Sorry. Yet another boiler thread....

Help Support

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
I don't know if it's true, but a plumber once told me that once the PRV had vented, it would never seal properly again. Could be ballcocks, I'm not a plumber..
After a while, they do stop working properly. I have a PRV as I have a wood boiler, unvented. the first one worked for several years before I had to replace it. It is quite easy for a wood boiler to overheat, all it takes is a brownout.
We had a whole new system installed about 12 years back and got this error from time to time (with a GlowWorm boiler). On investigation we had several of the issues others have mentioned, and fixed them, but it still happened from time to time. Eventually the penny dropped that the expansion vessel by the cylinder (as distinct from the one in the boiler) was too small, so the pressure was varying too much when the boiler fired up. We added another expansion vessel and it stopped happening. YMMV though :)
Another very common fault with expansion vessels is the flexible tube or small bore pipe that connects the expansion vessel to the heating side of the boiler can become blocked with sludge effectively sealing off the expansion vessel and over time it loses its charge . If the engineer re pressures the vessel incorrectly then he will get a reading of 0.5. To 1bar after 1 or 2 presses of the pump . The heating side (rads needs to be @ 0 bar and a drain of or other fitting left open ) with the pump connected to the vessel the first few pumps may see water coming from the open end but this is normal ( sometimes it’s just condensation) but the pumping continues until you achieve the correct pressure. The pressure gauge on the boiler should still show 0 bar . If after a pump or 2 your pressure is showing 1bar or above then the connection to the vessel is most likely to be blocked . Simply remove any pressure from the vessel and remove the flex or pipe and unblock or replace it . It’s also good practice to replace the shraeder valve as again many of these leak and cause the vessel to lose its charge. Once completed the boiler should be put into operation and the prv should no checked and replaced if any significant drips are observed. I have to say that in all my years of working as a gas engineer I lost count of how many cust got ripped of for non existent leaks or had new boilers because of pressure loss issue’s that the so called engineer could not diagnose correctly.
All interesting stuff, in France the water pressure is generally 3 bar so the security valve is set to 7 bar.
It been a while (approx 5 years) when I was last a plumber/gas engineer, now a picture framer but my thoughts.
Someone mentioned that if the PRV opened it might not reseal completely, this is correct as debris can be present on the faces of the rubber seal and the brass faceplate it seals against. Expansion vessels do need pumping up usually after approximately 3 years but could be sooner, the correct way to do I this is isolate the flow & return under the boiler ( assuming they turn ok and fully close) you then empty the boiler via the drain off not the PRV ( or risk of contamination and failure to close properly) you then connect a foot pump or decent bike pump with a pressure gauge to the expansion vessel valve and pump up to about 0.8 bar or amount stated in manual, leave the drain off open while you do this as the pumping up will force water the has accumulated in the expansion vessel out. Disconnect pump,Then close drain off and open flow / return valve and you may have to top pressure up to manufacture value (think it was about 1bar) the PRV is designed to open at 3 bar if a over pressure state occurs. I seem to recall sometimes pressure could be lost by crossover in the domestic hot water plate heat exchanger due to cracks internally in the unit.
I had the crossover issue with the DHW heat exchanger on our Glow Worm, although it doesn't cause a loss of pressure, it actually takes your boiler pressure up to that of the mains. I initially thought it was the external filling loop, but it carried on gaining pressure with the external loop physically removed. Then thought ok, maybe it's own loop, which I have never used, is passing. But no, even with that sealed of it was still gaining, Then the penny dropped. Turned off the mains supply to the boiler and no more gain which confirmed it. Pretty rare I would think. Fortunately very easy to change on these.
Personal experiences include an expansion vessel diaphragm failing (after many years) requiring a replacement not just a pump up every few years.
Expansion vessel problems cause overpressurisation so the pressure relief valve operates repeatedly - the pressure release valve doesn't seal properly once this begins (don't test them, leave well alone) so it drips and loses pressure. Just buy a like for like branded replacement as they are easy to swap.
After 20+ years of the same boiler I recently picked up on a very slow loss of pressure being caused by a drip from the system drain point. The rubber washer must be perishing. For now, tightening it up has stopped the very intermittent drip and the pressure loss but I'll need to replace it next time I'm working on the system.
I don't know if it's true, but a plumber once told me that once the PRV had vented, it would never seal properly again. Could be ballcocks, I'm not a plumber..
As a retired industrial pipe fitter I can confirm the PRV is likely to not work efficiently but not a definite. Normally when a PRV blows off it is normally sent for resetting, infact on some plants I have worked on they are sent on a regular basis even if they haven’t blown off. I doubt if this happens in domestic plumbing but might in commercial plumbing. If you have any doubt I would get a new one fitted as they can weep.

Also the PRV needs to be vertical or it can weep. We fitted one to a steam main at Batchelots foods years ago in Worksop. Couldn’t get the PRV vertical due to roof space and it wept water so we had to modify the pipework.
Last edited:

Latest posts