Reusing An Existing Central Air System
There are benefits to keeping the old, but check off these important boxes.
How often has an AHU or system been reused as part of a building renovation project and/or to expand the area served by the associated unit to provide additional air conditioning or heating? It happens probably everyday somewhere, and yet I can’t say I have ever seen a design checklist to standardize the process and to help ensure the successful reuse of a unit and/or system.
Here is an example of reusing a central air system where the building owner’s design engineer convinced the owner to keep the duct distribution system and purchase a new, larger capacity AHU.
The application is a hospital in need of increasing the number of air changes within the operating room suite of six ORs. The original installation years ago was based on 12 air changes with prefilters and final filters upstream of the supply air fan. The system was a constant volume system with individual reheat coils and associated duct humidifiers serving each OR. Other support rooms had individual reheat coils. The new central air unit was capable of delivering 24 air changes; the contract’s scope of work began at the outdoor air intake and terminated at the unit’s supply air discharge. The system’s total static pressure was increased from 4 in TSP to 7 in TSP.
The design engineer was fortunate to avoid the pitfalls of reusing the central air unit, but he overlooked the impact of the new supply air quantity, increased duct velocity, and increased duct pressure. The errors and omissions quickly began to surface when the unit was turned on and the fan quickly ramped up. Metal flakes laying on the inside bottom of the supply air duct were blown into the rooms by the increase in duct velocity as the supply air ductwork began to expand out on all four sides from the increase duct pressure. The problems didn’t stop there, but I will.
Design issues like this can potentially be avoided by having an AHU and/or system reuse checklist developed from the following segments:
1. Basis of design
a. Written and mutually agreed upon by owner and engineer.
2. List of existing conditions
a. Building (hours of operation, etc.)
b. Duct distribution (e.g., SMACNA sheet metal classification, etc.)
c. Unit (ASHRAE Standard 431-2014, etc.)
3. Existing issues & concerns
a. Unit has excessive vibration and/or air leaks
b. Signs of corrosion inside and/or outside unit
c. Air filter requirements do or do not meet current standards
4. Go or no-go decision. Is this approach feasible to implement?
If the project is a “Go,” continue developing/using the checklist:
5. Distribution system
a. Take existing condition duct pressure test and then compare estimated new duct pressure requirements, etc.
b. During existing pressure test document air leakage (e.g., specify new duct leakage requirements, etc.)
c. Assess downstream obstructions (e.g., existing heating coil size and capacity)
d. Record video inside of ductwork (e.g., loose duct lining)
e. Clean ductwork (e.g., return air ductwork tends to be very dirty over time)
f. Survey duct damper conditions (e.g., fire damper partially closed)
6. The unit
a. Complete infection/contamination test (e.g., signs of mold)
b. Seismic supports in place (yes) (no)
c. Insulation inside (e.g., none)
d. Unit tight (e.g., airtight)
e. Unit noisy (e.g., bearing noise)
f. Corrosion (e.g., inside)
g. Energy conservation opportunities (e.g., VSD)
h. Current codes and standards (e.g., increase filter efficiency)
i. Air velocities (e.g., potential for moisture carry over)
j. Fan curve (e.g., maximum available cfm)
7. Addressing existing unit/system deficiencies
a. Chronic freeze-stat shutdown (e.g., air stratification)
b. Snow entering the unit
c. Inadequate access (e.g., filter removal)
d. Excessive duct discharge static pressure (e.g., 90-degree elbow at outlet)
e. Inadequate capacity (e.g., coil inlet-outlet piped incorrectly)
a. Temporary services needed (e.g., temporary AHU needed)
b. Accessibility (e.g., to ductwork above ceilings)
The draft of this preliminary designer checklist can easily be increased to cover a broad range of issues, concerns, requirements, and compliance. Once completed, the checklist can always be continuously improved based on lessons learned and other people’s input, so that reusing a central air unit/system will be a cost-effective solution.