Unvented Water Heaters and Expansion Vessel Kits

Many customers ask us if they need to buy an expansion vessel with their unvented water heater tank purchase. The answer is Yes or No. As water heats in the tank it expands. This expansion can cause the heater tanks, connections or pipes to fracture and split, causing a catastrophic failure of the heater.

So, you need to consider your setup and you may (or may not) need an SF3 expansion vessel kit.

Note: Expansion vessels are not required if you have a vented tank or instantaneous water heater, regardless of its size and capacity, you do NOT need an expansion vessel. This is because an instantaneous water heater does not store any water and so there is nothing to expand and the vented tanks will vent from the vented taps or spout outlet so are an open outlet system.

You only need to check if an expansion vessel is required if you are purchasing an unvented water heater.

For 30 litres or more, you will need an expansion vessel every time, usually these size tanks will have a vessel packaged with the water heater when you buy it.

If the unvented water heater is less than 30 litres, then this is the only time you need to check your install.

The Check is as follows. You need to measure the distance of the pipework from the cold inlet of the water heater to…

1. the nearest cold water draw off “Cold Tap” OR
2. the nearest check valve “non-return valve” which is one way so will stop reverse flow.

NOTE: So… You are measuring the pipe length from the cold inlet, usually to the nearest cold tap. Sometimes from a previous install there may be a check valve which would also affect the backflow. These should be removed if you are not using a vessel. Or left on if you are fitting a vessel.

This run of pipe will need to be a certain length depending on your tank capacity. It will expand and take strain away from the tank as the water heats up and pushes back down the cold pipe.

Recommended lengths of cold pipe from cold inlet on tank to nearest cold tap is as follows:

5 litre water heater – 1.4 metres of 15mm pipework from tank to tap.

10 litre water heater – 2.8 metres of 15mm pipework from tank to tap.

15 litre water heater – 4.2 metres of 15mm pipework from tank to tap.

If your cold line pipe-run is less than this, you will need an expansion vessel kit. See SF3 on our search bar.

They come with a check valve to stop the water expanding past it thus forcing it into the expansion vessel. The SF4 option kit also come with a pressure reducing valve for high water pressure areas where your mains IN water pressure is above 4bar approx. The SF4 kits are essential in preventing any high water pressure filling your expansion vessel even before your water heater is switched on.