hvac systems investigation & retro...

P.S. 260 Q / P.S. 307

40-20 100th Street

Queens, NY 11368

HVAC Systems Investigation

& Retro Commissioning

NEBB Certified in Building Systems Retro-Commissioning

Airpath Engineering, PC40-08 Oser Avenue

Hauppauge, New York 11788

http://www.airpath.us/index.html

Goals of Investigation

1. To verify RTU-1 and RTU-2 system performance

2. To determine if RTU-1 and RTU-2 can deliver design air flow

and pressure to all connected VAV terminals

3. To investigate how air system diversity affects flow and cooling

performance

4. To report the deficiencies that limit system performance

5. To verify and/or correct flow parameters and calibrations of the

VAV air terminals

Information From Previous Balancing Report

The previous balancing firm reported that the total

connected air load is significantly higher than the specified

capacity of each unit

It was also reported that not all VAVs are able to achieve

design flow

All inconsistencies that have been reported have been verified

by Airpath

Issues Limiting RTU Performance

There are two separate factors that are creating excessively

large pressure drops in the RTU systems.

1. The gas heat section of RTU-1 and RTU-2 both have an

inherently high air friction loss that was not accounted for in

the fan total static pressure from internal component air

friction calculation. This is mainly a problem in RTU-1.


There are two separate factors that are creating excessively

large pressure drops in the RTU systems.

2. The sound trap in the discharge ductwork of the RTU-1

system is installed in a position that produces high air

friction loss and limits the flow to some of the vertical

supply air risers.


Significantly low supply air duct pressure is preventing many

terminals on the RTU-1 system from achieving design flow,

including FPB-108, FPB-207, FPB-212, FPB-312, FPB-410, FPB-

506, FPB-508, and FPB-510.

RTU Gas Heat Section Pressure Drop

• The current gas heat section design has an inherently large internal air

friction loss, much greater than the published submittal value of 0.19”wc.


• The pressure drop measured by Airpath was 0.88”wc.


• The current design does not provide good air flow through the gas

heat pipes, a significant portion of air goes above or below the pipes.

RTU Gas Heat Section

Temporary Adjustment for Testing• To test the capabilities of the system, Airpath adjusted the sheet metal baffles to

reduce the air friction loss through the gas heat section to simulate the anticipated 0.19”wc air friction.


Temporary Adjustment for Testing• After making this adjustment, the pressure drop across the gas heat

section was measured to be 0.27”wc.


Temporary Adjustment for Testing• This adjustment increased the total airflow of the RTU-1 system by 2,000

CFM. However, the discharge static pressure was still to low to provide adequate pressure at each of the VAV terminals.

RTU Gas Heat Section Proposed

Solution To Reduce Internal Air Friction

• In order to solve this issue, Airpath suggests that the sheet metal

baffle be replaced by a bypass damper.


Solution To Reduce Internal Air Friction• The bypass damper can be controlled in such a way that it is open for

all non-heating modes when the airflow through the system is higher.


Solution To Reduce Internal Air Friction

During heating mode the damper would close to direct the air flow

through the gas heat pipes.

PS 260 5th Floor Shop Drawing (SK-5-2, SK-5-3) with sound trap

RTU-1 Sound Trap

• The RTU-1 discharge duct has a sound trap immediately before the

system branches out to the four risers that run down the building.


The sound trap greatly limits airflow to the branches, particularly the

branch that goes to riser 4.


Sound trap is not specified in the original design by Cosentini

Engineering Associates

PS 260 5th Floor Contract Drawing (M-105)

Drawing does not specify a sound trap

Why was a sound trap added?

Is it possible that the pressure loss of the sound trap was not

anticipated in the original supply air duct design external static

pressure calculation by Cosentini Engineering Associates

RTU-1 Sound Trap Solution

Airpath believes that the sound trap core may be removed

internally and the duct can be relined with new duct liner.

Removing the core of the sound trap will allow the airflow to

be correctly distributed among the four risers, and also

reduce the air friction loss in the duct… more air pressure

shall be available at the vertical risers.

RTU Rooftop Misplacement

• It was reported to Airpath by the SCA that the RTU-1 and

RTU-2 units were misplaced in each other’s rooftop locations

during the initial installation

• It was originally believed that the only difference between

the units was the return fan and return fan motor

• Airpath discovered that the supply fan pressure class, as well

as the supply fan motors, were also different

RTU Rooftop Placement, continued

As previously mentioned, the total connected air loads do not

match the designated air flow for the units installed

Unit Placement Initial Flow Capacity

from Submittal

Actual Total ConnectedVAV Load

Actual Supply FanMotor HP

Actual Supply FanSize/Class

Manufacturer Supply Fan Max CFM*

RTU-1 North Roof 19,800 29,270 20 40.25” Dia.Class I

24,000

RTU-2 South Roof 21,400 24,620 25 40.25” Dia.Class II

26,000

•- Supply Fan Maximum CFM is the maximum flow rate that the fan of the specified class can produce at the required fan total static pressure

Consequences of RTU Reversal

• Unfortunately, although the sizes of the supply fans in RTU- 1 and 2

are identical, RTU-1 specifies a class II fan. This fan is currently

installed in RTU-2, and the class I fan for RTU-2 is in RTU-1.

Therefore the present RTU-1 has a class I fan, where a class II fan is

required.

• This limits the maximum air flow capability of RTU-1. The only way

to achieve design flow would be to replace the fan and motor. This is

an impractical solution

• System performance can be improved by resolving the previously

mentioned issues, but it will likely never be able to fully satisfy all

terminals with the currently installed fan

Alternate solution

• Airpath suggests isolating one branch of the RTU-1 system and

connecting it to a new smaller rooftop air conditioning unit. The

branch that connects to riser 4 as well as 3 of the 5th floor VAV

terminals is preferable. A new 15-20 ton rooftop air conditioner

would be able to satisfy the connected VAV terminal load of 7450

CFM. A 15 ton RTU will have an 80% diversity, a 20 ton RTU

will be evenly matched to the outlet load.

• The currently installed fan in RTU-1 will be able to satisfy the

other 3 risers (21,820 CFM) without much diversity.

Suggested supplemental RTU rooftop position

Suggested duct connections on the 5th floor for the supplemental RTU

Fan Powered VAV Inlet Dampers

• Airpath discovered volume dampers installed on the inlet

ductwork of many of the FP VAV terminals on the second floor.

These dampers are unnecessary and restrict airflow to the

terminals.

• Dampers are shown on the Blue Diamond sheet metal shop

drawings but not specified on the contract drawings.

• Airpath fully opened these to maximize airflow to the terminals.

Volume dampers located on FP-VAV terminal inlet duct

PS 260 2nd Floor Contract Drawing (M-102b)

Drawing does not specify volume dampers at FP-VAV terminal inlet

PS 260 2nd Floor Shop Drawing (SK-2-2)

Drawing shows volume dampers at FP-VAV terminal inlet

Fan Powered VAV Terminals

in Need of Repair Fan speed controls of FPB-206 are broken off, fan is currently

300 CFM over the design flow of 900 CFM.

Malfunctioning fan motor controller on FPB-305, fan is not currently running.

FPB-308 is not communicating with the BMS, the flow set point is stuck at 0 CFM and cannot be adjusted.

Malfunctioning fan speed controller on FPB-505, the fan is currently set to low speed and cannot be adjusted.

FPB-414 is unable to achieve design minimum air flow, the internal VAV damper appears to close (verified by Airpath) but the damper may not be seating fully closed

Fan motor does not energize on FPB-507, possibly due to a fan controller malfunction

Results

Airpath believes that addressing the issues that have been

presented, both the RTU-1 and RTU-2 systems can attain

dramatically increased performance and that each of the

connected FP VAV terminals will be able to properly

maintain the temperature in the building.

Summary of Possible Upgrades1. Relocate the upper internal gas heat section baffle and install a

bypass damper below the gas furnace in the configuration

illustrated by Airpath, and approved by the manufacturer.

Control the bypass damper to close when the RTU is in heating

mode (furnace on)

2. Remove the sound trap core in the RTU-1 discharge ductwork

and reline the duct

3. Isolate the branch of the RTU-1 supply duct connected to riser

4 and the 3 VAV terminals, and install a 15 ton RTU dedicated

for all the connected air terminals in this isolated section

4. Repair miscellaneous terminals listed in the report

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