[morke]

I've got a 3-ton system (not sure of blower capacity)with 3 zones set up in a radial plenum/duct design. The 3 zone plenums are near the air handler and flex ducts radiate out to the vents. Zones are: main floor, sun room (on same floor) and basement. I've got the basement totally shut off and still am getting very little flow to the main floor and sunroom registers. I think there are just too many for the blower size -- about 14 vents total for the two zones. Also, some of the ducts are 25' - 30' long and mainly seem to be 6". I think the long ducts are also restricting flow.

Any comments/suggestions?

 

[lentz]

Is this a new sysem? You might call a HVAC company in your area and let him tell you what is wrong. He will be able to check and see if you have enough CFM to heat your house. Sounds like you might have a bad duct design. Or maybe the fan could set on high speed to help.

 

[morke]

Yes, a new system. I've actually got (3) other units in the house (it's a big house) that seem to work fine, and the unit in question was okay until the temperature dropped (but it's always had low air flow per register). The HVAC contractor that designed/installed it is going to come look at it but I wanted to gain more knowledge first. I'm not an HVAC professional but I'm a mechanical engineer, and the long length/small diameter of some of the ducts seemed questionable to me. I'll see what he says. A higher fan speed is a good point -- they actually lowered the fan speed on my upstairs unit as it was blowing too hard. I'm thinking the problem unit is set to the medium speed.

 

[seanddd]

I have been running flex ducts without much problem up to 10 ft, with the duct fully extended and no kinks or sharp turns. Flex ducts has a high friction loss, especially when not properly installed, nevertheless 25-30' run is not acceptable, regardless of the diameter, sheet metal main trunks should be installed. As to capacity, for recent new constructions, we have been putting in 3-ton units for 2-floor 2600sf 16 vents, but primarily in sheet metal. Fan speed can be adjusted to compensate a little, check the static pressure.

If you have an exposed basement and enough ceiling space, it should not be too hard.

I've got a 3-ton system (not sure of blower capacity)with 3 zones set up in a radial plenum/duct design. The 3 zone plenums are near the air handler and flex ducts radiate out to the vents. Zones are: main floor, sun room (on same floor) and basement. I've got the basement totally shut off and still am getting very little flow to the main floor and sunroom registers. I think there are just too many for the blower size -- about 14 vents total for the two zones. Also, some of the ducts are 25' - 30' long and mainly seem to be 6". I think the long ducts are also restricting flow.

Any comments/suggestions?

 

[udarrell]

A radial duct system with very long & short flex duct runs, is IMO - difficult to balance airflow; some rooms will tend to be starved.

There may be a number of reasons why the overall airflow seems to be way too low! You need a good airflow tech... - Darrell

 

[beenthere]

Also may be short on return.
Can't supply more then the air handler has coming into it.