would it b o.k to tee into an existing rad to pipe to an additional radiator. The size of the pipework is 10 mm and the extra rad is approx 3.5 metres away in the same room downstairs. The boiler is oil fired,and open vented, with f@e tank. i dont know how many rads the property has but its a 2 up 2 down terrace house. Would the rad need to be sized?