Please help - actual dual-flow kitchen mixer taps?

[DIY10291]

Hi. Today my kitchen mixer tap broke (head snapped off the cartridge). Pic of the tap:

From a quick Google, it's an obscure cartridge so I'm better replacing the tap. However, I have an old heating system with a Baxi back boiler and a cold tank in the loft and hot water in upstairs airing cupboard. According to a post I made last year (https://www.diynot.com/diy/threads/replacing-old-designs-of-taps-with-new.612100/ - thanks @Madrab and others) I need a dual flow mixer tap to avoid backflow (I'm not too concerned about output pressure; the dishwasher does the washing up and I wash my hands in the bathroom, so it's mostly just for cold water). However, between Screwfix, Wickes and Toolstation it's almost impossible to find a kitchen mixer tap labelled as dual flow. I found and bought https://www.wickes.co.uk/Wickes-Toba-Monobloc-Kitchen-Sink-Mixer-Tap---Chrome/p/180982 : "Dual flow tube spout", "Suitable for both high- and low-pressure water systems" and min pressure of 0.2 bar, which sounds good. It also has the design I wanted (separate hot and cold levers):

However, I opened it, unscrewed the aerator and looked in. I can't see any separate channels, and I was able to push a piece of cardboard through about half of the way (the length of the cardboard, which was the diameter of the spout, ish). Does this mean the tap is unsuitable and was mislabelled in three places? And if so, are there any actual options for a dual-flow tap these days from somewhere like Wickes, TS or SF? I found https://www.magnettrade.co.uk/products/axiom-dual-flow-tap-chrome/ but doesn't look like they sell to DIYers.

And on installation, the current setup is:

To remove the current tap, I just undo the nuts on that bolt using https://www.wickes.co.uk/Rothenberger-Monobloc-Tap-Spanner-Set/p/118702 right? And the spiral clamp thing will just come off? And then for the water inlets, it is all in rigid copper now, no flexi hoses. It looks like the pipe is tapered from 15mm to M8(?) using something like https://www.showerspares.com/search/bristan-15mm-copper-pipe-and-m8-adaptor-pair-sk420013/ - so should I use this tapered copper pipe, or should I cut it once it gets to full 15mm (above or below where the 15mm-to-m8 is currently soldered, just below where the picture ends?) Will I need a double check valve? My current plan was to use the flexible tails it comes with, along with https://www.toolstation.com/flat-faced-male-straight-isolating-valve/p95064 to have an isolator too, but it would be cleaner to have just one piece - something like https://www.wickes.co.uk/Primaflow-...onnector---15-X-12-X-300mm-Pack-Of-2/p/160136 to connect the 15mm copper to the tap directly, but I can't find one of those with an isolator valve. Although if I need a double check valve I'll need to change plan again, and maybe skip the isolator, as there's not too much room under the sink.

Would really appreciate any help please!

 

[Harry Bloomfield]

My understanding, is that - stored water and fresh water should never be allowed/able to mix, which means the spout needs to separate the two, all the way to the exit of the spout.

 

[Madrab]

The fact that they state that the tap has a dual flow spout should mean that there are 2 separate flows into the (low pressure) spout to stop the water from fighting each other in the body of the tap, with the higher pressure always winning out. When you look at the exploded diagram though, you can see that there is an adapter that mixes the 2 flows, one outer flow and one inner, just after the body at the bottom of the spout which will probably achieve the same thing. It means that it would act like a venturi rather than the flows being able to oppose each other.

Old designs of dual flow would have the separate pipe running internally up to the end of the spout and then mixed at the end. I'd give it a go and see how it runs.

Yes - loosen the nut and the metal C clamp and rubber washer will drop off. Undo one of the copper tails and the old tap will lift out. If the new tap has flexi's then how that connects to the supply pipework will be different and that should really have ISO valves. You can then file down the outside edges of the ISO's for the flexi's to fit to (to avoid the sharp edge of the valve cutting into the rubber seals in the flexi's) or use tap tail connectors in the ends of the ISO's for the female flexi's to fit on to.

 

[DIY10291]

The fact that they state that the tap has a dual flow spout should mean that there are 2 separate flows into the (low pressure) spout to stop the water from fighting each other in the body of the tap, with the higher pressure always winning out. When you look at the exploded diagram though, you can see that there is an adapter that mixes the 2 flows, one outer flow and one inner, just after the body at the bottom of the spout which will probably achieve the same thing. It means that it would act like a venturi rather than the flows being able to oppose each other.

Old designs of dual flow would have the separate pipe running internally up to the end of the spout and then mixed at the end. I'd give it a go and see how it runs.

Yes - loosen the nut and the metal C clamp and rubber washer will drop off. Undo one of the copper tails and the old tap will lift out. If the new tap has flexi's then how that connects to the supply pipework will be different and that should really have ISO valves. You can then file down the outside edges of the ISO's for the flexi's to fit to (to avoid the sharp edge of the valve cutting into the rubber seals in the flexi's) or use tap tail connectors in the ends of the ISO's for the female flexi's to fit on to.

Great, thank you. Perhaps many of the taps available now that state they can handle 0.3 bars but claim to be single flow have a similar mechanism to prevent backflow?

Sorry a few more follow-up questions:

1) If I install it and it seems to be running fine, how would I be sure I'm not getting backflow still?
2) If the adapter works as expected, that means no check valves needed?
3) If I do away with the current 15mm copper tapered to M8, and cut it at a 15mm section, can I use the supplied tap connectors (1/2", attach them to https://www.toolstation.com/flat-faced-male-straight-isolating-valve/p95064 for both hot and cold, then attach that to the 15mm hot/cold inlets, so no filing required (because it is flat faced) and no tap tail connectors required because it is straight tap -> 1/2" -> 15mm -> pipe?)

Thanks again

 

[muggles]

You need to ensure any tap you fit is WRAS Approved

 

[Johntheo5]

1) If I install it and it seems to be running fine, how would I be sure I'm not getting backflow still?

One (maybe crude) way is to open the cold (mains pressure) tap fully and then open the hot fully and see does the flowrate increase, it does on my 15 year old Franke Moselle, twin tap, bi-flow spout with swival aerator, it has 15mm copper tails.

 

[Madrab]

Great, thank you. Perhaps many of the taps available now that state they can handle 0.3 bars but claim to be single flow have a similar mechanism to prevent backflow?

Not really - any fitting or appliance that claims to have backflow protection would need to use a certain mechanism to facilitate that, check valves/air gaps etc. Just because a tap can maintain a flow at low pressure just means that it uses an internal design and parts that are less restrictive to flow.

As suggested, any appliance connected to the mains water should be WRAS approved.

Are you talking about backflow in the case of unbalanced supplies and say the cold mains impacting on the hot? If so all you can do is what you are doing by using a tap that's design aids in doing that

can I use the supplied tap connectors

What are the current taps copper tails attached to?

 

[DIY10291]

Not really - any fitting or appliance that claims to have backflow protection would need to use a certain mechanism to facilitate that, check valves/air gaps etc. Just because a tap can maintain a flow at low pressure just means that it uses an internal design and parts that are less restrictive to flow.

As suggested, any appliance connected to the mains water should be WRAS approved.

Are you talking about backflow in the case of unbalanced supplies and say the cold mains impacting on the hot? If so all you can do is what you are doing by using a tap that's design aids in doing that

What are the current taps copper tails attached to?

Ah - the Wickes tap, the only dual flow I could find, doesn't say anything about it being WRAS approved!

Yes - I'm concerned about the cold mains heading back into the hot supply - not sure if the venturi adaptor you noticed will be sufficient to deal with that, or if I need a double check valve on the hot (and cold?) too.

The current pipework is 15mm copper which tapers (I mean gets thinner) into the tap directly - looks like one of these things: https://www.showerspares.com/search/bristan-15mm-copper-pipe-and-m8-adaptor-pair-sk420013/ . But I'm guessing this time it'd be easier to cut away below where this is soldered in and replace with flexis (+ isolators +possibly double check valves)

 

[Harry Bloomfield]

Ah - the Wickes tap, the only dual flow I could find, doesn't say anything about it being WRAS approved!

Yes - I'm concerned about the cold mains heading back into the hot supply - not sure if the venturi adaptor you noticed will be sufficient to deal with that, or if I need a double check valve on the hot (and cold?) too.

WRAS will be more concerned, with your stored water, from your cold tank, being able to get back into the mains. If the mains pressure goes off, and you turn on your hot tap, blocking the spout, your contaminated hot water could make its way back into the mains.

 

[Madrab]

Ah - the Wickes tap, the only dual flow I could find, doesn't say anything about it being WRAS approved!

As far as I know most if not all stuff sold by the big sheds would be WRAS approved

where this is soldered in and replace with flexis

Ah ok, so the current tails are soldered to the supply pipework, yep, need cut and then ISO's added. I wouldn't add double check valves, certainly not onto a gravity supply, as that could seriously impede the flow, pressure dependent, singles or even an NRV and only as a last resort.

 

[Justin Passing]

You need to ensure any tap you fit is WRAS Approved

That's what I would have thought, but surprisingly it's not a Requirement.
Typical Brit Plumbing sloppiness.

Water Regulations Approval Scheme Ltd Product Approvals

WRAS Product Approvals In this section you can search for existing Product and Material approvals and download resources for existing and secondary applications. Please choose

www.wrasapprovals.co.uk

 

[Madrab]

That's what I would have thought, but surprisingly it's not a Requirement.

Not sure where it says it's not a requirement - it says that :-

The regulations impose a legal duty on everyone to use suitable water fittings when connected to public water supplies. Plumbing products are considered suitable, if they pass appropriate material and mechanical regulatory performance tests. Remember, if a product in your home or any property is found to be non-compliant with these regulations, you may be asked to replace it with a compliant product, costing you both time and money. In some cases, you might even be prosecuted and fined.

One sure way to do that is ensure it is WRAS approved?? I don't know of any other body in the UK really that tests and provides that approval??

 

[Justin Passing]

Not sure where it says it's not a requirement

In thereference I gave, it sez....
. "So, whilst a WRAS approval isn’t mandatory" ...

so the WRAS mark may not be there.

 

[Madrab]

In thereference I gave, it sez....
. "So, whilst a WRAS approval isn’t mandatory" ...

so the WRAS mark may not be there.

Yeah but the whole sentence says :- So, whilst a WRAS approval isn’t mandatory the testing of the products is
and the only place that facilitates that type of testing is WRAS I believe, if it passes it gets WRAS approval.

 

[Justin Passing]

yes I can read - it does not need the approval!