Combi system losing pressure

15 Feb 2022
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United Kingdom
Has anyone known a Grant 26 combi (external) to leak internally into the flame chamber?

For the approx four weeks, every morning the boiler has lost all it's pressure. I refill the system to 1.2 bar, and it kicks back in, and works fine - rads toasty warm and hot water is hot! This may last two hours, or 6 hours, or until following morning; then no pressure.

No signs of leaks at the boiler, round any radiator, or visible pipe work. As I have a full basement, can view nearly all ground floor pipe and first floor leaks would of stained the ceilings by now. I can only think that the 'flame / burn chamber' has split and is leaking in and evaporating away as condensate. Have removed the baffles & burner etc and cant find away signs.

Background - External Grant Eco 26 (2008) oil fire that has worked 'perfectly' during the three years I've lived here. As the property was empty for 7 yrs prior, I have completed the following
- replaced burner / blower assembly
- replaced PRV, flow switch, pressure switch
- replaced one way valves and auto air valves
- replaced PCB, white and black sensor
- replaced heat exchanger and thermal mixing valve

Over three year project the property was piped with new 22mm copper F/R, 15mm copper to new rads. 2 x heat zoning valves & by pass valve fitted.

Any advice appreciated before dropping £3k on new boiler
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Are internal leaks on oil boiler heat exchangers know? Yes.
Starting to see some of early Grant's need replacing.
I would disconnect it from system and put a pressure test on it
Not above normal working pressure though!
once you have reset the pressure, then turn the heating on, what does the pressure guag read after an hour of being running
@iammcd - refilled system to 1.4 bar at 07:10 and at 08:10 was reading 2.4 bar.
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Just be certain your not loosing pressure through prv . Some of the Grant boilers from that era had a 2.5 bar prv fitted your very close to that limit.
I take it you have checked expansion vessel?
@Exedon, thanks for the pointer. Hadn't even been checking pressures during the day, was suck in a mental loop that I had a leak. Did a PRV leak off test, and yes the PRV is opening. Will test the tank when the system is cold, and probably just replace as maintenance. Thanks
sorry for delay in updating. Have checked the expansion vessel and it's full of water! New has been delivered and will fit once it stops peeing it down. Only problem with external oil boilers, the only breakdown when it's raining or snowing.
Thanks for everyone's assistance, new expansion vessel fitted and all is well.

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