Double check valve size

[Gone West]

On a previous thread @Nickfromwales gave me this info:

 

If you tick the boxes, so.....

1) incoming mains into propery

2) stoptap of whatever guise

3) double check NRV

4) drain off cock 

 

So first question is, can 2 and 3 be swapped around? and secondly is there any advantage to using a 28mm double check valve instead of a low loss 22mm DCV wrt flow rates.

 

[Triassic]

Why the double check non- return valves? 

[Jeremy Harris]

I can't see a problem with swapping the double NRV over with the household stop tap, all that matters is that you need to be able to turn off the supply to service the NRV if need be, and that can be done by the water company stop tap. 

 

The double NRV is a water company requirement, to positively prevent backflow from the house to the water main, as a measure to reduce the risk of contamination of the water main water if a reverse flow condition could arise (low, or non-existent, water main pressure due to it being turned off).

[Nickfromwales]

2 hours ago, PeterStarck said:

On a previous thread @Nickfromwales gave me this info:

 

If you tick the boxes, so.....

1) incoming mains into propery

2) stoptap of whatever guise

3) double check NRV

4) drain off cock 

 

So first question is, can 2 and 3 be swapped around? and secondly is there any advantage to using a 28mm double check valve instead of a low loss 22mm DCV wrt flow rates.

 

If you have poor pressure / flow then I'd deffo go for a 28mm stopcock and likewise with the NRV. Cost is negligible so yes to that. 

Why do you need / want to reverse 2&3?  My gut feeling is you may get the water board moaning in case the NRV failed and you couldn't stop the water loss. After that I doubt it makes any difference TBH, just unorthodox. 

[Gone West]

 

4 hours ago, Nickfromwales said:

Why do you need / want to reverse 2&3?

 

@JSHarris & @Nickfromwales. Thanks for that. The area where the mains enters, and the other gubbins is, including the water softener, is a bit cramped. I could fit the DCNRV just after the mains pipe but because the water softener is in the way it would be unwise to fit the stop cock there as it would be difficult to access quickly in an emergency. Do NRVs fail very often, I've never lived in a house with one before.

[Jeremy Harris]

The old "floating washer" stopcock is supposed to work as a NRV, at least when new.  The design allows the part that has the washer on to move back and shut off the supply under reverse flow conditions.  In practice this doesn't work well, especially when the valve has been in use for a while, as the floating piston gets stuck into the drilling in the end of the main shaft.

 

I'm pretty sure that the water companies requirement to fit a proper, spring loaded, double check valve, as a backflow preventer, arose because the standard design of household stopcock just doesn't do a good enough job on it's own.  To some extent the water companies are going a bit OTT by demanding double check valves on all properties, because the regulations don't actually require backflow prevention for domestic drinking water supplies, only for supplies where there is a risk of contamination.  The only real contamination risk from a domestic supply is if there's an open tank connected without an air gap, something I've never seen.  The reason that cold tanks, header tanks and cisterns use a high-level fill point, with an air gap between the water entry point and the highest level in the tank/cistern is to prevent a backflow siphon.

[ProDave]

I had to fit double check valves. I have three of them. All fitted in the 25mm mdpe before the stopcocks (I use mdpe in, copper out stopcocks) One for the standpipe, one for the static caravan and one where the mains water enteres the house.

 

Scottish Water would not make our connection until I had fitted the NRV's for the standpipe and static 'van but once fitted made the water connection, so they must be okay with them being before the stopcock. You can of course shut it off at the boundary box if you need to change the NRV.  I don't think anyone is ever going to come and check the NRV is present in the house, but it is there should anyone wish to see it.

[Triassic]

I've just replaced a 3/4 inch steel supply main with 32mm plastic. At no point did the water company mention the need for a NRV!

[Jeremy Harris]

Both the Water Supply (Water Fittings) Regulations and Scottish Water Byelaws require backflow prevention, in accordance with the risk posed.  The risks are categorised in the regs, and a supply where there is only potable water on the user side doesn't require (by law) any check valve at all.  The law requires a single check valve where the user side has a fluid that may have a slight taste, odour or temperature difference to the supply, a double check valve where the user side has a fluid that contains chemicals of low toxicity or health risk and an air gap where the user side may contain toxic or carcinogenic substances.

 

However, some water companies ignore the law (both here and in Scotland) and seem to insist on double check valves even though they are not required.  As water is a monopoly, and users have no choice, then all you can do is comply with the water companies demands.....................

Edited June 3, 2017 by JSHarris

[Triassic]

Thanks for the clarification, clearly ours is a private potable water supply, so no NRV required. 

[Gone West]

14 hours ago, JSHarris said:

 

The law requires a single check valve where the user side has a fluid that may have a slight taste, odour or temperature difference to the supply, a double check valve where the user side has a fluid that contains chemicals of low toxicity or health risk and an air gap where the user side may contain toxic or carcinogenic substances.

 

I guess then properties that have a water softener should have a single check valve at least.

[Jeremy Harris]

15 minutes ago, PeterStarck said:

I guess then properties that have a water softener should have a single check valve at least.

 

 

Yes, but I think there is already one on the inlet side of a water softener, usually.  I think the water companies are just specifying that double check valves be fitted to all supplies as a way of not having to assess the risk for individual properties, which makes sense, from their point of view.

[ProDave]

Of course all this could be solved, at a stroke, if the boundary boxes that the water company supplied and fitted had a double check valve, instead of the single check valve currently used.

 

Before the water gets to my house it passes through a single check valve in Scottish Water's boundary box. A second single check valve in my own boundary box, and then a double check valve in the house.  Madness.

[Nickfromwales]

Yup. That is nuts.

 

21 hours ago, PeterStarck said:

Do NRVs fail very often,

Not often no. Most issues are created by not flushing debris out of new pipework, mdpe burrs / flux / solder droplets etc which accumulate in the throat of the NRV and then get caught in the moving part when water passes through. That crud then holds the NRV open thus rendering it useless. 

Cleaning after commissioning is vital, about a month into normal use would suffice, and more so with any pressure reducing valves.

With a large or complex install I would recommend a strainer but proper flushing and the one month clean out should suffice.