#1346 90.1 PRM Fan Pressure Drop Adjustment

[Lukas Hovee]

Regarding implementation of Table 6.5.3.1B fan pressure drop adjustments:

Building Descriptor 'Pressure Drop Adjustments indicates baseline pressure drop adjustments are calculated as:
A = SUM(PD x CFMd/4313)

Where PD is calculated for each individual PROPOSED hvac system, and
CFMd is the design (autosized) CFM for the BASELINE system.

There is not a 1 to 1 correspondance between proposed and baseline systems, i.e. a baseline system may be made up of thermal zones which were originally served by multiple proposed systems. How are we to calculate A for cases where some of a baseline system's zones were served by proposed systems eligible for pressure adjustments while others were served by proposed systems eligible for different, or no pressure adjustments?

[Lukas Hovee]

Additional complications to the above issue:

Thermal zones served by a single PROPOSED system may assigned to different BASELINE systems.

Two possible approaches below:

1) Simpler - If the proposed system serving a thermal zone is eligible for a pressure drop adjustment, the baseline system serving that zone is also eligible for the adjustment. The adjustment is calculated for the entire system CFM

2) More complex - If the proposed system serving a thermal zone is eligible for a pressure drop adjustment, the pressure drop adjustment for the baseline system is calculated as:

   A(baseline, system) = SumOverZones( PD(proposed, zone) * CFM(baseline, zone) /4131)

This second approach is somehwat consistent with the notes to table G3.1.2.9, the only difference being that the A is calculated for the thermal zone rather than the system.

A further complication:

Some credits are really only applicable to PART of a system's airflow. For instance, a 500 cfm laboratory hood may be in a thermal zone that is conditioned by 2000 cfm of air. Table 6.5.3.1.1A indicates that A is calculated based on the cfm of the DEVICE, rather than the system. In this case, we could have the user input the DEVICE cfm directly (with a check to insure it is <= the system cfm), but not clear how this would translate to calculating A from the baseline cfm?

[Supriya Goel]

Luke-
My apologies for the late reply. Option 2 would work fine. A few comments below-

1. I agree with your recommendation to add input for flow through device. we can ask the user to select the filtration credit and then specify the corresponding CFM
2. To calculate A for the baseline system, we would calculate the ratio of CFMdevice and CFMzone for the proposed building and use the same proportion for the baseline.
3. Ratio = CFMdevice-proposed / CFMsupply-zone-proposed
4. CFMdevice-baseline = Ratio * CFMsupply-zone-baseline
5. If (in the proposed model) the airflow through the device = zone air flow, then the baseline airflow through the device = baseline design zone airflow
6. In the case of exhaust, the baseline exhaust airflow = proposed exhaust airflow. So for all exhaust related pressure drop credits, the CFM device is equal to the user input for the proposed.
7. The pressure drop adjustment for 'Fully Ducted Return and/or exhaust air systems' should be broken into 2 pressure drop credits, so that we know which case is supply and which is exhaust.

[Nikhil Kapur]

Issue assigned to ASHRAE 90.1. Issue can be reopened when development on ASHRAE ruleset begins.