What does a combination valve on a water cylinder do?

A combination valve is an important component on many unvented hot water cylinders. It normally combines several functions into one valve assembly, including pressure reduction, non-return (check) protection and pressure regulation of the incoming mains water.

Its job is to make sure the water entering the cylinder is at a suitable pressure and to prevent hot water from flowing back into the mains supply.

If the combination valve is faulty, you might notice:

- Poor or fluctuating hot-water pressure.
- Water coming from the expansion or discharge pipe.
- The cylinder pressure becoming too high.
- Knocking or water hammer when taps are opened or closed.
- Reduced hot-water flow.
- Are they expensive to replace?

The valve itself isn't normally one of the most expensive components on an unvented cylinder, but the cost varies considerably depending on the make and model of cylinder and the exact valve required. There is also the labour involved, because the cylinder needs to be isolated, depressurised and the system recommissioned correctly.

I'd be wary of replacing it simply because someone has said it is faulty. A G3-qualified engineer should diagnose the problem and establish whether the combination valve is actually causing it. Sometimes a pressure issue can be caused by another component, such as the expansion vessel or expansion relief valve.

See: https://easyflow.co.uk/articles/what-is-a-combination-valve-understanding-unvented-cylinder-components

And for pricing, See: https://easyflow.co.uk/price-advantage