Boiler Pressure Questions... too low then too high!

ma9gm

Hi,
I was hoping someone would be able to help me understand what might be going on with my boiler. I have a baxi combi boiler duo tec 40 he. It's about 10 years old. When the boiler is off, the pressure sits at just below 1 bar, however, when the boiler is on the pressure goes up to just under 4. If I lower the pressure anymore the boiler won't start because it tells me the pressure is too low. But 4 seems way too high. I'm just a bit confused by it. Anyone got any idea what might be going on? Thanks in advance.

Ebe_Scrooge

Usually the pressure should be about 1 bar cold, and will increase to about 1.5 or 2 bar when it's hot.  If it's getting to high when it's hot, this might point to a problem with the expansion vessel or the pressure relief valve.  Probably worth getting it looked at by a heating engineer.

ma9gm

That's really helpful. So looks like there could be an issue with the expansion vessel. I'll get someone to have a look at. Thank you

JohnB47

Yes, it does sound like the expansion vessel needs repressurised. I did mine myself, on my old boiler, using a pressure guage and a bicycle pump. If your pressure has gone up to about 4 bar, it's possible that the pressure release valve (PRV) has also activated. This allows the system to vent some of the pressure and as a result, some water, usually through a copper pipe fed through the wall at the back of the boiler. If you can find that pipe and you see a drip, or signs that it has dripped, then the PRV will need to be changed too. Even though it reseats when the pressure drops, the stuff it vents is usually dirty and stops the seal closing properly, so it can drip constantly from then on. A plumber will know to check this.

ma9gm

Oh, that's interesting. I didn't know I could re-pressurise. Is that something I can simply google? The cooper pipe looks dry and doesn't look like it's dripping, but I can keep an eye on it.

ma9gm

Actually, I found a video that shows what needs to be done. It doesn't look too complicated.

ma9gm

Would the big swing in pressure suggest the pressure has been lost in the vessel, or that the pressure is too much in there?

JohnB47

I am not a plumber but I'm pretty certain that the symptoms you describe are due to the loss of (air) pressure in the vessel. Alternatively, the rubber diaphragm separating central heating water and the pressurised air could be punctured. Either way, the vessel is likely full or almost full of central heating water and hence there's no air there to absorb the extra pressure when the system is running.
You've been lucky if your PRV hasn't activated.

ma9gm

Did it and that seemed to have fixed the issue. When the boilers off it sits half way between 1&2 bars. Then now when it's on, it sits just above 2 bars. Thank you so much for your help.

Rodders53

ma9gm said:

Did it and that seemed to have fixed the issue. When the boilers off it sits half way between 1&2 bars. Then now when it's on, it sits just above 2 bars. Thank you so much for your help.

That's possibly a tad high?  Consider reducing to 1 bar cold rather than 1.5 bar to keep the hot under 2 bars? 
https://www.baxi.co.uk/our-boilers/combi-boilers/baxi-duo-tec-combi from the Installation manual:
Central Heating Primary Circuit Pressures 
Safety Discharge 3 bar 
Max Operating 2.5 bar 
Min Operating 0.5 bar 
Recommended Operating Range 1-2 bar 

The pressure vessel should be pre-charged to a minimum of 0.5 bar as that is the pressure Baxi provide them set to when new.  Although up to 1 bar is not an uncommon pre-charge figure.

ma9gm

Rodders53 said:

ma9gm said:

Did it and that seemed to have fixed the issue. When the boilers off it sits half way between 1&2 bars. Then now when it's on, it sits just above 2 bars. Thank you so much for your help.

That's possibly a tad high?  Consider reducing to 1 bar cold rather than 1.5 bar to keep the hot under 2 bars? 
https://www.baxi.co.uk/our-boilers/combi-boilers/baxi-duo-tec-combi from the Installation manual:
Central Heating Primary Circuit Pressures 
Safety Discharge 3 bar 
Max Operating 2.5 bar 
Min Operating 0.5 bar 
Recommended Operating Range 1-2 bar 

The pressure vessel should be pre-charged to a minimum of 0.5 bar as that is the pressure Baxi provide them set to when new.  Although up to 1 bar is not an uncommon pre-charge figure.

Oh ok. I'll drop the water pressure a bit then. Get it down closer to 1.