Mixer taps and WRAS

[Tricky Dicky]

Quite a while back I recall that to meet WRAS regulations mixer taps had to have back flow valves and one that we installed at our last house actually came with two back flow valves in the package.

I have not seen mention of this with modern mixers. So my question is do modern mixer taps have the back flow valves integral like some of the bib taps or are you still required to fit the back flow valves? If a mixer or tap says WRAS approved is that an indication that the whole thing complies with back flow valves included?

[John Rumm]

It depends a bit on the type of mixer. Many mixer taps are concentric spout designs, where the mixing happens after the water has left the tap. This is common in kitchen sink mixers for example. These don't require additional back flow prevention.

When you have a tap that mixes internally, and you have one side fed directly from the rising cold main, then you should have a non return valve on the cold side. If you are mixing mains cold and gravity fed hot, then you will need one on both sides to prevent it forcing cold mains water back into your cylinder!

(I expect in reality there are loads of internal bathroom basin mixers out there being fed with mains pressure hot and cold from a combi boiler that lack backflow protection)

[The Natural Philosopher]

I suspect that modern mains pressures are now so low they are no longer needed

[Tricky Dicky]

TNP the purpose of the back flow valves is to prevent contamination by mixing incoming mains with DHW. With taps this is not a problem but it depends on how the two are mixed with a mixer tap. As far as pressure goes we are fine our water works are a mere few hundred metres away on a hill across the valley.

[Fredxx]

I've often wondered why there isn't a requirement to fit a double check valve in the DWH circuit before cold water enters a combi boiler.

[NY]

I can see the need if the tap is fed from the mains (cold) and gravity (hot), but is WRAS saying that a valve is needed if both cold and hot are from mains (either hot via a mains-fed combi boiler that heats on demand, or from a cylinder that is mains-fed)?

[John Rumm]

On most combis the cold main only flows through the PHE, and a tap - so no direct way it can get contaminated...

(mixing devices excepted)

[John Rumm]

I have not looked at the WRAS requirements... can't see an obvious need on a combi, and most (larger) unvented cylinder include a non return valve as a component of the main inlet control valve

(and small unvented cylinders may actually need no check valve on the inlet so that they can deal with water expansion by allowing it to push some water so back into the cold main)

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[Max Demian]

You might leave the filling loop on after repressurising the CH system, and the valves fail.

[John Rumm]

Filling loops seem to go for belt, braces, and tin knickers... (i.e. a tap, a non return valve, and a detachable hose).