Combi Boiler & Pressure

[DIYm1]

Guys

I am still learning about boilers and central heating system.

My W Bosch 28i may need re-pressurising - a layman's educated guess?
Please advise if you can:

Pressure fine at 1 bar most of the time.
In evening sometimes, it drops to just below it but always go back up to 1bar eventually.
Hot water working fine.

Now that C Heating is at full blast in most rooms of the house, I have noticed that when the boiler starts days for the C Heating, pressure drops from 1bar to 0.75 bar (The drop is normal from what i have been reading with the pump coming on - unless I am totally wrong). It stays this way however and only goes back up slowly with time - 1-2hours - to 1bar.

Central heating working fine - first and second floor
Radiators in loft however only hot at the bottom (have just started to use them recently)

Is the significant pressure drop from boiler normal? Is it related to the cool radiators in the loft which I have only just recently started using?
I don't think the pressure drop was as big as now before I started using the loft radiators which are not working efficiently anyhow.
Shall I re-pressurise the boiler to say 1.5bar?

Thanks!

 

[EliteHeat]

You have a little air in the system that is being compressed when the pump starts. As the water heats up the volume expands hence the small pressure rise.

Just bleed all your rads and repressurise to 1 bar when cold.

 

[DIYm1]

Thanks Elite... I am slowly getting the picture
Will need guidance on having the combi boiler re-pressurised later on - Will start a new thread to seek expert advice (I am doing some research/reading to undertand how these things work).

But first,
I need to sort the air out - Still figuring out where the bleeding valves are on 2 radiators out of 3 in the loft.... (see other thread).

 

[sm1thson]

filling loop instrctions here (hopefully they apply to your boiler aswell -if not it will give you the idea then check the manual -can be found online if you dont have the original)

some people say 1bar some say 1.5 (so i set mine to 1.25!) note once youve bled it seeing the temp go up by 0.5bar is normal. only fill when cold (ie dont put cold water into a hot boiler!)