Fire regs sh!t show and now I need a new priority demand valve

[Adsibob]

Our BCO made us get a Fire Safety Report. The Fire Safety Engineer required us to install a sprinkler system on the ground floor of our house. This was because we had an open plan downstairs which meant the kitchen was open to the staircase AND we were doing a loft conversion.

 

The spinkler company, iMist, made us install a Priority Demand Valve (PDV) on our incoming water main so that in the event of a power cut, the sprinkler system's tank is given priority. This is really annoying as although we've only had one power cut in the 3 years we have lived at the property, for the 6 hours or so that the power was off we had no hot or cold water in the house (I moaned about it on this Forum and I think @ProDave or @Nickfromwales and others agreed is was ridiculous plumbing situation). I complained to iMist and they shrugged their shoulders and said that's the law and I cannot break it. Turns out that they may have been wrong as they are now saying something different, but that is the subject of another post...

 

So we've lived with this PDV and apart from that power cut no problems, until... yesterday morning it melted! There was an unpleasant chemical burning smell and no water in the house. I checked the fuseboard and the circuit on which the Priority Demand Valve is wired had tripped. I tried to put the circuit back on but it refused. Further investigation led me to realise the priority demand valve itself had malfunctioned. All the plastic sheathing around the bit that connects the electrical cable to the solonoid valve had melted. So the problem I had was that there was no water in the house and no way to easily fix it without a replacement PDV, as there is no bypass plumbed in, presumably to comply with the fire regs or imist's requirements in that they don't want you to be able to bypass this piece of "safety kit" which ironically in our situation could have caused a fire!

 

Nobody is accepting any blame of course, but it was very unsatisfactory as there is no way to bypass that valve, this being apparently a requirement of the fire safety regs.

 

The PDV is this cheap crappy unit made in China: https://www.amazon.co.uk/dp/B071NW44B1?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0  I had no idea a proper solonoid valve costs over a £100, so incredible they can make any profit on the mere £30 asking price. 

 

Luckily I managed to got the foreman from our original build back and he has rigged a temporary fix, by disconnecting the melted wiring and removing a spring from inside the PDV so it is now permanently open rather than permanently closed. There is a tap that enables us to empty the pipe it's fitted on, so no huge mess of water, though it's not clear to me that there won't be a huge mess when we actually remove and replace it.

 

Anyway, my two questions for the Forum are:

 

1. Is there a really good quality PDV (which when powered is open, but when power is lost is closed) which has the same 84mm (3.30") dimension as shown here:
2. Is this actually safe, even in a non-chinese crap brand? The design seems flawed, in that it is constantly being powered to resist almost 4 bars of pressure, 365 days a year? Seems nuts to me! Although the water passing through it is cold, it sits about 30cm below our boiler, in a boxed in cabinet, so it must get pretty warm in there. Is this an acceptable fire risk??? The gas pipe feeding the boiler is also very nearby. I just can't believe this thing melted the way it did. The noxious smell could be smelt in the bathroom directly above, although that's perhaps not that surprising given all the water risers go from that area up to the bathroom and beyond, so the wall that houses those risers providers for plenty of space for the smell to travel through.

Edited March 20 by Adsibob

[Mr Punter]

I am not a fan of iMist with pumps, tanks, electrical components, annual servicing and high purchase price.

 

I have a normal sprinkler system running from the mains.  I don't have it serviced so there are no ongoing costs.

[Adsibob]

1 hour ago, Mr Punter said:

I am not a fan of iMist with pumps, tanks, electrical components, annual servicing and high purchase price.

 

I have a normal sprinkler system running from the mains.  I don't have it serviced so there are no ongoing costs.

I can now see the benefits, but alas without the benefit of a Time Machine and the right advice at the time, not really something I can switch to now. So I’m stuck with the annual servicing. But I shouldn’t be stuck with a crappy PDV. Can you recommend a better one?

Edited March 20 by Adsibob

[Nickfromwales]

5 hours ago, Adsibob said:

I can now see the benefits, but alas without the benefit of a Time Machine and the right advice at the time, not really something I can switch to now. So I’m stuck with the annual servicing. But I shouldn’t be stuck with a crappy PDV. Can you recommend a better one?

Firstly, do you know if you have the privilege of deciding what this bit of kit will be replaced with?
 

Can you interfere with a completed, certificated installation for automatic fire suppression with a bit of kit you asked people here about? 
 

I suggest (if you haven’t already but I assume not) that you look at approved devices for this application before doing anything, as this is quite a grey area imho and borderline ‘dangerous territory’.

 

Next up, I’d choose a mist system over a sprinkler system without a second thought. A sprinkler is a dumb system with a huge appetite for indiscriminate water flow without any form of control or moderation, so you can expect significant damage to the property; long after a fire is extinguished the water will just keep on flowing……and flowing…..and flowing. (Repeat until you get home).  
 

Mist systems have “electrical components” so they become intelligent. The last system we installed was from a great company, excellent consultancy and clients were very happy with it (when they were given sufficient information to understand the difference) and how it functioned.

 

A mist system will suppress fire for 45 mins (may differ between manufacturer or specific circumstances) and then shut off, with an assumed volume of circa 120L being injected into the dwelling, but only from the head or heads that have been activated in the immediate locality of the event. This, I am told, is the equivalent of you filling your bath to the usual level and then tipping it over. Once. That’s the “damage” by water beyond that of the fire before being extinguished.

 

Absolute night and day better system. Hands down. 

 

As for cost, there’s very little in it if you choose well and shop around, so no argument against one at all.

 

Consider that, for a mist system, you can run off a 15mm cold supply without issue, whereas a sprinkler will need a min 32mm cold mains (so no chance of retrofit on an existing supply) and if the local supply from the street is poor you will need tanks, pumps, and Christ-knows-what else then a mist system very soon makes more sense. 
 

What doesn’t make sense is, that for certain distances from the street, a mist system is ‘not permissible’. Ffs. They love to make this simple, don’t they 🤷‍♂️🤦‍♂️.

Now, as it’s getting late, one last kick in the cock if I may.

 

The mist system we just completed, did NOT require a PDV, as these are usually only associated with a sprinkler system, so I am sat here asking WTF?!?!

 

AFAIC you shouldn’t need one! The cold mains should feed your mist system and then go on to the domestic stopcock and downstream plumbing system. The mist needs very little volume and flow, the whole beauty of it, so I’d be asking some questions as to why you have / need one at all.

 

Not sure about iMist, but 2 companies I have used have NOT specified a PVD. 

 

[Adsibob]

1 hour ago, Nickfromwales said:

Not sure about iMist, but 2 companies I have used have NOT specified a PVD. 

I wish I had gone with those two companies.

 

 Just to clarify: I am not considering changing systems. I want to keep the imist, just either take the PDV out altogether (with imist’s consent… it seems they may be amenable to this); or I want to install the best PDV money can buy (within reason) so that this never happens again. So that I can decide which route to go, grateful if anyone who knows plumbing fittings could let me know which PDV manufacturers are best

Edited March 21 by Adsibob

[Nickfromwales]

6 hours ago, Adsibob said:

I want to keep the imist, just either take the PDV out altogether (with imist’s consent… it seems they may be amenable to this)

Push to remove it. If it fails you’re fecked whichever way.

The beauty of the mist system is that it consumes very little water to do the same job, vs sprinklers, so even if you were running the bath the mist system should still function.

No need for that valve afaic. I’ve tried to find the installation guidelines for the one we did but next to no signal where I am atm so my email is pants.

[Mr Punter]

9 hours ago, Nickfromwales said:

Next up, I’d choose a mist system over a sprinkler system without a second thought. A sprinkler is a dumb system with a huge appetite for indiscriminate water flow without any form of control or moderation, so you can expect significant damage to the property

 

Really not as much water as there would be if the fire service used their hoses, which is where the other 99% of houses are.

[Adsibob]

4 hours ago, Nickfromwales said:

The beauty of the mist system is that it consumes very little water to do the same job, vs sprinklers, so even if you were running the bath the mist system should still function.

So apparently, after a helpful call with imist today, the minimum requirement for the water supply is 14 litre per minute, which I just meet. I can only remove the PDV altogether if I have 25 lpm, which I definitely don’t have and never will have - we have already got Thames Water to increase our flow rates at a crazy price of c. £2k! As an alternative they are happy for me to swap out the PDV with one that only draws power to close, rather than the inverse which I have now, ie draws power to open. They are sending me a suggestion for such a PDV. In the meantime they have advised me to decommission the imist pump until this is sorted because the swapping in and out of valves can cause a sudden pressure influx into the pump which can cause a false trigger 😱

 

 luckily that is apparently very easy to do, so they’ve said I can do it myself and are sending me a doc explaining how. Let’s see…
 

 

Edited March 21 by Adsibob

[Nickfromwales]

1 hour ago, Mr Punter said:

 

Really not as much water as there would be if the fire service used their hoses, which is where the other 99% of houses are.

But the fire brigade would turn their hoses off when the fires extinguished. 
 

An unattended sprinkler system will put water into the house until you switch it off manually at the stopcock.

[Mr Punter]

40 minutes ago, Nickfromwales said:

An unattended sprinkler system will put water into the house until you switch it off manually at the stopcock.

 

True but compared to an unattended fire the damage would be far less.  I imagine if the fire brigade attended and the fire was extinguished they would turn off the sprinkler supply.

[Adsibob]

imist have recommended this PDV instead:

 

https://www.mmcwebshop.com/products/f59dc1f60e/35372000007965040
 

it’s 3/4 BSP, is that going to be compatible with my pipework, which is compatible with the dimensions of the PDV shown in the image in my OP, sorry I’m being really daft. 

[sharpener]

On 20/03/2025 at 15:53, Adsibob said:

The design seems flawed, in that it is constantly being powered to resist almost 4 bars of pressure, 365 days a year? Seems nuts to me! 

 

The suggested replacement looks very similar in design to the 1/2 in pilot-operated solenoid valves in our rainwater supply system. They are energised 24/7 and have worked for years. One turns on the rainwater provided the tank is not empty and the other shuts off the mains water against the incoming mains pressure. So I think the concept is sound.

 

I have on one occasion in 17 years had to dismantle one valve and clean the pilot orifice which was a bit tricky. 

 

 

Edited March 22 by sharpener

[Adsibob]

Thanks @sharpener.

 Another reason for preferring the “normally open” version is that in the event of a power cut it will stay open I believe, thereby making sure we don’t lose power and running water at the same time.

I found a 1” version of the one recommended by imist https://www.mmcwebshop.com/products/a0e1dbe8f9/35372000019718077 so have gone with that. It cost 7 times the price of the cheapo made in China crap my builder originally installed so hopefully will last a lot longer than the 2.75 year life we got out of the cheapo one.

Edited March 22 by Adsibob