Worcester 28ci pressure relief valve

[mpg]

I have replaced the prv in my boiler and may have cocked up as the old one was leaking i just opened up the old one to drain the system fitted the new one topped up the system to 1 bar all ok.

put on the heating last night and the pressure went up to 2 bar and opened the 3 bar prv so now a steady trickle of hot water is being released

what do you guys think i have done apart from being an idiot




cheers
matt

 

[gas4you]

Sounds like a faulty prv. Shouldn't open until 3 bar.

Possibly your pressure gauge is blocked with some crud and not reading accurately so you have inadvertantly over pressurised it.

 

[mpg]

well i`ve had heating on for half hour or so and its let some pressure out and its sitting at about 1.8bar so lets say its over reading by about a bar if i drop it down when hot to about 1.3 bar hot or .05bar cold does that sound like it may work

 

[Gasguru]

Drop the pressure down to zero and check the expansion vessel air pressure - should be about 0.8 Bar. Then presurise boiler to 1 Bar with radiators COLD and boiler OFF.

Not sure what make of PRV is fitted to your boiler but if you just changed the plastic "head" of the valve is very easy to adjust the valve out of calibration when screwing the valve back in if your not familiar with the valve constrution.

 

[scatmanjohn]

Faulty/partially blocked pressure gauge tube is my guess.

Fairly common on this one.

 

[scatmanjohn]

Not sure what make of PRV is fitted to your boiler but if you just changed the plastic "head" of the valve is very easy to adjust the valve out of calibration when screwing the valve back in if your not familiar with the valve constrution.

Not sure how it would be possible to adjust the calibration by changing the head.

Can you explain more Gasguru.

 

[mpg]

The prv is a the push in type with the red screw top. im just waiting for it all to cool down before testing.

 

[Gasguru]

If just the plastic head of the valve is replaced (grips on the black part) when the new one is screwed in they can go out of calibration.

This is the common Caleffi but many others are of similar construction.

http://www.caleffi.com/caleffi/en_I...ude/~applications~catalog~serie.jsp/index.sdo

When the PRV is calibrated in the factory thread locking compound is not put on the threads beneath the cap and its very easy to end up "recalibrating" it when replacing.

 

[mpg]

Thats pretty much what mine looks like and i replaced the whole thing by removing the locking pins pulling the old one out and pushing the new one in and replacing the pins.

its stopped dripping now(hope its now set its own pressure)

I`ll check it again in the morning, I let the whole sytem cool and then cranked the heating up full and its been fine

 

[scatmanjohn]

When the PRV is calibrated in the factory thread locking compound is not put on the threads beneath the cap and its very easy to end up "recalibrating" it when replacing.

still confused about how that can happen

 

[Gasguru]

Because its not uncommon to just swap the head as the brass body may be difficult to get out or siezed eg in the Puma the PRV is locked into place by a small hex grub screw that can sieze.

Therfore it is not uncommon for just the head to be swapped over. However as the new head is screwed in the spring pressure adjusts as there is no thread lock compound on the thread.

 

[gas_man_gaz]

cant see how changing the head on a prv can affect the calibration unless its not fully screwed in?

 

[Gasguru]

Ok next time you've got an old Caleffi prise out the metal cap that's pressed in the end of the valve head.

You will then see how it is calibrated.

When you unscrew or screw the head back into the brass body the spring rate is easily adjusted (out of calibration) as the sealing washer (top hat shape) contacts the seat and the plastic continues to rotate.

This goes for several other variants of safety valves too.

 

[scatmanjohn]

Gasguru...i took one to bits today and am still a bit confused.

Under the metal cap is just a pin with a retaining clip which stops the red cap from coming off.

So i went further and took off the retaining clip and removed the red cap.

Under there is a section that screws into a plastic housing.

As far as i can make out that is the only adjustable bit in there.

Is that what is used to set them up during manufacture?

I can't get my head round how it could affect the calibration by just changing the head.

I'm not saying you are wrong, i'm just interested in how they are put together.

 

[Gasguru]

Here we have the most common Calleffi PRV head (note the top hat seal is missing). Next is another Calleffi head often found on Ferrollis and Saunier Duvalls (the little lifting lever is missing on this one although they are not always fitted). Many other manufacturers are similar in design though.





Diecast alloy part A screws into plastic body part B and is set to give correct lift pressure.





When the valve is calibrated Part A is adjusted in Part B to give the correct lift pressure. However, A is not locked to B with threadlock during manufacture. If you unscrew or screw just the head in or out then Part A can turn in Part B changing the lift pressure. It will occur if you allow the top hat seal to remain in contact with the brass valve seating whilst turning the valve head. Set the valve open by turning cap as far as it will go before springing closed. This will pull the top hat off the seat, then carefully screw in valve to preserve the original setting.