Your radiators aren't the only things warming the house.
Although not specifically intended as a heat emitter, the piping connecting the components of a hydronic system certainly does release heat to its surroundings. Sometimes we disregard the heat output of piping, thinking it's trivial relative to that being released by the "real" heat emitters. We may even point out that if the pipe travels through heated space the heat it releases is not really "lost" heat, it's heat delivered to the space by the piping rather than the intended heat emitter.