Basin mixer tap - hot water flow minimal

[A Manuel]

I had a basin with separate hot water and cold water taps which functioned fine. Gravity fed system with hot water from cylinder and cold water from mains . With a bathroom upgrade, installed a mixer tap - Grohe Get Mono mixer tap (0.2 bar - 5bar) with flexi tails. The hot water flow is very low so the mixer tap does not work. Plumber thinks this is due to restricted size of hot water flexible metal hose but unable to change this to the size of the copper pipe. Looked at Bristan Capri but here again there are restricted size tails. I have tried to turn the mains water stop valve to a minimum setting and this seems to work a bit better but the overall water out of the mixer is reduced to a minimum to get the water warm. Is there any solution to this issue - any other mixer taps that would replicate the performance of individual taps ? any larger size flexi hoses (leads) which do not restrict flow. (Or means of replacing flexi pipe with copper pipe same size as supply pipe) Thanks for your help

 

[JCplumb]

A normal mixer tap on flexis (or on solid tails for that matter) simply doesn't work on a gravity hot/mains cold system.
Your plumber should have known that.
You can reduce the mains pressure and flow to the cold side, but you will still have a low flow. Seperate taps (like you had before) are the best option, failing that look for some dual flow mixer taps.
You may also be getting back flow from cold to hot pipework because of the tap you have in now, is a check valve fitted on the hot feed pipework to prevent this? It can cause the storage tank for your cylinder to overflow.

 

[JCplumb]

Oh and ignore info such as ''suitable for 0.2-5 bar'' It's presuming you have balanced feeds, which you don't.

 

[A Manuel]

A normal mixer tap on flexis (or on solid tails for that matter) simply doesn't work on a gravity hot/mains cold system.
Your plumber should have known that.
You can reduce the mains pressure and flow to the cold side, but you will still have a low flow. Seperate taps (like you had before) are the best option, failing that look for some dual flow mixer taps.
You may also be getting back flow from cold to hot pipework because of the tap you have in now, is a check valve fitted on the hot feed pipework to prevent this? It can cause the storage tank for your cylinder to overflow.

Thanks very much for this response - clearly you are knowledgeable on this matter. On the internet I can see Bristan club dual flow basin mixer - presumably this should work (I do not want to change the basin now). No check valve is fitted now (to normal mixer with flexis); there must have been some back flow as it takes ages for hot or warm water to flow. Will check valve be required for dual flow valve ? Thanks once again for your response

 

[Ric2013]

Check valves are a legal requirement in your situation, but taking a long time to get hot if you are running hot only is not due to lack of check valves. In fact, with a check valve, the flow would be even worse and the pressure would struggle to open the check valve at all. Even some kitchen mixers with separate waterways can take a while to warm up as the low flow on the hot side can take a while to clear the cold water that is sat in the pipe from the hot water cylinder to the kitchen tap.

If, by dual flow valve, you mean there is no mixing in the tap body as there are separate waterways, then no check valves are required, but when I look at the instructions, having googled 'bristan club dual flow basin mixer', this tap does not appear to be separate waterways - and the instructions specify check valves are required. So I'm not quite sure what Victorian Plumbing or Plumbcenter mean by 'dual flow' as Bristan does not appear to use this term.

I think most bathroom mixer taps do allow in-body mixing as they are designed to have both hots and colds at low pressure. But bath taps with separate waterways do exist, so you may yet find what you need. I recommend you contact Bristan and ask them if the club allows in-body mixing or whether it has separate waterways as there may be a special version I have not found online.

 

[A Manuel]

thanks very much for this explanation. I understand the problem and will contact the manufacturer like Bristan now. Thanks once again

 

[JCplumb]

There's a Q and A at the bottom of the following page confirming that the Bristan Value Club is dual flow and is suitable for your situation.
I ended up drilling new tap holes in my basin a couple of years ago because I couldn't find a dual flow mixer under £100, so this is a no brainer.
Bristan Value Club Dual Flow Basin Mixer Tap With Pop Up Waste - VAC DFBAS C MT

 

[A Manuel]

thanks very much, I have ordered this item now. Let us hope it does the job

 

[WaterTight]

A normal mixer tap on flexis (or on solid tails for that matter) simply doesn't work on a gravity hot/mains cold system.

But it would work in her situation. From memory I think all Grohe taps are for high pressure only. Any mixer tap rated 0.1-0.3 bar from any of the sheds will work fine on her system. Copper tailed and rated for 0.1 bar would be ideal but I bet even 0.3 with a flexi would still be reasonable flow as long as it's a CWSC in the loft and not a fortic in an adjescent cupboard.

Acceptibility of water mixing in spout/regs/check valves are a separate issue and obviously worth getting right but wasn't the original concern.

 

[WaterTight]

Incidentally if it's a shower then it is a different matter as precise temp control is critical. For washing hands/washing up gravity hot and mains cold in a mixer tap rated for low pressure system will work fine

 

[JCplumb]

I think the massive difference in pressures between mains fed cold (maybe 3+bar) and gravity hot (maybe 0.2bar) will mean normal mixers will not work well, the cold will overpower the hot and in some cases will back feed up to the cwsc and overflow.
The original post states that it's not working so safest bet is a dual flow imo.
I totally agree that some non dual flow taps on these types of systems work OK but it's situational. Been to loads of houses where builders etc. have installed basic mixers on gravity systems and it has caused nothing but trouble.

 

[Welder]

Any ceramic disc tap will struggle here. The best option is a traditional multi-turn hard piped with pulled bends and a full bore iso.

I'm not aware of any Grohe or HansGrohe that uses conventional valves. In addition, the flexis are often quoted with a bore size BUT the size quoted is the bore of the rubber hose. The brass fitting on the end considerably it down to something like 7-8mm. Hence hard piped 12mm MIN.

 

[A Manuel]

Any ceramic disc tap will struggle here. The best option is a traditional multi-turn hard piped with pulled bends and a full bore iso.

I'm not aware of any Grohe or HansGrohe that uses conventional valves. In addition, the flexis are often quoted with a bore size BUT the size quoted is the bore of the rubber hose. The brass fitting on the end considerably it down to something like 7-8mm. Hence hard piped 12mm MIN.

 

[A Manuel]

Bristan dual flow tap has now been fitted and works fine. It is genuine dual flow with no mixing in the tap, comes with copper tails. Thanks JC and others for helping me

 

[ShaunCorbs]

Thank you for getting back to us and updating us