Hot Water Cylinder Overflow Explained

As hot water cylinder experts dealing with installation, repair and service, we receive plenty of enquiries about overflowing cylinders. At first glance, any hot water cylinder overflow might be alarming when you first see it. But in most cases, it’s simply a case of normal thermal expansion and nothing to be concerned about. Your cylinder is just doing what it is designed to do.

When water is heated it expands. That’s what we mean by thermal expansion. When the water in your cylinder is heated from cold, the volume can increase by as much as 3%; in a hot water cylinder with a capacity of 180 litres, that is as much as 5.4 litres. That extra water has to go somewhere! Hot water cylinders are manufactured to relieve this expanded hot water through an open vent pipe on the roof, or via a relief valve and pipe to an outside wall or drain.

We know of some cases where these vent or relief pipes have been blocked off by people in an attempt to stop the dripping. They’re obviously unaware that it is perfectly normal. In some cases, it has had disastrous consequences. In instances where a secondary safety vacuum break was not fitted, the water cylinder has exploded, blowing roofs off houses and buildings moving on their foundations. These days, however, cold water relief valves are installed to the inlet side of the hot water cylinder to save hot water.

The dripping relief pipe you’re noticing is probably experiencing normal thermal expansion, but it could also have a faulty valve. Here’s a simple test to determine what os going on:

  • Turn off the power supply to the tank for five minutes.
  • If the dripping stops, this is normal thermal expansion.
  • If the dripping continues, the valves are probably faulty and you need to contact us

If you have any concerns at all about your hot water cylinder, contact us and we’ll come to your place and sort things out for you. As we’ve said before, we are hot water cylinder experts. In some situations, a minor repair will greatly improve the performance of your system. But, if we see significant damage due to wear and tear or just old age, then we might suggest that you invest in a new hot water cylinder – financially, you’ll be far better off in the long run by ordering a replacement.


As registered Master Plumbers our work is covered by the Master Plumbers Guarantee for 12 months, giving you peace of mind.