How to size a Feed & Expansion Cistern??

[ISM]

As the title suggests really, which will hopefully help me with my challenge below,

Have been working on a 8 bedroom house with an old CH System with its own boiler room, 75kW Gross Boiler (Ideal Concord SE 205), 24 radiators - 90% of them in column type, anything from 4" to 1/2" steel pipe running through the entire house (F&R - 1 pipe), both the CH & HW circuits are vented, No Cylinder stat and I believe the F&E tank is around 60/70 litres (total area of F&E tank L600 x W440 x H420).

I changed one radiator, but had to drain down the entire system (well it seemed like I did, as it took nearly 4 hours!!), had to back fill due to the feed pipe off the F&E tank was blocked, so admittedly I did over cook it with water initially, but drained down until the fill level in F&E tank was approx 100mm from the base and set the float valve to suit.

Gate valves control the CH & HW circuits so at the moment the gate valve for CH is closed, with just HW set to come twice a day and the boiler stat is sound. So, as you've probably guessed the system is venting over, but venting up through the feed pipe and not out the vent pipe. I can only assume it has been doing this all the time, to which the gardener mentioned it only happens in the summer, but my customer believes this is not the case.

So I believe the F&E tank needs to be bigger; to probably around 100 litres - any thoughts/best practice very welcome!!

 

[Billy Bob Bob]

gas pressures over set,stat over set or knackered,pump not well(or over set) (if fully pumped) system design issue rule these out first

 

[AWheating]

should be size so once filled the volume left in the tank between the water level and warning/overflow pipe is atleast 10% of the volume of water in the system. so unless you have volumes of rads, boiler and pipework to add up its a hard one to answer.