through the inlet of the geyser and backwards into the valve which starts to pick up a lot of dirt – and the warranties could be voided. Valves could be damaged permanently, and geysers could be damaged from over pressurisation if the PRV starts to act up because of dirt.

There are quite a few other negative symptoms:

    • Difficulty in mixing
    • Scalding, especially when you’ve got infirm people or small kids trying to have a shower
    • Tap damage
    • There is a significant risk of water hammer
    • Other valves, for example, thermostatic mixers will not function if you have unbalanced pressure on a line that requires a thermostatic mixer

“When you look at inline stock taps for example, like a shower – an old shower with two inline stock taps as shutoffs and the old jumper type stop tap with the correct direction of flow, the water pushes up the jumper upwards and outwards and the water is able to flow. As you lift the spindle and the jumper lifts, the water aids that action and helps to lift that spindle in the opposite direction of flow. Then you have water flowing from the wrong way backwards because of back pressure. Sometimes that spindle is loose and could result in the spindle going against the seat and not allowing any flow through or, as usually happens, it starts to judder and you have some serious water hammer issues.

“A lot of us do this: we install non-return valves on unbalanced systems as part of the replacement of geysers. When geysers are replaced, and it is an unbalanced pressure system, a non-return valve is installed to mitigate the effects of back pressure. Now that is certainly a step towards improvement of the situation and a help to the homeowner. But it’s important to know that it doesn’t constitute balanced pressure or mean that the system is now balanced.

“Just because a non-return valve has been installed doesn’t now make the system compliant and it’s not a remedy. It’s simply a band aid.”

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

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