G3 Registered, replacement control valve blowing off

[Jones82]

Hi Guys

Just wanted to pick your brains on this one. (I'm g3)

Customer has a 250 litre un-vented cylinder in his loft. He called me to the blow off dripping outside.

The expansion vessel had failed and it was fitted at the bottom of the cylinder on the floor of the loft. The control valve was also ancient so I replaced the control valve and re-piped the inlet so the expansion vessel was above the cylinder.

3 months on and the blow off is dripping again. With a new control valve and expansion vessel.

The temperature and pressure relief valve appears sound. When I disconnected the new blow off on the control valve its definitely coming from there.

The un-vented cylinder is quite old. 15 years +

Would a split coil present like this? They have been losing pressure on the heating side.

This would be my first split coil on unvented, I've seen quite a few open vented but not on a sealed system.

Thanks

 

[EvilDrPorkChop]

If you had a split coil on the unvented, your Heating would be increasing pressure not loosing. As the pressure of the cylinder is higher than the heating, so therefore would be introducing fresh water into the heating system. Unless of course your standing pressure is lower than 1.5bar.

Don't really see where the expansion vessel was fitted (Height wise) as long as it was fitted correctly.

What replacement control valve did you fit?

The other thing is, is the house piped on balanced cold? As otherwise a bar shower or something could be back feeding.

 

[SimonG]

Have you checked charge on expansion vessel.

 

[Scott_D]

Is the expansion vessel big enough?
Secondary return?

 

[SJB060685]

Problems causing PRV to discharge:
.Incoming pressure too high, ie, pressure reducing valve failed, or not one installed.
.Expansion vessel too small, lost some or all charge or too much charge.
.Defective PRV or bit of crap stopping from sealing properly.
.Unbalanced cold supply with mixer tap etc causing back pressure.

A split coil will just fill heating system due to the large pressure differential, unless of course as mentioned above cylinder static pressure is below heating system pressure but this wont happen.

 

[Jones82]

Thanks I’ll look into back pressure. It’s looking like a tap or shower may have failed.

The control valve and expansion vessel were replaced like for like recently. I’m sure both are correct


One of my pet hates of unvented is expansion vessel at bottom of the cylinder as that means you have to drain the whole cylinder to repair or replace it.

 

[Scott_D]

Thanks I’ll look into back pressure. It’s looking like a tap or shower may have failed.

The control valve and expansion vessel were replaced like for like recently. I’m sure both are correct



One of my pet hates of unvented is expansion vessel at bottom of the cylinder as that means you have to drain the whole cylinder to repair or replace it.

Fit a valve and DC so you can work on it easily next time

 

[Jones82]

Just popped in on the way home and the balanced cold of the control has been capped. So the back pressure theory is looking likely.

I just had a thought, could I just fit a 22mm double check valve on the dhw draw off?

Im also thinking of fitting a prv after the mains stopcock as it’s easily accessible in the garage.

 

[SJB060685]

Just popped in on the way home and the balanced cold of the control has been capped. So the back pressure theory is looking likely.

I just had a thought, could I just fit a 22mm double check valve on the dhw draw off?

Im also thinking of fitting a prv after the mains stopcock as it’s easily accessible in the garage.

Yes a DCV on HW outlet will stop any back feeding, if it doesn't solve the problem you know it lies elsewhere. Install a PRV on incoming mains will at least balance the supplies.

 

[EvilDrPorkChop]

Just popped in on the way home and the balanced cold of the control has been capped. So the back pressure theory is looking likely.

I just had a thought, could I just fit a 22mm double check valve on the dhw draw off?

Im also thinking of fitting a prv after the mains stopcock as it’s easily accessible in the garage.

I'd fit PRV on Mains first and see if it cures it (Set at same a cylinder PRV), as in theory pressures will be balanced then theres little chance of back feeding. Which cylinder is it?

I've had the same before and done what you suggest with PRV on mains and it solved my issue.

Double Check valve on Hot Outlet would be last resort for me.