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Unvented Cylinder Remedial Work


We were recently called to a property in Braemar to have a look at a bucket that was continuously overflowing. What we found was dangerous and had to be fixed ASAP.

What’s wrong?

Unvented cylinders are dangerous if not installed properly. If all the safety mechanisms aren’t in place they can cause the cylinder to explode and the explosion is said to be worse than that of gas. All installations should have the following:-

  • Suitably sized expansion vessel
  • Adjustable thermostat on the immersion
  • Overheat cut out function on the immersion
  • Pressure and temperature release valve (PRV)
  • Pressure reducing valve
  • Tundish connected to the PRV and pressure reducing valve, suitably terminated

The installation we attended didn’t have a expansion vessel and the tundish was left to discharge into a bucket.

So what was happening?

Because there was no expansion vessel, as the water in the cylinder was heating up and expanding, the extra volume had no-where to go (the expansion vessel would usually take care of this) and as a result would trigger the PRV and discharge into the bucket. This happened every time some hot water was used. The bucket would then overflow if it wasn’t emptied in time.

The Solution

We fitted an expansion vessel on the cold water inlet, moved the pressure release valve to a higher location in order for the pressure release valve to discharge correctly. A McApline Mactun was used and correctly fitted to discharge into a waste pipe.


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