Central Heating Problems - is there a leak?

[bobby]

5 years ago, BG replaced an old boiler with a Glowworm Micron 60FF wall mounted boiler.

We went on holiday for a couple of weeks and as usual I turned off the central heating at the controller and turned off the water and gas as usual.

When we got home, I turned on all the services and noticed the tank feed for the central heating needed filling excessively. Turned on he central heating and there was air in the system which I bled successfully. Heating OK for a couple of days after which the pump was very noisy.

Called in BG (we're under their maintenance contract) who diagnosed air in the system. He fitted a new pump, a new boiler circuit board and opened up the 22mm feed piped from the boiler to the pump. Three visits later the engineer recommended a power flush costing an exhorbinate amount (£679!). I cleaned the system with Fernox heavy duty restorer as per manufacturer's instructions - the water in the system seemed very clear after draining & multiple flushing. The system was no better with the pump sill noisy and lots of air even after bleeding.

I let the system cool all day, tied up the ball cock in the vented header tank and measured the water in the vented header tank using a dip stick. After 3 - 4 hours the water level had gone down by 9" or so. There is no visible sign that water is escaping anywhere in the central heating system, although some pipes are run under solid floors.

The question is what should I do next? Spend the money with BG or try Fernox's leak sealer?

One other thing, from the vented header tank there is only a 22mm feed pipe to the boiler feed line which serves also as a vent. BG tell me this is normal practice but every diagram I've seen has a seperate vent pipe. Should I fit one?

Many thanks