PRV leaking a lot

[mave]

The PRV on my Radiant combi boiler decides to pour out loads of water every so often.

What is the most likely cause?

I've also lost the instructions. Is there a site where I can download a new copy?

 

[oilman]

Do you have a pressure gauge on the system? what does the gauge indicate just before the PRV blows? (I know this will not be easy to say as you probably cannot predict when the PRV will blow) The PRV may be at fault, but more likely is the expansion vessel diaphram has failed and so when the system water expands, it makes the radiators and boiler swell until the pressure causes the PRV to go. To check this, operate the PRV so the system pressure is at zero, then depress the air valve in the expansion vessel for a moment to see if air comes out. This is to save your tyre pressure gauge from becoming filled with water. If air comes out, check the pressure with the tyre pressure gauge, and it should be between 1/2 and 1 bar (7 to 14psi).

Let me know what happened.

 

[mave]

Thanks oilman.

The PRV was discharging onto the kitchen worktop, so I have now routed that outside, so the problem is not quite so drastic.

When the heating warms up the pressure increases from 1.5 to nearly 3 bar. The pressure is generaly unstable and fluctuates when the HW is turned on. Am I right in thinking this shouldn't happen?

Sounds like it probably is the diaphram. I will do the check that you suggested. If so, does that mean a new vessel or just a new diaphram?

 

[oilman]

Some (but none that I've seen) have repalceable diaphrams, most need to be replaced. If yours is a gas bolier it may be difficult to get to, but there are guys on the forum who know better than I. If it is difficult to replace you can add an external one, and then connect the pipe that goes to the existing vessel to the new one, or tap into the return pipe near the boiler and add an external vessel that way. perhaps you should do this anyway as your existing vessel may be too small. in general terms it should be 10% of the total system volume. You can find radiator data on the web, and then add the volume of the pipes and the boiler. You would do no harm to have a bigger expansion vessel than you need, it reduces the stress on the diaphram. Air pressure should be 1/2 bar for a 2 storey system, then pressurise the water to 0.7bar when cold. The pressure may go up to about 1 to 1.5 bar when hot. Any higher and you need a bigger vessel.