I heard that turning off some rads in a house is not a good idea. In this case there is a combi oil boiler (sealed room type), with about 13 rads, and a somewhat oversized boiler. Each rad has a seperate hot in pipe and a return pipe. (I think its called a two pipe system). Anyway is it true that it is better to leave all the rads on full, or is it ok to to reduce some of them with the manual valve (not thermostatic type), Thank you.