Slow pressure drop on worcester bosch boiler

[CarloHeatingEng]

One of your calculator inputs is off. You put the safety valve in at 10 bar, but on a sealed heating system like yours it's 3 bar. With 3, your other figures come out at roughly 7 to 8 litres, so a 12 litre vessel is big enough on size.

 

[TryAgain1]

One of your calculator inputs is off. You put the safety valve in at 10 bar, but on a sealed heating system like yours it's 3 bar. With 3, your other figures come out at roughly 7 to 8 litres, so a 12 litre vessel is big enough on size.

Yes correct. OK I won't bother getting an EV. Least I checked. Thanks

 

[dilalio]

Yes correct. OK I won't bother getting an EV. Least I checked. Thanks

Checked!
Lol, done that in 5 minutes, without all the armchair engineering x

 

[TryAgain1]

Checked!
Lol, done that in 5 minutes, without all the armchair engineering x

It's a comfortable recliner

 

[MeldrewsMate]

Before adding expansion vessels find and cure the leak. One thing at a time.
Have you tried the boiler isolation and topping up overnight test? What was the result?
Re: gurgling noises. Is the pressure gauge attached to the boiler? If so (and in a bungalow) 1 bar is more than enough. Have you determined that the pressure gauge is giving reasonable readings?

Did the boiler pressure stay constant during our long hot summer, ie when the heating was not in use?
Have you inspected the glands around lockshield valves?

Have a look at the condensate trap or drain. Is it stained brown?

Finally, if you determine that the leak is from the radiator side then consider having a high pressure test done. In this test the rads are isolated from the boiler as instructed above by others. A test pump is attached to the pipework or to a radiator and the hydraulic pressure is raised by use of a hand operated lever, up to 10 bar. Very little extra water is added to achieve this, so a small leak will cause a rapid loss of pressure. I have used this method to find pinholes in radiators, leaks which only open up with elevated pressures (such as when the rads are hot). The resultant slow leak evaporates and shows no external signs, especially not when it leaks on the wall side of the rad. Bathroom rads are the most likely 'rotters'.

MM

 

[Johntheo5]

Yes correct. OK I won't bother getting an EV. Least I checked. Thanks

A 2 storey house needs a minimum EV pressure of 0.5bar to reach the top of the upstairs rads, I would suggest 0.75bar as the minimum. To get some idea of any leakage of air or water then there should be a water reserve in the EV when cold, if the Ev is filled to 1.25bar (cold) then there will be a reserve of 2.67L in a (12L) EV, a 140l system will expand by 3.14L, 2.24%, in rising from 1.25bar at 10C to a final pressure of 2.39bar at 70C, no fear of the PRV feathering at that pressure. If you don't fill to 1.25bar and leave the cold pressure at 0.75bar, then the final pressure when hot will only be 1.37bar, OR a smaller EV can be used to allow the final pressure to rise to 2.39bar, some, especially gas boiler manufacturers suggest this as they can shoehorn in a smaller EV or it does away with the need for a external EV.

 

[TryAgain1]

A 2 storey house needs a minimum EV pressure of 0.5bar to reach the top of the upstairs rads, I would suggest 0.75bar as the minimum. To get some idea of any leakage of air or water then there should be a water reserve in the EV when cold, if the Ev is filled to 1.25bar (cold) then there will be a reserve of 2.67L in a (12L) EV, a 140l system will expand by 3.14L, 2.24%, in rising from 1.25bar at 10C to a final pressure of 2.39bar at 70C, no fear of the PRV feathering at that pressure. If you don't fill to 1.25bar and leave the cold pressure at 0.75bar, then the final pressure when hot will only be 1.37bar, OR a smaller EV can be used to allow the final pressure to rise to 2.39bar, some, especially gas boiler manufacturers suggest this as they can shoehorn in a smaller EV or it does away with the need for a external EV.

It's a 1 storey bungalow

 

[TryAgain1]

Before adding expansion vessels find and cure the leak. One thing at a time.
Have you tried the boiler isolation and topping up overnight test? What was the result?
Re: gurgling noises. Is the pressure gauge attached to the boiler? If so (and in a bungalow) 1 bar is more than enough. Have you determined that the pressure gauge is giving reasonable readings?

Did the boiler pressure stay constant during our long hot summer, ie when the heating was not in use?
Have you inspected the glands around lockshield valves?

Have a look at the condensate trap or drain. Is it stained brown?

Finally, if you determine that the leak is from the radiator side then consider having a high pressure test done. In this test the rads are isolated from the boiler as instructed above by others. A test pump is attached to the pipework or to a radiator and the hydraulic pressure is raised by use of a hand operated lever, up to 10 bar. Very little extra water is added to achieve this, so a small leak will cause a rapid loss of pressure. I have used this method to find pinholes in radiators, leaks which only open up with elevated pressures (such as when the rads are hot). The resultant slow leak evaporates and shows no external signs, especially not when it leaks on the wall side of the rad. Bathroom rads are the most likely 'rotters'.

MM

See post 17 which I have pasted in:
'Boiler heating loop isolation test carried out from 2100 11/10/26 to 0900 12/10/26. On 11/10 I increased pressure to 1.5 bar in heating loop then isolated Flow/Return (2m from boiler) 2100 turned boiler off. Inspected 0900 12/10/26 pressure had dropped to just below 1 bar (boiler turned off all night, temp about 20/30 degrees). When turned boiler back on the temperature crept up from about 20/30 degrees C to 70 degrees C with the final pressure about 1.6 bar.

So the pressure swings from 1 bar (boiler turned off) to 1.5/1.6 bar (boiler fired on).' The pressure gauge is attached to the boiler and as of now I only top up to 1 bar. I don't know what the PG readings are other than occasional checks; generally PG readings are within green range (0.75 to 2 bar) exceptions seem to be early morning when the PG reading is at 0.75bar (close to red). During 2026 heatwave from April to Sep 2026 the boiler was not fired on and the PG readings stayed constant at 1.5 to 1.3 bar. I have inspected every lockshield valve I cannot see any leak The condensate pipe is white ABS and terminates outside into gravel and the termination endpoint (type of metal cowl) is not wet and covered in cobwebs so no sign of being used. Can hep2o polybutylene pipes have this 10 bar high pressure test? Is boiler isolation from the rad pipework using MagnaClean isolation enough for isolation or are full ball or lever valves needed? I have heavily clipped the hep2o pipework

 

[Johntheo5]

It's a 1 storey bungalow

If you use a precharge pressure of 0.5bar and a filling pressure of 1.0bar, the water reserve is 3.L and the max pressure in the vey unlikely event that the whole 140L reaches 70C will still only be 1.93bar and normally with 1.0bar/1.5bar pre&fill pressures, wouldn't expect much higher than 1.8 to 2.0bar.

 

[CarloHeatingEng]

Hello, that sum only shows a 12 litre vessel is big enough. It doesn't tell you whether yours is still holding its charge.

Your summer reading is the useful clue: it held 1.3 to 1.5 with the boiler off, so it isn't losing water sitting cold. Whatever's going, goes when it heats up. So I'd still check the air charge on the existing vessel with the water side drained, as before. If water comes out of the Schrader valve, the diaphragm's gone. And keep the bag on the PRV outlet for a week of normal heating, not just one night, because a vessel that's lost its charge can let the PRV lift briefly when it's hot.