Ductwork is Very Important to HVAC System PerformanceProper duct sizing has much to do with the perf...

1. Designing an HVAC System
Ductwork is Very Important to HVAC System Performance
Proper duct sizing has much to do with the performance of your HVAC system. Often when people
complain of having colder rooms or areas of their home, it is a result of improper duct sizing or
register placement.
A duct-u-lator is often used in sizing duct these days though there is what is called a manual "D"
formula for doing this as well. In order to size duct properly, we must start with the plenum, the duct
work installed directly on top of the furnace and coil. This must be sized to handle the total amount
of air that is produced by the furnace. From there, we decide how much is going in one direction and
how much is going in the other. The first 2 pieces of horizontal supply duct will now be sized to
handle that amount of airflow. As supply runs are taken off of this main trunk line, we will deduct
that amount of air that is leaving the trunk and down size the trunk line to carry the remainder. This
shrinking of the duct as it goes down the line helps to maintain the proper amount of air pressure
and velocity until we finally reach the end of the duct where it will be sized to hold whatever air is
left.
The return duct works the same however the last piece of duct is the one located near the furnace
since this is where the air finally returns to the unit. This last piece is often referred to as the
collector and must be able to handle the entire amount of air returning to the unit. The duct again
will shrink as it goes away from the unit but in this case it is because it's collecting more and more
air as it returns to the unit as opposed to the supply which is giving up air as it leaves the unit.
NOTE: Most supply runs will be ran out to the register in 6" piping. Some larger rooms may use 7"
piping. As for return air runs, it's common that the entire joist space coming from the return register
to the duct is sealed off to carry as much air as possible. "You can never have too much return air"
some may say and it does help the unit to run quieter and with more ease.
Example of Duct Sizing Mathematics
Let's say you have a 2,000 (5 tons) CFM fan motor and the system has 10 heat runs going each way
off the unit. You will need a 1,000 CFM to go each way for even flow and that 1,000 CFM will mean
each run will get 100 CFM to it. Again, this is an example but let's go with 20" x 8" duct each way. In
the first 8ft. of duct you take off 4 - 6" supply runs each supplying 100 CFM. Now, instead of needing
your duct to carry 1,000 CFM, you only need it to carry 700 so we shrink it down to the proper size
to maintain airflow. You would keep doing this as you go down the line until all the supply runs have
been fed. The same train of thought would apply to return air ducting however you'd be looking to
draw around 200 CFM per return. These are nice easy round numbers but the truth is if you only
had 20 supply runs, your 2,000 CFM fan is likely a bit too big and wouldn't be working as well as it
could.
Where to Locate Supply and Return Registers
Choosing the right location for your registers is important to how well your home evenly heats and
cools. It is also important to your energy bills.

2. Putting supply registers in the middle of a room isn't a good idea though you'll find them there often
in older homes. Now in the defense of our predecessors, they were working with gravity heat as
opposed to forced air but this design allows the cold or heat to penetrate too far into our homes
before being confronted. Registers are to be strategically placed around the perimeter of the rooms
in our homes. Under windows, beside doors, etc...this is to stop the elements from getting much past
our exterior walls and to attack at the coldest points of those exterior walls. Another added benefit
of this placement is that it is less likely to be covered up by furniture and such.
As for our return air registers, they are better placed on the interior walls as far as possible from our
supplies to help draw the supplied air through the room. It is again important to try to place them
where they will not be covered up. As I mentioned above, you can never really have too much return
air and when we choke these off, we starve our furnace fan for air. Think of your furnace somewhat
like a lung only that it can inhale and exhale at the same time. We cannot do that and your fan
cannot do them separately. Try to exhale for an extended amount of time without bringing any air
back in...doesn't work out to well does it? This is what the fan is feeling if it doesn't draw air in and
in time, it will overheat, work too hard, and ultimately it will break down.
NOTE: This is also where our furnace filter comes in. You can have all the return air in the world but
if your filter is clogged, it means nothing because the fan cannot draw air from them through a
clogged filter.
Are You Ready to Design Your Home's HVAC System?
Okay, I highly doubt that anyone is going to sit down with the wife and kids to work out a manual "J"
or run the numbers for their exact BTU needs, but I do hope that if you're considering building a
home or having a new system put into your existing home, this information will come in handy.
Knowledge is power and if you know even the slightest bit about the principles of HVAC, you will
feel more secure in the decisions you make and knowing that your eyes are wool free when dealing
with contractors.
Happy home-owning!