Applies To
Rooftop packaged units fitted with a fresh air hood or outside air intake arrangement.
Example applications include RTPU systems where the contractor requires dimensional information to fabricate an external plenum or filter access arrangement.
Purpose
This article provides general guidance on the fresh air hood drawing for RTPU units and explains how the drawing may be used by contractors, installers and service teams.
The fresh air hood drawing is typically required when a site needs to confirm fresh air intake dimensions, hood location, opening size, fixing points or clearance requirements.
When This Drawing May Be Required
The fresh air hood drawing may be requested for:
| Requirement | Purpose |
|---|---|
| External plenum fabrication | To allow a contractor to manufacture a plenum to suit the unit fresh air opening |
| External filter installation | To provide filter access without removing the return air panel |
| Site coordination | To confirm available space around the rooftop unit |
| Mechanical contractor review | To verify dimensions before fabrication or installation |
| Replacement or retrofit work | To match an existing fresh air hood arrangement |
| Access improvement | To support easier maintenance and filter replacement |
General Description
The fresh air hood is designed to provide outside air entry into the rooftop packaged unit while helping protect the intake from direct weather exposure.
Depending on the unit configuration, the fresh air hood may be connected to the return air or mixing air section of the unit. On installations where external filters are required, a contractor may use the hood drawing to design a plenum or filter housing upstream of the unit.
Important Notes
Before using the drawing for fabrication or installation, confirm the following:
- The drawing matches the exact unit model and serial number.
- The fresh air hood arrangement is the same as the unit installed on site.
- Site measurements have been taken and verified.
- Any fabricated plenum does not restrict airflow.
- Filter selection does not create excessive pressure drop.
- The installation does not block service access panels.
- Weatherproofing and drainage are considered.
- Any modification complies with relevant site, mechanical and safety requirements.
The drawing should be treated as a dimensional reference only unless it is specifically issued as an approved fabrication drawing.
Contractor Guidance
When fabricating an external plenum or filter box, the contractor should confirm:
| Item | Check |
|---|---|
| Opening size | Match the fresh air hood or unit intake dimensions |
| Fixing method | Confirm available fixing points and avoid drilling into critical components |
| Filter access | Ensure filters can be safely removed and replaced |
| Airflow | Avoid sharp restrictions or undersized plenums |
| Sealing | Seal all joints to prevent water ingress and air leakage |
| Drainage | Ensure rainwater cannot be directed into the unit |
| Clearance | Maintain access to return air, electrical and service panels |
| Structural support | Do not place unsupported weight on the unit casing or hood |
External Filters
If the purpose of the plenum is to allow external filters to be fitted, the contractor should ensure the filter arrangement is suitable for the unit airflow.
Key considerations include:
- Filter face area
- Filter depth
- Filter grade
- Pressure drop when clean and dirty
- Safe access for routine maintenance
- Weather protection
- Air bypass prevention
- Secure filter retaining method
A poorly sized filter bank can reduce airflow and may cause nuisance airflow alarms, poor performance, icing, high pressure faults, or compressor protection issues.
Information Required Before Supplying a Drawing
When requesting a fresh air hood drawing, provide:
- Unit model number
- Serial number
- Site name and location
- Whether the drawing is required for replacement, fabrication or access modification
- Photos of the installed unit and fresh air hood
- Confirmation of whether the unit has a mixing box, economiser or standard return air section
- Any specific dimensions the contractor requires
Recommended Response to Customer or Contractor
Use the following response when supplying the drawing:
Please find attached the fresh air hood drawing for reference.
The drawing should be reviewed against the installed unit before fabrication. Site dimensions must be verified by the contractor to ensure the proposed plenum or external filter arrangement suits the actual installation and does not restrict airflow, block service access or create a water ingress risk.
Troubleshooting Considerations After Modification
If an external filter plenum is fitted and the unit later experiences operational issues, check the following:
| Symptom | Possible Cause |
|---|---|
| Reduced airflow | Filter or plenum restriction |
| Supply fan alarm | Excessive pressure drop or airflow restriction |
| Filter alarm | Dirty filters or incorrect differential pressure switch setting |
| Poor cooling or heating performance | Reduced air volume through the unit |
| Water ingress | Incorrect hood/plenum sealing or drainage |
| Noise increase | Air velocity too high through plenum or filter bank |
| Frequent maintenance complaints | Poor filter access design |
Escalation Information to Collect
If support is required after installation or modification, collect:
- Unit model and serial number
- Copy of the drawing used
- Photos of the fresh air hood and fabricated plenum
- Filter size, quantity and grade
- Measured pressure drop across the filter bank
- Supply air and return air measurements
- Any active alarms
- Confirmation of whether airflow was commissioned after modification