Shore bar valve replacement

If your home is only around 5 years old, it's likely you have an unvented hot water cylinder (pressurised system) rather than a traditional cold water tank. In many installations there are isolation valves on the hot and cold pipework feeding the shower, but they're often hidden behind the wall, in a ceiling void, or in an airing cupboard near the cylinder.

Simply turning off the mains water and opening the taps will stop the water supply, but it won't necessarily drain an unvented cylinder completely, as it remains full of water under normal conditions. This won't normally damage the boiler or cylinder, but you should avoid running the hot water or heating system while the water supply is isolated.

Before draining anything, I'd recommend checking for isolation valves on the shower supplies or near the hot water cylinder. If there aren't any, it's often worth having them fitted while the mixer valve is being replaced, as they'll make future maintenance much easier.

If you can let us know whether you have a combi boiler or an unvented hot water cylinder (such as a Megaflo), or upload a photo of the cylinder and boiler, we can advise on the safest procedure.

Usually if have an unvented hot water tank with a combi when you turn off mains water supply this normally stop hot water..
If not should have an inlet and outlet isolating valve fitted....to be safe when turning water back on ensure have a hot water tap open to vent any air

Turn off the mains water supply, and open the taps to relieve pressure. The boiler’s heating circuit won’t be affected if you have an unvented cylinder. Isolation can be checked through the valves near the cylinder or pipework for the shower. If there are no valves, you may need to drain down before replacing the shower mixer bar valve

Simply turning off the mains water and opening taps will stop the water supply, but it won’t necessarily drain the cylinder completely, and you generally don’t want to drain an unvented cylinder unless necessary.

Yes, you can shut off the main water supply and drain down to change the shower valve.

It won’t damage the boiler or cylinder, but:
• Turn off the boiler and immersion first
• Close the main stopcock
• Open taps (including the shower) to drain pressure
• Expect some residual water from the tank

Most pressurised systems (unvented cylinders) do have isolation valves nearby, but if you’re unsure, shutting the main and draining is the safe method.

You should be fine to isolate the mains water and drain the system down to replace the shower bar valve. On a modern pressurised system, closing the main stopcock will stop the incoming supply, and once you open taps to relieve pressure, the system will drain back without damaging the boiler or tank.
Most unvented cylinders (pressurised tanks) will have their own isolation valves on the inlet, but if you’re unsure, the safest method is to turn off the mains, open the lowest taps in the house, and let the pressure drop before removing the valve.
Just be aware there may still be a small amount of residual water in the pipework when you remove the bar valve, so have towels or a bucket ready.
If you’re not confident or can’t locate isolation points, it’s always worth getting someone in to avoid any unnecessary leaks.

For a modern UK system with a pressurised hot water cylinder, you should be careful before removing a shower bar valve.

Turning off the mains water supply will stop the cold feed, but the system may still hold pressure from the cylinder. In most setups, there is usually an isolation valve on the cold feed to the cylinder, and sometimes on the hot outlet as well, but this depends on the installation.

Before starting, I would recommend:
• Turn off the mains water supply
• Switch off the boiler and hot water system
• Open taps to relieve pressure and drain down the system

If there are no visible isolation valves for the shower, you may need to drain the system partially. However, it’s important to avoid running the cylinder dry or causing airlocks.

If unsure, it’s safest to have a qualified plumber carry out the work, as pressurised systems can be sensitive and need proper handling.

• Turn off main stop tap.
• Open lowest cold tap to drain pressure.
• Open a hot tap too.
• This won’t drain or harm the unvented tank.
• The cylinder stays full and safe.
• You can now swap the mixer.
• Expect only a small bit of water in the pipes.

Turn boiler/immersion off first. If it’s a pressurised/unvented cylinder, you usually don’t need to drain the cylinder to change a bar mixer. Best is to find and close the hot + cold isolation valves feeding the shower (often in an airing cupboard, behind an access panel, or under the bath).

If there are no local isolations, you can shut the main stop tap (or the service valve on the cylinder’s cold inlet group if fitted) and then depressurise by opening the shower and a cold tap until flow stops. That won’t damage the cylinder/boiler, but don’t run the boiler with the water isolated. Refill slowly and check for leaks.

Avoid disturbing any safety/relief valves on an unvented cylinder—get a qualified plumber if unsure.