System Boiler Pressure drops


Postby zyxxix » Tue Feb 10, 2009 3:46 pm

I have a Voker Mynute 10 System Boiler with a poor history:

From new it was installed incorrectly: the bolier was wired to be always on, only being turned off by the high limit thermostat. This also hid the fact that there was a leak in the heat exchanger itself of all places, so we could never find the leak- the water just evaporated away. I only discovered water leaking out of the unit after an accidental overfill whilst powered off.

Anyway, the pressure has dropped suddenly again (about 2 years since repair) and it won't hold for any length of time, although it still heats the water relatively well. Problem is, no gas engineer in the local area wants to touch this - complaining the leaks "could be anywhere" and you can't "get under the floor in these houses".

OK, so I had to debug this problem the last time and TELL the engineer what to do, so I need some advice on how to find the fault myself and tell him again (sigh).

1) Can't believe there's a leak upstairs, or the celing would be wet - it must be a significant leak - I could add water every 2 hours, yet I can leave it for days - it's 0 bar on the gauge when cold every time now.
2) I've touched all the radiator joints when cool/warm and pumping hot and not found anything there either.
3) Next. I'll lift the floor hatch to the section that houses the valves for CH or Water - but if no leak there, then what? Is there a technique to debug e.g. identify it the leak is on the Water or CH side etc?
4) Safety valve seems to be ok - not leaking as sometimes happens, so I'm told
5) Expansion tank won't hold pressure either - can this be the cause or just the effect of the low water pressure? I topped up the water to 1.5 bar, and sometime later observed it had hit 3.0 bar and the safety valve must have been operating - is this because of lack of expansion tank pressure?


Any advice appreciated - even to recommend a good gas engineer in Ayrshire! :roll:
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Postby htg engineer » Tue Feb 10, 2009 6:10 pm

Firstly the pressure gauge is for the CH only not the hot water section of the boiler.

If the pressure rises to 3 bar the PRV will operate and the pressure will drop. If the pressure rises to 3 bar with the CH on then yes the expansion vessel is at fault. If it rises to 3 bar with heating off, could be the plate heat-ex (combi's).


htg
Last edited by htg engineer on Wed Feb 11, 2009 9:34 am, edited 1 time in total.
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Postby zyxxix » Tue Feb 10, 2009 9:44 pm

Thanks will try to lead the eng to consider the expansion vessel.

[quote="htg engineer"]Firstly the pressure gauge is for the CH only not the hot water section of the boiler.
htg[/quote]

But surely as the boiler just heats a single source of water and this output is connected to 2 different zones via zone valves it can't distinguish the pressure for CH or HW? The HW is still in a tank on the first floor, and that is heated by the HW pipe from the boiler - the HW in the tank is not being heated directly, or am I missing something?
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Postby htg engineer » Wed Feb 11, 2009 9:34 am

"Is there a technique to debug e.g. identify it the leak is on the Water or CH side"

The pressure gauge tells you the system pressure, that's the pressure in the boiler, radiators and pipework. This is not for the pressure of the cold mains, hot water cylinder, hot water pipework.

If the pressure drops you have a leak either on the boiler, on a radiator or pipework on the pipework to the cylinder or the heating coil in the cylinder.

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