Diagnose my Tundish Problem ?

[Spinny]

I am getting occasional water discharge through the tundish on my mains pressure unvented cylinder with gas boiler.

(Not 100 percent certain, but this appears to occur when the hot water is on and has been on for a while.)

The mains pressure is about 3.5bar. There is a Caleffi combination Pressure Reducing Valve fitted and it reduces the pressure to slightly below mains. There is also a temperature/pressure relief valve on the Gledhill cylinder. (See photos). +Both are connected to the same discharge pipe and single shared tundish.

- the water discharge is not hot - I have stuck my finger in it.

- I have temporary outside style hot and cold taps piped from the system just 2-3m away from the system. When the tundish runs, the water pressure at the cold tap is about 3.5bar/50PSI. The water pressure at the hot tap is very high at about 6.2bar/90PSI.

- When I open the hot tap the hot water blasts out and I leave it to run for a few minutes to release this pressure. Reconnecting the pressure guage after doing this shows the pressure at the hot tap has then fallen to about 3.5bar/50PSI and the tundish stops running.

- If I open the cold tap it momentarily blasts out - just for a second or so - then runs normally.

 

In the 2nd Photo the pipe connections are: 

- 22mm pipe into the top is the mains water supply

- 22mm pipe out on the right with right angle bend upwards is I believe the balanced cold water supply

- 22mm pipe out on the right below the balanced cold runs to an expansion tank mounted on the wall (Note green stain on this pipe - joint weep ?)

- 22mm pipe out at the bottom provides the cold water feed to the base of the unvented cylinder

The grey plastic is the mains feed pressure reducing valve.

The red plastic is a 6 bar pressure relief valve outputting to the tundish (So I presume this is what is opening under excess pressure conditions)

 

Presumably the red valve is opening as a result of back pressure from the cylinder ??

 

So what is causing this ? Why is the cylinder pressure rising so high, and does this indicate some problem with the expansion vessel, which is presumably there to provide hot water expansion capacity and avoid over pressure ?

 

PS Having a 6 bar and 60deg C 250L hot water bomb in the house would seem like cause for concern ?

 

Help, explanation, answers, how to get it fixed etc greatly appreciated...

 

(PPS Currently have the Hot Water timer on for 2 hrs in the morning and 2 hrs in the evening - temperature of the hot water from hot tap appears to be 60C as expected. But the over pressure seems like it may perhaps be linked to having the hot water timer on. - though it doesn't occur every time - have only noticed the tundish running maybe once every few weeks.)

 

 

 

Edited 11 hours ago by Spinny

[Nickfromwales]

Expansion vessel having no pre-charge is the usual suspect here.

 

It seems the heating of the cylinder is creating expansion, and there’s nowhere for it to be absorbed.

 

Time to isolate the expansion vessel and check the charge pressure so we can help you through the process of elimination. 
 

Regarding the ‘bomb’ issue, relax.

 

There’s been a failure of some sort, and the safety features have done their job; the red T&PRV had opened to relieve the buildup of pressure in the cylinder, functioning entirely as intended to keep you safe