Gas and Oil Home Heating Furnaces - HVAC system too small?

I have a HVAC system that doesn't quite perform very well. The vent air flows are inconsistent. I have crawled into the attic and sealed all joints, but I have a couple of vents with very little air coming out. One is an addition. I sealed the vent and it made very little difference in the airflow of the other vent. Is a household air conditioning system supposed to kind of pressurize or is it just air running down the vent to exit where it may. From inspection of the main plenum with the aforementioned vent sealed, mine seems to be just running down the ductwork. Is my blower too small, or something else. Any help appreciated.

by checking air filter,belt on blower(if it is belt driven),the condition of the evap. coil and the squirrel gage vanes to make sure they are clean.Remember air flow is like water flow.If you have 5 garden hoses off of one spigot and everyone uses water at the same time the water pressure out of the hose decreases proportionally.Same with air flow.Your blower can only put out so much CFM.If you add extra diffusers or duct you are robbing it from an other area.There is one possibility.Your blower is running at a speed other than high.How long has this problem existed?PDF

Duct is got pressure. If you have any supplies off of the very end of the duct, you will lose most of your pressure. Give it a check.

Thank you for your replies. I will check the fan and registers this weekend as you suggested. Thanks again, Glenn

As before,thanks for the replies to my posts.I have checked the blower and ducts,no obstructions.The blower seems to be working fine,I dont really know how to check if its at the proper speed.Same one its been at for a couple of years,I would guess.Sounds the same anyway.This is a twenty year old house by the way,that had a lot of problems with lack of insulation and such that I have corrected.I do have three vents at the end of the run in the family room.The aforementioned added vent runs off of one of these.The added vent is one that has problems with airflow.I closed the family room vents and had a small increase in the flow to the added vent but not much.The vent is not clogged with anything(Its very short and I can see a light shined from one end to the other)I did find a damper in the main plenum where it tees from the main rooms to the bedrooms.I adjusted this and it also helped a bit.Still not acceptable.I guess my question is this.Do extensions off of the main duct(they come off at right angles) have to have something(main duct is straight)extended into the duct(a scoop of some sort)to gather the air into the extensions or do you think my system is just not adequate to the task.Sorry about the length of the post but I thought maybe the more info I give you the better answer I might get. Thanks again, Glenn

Grasping at straws now.. Not knowing anything about the design, duct sizes, unit cfm, etc. I tend to think that the extra load added on to the exsisting system is just to much for it. Have a load calculation done on your home and it will give you the answer.

You seem to be indicating that the main problem is to the addition. You said that the duct to the addition is extended off of another branch line. This is a problem. You cannot expect the end of a branch line to magically have enough air supply to feed an additional 50 or 500 square feet, it's not in the original calculation, it's not there. For an addition you will need a new line from the plenum to the addition. If your cut in branch duct is at a true right angle such as a "T" and not a "Y" you will have great trouble getting air into it. The angle of the turn itself represents the equivilant friction of 30 to 50 feet of strait duct. 20 year old house? What type of duct do you have? Flex? Metal? Fibergalss? Are you being careful not to disturb any asbestos?