High Pressure double duct system?

Dave F

I am troubleshooting an existing HVAC system. 1950 installation. One of the new suite buildouts does not get enough airflow. The system is a double duct, with a dwdi centrifugal fan discharging into a hot deck and adjacent cold deck. The fan is delivering 6" of static pressure at the discharge. By the time the air travels down the main duct riser, there is only 2" of static left. At the base of the riser, a 12" ducts feed a large portion of the floor. This leaves little (if any) static available for the double duct mixing boxes). Apparently there have been no complaints by any of the other tenants in the building. It seems that, in order for a sufficent volume of air to be delivered to the double duct mixing boxes, the duct velocity would be enormous. Most of the original double duct boxes have 5" diameter hot and cold ducts connected to them.

Does anyone have any insite as to how these systems were designed?

Were low temperature systems being installed in the 1950's? (The system is not being utilized in this manner at this time).

Perhaps the system is relying on the fact that, even during peak cooling, some of the air is flowing through the hot duct, thereby reducing the flow velocity in the cold duct?

Did the original field fabricated double duct mixing boxes only have a small pressure drop?

Any insite would be greatly appreciated.

Dave_4

get a balancer in there to help troubleshoot

you shouldn't be loosing 4" s.p. between the fan and the floor branches......check the filters on the main fans-they could be eating up 3/4" static...clean the HW & CHW coils-another 1/4"....re-seal main hot & cold deck duct joints....controls on the dual-duct boxes may allow for simultaneous heatig cooling....this should only happen during heating mode(if at all)....the newer controls & box actuators (titus, kruger, envirotec etc....)only need 1/2" at the box inlet plus whatever you need downstream of the box....a 5" inlet will give you 350 cfm at a reasonable noise level....fi you drop the temperature of the supply air, you'll get more cooling with less airflow, but verify the other floors won't need all their settings re-calibrated......

Don_2

Might be

> you shouldn't be loosing 4" s.p. between the fan

> and the floor branches......check the filters on

> the main fans-they could be eating up 3/4"

> static...clean the HW & CHW coils-another

> 1/4"....re-seal main hot & cold deck duct

> joints....controls on the dual-duct boxes may

> allow for simultaneous heatig cooling....this

> should only happen during heating mode(if at

> all)....the newer controls & box actuators

> (titus, kruger, envirotec etc....)only need 1/2"

> at the box inlet plus whatever you need

> downstream of the box....a 5" inlet will give you

> 350 cfm at a reasonable noise level....fi you

> drop the temperature of the supply air, you'll

> get more cooling with less airflow, but verify

> the other floors won't need all their settings

> re-calibrated......

Don_2

maybe

> you shouldn't be loosing 4" s.p. between the fan

> and the floor branches......check the filters on

> the main fans-they could be eating up 3/4"

> static...clean the HW & CHW coils-another

> 1/4"....re-seal main hot & cold deck duct

> joints....controls on the dual-duct boxes may

> allow for simultaneous heatig cooling....this

> should only happen during heating mode(if at

> all)....the newer controls & box actuators

> (titus, kruger, envirotec etc....)only need 1/2"

> at the box inlet plus whatever you need

> downstream of the box....a 5" inlet will give you

> 350 cfm at a reasonable noise level....fi you

> drop the temperature of the supply air, you'll

> get more cooling with less airflow, but verify

> the other floors won't need all their settings

> re-calibrated......

Paul_6

check the duct

run over real good. look for access doors where it goes through walls, open them up and look for fire or smoke dampers that may have fallen shut, or failed partially closed.get a good light and a Big inspection mirror and poke around real good. lol Paul.