Combi Boiler Low Pressure when cold, high pressure when hot.

2 Dec 2009
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United Kingdom
I have a Vaillant Turbomax Plus 828e.

All radiators are bled.

When the system is cold the bar pressure reads 0.8, but when running it climbs over the recommended 2.0 bar, reaching 2.2-2.3 bar, last year it never peaked above 1.5 when hot - any ideas why it should change this year.

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Ahh, is the 'expansion vessel' a green rubber bag you inflate slightly with a bicycle pump ?
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Hi, thanks for the reply, I remember now, a few years ago Vaillant instructing me to simply use my bike pump, and to let a bit of pressure out before hand.
Your FAQ suggests 'repressuring', - do I need to go through all that just to put a bit of pressure back in ?
Trebor2Go";p="1427617 said:
do I need to go through all that just to put a bit of pressure back in ?
And what about the excess water? Where will it go?
OK, well I found someone had turned the heating temperature dial up to full, which I hadn't noticed, would this have caused the Pressure dial to read above 2.0 Bar ? the temp. dial is at the 3/4 position now.

I still investigated the Expansion/Pressure Vessel, when I pressed the 'tyre valve' in no water (apart from maybe some condensed moisture) came out, but also no air seemed to come out either though - should I have expected a hiss of air at all ?
With the system hot I connected a bicycle pump and pumped 5-10 times.
Now when running hot the bar pressure measures roughly 0.75 Bar.
When cold bar pressure is reading 0.25 Bar.

I'm confused, the systems not leaking (not that I know of) - so where has all that water/pressure gone since pumping the bike pump. I know the min cold pressure should be much higher than 0.25bar. The installation manual doesn't give any Min. Bar Values for cold, is it 1.1 Bar ??
Thanks Kev,
If thats the procedure then I'll do it, let you know, mustn't forget my Fernox inhibitor, I've had some in the workshop for a few years, does it go off ?
The water won't have gone anywhere (unless you have a leak).
Water does not compress, air does!, You've pushed a bit more air in the air side and its pressure rises slightly. You've not created a larger air space for the excess volume of water, because you can't, until you remove some of the existing water.
The vessel is there to accommodate the extra volume as the water expands. The extra depends on the total volume of water in your system and the rise in temperature.
A system where the pressure starts at 1bar and rises to 2bar is normal. The problem arises when the higher pressure reaches 3bar which is when the pressure relief valve operates. So you would not want to start at 2bar and rise to 3bar.
I followed the procedure, much better, but I'm unsure of the pressure in the expansion vessel - its roughly 10psi, the system only seems to expand by 1 Bar when its running hot ... does that sound OK - or could I have pumped too much air in.
Cheers Kev, last question, I've dropped about 20 litres out of the system (its got 8 rads in a Semi detached house) to do what was required, the system already had rust inhibitor in it, but I've only topped up with fresh water - should I put some more inhibitor in - or will it be OK as it is.
put some in you can't overdose it

unless you go really silly

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