One thing that confuses me on these collectors - I see that folks put the fans pulling the hot air out of them. Yet we often preach that it is "better to push cold air to the stove". It would seem to me that perhaps some of the same logic may apply in terms of being easier to push dense cold air into the collectors than to pull the 'thinner' warm air out. Can anyone explain the logic here? Is there any particular logic one way or the other?