Do Pressure and Temperature Relief Valves Fail - or is there another problem ?

[Blooda]

I have a 90^oC and 7 Bar Pressure and Temperature Relief Valve on DHW tank.  Part of a ASHP supplied system.

Last week during legionella cycle, I noticed water was pouring through, it took a few turns to reseat the valve and stop the flow. 

During the week, heating the water to 43^oC there was no problem.  But during yesterday's  legionella cycle, it let off again, but reseated. But the water temperature only got to 60^oC instead of the 70^oC.

 

There is air coming out of the valve on top of the potable water Expansion vessel.

Can't for a minute think the temperature is getting to 90^oC [as ASHP supply], so temperature sensor should be reading ok.

 

So as per the title, do these fail ? or is there anything else to check before I change it. 

 

 

[ProDave]

It sounds more likely the air bladder in the expansion vessel has failed and on the higher heat cycle it has ran out of expansion capacity and the over pressure valve has opened.

[jack]

I've had this problem repeatedly over the last 8 years.

 

First time I noticed it, it was exactly as Dave said (expansion vessel bladder failed).

Replaced the expansion vessel, but it still leaked when the immersion diverter was on for any length of time (albeit not as badly).

 

Had a G3 service, and one of the temp/pressure safety valves was found to be leaking. That was replaced, but I still have the issue, and I notice it's gotten worse again recently.

No help to you of course, but in answer to your question, yes, they can fail!

[Temp]

Yes they can fail and once they open they seem to open or leak more easily.

[jack]

3 minutes ago, Temp said:

Yes they can fail and once they open they seem to open or leak more easily.

 

I've heard from more than one source that testing them (or having them open due to an over-pressure/temperature event) is the quickest way to cause them to fail.

[Conor]

Replace the valve then disable your legionaries cycle.

[JohnMo]

Simple flow chart to diagnose issue. When was the last service completed? You need to know what you are doing with UVC get a certified (G3) person in to sort it out.

 

[John Carroll]

11 hours ago, Blooda said:

I have a 90^oC and 7 Bar Pressure and Temperature Relief Valve on DHW tank.  Part of a ASHP supplied system.

Last week during legionella cycle, I noticed water was pouring through, it took a few turns to reseat the valve and stop the flow. 

During the week, heating the water to 43^oC there was no problem.  But during yesterday's  legionella cycle, it let off again, but reseated. But the water temperature only got to 60^oC instead of the 70^oC.

 

There is air coming out of the valve on top of the potable water Expansion vessel.

Can't for a minute think the temperature is getting to 90^oC [as ASHP supply], so temperature sensor should be reading ok.

 

So as per the title, do these fail ? or is there anything else to check before I change it. 

 

 

Can you post the volume of both the HW cylinder and the expansion vessel and the pressure of the cold water to the cylinder or the PRV setting, normally 3.0 bar.

 

[Mike]

13 hours ago, Blooda said:

do these fail ?

The Flamco ones I recently used recommend that they be replaced every 10 years to reduce the risk of them failing.

 

 

 

 

[Nickfromwales]

2 hours ago, John Carroll said:

Can you post the volume of both the HW cylinder and the expansion vessel and the pressure of the cold water to the cylinder or the PRV setting, normally 3.0 bar.

 

Manufacturers often give the correct vessel > UVC  A more relevant question would be whether these were delivered as a pair by the manufacturer, or sought interdependently (therefore possibly the EV was / is undersized). I routinely oversize, particularly if there is a hot return circuit (HRC) as the additional volume of heated water in the pipework can often tip the scales. THis is usually stated in the MI's, so should be mitigated by design anyways; but there are plumbers and there are plumbers......

Domestic cold mains rarely exceed 6 bar static, doubt this is relevant here due to the pattern stated.

[John Carroll]

HW cylinders will generally have a EV sized to not less than 10% of the cylinder volume, and are installed with a PRV (pressure reducing valve) set to 3.0 bar, the EV should then be precharged to 3.0 bar, (2.8 bar is recommended, as it apparently helps to prevent water hammer). If the OPs EV is sized to this 10% minimum then the air end pressure would have had to fall to ~ 0.3bar to cause the T&PRV to lift at 7.0 bar when the cylinder temperature reached 60C.  Spraying the air end schrader valve and where it exits the EV with soapy water will often show this, the cylinder then has to be de pressurized and the air end pressure checked, if its down to 0.5bar or less and no leakage has been detected then maybe get it recharged to 2.8/3.0 bar especially if it hasn't been checked at the last service.

Edited March 2 by John Carroll

[Blooda]

Hi,  thanks for the replies over the weekend I will knock a circuit diagram later and post. 

 

[Blooda]

Sorry for the delay .

Here is the sketch -  i think it is right - the cold water gets a bit lost in ceiling and behind the tanks.

Cheers

 

 

[JohnMo]

I would have thought the accumulator would be on the mains side. You are in effect trying to reverse flow the balance side of the system. You basically have a big accumulator attached to a small expansion vessel via the combination valve.

 

 

 

 

[Blooda]

Let me check again, things go into the ceiling, so loose track...  but by closing and opening valves should be able to work our what is what.  

Another job for the weekend. 🤔