Frequent filling of closed central heating system

[lxbn]

About 2 years ago when the loft was built and a megaflow installed, I had my gravity CH system converted to a closed system. The boiler, a Worcester Bosch CBi24, remained as it was quite new and suitable for a closed system (the megaflow system came with with an expansion vessel of 8 litres). The radiators in the house were increased because of the loft to 20 in all.

The system was fine, but over that time I have had to use the filling loop to fill the system every now and again. This was to be expected as there was some air in the system which needed to be bleed out. But after 2 years I would have expected that almost all of the air would by now have been expelled from the system, but it seems that I have to fill up the system almost every 2-3 weeks or so. There does not seem to be any leaks as by now with the amount of water filled in that would have been obvious with damp patches appearing, but there are none.

I've noticed that the gauge, which should be at 1 bar, climbs when the heating is on and settles back to below 1 bar after a few days. Could the problem be that the expansion vessel is a bit small for the installation. The Heatrae engineer said this might be a possibility and that a vessel about twice the size should be installed instead. It that a reasonable possibility and could there be others. The system vents excess water in the system but I haven't noticed that as the pipe leads outside the house and not easy to observe.

 

[twgas]

Is the 8 litre exp ves a white or red one, Megaflows have an air bubble so don't need a seperate vessel. Some people, however, call other makes of unvented cylinders megaflows, so it may need the exp ves.
If the 8 litre ves is a red one connected to the heating system it probably is too small.

 

[45yearsagasman]

8 litres does sound a bit small,but vessel size is dependant on system size.Have you checked the exp vessel pre charge? This is normally on a data badge on the vessel.If in any doubt there is a calc to determine vessel size and I'm sure some of the system design experts on here will be pleased to help.A cb24 probably means that you have a medium sized system and that 8 litres is not enough.

 

[lxbn]

The vessel is a red one and came with the Heatrae Megaflow unit as a packaged standard option (not tailored for the installation but what would be generally suitable) for converting the CH system to a closed system but not meant for the megaflow unit itself.

Sorry 45yagm, I didn't look at the badge and in it's current location at the back of a top shelf that might not be easy although a Heatrae engineer did come on-site a few months back to sort out a vibrating valve issue and suggested that the vessel might be a bit small. He said that there was a formula for calculating the size required that involved measuring the quantity of water in the system. But with a mix of older and newer, double and single and various sized radiators/towel rails not counting the pipework, for me that would have been difficult.

Is there some sort of basic guide for for estimating the amount of water in a radiator just by it's size? Most of the pipework is 15mm so with a rough idea of how the pipework is routed I can roughly estimate the volume in the pipework. Thanks for the replies.