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Blowing up a hot water cylinder

I made this GIF from an experiment done by Myth Busters to show the extent of damage a hot water cylinder can do when there is no pressure relief. blowing-up-hwc

The image above reinforces the importance of using individuals that understand the physics of hot water and the effects an installation can have on its surroundings.

So at what point will a cylinder explode?

In principal it’s simple to understand. Heating a substance causes molecules to speed up and spread slightly further apart, occupying a larger volume that results in a decrease in density. So “larger volume” means “more space” which is limited to the size of the hot water cylinder.

If hot water is not used and your thermostat fails, your immersion element will continue heating your water (to boiling point) until such time one of the pressure relief valves activates and relieves the pressure. Every installation should include two relief valves, one on the cold water side and one on the hot water side.

This regulation was put in place around 1997 meaning that any cylinders installed prior to that may only have one source of pressure relief. Hence the low pressure vent copper pipe going through the roof.

This is where it gets tricky. Many installers dont bother replacing older valves when replacing an old hot water cylinder with a new one. Others have managed to install a tempering valve on a low pressure system right into the overflow pipe, blocking the only means of pressure relief. The hot water system then becomes a time bomb which may result in something pictured above.

Sometimes its not the plumbers’ fault but rather the homeowner who sees a pipe dripping on the outside of a house and thinks.. “Oh, this should not be leaking” and decides to “blank” it off by any means available, creating the perfect conditions for a catastrophe. My recommendation on this is, see a dripping pipe? Please call the plumber.

If you are unsure about what valves you need or already have, send us an email or give us a call for help.


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Cold Water Expansion Valves, Also Called Energy Saving Valves

Very often we get clients calling us saying there is a pipe that is constantly dripping outside their house and they want it fixed so that it stops. The first thing that we do Is try to educate the clients on what the role of this valve is along with the rest of the valves on the hot water cylinder. So what does a Cold Water Expansion valve do?

When your cylinder is filling up with cold water at a high pressure after; let’s say you have had a hot shower, the element comes on to heat the water back up to its pre-set maximum temperature of around 60 degrees Celsius.

The water being heated causes molecules to expand and increase pressure exponentially in an already high pressure hot water system exceeding normal operating pressure.

The Cold Water Expansion has 2 roles in this situation,

  1. it’s a safety mechanism that releases excess pressure from your hot water cylinder as it heats up to avoid over pressurizing protecting the system as a primary means of release. The secondary means of release is the TPR valve which activates when the CWEV does no longer copes.
  2. Whilst this valve acts as a pressure relieve valve, Its primary role is power energy saving. It is installed on the cold line, typically close to where the cold water enters the cylinder. The pressure relief is set below the primary relief (in KPA) allowing it to drip cold water instead of hot water when expansion occurs before the other valves at the top of your cylinder (TPR valve). By doing so, it reduces hot water waste and results in a lower energy bill.

On hot days with low hot water consumption, the cold water expansion valve can run up to 10 liters per hour! Remember, that by doing so you are saving on power. The alternative would be, less water wastage at a higher pressure via the TPR valve but a higher power bill. The logic is, water is cheaper than power.

Whilst the valve is designed to drip while the cylinder is heating, if yours is dripping excessively and you believe this is not normal, it may indicate it’s time get us in to replace the valve or investigate other issues that may be forcing the valve to drip.

How do I know my cold water expansion valve is working?

Check that the valve on-top of your cylinder (TPR) is not discharging any water because if it is then the cold water expansion valve isn’t doing its job. Also, if you manually turn on the cold water expansion valve to discharge water (turn blue knob) look for discoloration of the water as this indicates the valve is rusted internally or the rubber seal has perished and is no longer effective.

Why not book in a hot water cylinder service to do a full system check? Call us today.