Non-return valves for mixer tap

[LlMikeybll]

Hi all, I'm planning on fitting my new sink which has a mixer tap. I'm using an unvented system, will this still require non-return valves fitted to the taps connections?

 

[Stigster]

Edited -

Sorry, for some reason I read "vented" instead of "unvented" first time I looked and answered. With cold mains feeding the tap and also the unvented cylinder you should not need non-return valves. Unless you have very high pressure on the cold feed it should be fine.

 

[LlMikeybll]

Sorry, for some reason I read "vented" instead of "unvented" first time I looked and answered. With cold mains feeding the tap and also the unvented cylinder you should not need non-return valves.

I'm using an unvented

Edited -

Sorry, for some reason I read "vented" instead of "unvented" first time I looked and answered. With cold mains feeding the tap and also the unvented cylinder you should not need non-return valves. Unless you have very high pressure on the cold feed it should be fine.

The cold comes straight from the mains and the hot from the cylinder, the cold mains is at high pressure though. Would the pressure from the cold not cause backflow to the cylinder or is there usually a non return valve at the cylinder to prevent that?

 

[ShaunCorbs]

Nope non needed providing the tap is suitable for the uk

 

[Stigster]

I'm using an unvented

The cold comes straight from the mains and the hot from the cylinder, the cold mains is at high pressure though. Would the pressure from the cold not cause backflow to the cylinder or is there usually a non return valve at the cylinder to prevent that?

Yes, there's a non-return in the combination valve (aka "Group Inlet") which will not allow water to feed back to the cylinder no matter how unbalanced the pressures are in the hot and cold feeds.

The issue occurs when you have cold mains and gravity (tank fed) hot, the cold can overpower the flow of hot water inside the mixer tap (even then that depends upon the tap design and how the waterways are made) and feed back up to the header tank and cause it to overflow.

 

[Plumb Tidy]

Yes, there's a non-return in the combination valve (aka "Group Inlet") which will not allow water to feed back to the cylinder no matter how unbalanced the pressures are in the hot and cold feeds.

The issue occurs when you have cold mains and gravity (tank fed) hot, the cold can overpower the flow of hot water inside the mixer tap (even then that depends upon the tap design and how the waterways are made) and feed back up to the header tank and cause it to overflow.

What if the tap was a single flow mixer tap would the high pressure cold not force its way back through the hot and back to the cylinder? Its normally what happens in bar mixer showers that's all if the cold is not taken off the balanced feed from the combination valve. I came across a domestic property in south wales where the mains was over 12 bar it almost broke my gauge!

 

[Stigster]

I don't see how any pressure of cold mains could push water back to the cylinder because of the non-return in the group inlet. That would need to let by for that to happen and given the design of the non-return I cannot see how water could backfeed through that provided it was in good working condition.

The harder the water pushes against the non-return, the harder it pushes it shut and keeps it shut so under normal conditions, even 12 bar cold mains, I can't see that happening.

It would set off the expansion relief port though I suppose. If the pressure were that unbalanced then a check valve is not the answer, pressure reducing valve on the mains would be the thing to do if you had 12 bar coming in.