Baxi 105e pressure

5 Feb 2005
Reaction score
United Kingdom
The pressure on my Baxi combi 105e instant is 1 bar when idle and rises to 1.8 bar when CH is on. With the CH off and hot water running (including preheating) the pressure drops to around 0.6 bar.

Would appreciate thoughts on why the pressure is dropping so low.

Have just fitted a new expansion vessel as the old one was leaking internally.


Sponsored Links
when the c/h is on, the water will expand, giving you the presure rise. but when your h/w or preheat is on, the pump causes a negative presure on the presure guage hense the drop in presure.

its all normal
Can you explain what negative pressure is?
I don't understand it. Is it pressure in a different direction
the pump will give a positive presure in front of the pump, cause its pushing the water. and a negative pressure behind the pump, cause its sucking the water through. the presure guage is behind the pump, so when the h/w or preheat are in use, this causes the presure to reduce behind the pump, hense the presure dropping.
Sponsored Links
From 1 to 1.8 is a big difference under normal circumstances. Might be that your installer forgot to pressurise your new expansion vessel.
depending, if the rise is instant, ie as the boiler kicks in to c/h. but if the rise is during the temperature rise, its perfectly normal. they can go upto 2-3
Most boilers and all combis have a safety valve (PRV) set at 3 Bar. Any pressure over 2 Bar is probably too high UNLESS the boiler is at the bottom of a very tall building. Large swings of pressure usually indicate a problem and CANNOT be caused by the pump - most pumps operate with a 'head' of 5 metres or so. This is equivalent to no more than 0.5 Bar.
FWIW minw sit's at 1/1.2 cold and rises to 1.8 hot
3 floors and 14 rads (no small ones ) ;)

having said that i got a proper plumber to fit it :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL:
thanks guys for the advice. I installed the expansion vessel which was pre pressurised to 0.8 bar. I understand the baxi needs a minimum pressured vessel of 0.5 bar.


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links