The delivered efficiency of the system is largely dependent on the efficiency of the duct system – how well it is designed and installed.
Here are some of the common factors which characterize an inefficient heating/cooling duct system and may be contributing to your high energy bills.
a. Not enough return air. The air going back to the furnace has to equal the air being delivered to the building in order for the furnace to operate the way it should. Too often a small return is asked to serve a large space resulting in the over-working of the system and an uncomfortable house.
b. Dirty filters. Dirt interferes with air getting back into the system.
c. Leakage. It’s estimated that up to 25% of all conditioned air ends up in attics and crawl spaces because of leakage.
d. Twisted, crooked ducts. Flex ducts are not always installed correctly. Smooth, straight runs with as little bending as possible allows air to flow with minimum restriction.
e.. Remote plenums. These are duct board boxes in the system which we call “pressure drop boxes”. Air is a fluid and anything that gets in the way of its flow tends to cause the air to eddy and pool and not get where it is supposed to be. Sometimes they blow apart. Since they are located in an attic or crawl space, you can imagine the results.
f. All too often we find disconnected ducts.
g. Improperly sized ducts. This is especially critical with air conditioning system since cold air needs larger ducts than hot air does. If enough air is not getting across the coil which is located with the furnace the coil can freeze – a catastrophic failure.
It’s estimated that only 57% of the capacity of a furnace or air conditioner is delivered to the building the system is supposed to be conditioning. With proper diagnosis and duct re-hab it’s possible to get over 90% efficiency in the system.