Sorting out expansion vessel and overtemperature valve

[jack]

The overtemperature/pressure valve on my pressurised DHW tank has started leaking (again). This is the model:

 

 

My understanding it that it's a combined temp/pressure release valve (there's a separate 3.5 bar overpressure valve connected to the same tundish).

 

I also remember reading that these types of valves have a tendency to leak after they've been triggered, or even after testing.  

 

This is the second time this one has failed. Last time, the bladder on the expansion vessel had failed without me realising, leading to higher than usual system pressure. However, I don't now whether that was the cause of the valve failure. Our static pressure is less than 3 bar, so I doubt the system has ever seen anything like 4.5 bar, even with the immersion diverter working.

 

I wonder whether the immersion diverter is occasionally causing the temperature near the top of the tank to exceed the trigger temp for this valve?

 

I recently installed the relay module on our Eddi immersion diverter, which will let me control the maximum temperature better. I also have a couple of spare pockets that I could use for something like a solar mixing pump, but I'd rather avoid any more complexity. 

 

So my questions:

• If you had to speculate, what do you think is the most likely source of failure?
• If it's likely to be temperature, are there brands of valve that are better at coping with being triggered and returning to closed without leaking?
• What is the most appropriate pressure for my expansion vessel?
• Anything else I should be thinking about?

 

Thanks as always.

[Nickfromwales]

Give it a dozen cracks open, and slammed shut. See if that clears any debris which is what is usually the issue.

 

Give that a whizz and report back.

 

Immersion very likely the culprit, or not enough expansion volume. 
 

Do you have an HRC circuit on the DHW?

[jack]

Thanks Nick.

 

56 minutes ago, Nickfromwales said:

Give it a dozen cracks open, and slammed shut. See if that clears any debris which is what is usually the issue.

 

I tried that a few times earlier, but could try it a few more times and see what happens.

 

56 minutes ago, Nickfromwales said:

Immersion very likely the culprit, or not enough expansion volume. 
 

Do you have an HRC circuit on the DHW?

 

No HRC circuit.

 

I just checked and the expansion vessel had no pressure. Bah! So likely that, possibly made worse by the immersion diverter.

 

I need to put a reminder in to check the pressure in the expansion vessel more regularly. 

[Nickfromwales]

3 minutes ago, jack said:

Thanks Nick.

 

 

I tried that a few times earlier, but could try it a few more times and see what happens.

 

 

No HRC circuit.

 

I just checked and the expansion vessel had no pressure. Bah! So likely that, possibly made worse by the immersion diverter.

 

I need to put a reminder in to check the pressure in the expansion vessel more regularly. 

I’d double the amount of volume, by adding to it with a second vessel plumbed in in parallel. That’ll give you ample headroom for immersion higher temp top ups, as tbh you don’t want to lose out on those really.

 

TBH x2, I’d just install 2 new matching units and draw a line under it, in case the existing has become fractured. 

[Nickfromwales]

One big one obvs if you have space. 

[jack]

59 minutes ago, Nickfromwales said:

Give it a dozen cracks open, and slammed shut. See if that clears any debris which is what is usually the issue.

 

Give that a whizz and report back.

 

Did it ~15 times aggressively but no luck.

Any recommendations for an alternative brand?

 

38 minutes ago, Nickfromwales said:

One big one obvs if you have space. 

 

What's "big" in this context? It's a 22 L tank for a 250 L unvented cylinder. Minimum appears to be something like 4-5%, but rule of thumb is 10%, so it's above the minimum but a little short of the rule of thumb. 

The bladder on the original one died a few years ago, so I replaced it with a new one with a replaceable bladder. I'd ideally rather hang onto that than replace it again, although if it's important, I'm pretty sure I have the space for a larger one. Any brand you'd recommend?

[Nickfromwales]

Zimlet are popular, and tbh most mainstream names prob just stick their mark on these anyways. 
 

Once you sort the expansion, I’d say just double it, you’ll be able to fit the same T&PRV (Reliance is industry standard) and the issue will go away.

[Nickfromwales]

What’s the immersion set temp?

[jack]

1 hour ago, Nickfromwales said:

What’s the immersion set temp?

 

I'd need to check, but definitely nothing like 90 deg.

 

The issue is that the immersion is quite low on the tank (the upper immersion died and I've never gotten around to replacing it). By the time that lower region of the tank gets hot enough to turn off the immersion, there's been a lot of time for the water at the top of the tank to get very hot due to convection. That was my thinking when I mentioned a destratification pump.

[jack]

21 hours ago, Nickfromwales said:

Give it a dozen cracks open, and slammed shut. See if that clears any debris which is what is usually the issue.

 

Right, I've had a bit of time this morning to look more closely at this.

 

It's actually the pressure relief valve that's failed. The 15 mm pipework from outlets of the PRV and the PTRV joins before the tundish (I assume that's acceptable?), so it isn't possible to tell which one's failed without disconnecting the pipework from the valve outlets. 

 

The failed valve is a Reliance 102-series 3.5 bar PRV. Apparently that's a "cold water only" valve. I think this was the valve originally supplied with the cylinder, but it's also possible I asked that it be replaced at the same time I had the TPRV replaced a few years back. Either way, it doesn't appear to be the correct valve for this application.

 

Annoyingly, none of the Reliance valves designed for hot water systems come in a 3.5 bar variant. There's 107 series that is allegedly for "cold water use only", but it has a 110° max working temp. Shame I didn't realise what was going on yesterday or I could have given their technical department a call to see what the deal is there.

 

On the assumption that I shouldn't increase to a 4 bar version, and a 3 bar version is too low given I'm already having issues with the 3.5 bar version, I may need to look elsewhere. Annoyingly, I can't see any obvious options with the same form factor, inlet fitting, and outlet fitting. If the Reliance 107 model mentioned above isn't suitable, it might be time to replace both valves with a different brand and adapt the outlet pipework to suit.

 

Anyone have any thoughts?

[Nickfromwales]

24 minutes ago, jack said:

 

Right, I've had a bit of time this morning to look more closely at this.

 

It's actually the pressure relief valve that's failed. The 15 mm pipework from outlets of the PRV and the PTRV joins before the tundish (I assume that's acceptable?), so it isn't possible to tell which one's failed without disconnecting the pipework from the valve outlets. 

 

The failed valve is a Reliance 102-series 3.5 bar PRV. Apparently that's a "cold water only" valve. I think this was the valve originally supplied with the cylinder, but it's also possible I asked that it be replaced at the same time I had the TPRV replaced a few years back. Either way, it doesn't appear to be the correct valve for this application.

 

Annoyingly, none of the Reliance valves designed for hot water systems come in a 3.5 bar variant. There's 107 series that is allegedly for "cold water use only", but it has a 110° max working temp. Shame I didn't realise what was going on yesterday or I could have given their technical department a call to see what the deal is there.

 

On the assumption that I shouldn't increase to a 4 bar version, and a 3 bar version is too low given I'm already having issues with the 3.5 bar version, I may need to look elsewhere. Annoyingly, I can't see any obvious options with the same form factor, inlet fitting, and outlet fitting. If the Reliance 107 model mentioned above isn't suitable, it might be time to replace both valves with a different brand and adapt the outlet pipework to suit.

 

Anyone have any thoughts?

Ok. Exactly where is this valve. Is it the cold mains ‘blow off’ on the control group block? 
 

If so, it should be minimum 5 bar or even 6 bar, as factory. 

 

[jack]

It's on the cylinder.

 

To save me going and taking a photo, this is how it came from the manufacturer (McDonald Cylinders):

 

 

The one on the left is the PRV, which is what's failed. Interestingly, this diagram shows a light blue handle on the valve, which accords with the 107-series ("cold only", but rated to 110° C) Reliance valve I mentioned above.