Unvented PRV dripping intermittently - advice please?

[Masood]

As per title really - there is an intermittent drip into the tundish. What I've checked:

Water dripping is cold - so rules out the TPRV, must be PRV
Only happens when cylinder is hot so presuure / expansion is high
The exp vessel membrane is not ruptured as no water comes out of Schrader valve
However - when I looked last week, there was zero pressure
I pumped pressure up to 1.1 bar
I changed the PRV internals but still dripping

Any ideas???

 

[townfanjon]

Expansion valve itself ??

 

[stevetheplumber]

have you disconnected the pipework to prove which valve is passing

 

[gassafe]

1.1 bar is low? Normally 3 I thought!

Sounds like it needs a new PRV on the cold normally 6 bar one, unless you can snap it round & reseal itself!?

 

[gassafe]

Plus the last prv's iv done have had pitted bodies, like a dripping tap base when it needs reseating! Maybe that?

 

[Masood]

have you disconnected the pipework to prove which valve is passing

No mate, as D1 pipework is very short and the drip is cold. As it only happens when cylinder is hot, surely if TPRV was passing, it would be hot water dripping out?

 

[Masood]

1.1 bar is low? Normally 3 I thought!

Sounds like it needs a new PRV on the cold normally 6 bar one, unless you can snap it round & reseal itself!?

Original PRV on cold was 3bar. Are you sure you're not thinking of TPRV?? Tried snapping it round, made no difference so swapped out the innards.

If the seat is pitted, why does it not pass all the time?

 

[gassafe]

Yeah you'd know if it was warm water!?

 

[townfanjon]

Mas, if there was no pressure in the ex vessel last week , the ex valve must have been operating for how ever long it had no pressure , like one in a boiler , they dont like it . As I said its first thing I would go for

 

[gassafe]

Original PRV on cold was 3bar. Are you sure you're not thinking of TPRV?? Tried snapping it round, made no difference so swapped out the innards.

If the seat is pitted, why does it not pass all the time?

1.1 was referring to the expansion vessel pressure?

 

[Masood]

1.1 was referring to the expansion vessel pressure?

Yes..

 

[gassafe]

Yes..

That's what I was referring too as low lol, I thought unvented cylinders had 3 bar normally pre-charge!

 

[Masood]

That's what I was referring too as low lol, I thought unvented cylinders had 3 bar normally pre-charge!

I'll double check. I asked on here because I had forgotten and was told 1.1 ish..

 

[gassafe]

What cylinder is it? All the unvented such as gledhill or range are 3 bar pre-charge on expansion, with 3 bar reducing valve on inlet, 6 bar blow off on cold & 9 on the tprv or there abouts...

 

[Best]

Yes, I agree with above. The expansion vessel precharge gas pressure is to be just slightly below the pressure of the unit when cold, - just a tad under 3 bar.
Try charging it up to near 3 bar, but the pressure relief valve might have the washer inside it now damaged a bit also.
Remember that the potable vessels often require replaced - they don't last forever. They are not too expensive. Telford recommend their units to have the vessel replaced in 5 year

 

[Masood]

Thanks all! I will try repressurising to just under 3 bar and replacing the whole valve...

 

[GrahamM]

What cylinder are we talking about here?

If the E/V is an external for the cylinder itself it will be generally 3bar pressure as mentioned but if it is on heating side you will be probably in region of 1bar or bit more if very big house.
If the TPRV is just passing and no more you will not necessarily notice it being hot.

 

[rocketmanbkk]

Did you fix it Mas?

What vessel was it?

I'm fixing 2 this weekend, with a man whose suitably qualified.

They're both megaflo

Both have hot water running & then stopping

Diagnosis is valve not letting enough water by to refill system as quick as its emptying.

In these I was told the the expansion vessel is inside & not fixable

Is this right?

 

[tolly]

Did you fix it Mas?

What vessel was it?

I'm fixing 2 this weekend, with a man whose suitably qualified.

They're both megaflo

Both have hot water running & then stopping

Diagnosis is valve not letting enough water by to refill system as quick as its emptying.

In these I was told the the expansion vessel is inside & not fixable

Is this right?

Megaflo's have an internal air gap/bubble to accomadate for the expansion when heated, follow the instructions on the label on the side of it if you need to regenerate bubble if TPRV is dripping. Although it wont work all the time and sometimes they dont hold there bubble for long and you will need to install a seperate potable EV.

Your diagnosis sounds wrong to me though, its probably dripping because theres no air gap for expansion so as its heated up and the pressure rises within the vessel it opens the TPRV till the pressure drops slightly, then this process repeats itself.

 

[rocketmanbkk]

My 2 are fixed & working. Both had combination valves renewed & all good. Enjoyed my time with my friend, re did some pipe work, soldered for the first time in over a year.

2 showers next Sat with him.