monobloc check valves

[wetfeet]

I'd thought that a kitchen monobloc mixertap, by law, must be duel-stream. But one plumber told me that for a monoblock kitchen mixer
tap fed from a combi I need a double check valve in the hot pipe because of the pressure differential between cold and hot. He also said that
if a washer-dryer is tee'd off of the cold pipe only, I only need a single check valve (+service valve) between tee and washer-dryer for that.

Another plumber, however, said I need no valve in the hot pipe, and a double check valve in the cold pipe, in front of the tee.

Does anyone know the right answer to this?

wetfeet

 

[Bahco]

Monoblock taps (if I remember correctly ) mix the water internally.
There is no need to fit double check valves as these taps have a type b air gap between their outlet and spillover level of the sink.
Single check valves are normally used for low pressure supplys from a cistern.
I have not heard of a washer having a DCV fitted and would like to invite other views (Chris R should know)

 

[ChrisR]

There isn't a pressure difference between H & C if you have a combi - there's flow restrictor in the hot, but that's not the same as a pressure difference.

Some taps do mix the H and C in the spout, so technically you'd need check valves to prevent the possibility of a spoutful of grade 2 (heated) water going back into the main.

Washing machines - most do have single or double check valves and/or air gaps in the input so you don't need one in a domestic situation. If you were washing wound dressings though you'd need a break tank (cistern) to prevent backflow of cat 5 water .

There are other details which escape me at the mo....

 

[wetfeet]

Ok, maybe he (it was the plumbing adviser in a B&Q warehouse) said difference in "flow rate" rather than pressure. But anyway, you're saying I don't need a valve in the hot, and only, theoretically, a single CV on the cold in front of the tee?

Btw, I thought that the monoblocs that mix the hot and cold in the spout were the dual stream ones - which didn't need CVs, but that the ones that mixed in the body of the tap did?

 

[wetfeet]

If anyone's interested, I emailed WRAS. This is their reply:
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(1): Regulation 4 Requirements for Water Fittings requires that every fitting in to an appropriate British Standard, European or some othe national specification which provides an equivalent level of protection and performance and conforms with the terms of a specification approved by the Water Supply (Water Fittings) Regulations 1999. (If it does not then it is illegal for you to fit it, also you may risk prosecution).
(2):If it is an approved fitting then you will need to ensure you have a balanced pressure across the valve.
(3): If the mixer has an AUK3 air gap then the water industry deems that the particular requirements of the Regulations will be met if there are no check valves on either side of the supplies to the mixer tap where the water mixes within the body and the supplies are on balanced pressure.
(4): Where the pressures are unbalanced then single check valves fitted on the inlet cold & hot water supply to the mixer will be required.
(5): The hot & cold supply to the Washing Machine will require double
check valves fitted upstream.ie: washing machine tap, double check
valve.
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Comments/questions.

What do they mean by "approved fitting", and equal pressure across the valve? What valve?

AUK3 gap means, basically, the spout is some distance above a fluid category 4 or 5. Category 3 includes domestic "washbasins", so do domestic sinks -with the associated food preparation, meat washing etc.- become category 4?

By "balanced", I assume that means the combi is supplied by mains, as is the cold(?)

Regarding the double check valve for the washing machine; since the machine itself must, I assume, at least have a single check valve, will just an extra single check valve between tee and washing machine suffice?

 

[ChrisR]

Comments/questions.

What do they mean by "approved fitting", and equal pressure across the valve? What valve?

Approved = just that - you will have noticed the WRAS symbol occasionally. One anomaly is that if a fitting is CE marked then it should be of the required standard, but a lot aren't!

AUK3 gap means, basically, the spout is some distance above a fluid category 4 or 5. Category 3 includes domestic "washbasins", so do domestic sinks -with the associated food preparation, meat washing etc.- become category 4?

No. eg Domestic bath water is cat 3 even though a baby will "foul" it! The volume of nasties seems to be taken into consideration. Commercial bathwater is automatically cat 5

By "balanced", I assume that means the combi is supplied by mains, as is the cold(?)

Yes. NO need for check valves.

Regarding the double check valve for the washing machine; since the machine itself must, I assume, at least have a single check valve, will just an extra single check valve between tee and washing machine suffice?

You can't assume, you technically need to find out whether your machine has dcv's in or not. Recently on a water authority inspection of a place with 8 bedsits I provided a dcv on the wm taps, but was told I didn't need it because it wasn't commercial. The inspector assumed that all wm's had dcv's in "these days"!

I dont have dcv's on my own wm, and I don't know about my wm inputs, but I didn't see any dcv's when I changed a solenoid valve the other day.

I asked a WRAS techie once about dcv's, pointing out that whatever makes a SCV fail would be likely to make a DCV fail as well. He more or less said yes of course but try telling the wombats who write the specs!

 

[DaveJ123]

Message deleted.

 

[wetfeet]

Ok, thanks. I think I'm sorted with with what I want to do now.

But I still don't understand the AUK gaps business. Unless I'm misreading, the following quote is saying that while domestic washbasins and baths require AUK2 gaps(i.e. category 3), domestic sinks require an AUK 3 gap (i.e. category 4 or 5). But then the table defines an AUK3 gap as being no bigger than an AUK2. This plumbing is a dark art!

http://www.defra.gov.uk/environment/water/industry/wsregs99/guide/section6.htm

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Shower heads or tap inlets to baths, washbasins, sinks and bidets

G15.13    Except where suitable additional backflow protection is provided, all single tap outlets, combination tap assembly outlets, or fixed shower heads terminating over washbasins, baths or bidets in domestic situations should discharge above the spillover level of the appliance with a tap gap (Type AUK2) as scheduled in Table S6.1. For a sink in a domestic or non-domestic location, and for any appliances in premises where a higher level of protection is required, such as some appliances in hospitals or other health care premises, a tap gap (Type AUK3) is required, see Table S6.1.
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