HVAC ducts
 

I have an upflow forced air furnace to which I added a 3-ton air
conditioner. The uncased A-coil sits in the plenum above the furnace. I
have a question regarding airflow. Approximately 5 ½ feet above the top
of the furnace is a square right angle to carry the air horizontally
across the bottom of the first floor joists. Approximately 4 feet from
that right angle is a second square right angle directing the air into
the main duct for distribution throughout the house. My question relates
to these right angles. Would it be beneficial to the airflow to change
the second square right angle to a curved one? Would a curve not improve
the flow? My concern stems from the fact I have 3 X 10 registers
throughout the house, since originally there was no air conditioning. I
have been told I should have 4 X 10 registers for the A/C. I know of no
way I can make the change, therefore I’m looking to find any way I can
to improve the airflow. If there is a way for me to change the
registers, I’m prepared to do that. The unfinished basement portion is
no problem, it’s the finished area of the house that is a concern. I’d
appreciate any suggestions you might offer. Thanks.

"Don Phillips" wrote in message
news:FUMXd.16875$i6.4990@edtnps90...

I have an upflow forced air furnace to which I added a 3-ton air
conditioner. The uncased A-coil sits in the plenum above the furnace. I
have a question regarding airflow. Approximately 5 ½ feet above the top
of the furnace is a square right angle to carry the air horizontally
across the bottom of the first floor joists. Approximately 4 feet from
that right angle is a second square right angle directing the air into
the main duct for distribution throughout the house. My question relates
to these right angles. Would it be beneficial to the airflow to change
the second square right angle to a curved one? Would a curve not improve
the flow? My concern stems from the fact I have 3 X 10 registers
throughout the house, since originally there was no air conditioning. I
have been told I should have 4 X 10 registers for the A/C. I know of no
way I can make the change, therefore I’m looking to find any way I can
to improve the airflow. If there is a way for me to change the
registers, I’m prepared to do that. The unfinished basement portion is
no problem, it’s the finished area of the house that is a concern. I’d
appreciate any suggestions you might offer. Thanks.


Every 90 is the same as 10 feet of straight duct. Turning vanes MIGHT help
some.
Eventually the duct pressurizes and the air travels where the pressure is
lowest.

"Don Phillips" wrote in message
news:FUMXd.16875$i6.4990@edtnps90...

I have an upflow forced air furnace to which I added a 3-ton air conditioner.
The uncased A-coil sits in the plenum above the furnace. I have a question
regarding airflow. Approximately 5 ½ feet above the top of the furnace is a
square right angle to carry the air horizontally across the bottom of the
first floor joists. Approximately 4 feet from that right angle is a second
square right angle directing the air into the main duct for distribution
throughout the house. My question relates to these right angles. Would it be
beneficial to the airflow to change the second square right angle to a curved
one? Would a curve not improve the flow? My concern stems from the fact I have
3 X 10 registers throughout the house, since originally there was no air
conditioning. I have been told I should have 4 X 10 registers for the A/C. I
know of no way I can make the change, therefore I’m looking to find any way I
can to improve the airflow. If there is a way for me to change the registers,
I’m prepared to do that. The unfinished basement portion is no problem, it’s
the finished area of the house that is a concern. I’d appreciate any
suggestions you might offer. Thanks.


This is Turtle.

Taking out two square angle turn in a plenum box run to go to two curve turn
type turns would improve about 5% to 10 % but if you did not increase the
discharge duct sizes. Your effect would be really next to zero.

So Changing the discharge size duct sounds like the answer.

Now you do need a Manual D run on the duct work system to tell exactly what you
need. then a Manual J for your heating and cooling needed. Then you would know
for sure about everything.

TURTLE

I'm not familiar with 'turning vanes'. It sounds like something that
would be installed in the existing square right angle? If that is the
case, then would the curved corner (ie curved right angle turn) I
mentioned be as good or a better option? I'm thinking if the duct has to
be opened up to install vanes of some sort, it will probably cost me
more than to have a curved corner duct fabicated and installed. The big
question is whether or not I'lll see improved airflow...no matter which
option is chosen.


SQLit wrote:

"Don Phillips" wrote in message
news:FUMXd.16875$i6.4990@edtnps90...

I have an upflow forced air furnace to which I added a 3-ton air
conditioner. The uncased A-coil sits in the plenum above the furnace. I
have a question regarding airflow. Approximately 5 ½ feet above the top
of the furnace is a square right angle to carry the air horizontally
across the bottom of the first floor joists. Approximately 4 feet from
that right angle is a second square right angle directing the air into
the main duct for distribution throughout the house. My question relates
to these right angles. Would it be beneficial to the airflow to change
the second square right angle to a curved one? Would a curve not improve
the flow? My concern stems from the fact I have 3 X 10 registers
throughout the house, since originally there was no air conditioning. I
have been told I should have 4 X 10 registers for the A/C. I know of no
way I can make the change, therefore I’m looking to find any way I can
to improve the airflow. If there is a way for me to change the
registers, I’m prepared to do that. The unfinished basement portion is
no problem, it’s the finished area of the house that is a concern. I’d
appreciate any suggestions you might offer. Thanks.



Every 90 is the same as 10 feet of straight duct. Turning vanes MIGHT help
some.
Eventually the duct pressurizes and the air travels where the pressure is
lowest.

Don Phillips wrote on 09 Mar 2005:

I
have been told I should have 4 X 10 registers for the A/C. I
know of no way I can make the change, therefore I'm looking to
find any way I can to improve the airflow.

I'm not getting this at all. Have you run this system through a
full air conditioning season? If so, what problem did you
experience?

If the problem is that "someone" told you that you need bigger
registers, perhaps you should try living with the system for a
while to see how it works for you. You may have no problem at all.

The simple answer to "improving airflow" is to install a bigger,
more powerful fan. But that may or may not solve a comfort problem
if you turn out to have one.

--
Doug Boulter

To reply by e-mail, remove the obvious word from the e-mail address

First get someone to MEASURE air flow with a flow hood or other device,
NOT their hand. you should have around 400 CFM per ton. CFM=Cubic
Feet per Minute. Installing bigger ducts would help, but the entire
duct system needs increased, not just 2 fittings. A 90 degree elbow is
equal to from 30 to 80 feet of straight duct if it is rectangular duct
and 10 feet omly if it is round duct. note that to double air flow
with a larger fan, you have to have 4 times the duct static pressure
(square the increase) and 8 times the blower horsepower (cube the
increase in air flow). These are called the FAN LAWS. The only way
around that is a more EFFICIENT blower. but the best thing is to
measure the air flow first. Note also that you may want to install a
larger coil which would have a larger surface area and therefore a
lower pressure drop. But that may cause a humidity problem, depending
on where you live (climate).