Why aren't PRD/PRV devices made resettable?

[Bordonbert]

Ok, just a layman's view here. I understand fully the need for a PRD in a shower unit. I know the Physics of it all and I get how they work totally. But why nowadays is it a piddly little soft ball popping out of a hole and not a proper sprung loaded valve set to open at the same pressure the ball pops at? It is no problem to make a valve which will open cleanly at the required pressure but will reset itself if the pressure is released. We have these fitted in every car on the road in the header tank of the cooling system. If fault in the valve is considered a problem, it could even be paralleled with a soft ball type which blows at a slightly higher pressure to override the valve if it fails.

I would guess there could be a point made that the PRD blowing enforces diagnostic action to be taken as to why and a repair implemented but, in truth, will the average handyman not just replace it and trust to luck? Let's be honest, the only thing which can cause the valve to blow is blockage in the outlet pipe and head setup. That is easily spotted and fixed.

So why not fit proper valves which reset themselves when the fault over-pressure is removed? Is it purely to keep on selling PRDs? 😉

 

[Chuck]

I imagine that it's some combination of manufacturing cost and reliability. Probably more of the former...

 

[John.g]

I only discovered this after 15 years or so, I'm fairly sure that on my my Mira Sport the PRD rubber ball was actually pressed into a hole in the heating can, one wonders if the whole heating can required renewal if the ball blew out. At least its a separate device on my (now) Triton T80Z.

Yes, you do have to replace the heating can on the Mira sport according to the Shower Doc.

https://www.showerdoc.com/mira-sport-max-shower-spares/question/mira-sport-electric-shower-154183

 

[ShaunCorbs]

Cost basically cost pennies for the ball a resettable one would be pounds

 

[Bordonbert]

Absolutely agree with the cost idea. It never ceases to amaze me that companies still think they have to shave off every penny in order to sell a superior product. Development costs in this area are low as it is all "done technology" and is really just a repackaging of already existing parts. And I for one would definitely pay say £15 rather than £12 for a replacement part which is more or less a one off cost.

It's an oddity that nowadays a third party company hasn't gone down this route. Examples of small cheap pressure relief valves which open for a pressure event and simply close once the pressure drops are everywhere. I'm looking at plumbing types for central heating boilers, gardening types for pressure spray containers, brewing types for kegs and fermentation jars, air types for compressor vessels, they are everywhere and many cost less than the price of a replacement current one-off type. The range of pressures available is easily wide enough to include what we would need. Let's face it, it's a couple of plastic parts, a spring and a rubber washer and voila!

I can't help wondering if there must be an element of forcing the owner to address the cause before fitting a replacement which is sensible. However, how many owners simply push the ejected ball back in or fit a new sacrificial cap and just hope for the best? A diaphragm type with the diaphragm on a spring loaded clip on mechanism which blows off but can be clipped back into place would be a real step forwards. A unit with a lower pressure spring valve for short temporary issues and a higher pressure sacrificial ball/diaphragm valve override when there is a definite serious blockage is easy to design and implement and would do the job more permanently and cheaply in the long run.

I did have a thought. Looking into things recently I found that a standard electrical shower unit should not be used with a two option overhead rain/handheld type. That outlet configuration seems to be designed to only work with mixer taps fed from the hot/cold water. At least one issue seems to be that the selector knob of a two head type can be stalled in a mid position between the two heads where the flow is at least severely restricted which of course blows the valve. That is understandable as the restricted flow can't be allowed to happen with a fixed heat output electrical unit. But why not fit this type of valve to cope with that situation. The spring valve could cope with releasing enough water to keep the system safe even if the selector bridges the two. Or better, why not just design the selector to have a short blend step from one head to the other when switched?

Is this a $1M idea waiting to happen? Anyone with a 3D printer care to try it out?😁

 

[Chuck]

Is this a $1M idea waiting to happen? Anyone with a 3D printer care to try it out?😁

More like a $1M law suit waiting to happen. Safety reliefs where failure risks injury have to be 100% fail safe, which is why they are usually burst discs or some other non-resettable design. Resettable ones need period inspection and testing...

 

[Bordonbert]

More like a $1M law suit waiting to happen. Safety reliefs where failure risks injury have to be 100% fail safe, which is why they are usually burst discs or some other non-resettable design. Resettable ones need period inspection and testing...

But does failure risk injury with what I outlined? An additional lower pressure valve would not interfere with the action of the existing valve which they are happy to manufacture and sell Chuck. I'm not suggesting removing the current failsafe ball/disc type, only adding an extra layer to prevent that one shot type from needing to act at all.

 

[ShaunCorbs]

They know about it

Triton years ago had a defective batch of balls (slightly too small and would go if the handheld was too high and with the hose being on the short side would cause a high restriction) rather than address the issue the reps back then just handed us a longer hose and new prv units was cheaper to do that instead of changing the model / sending service agents out to retrofit

 

[Bordonbert]

A little more thinking made me realise that, if you made this a short Tee extension unit with a 1/2" female loose connector on one end, a male on the other and a 1/2" 90deg arm carrying a cheap PRV it will fit between the existing shower unit hose outlet and the hose itself like a short extension. It's then retro-fittable to any unit out there with a 1/2" hose! And as it is a plain straight through pipe it does not interfere with ANY safety function in the unmodded system.

 

[John.g]

A little more thinking made me realise that, if you made this a short Tee extension unit with a 1/2" female loose connector on one end, a male on the other and a 1/2" 90deg arm carrying a cheap PRV it will fit between the existing shower unit hose outlet and the hose itself like a short extension. It's then retro-fittable to any unit out there with a 1/2" hose! And as it is a plain straight through pipe it does not interfere with ANY safety function in the unmodded system.

Someone tested a Triton PRV , one of cheaper models,and it operated at 1.38bar.