Is it reasonable to add a 6th rad to a baxi bermuda system?

I have a baxi bermuda back boiler running five radiators
Two small rads, two medium and a large one.Now I have flushed and drained the system, it seems to run well as I am unable to hold onto the bottom of the rads for long without being scolded..:0)
The system is 25 years old


I would like to add an extra bathroom/shower room rad which I am told needs to be large.

There is a plate on the fire front that says the system outputs 20,000 btu's.

To find out if I can successfully add another rad, do I just add the btu capacity of each rad to see if it is within the capacity of the system? Or do I have to take heat losses , pipe runs and all sorts of other factors into account just as much? It seems that the system would be able to cope.

Also, is there any way I can improve the efficiency of the back boiler by cleaning it out or something?The flame seems the right coilour (yellowiish) and the system works o.k.

Any input would be most appeciated..

jg