Mixer tap- OK to connect one side only?

[Crofter]

I might need to put up some friends in the unfinished house (hope no Council Tax assesors catch me!) and the hot water system isn't bought yet, let alone installed. I'd like to have a working basin though. I'm planning on fitting a pillar mixer tap. If I plumb in the cold feed only, what will happen? Will water come out the unplumbed hot side??

[jamiehamy]

I've got the same setup just now (not living in it tho). 

 

Cold feed is fine and yes, get water out the hot but not quite as strong (I have TMVs for the hot supplies to some cold feeding through)

 

 

[Dudda]

You'll need to put an end cap on the hot flexible tail which isn't connected up to a hot water system or you can have cold water coming out it under the sink. Found this out the hard way!

[Nickfromwales]

13 minutes ago, Dudda said:

You'll need to put an end cap on the hot flexible tail which isn't connected up to a hot water system or you can have cold water coming out it under the sink. Found this out the hard way!

+1

[Nickfromwales]

If it's the kitchen sink tap, it may well have separate water ways all the way to the spout. Some mix at atmosphere and some mix in the spout, dependant on whether or not they're mixed pressure taps or not eg gravity hot & cold mains capable. 

[jamiehamy]

12 minutes ago, Nickfromwales said:

+1

+2 lol

[ryder72]

10 hours ago, Nickfromwales said:

If it's the kitchen sink tap, it may well have separate water ways all the way to the spout. Some mix at atmosphere and some mix in the spout, dependant on whether or not they're mixed pressure taps or not eg gravity hot & cold mains capable. 

Nick - shouldnt there be some sort of non-return mechanism to prevent backflow?

[Nickfromwales]

1 hour ago, ryder72 said:

Nick - shouldnt there be some sort of non-return mechanism to prevent backflow?

Only if the manufacturers instructions state so . Taps that mix in the spout or body almost always do, but taps which are stated fit for purpose with mixed pressure inputs can only do so by making sure the two bodies of water cannot ever meet. 

People may already have these type of taps and not even know about it. A quick check will reveal it.....

Turn the hot on only and see where the water comes out of the spout. If it's a pencil flow from the dead centre, and then changing over to cold is a ring flowing from the spout edges then you've a mixed flow tap. The hot and cold never actually meet with that type of tap, so no requirement for back-flow prevention ?

[ryder72]

9 hours ago, Nickfromwales said:

Only if the manufacturers instructions state so . Taps that mix in the spout or body almost always do, but taps which are stated fit for purpose with mixed pressure inputs can only do so by making sure the two bodies of water cannot ever meet. 

People may already have these type of taps and not even know about it. A quick check will reveal it.....

Turn the hot on only and see where the water comes out of the spout. If it's a pencil flow from the dead centre, and then changing over to cold is a ring flowing from the spout edges then you've a mixed flow tap. The hot and cold never actually meet with that type of tap, so no requirement for back-flow prevention ?

Sorry no I meant, I thought taps were meant to have some sort of inbuilt non-return arrangement by regulation. Is that not the case?

[Nickfromwales]

Not afaik. Some taps do come supplied with them, but that's usually because they've come from outside the uk.  

Fixed taps cannot get submerged so therefore there should be no need to worry about preventing back flow contamination, unlike say a shower with a hose & handset that could get left in dirty bath water and the problems then start, with the possibility for that water to be drawn back into the potable network in the house by syphoning. Regs only really cover you contaminating the network mains so the one place where back flow prevention is definitely required is at the cold mains stopcock for each property. 

I don't think the DNO give a monkeys about you drinking your own bath water, but they sure as hell don't want next door drinking it, should they be working on it for eg. 

Some manufacturers cheat and say their taps are suitable for connection to gravity hot and cold mains pressure cold, but do this by sending the tap supplied with inline non return valves ( NRV's ) which you simply screw onto the end of the supplied flexis. These are not proper mixed flow taps, and the cold mixing with the hot in the body is often enough to stop hot water getting out of the spout as the cold mains pressure overwhelms the gravity hot supply to the point that it A) simply won't flow out of the tap spout, or B) will start to force the hot water back up the hot supply pipework to the tank and back-fill the CWS tank ( to the point it starts coming out of the overflow if left long enough ). 

Showers typically come with NRV's fitted, because of the hose, but not always. Best to check for your own piece of mind but I don't believe it's, ( inside the domestic dwelling ), regs. Basically, afaic, its down to the plumber / installer to ascertain if the tap is suitable for connection to the type of system available, and fit NRV's accordingly if so required. It does however become a requirement under G3 to have these fitted ( retro fited in existing installations ) if you retro fit an UVC after having a gravity fed system. It is also a G3 / manufacturer requirement to fit a primary NRV on the hot outlet of the UVC when undertaking an UVC retro fit, ( where the house cannot be / is not suitably plumbed for 'balanced' hot and cold feeds ), to ensure there is a failsafe provision to prevent back pressurisation of the UVC in the event that the satellite NRV's ever fail. The latter must be done even if fitting a second, additional, ( failsafe ) PRedV at the cold mains stopcock, ( which cleverly creates a balanced household hot and cold system without altering any of the existing pipework, and negates the pita retro fitting of the satellite NRV's ).