How long before a PRV goes if the filler loop taps are left open?

Unregistered · Oct 13, 2015

On a combi boiler, if the taps on the filler loop are left open will the PRV fail very soon? Does it depend on how often the hot water or heating is used?

The PRV on my tenant's boiler has failed. It happened shortly after the heating was turned on for the first time since the summer. The plumber says it needs a new PRV and was caused because the taps on the filler loop are open. The tenant's say they have not touched the taps. The last time I know for sure when the taps would have been used was about 3 months ago when a new timer had to be fitted.

If the filler loop taps are left open, would it fail much sooner than 3 months, or was it only when the heating started that the open taps caused the pressure problem?

steadyon · Oct 13, 2015

The PRV (Pressure Relief Valve) is designed to let water out of the boiler and heating system if the water pressure in the system exceeds a pre-determined value, nearly always 3 bar on a gas boiler.

When they fail, it normally means that the valve does not shut when the pressure is reduced below the preset value, and usually happens because the valve's seat has deposits of limescale , magnetite or similar on it.

Leaving the filling loop open and connected will fill the system to the pressure of the mains supply. If this is more than 3 bar, then the PRV will operate and dump water to outside until the pressure drops below 3 bar.

If, as you surmise, the loop has been left with both taps open, the PRV will have operated at all times when the mains pressure was greater than 3 bar, and in theory the pressure relief could have been discharging water for 3 months. If it has failed it will be at some point during that period, but no-one can say with any certainty when. Putting on the heating would tend to increase the pressure, and if the mains pressure were insufficient to operate the PRV, then putting on the heating could have added that extra bit of pressure to make it discharge.

It is unlikely that changing the timer would have required the filling loop valves to be operated.

While changing the PRV I would advise changing the valve at each end of the filling loop to screwdriver or allen key operated ones (of good quality) and consider paint marking them when the pressure has been set so that you can tell if they have been operated.