There are 5 vents connected to our Carrier furnace. Two of the vents does not blow hot air like the other three. I looked in the attic and found that the two vents that do not blow well have two duct/hoses connected to each. The other three that do have good heat flow only has one duct/hose each. I could not see how the connection is made on the two vents that don't blow hot but I do know that we only have one vent to each of the area where the ducts/hose appear to go to. I don't understand the need for the double ducts and how that may contribute to the poor air flow. I am suspecting that somehow, the air flow is returning to the furnace via the extra duct? Please help. Thanks.

Your system is unbalanced. The ducts that have 2 outlets must be eaqual in size to the other three. You do' nt need 2 outlets on these unless the rooms are very large .also make shure there air returns on these two rooms!

Thank you for your answer but I don't think you quite understand. It is not that there are two outlets to those two rooms that do not blow hot air well. The two rooms are a master bedroom and the living room/kitchen/dining room ( all one very large room). Both of these rooms only have one vent ( outlet ) but they have two large hoses going to each of these rooms. I question the reason why there are two whereas I only have one outlet? Perhaps one of these hoses is the return back to the furnace? Should these not have separate outlets?