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Do I need an Expansion Vessel for my electric water heater?

Expansion vessel and check valveOne of the most common questions we get from customers about electric water heaters is whether they need to buy an expansion vessel with it. The reason that some heaters need to cope with this expansion is that when water is heated, it expands. If this is not allowed for then heater tanks or pipes will fracture and split, causing a catastrophic failure of the heater.

Thankfully, the answer is pretty straightforward, and can be easily summarised as below:

Firstly, if you have an instantaneous water heater, regardless of its size and capacity, you do NOT need any expansion vessel. This is because an instantaneous water heater does not store any water and so there is nothing to expand.

Secondly, if you have a vented water heater, there is no need to have any form of expansion vessel, because the expansion is taken care of by being vented to the atmosphere through the special vented taps.

So you only need to consider an expansion vessel if you have an unvented water heater.

If the unvented water heater is over 30 litres, you will need an expansion vessel, but it will come packaged with the water heater when you buy it (for certain larger heaters, the expansion vessel is actually incorporated in the heater itself, so there is nothing external to install).

If the unvented water heater is 30 litres or less, then this is the only time you need to get your tape measure out and do some checking. What you need to measure is the distance of the pipework from the water heater, BACK along the pipe run to either

  1. the nearest valve (such as a check valve or non-return valve which may prevent reverse flow), OR
  2. the nearest supply pipe to which a cold water draw off is connected.

NOTE: this is NOT the length of pipework from the heater to the tap, it is from the heater backwards along the inlet pipe.

This length of (15mm) pipe is where the heated water will expand into, and the length of pipe you need varies according to the capacity of the heater:

10 litre water heater – 2.8 metres of 15mm pipework.

15 litre water heater – 4.2 metres of 15mm pipework.

30 litre water heater – 8.4 metres of 15mm pipework.

This diagram (from the Heatrae Sadia manual) demonstrates one configuration:Heatrae Sadia Multipoint Expansion vessel diagram

If your pipe-run is less than this, you will need an expansion vessel, and these are available from different manufacturers as kits. They usually come with a check valve (which stops the water expanding past it, and so forces it into the expansion vessel). They may also come with a pressure reducing valve - you will need one of these if your water pressure is above about 4bar. They are essential in those cases because without them, the water will fill your expansion vessel even before your water heater has started working.