Worcester Combi - Bleeding - Topping Up?

You need the Sealed System FAQ:

David

On a newly installed system (6 years ago), I top up about once every 18 months when it drops from 1 bar to 0.75 bar (cold).

The system isn't ever completely sealed -- valve and diverter shafts for example couldn't be turned if they were completely sealed. However,

A small leak, contrary to what you might think actually lets air in to a pressurised system. Air is much less viscous than water and will pass through a leak much faster. Due to the temperature cycling of the water in the system and the consequent change in soluability and partial pressure of Oxygen and Nitrogen in the water, an air molecule just next to a leak will often see the not-quite-sealed heating system as a partial vacuum, even though the water is still under pressure, so the air is sucked in to dissolve in the water. When the system heats up, the solubility reduces forcing some of the dissolved air to turn back into gas and form bubbles, which then collect at high points. Oxygen in the water can also cause rusting of metalwork, which releases hydrogen, which can also collect in the high points. Inhibitors prevent this reaction, but are steadily used in doing so (and may also be lost by leaks and topping up dilution).

Many thanks - it looks like it will be worth trying a polythene bag around the end of the relief pipe.

Thanks - as I have said above I will try a bag on the outlet pipe to see if I am losing any there - I will also monitor accurate refill times/pressures

We have a FAQ for this. See below. SealedCH.

Many thanks to all previous responders and the FAQ:

Indeed, the plastic bag over the discharge pipe shows that there is a very steady drip from it.

I will open the release a couple of times to see if it clears it - other than that I will consider whether to get the valve replaced.