Single Panel Single Convector Type 11 Double Panel Single Convector Type 21 Double Panel Double Convector Type 22. Central Heating Panel Radiators Type 11 21 22 Heights 500 600 700mm Convector. HIGH HEAT OUTPUT DESIGN: Prorad Single Panel T11, Double Panel T21 and Double Convector Type 22 central heating compact radiators are a high heat output design which is one of the industry's top heat output radiators.