Delta (Δt) T500C is the recognised European standard for calculating heat output. Typically, this will assume that your boiler flow temperature is 750C; has a return of 650C and a room temperature of 200C. Delta (Δt) T600C usually assumes that your boiler has a flow temperature of 850C, a return of 750C and a room temperature of 200C. If your heat output is listed Delta (Δt) T600C and you need to convert to Delta (Δt) T500C then you should multiply the figure by 0.79.