By Eamonn Ryan
In a recent ASHRAE Journal Podcast, Kelly Cramm, P.E., Life Member ASHRAE, and mechanical engineer with Henderson Engineers, shared insights drawn from more than 40 years in HVAC design and ASHRAE involvement. Cramm, who has served on numerous technical committees including TC 9.1 and 9.10, offered a comprehensive look at how VRF systems work, common pitfalls and what engineers and contractors must do to ensure long-term system success.
This series of three articles is derived from ASHRAE Journal podcast episode 29 on the topic of ‘VRF basics and proper installation techniques’. This is part two.
From the design phase onward, engineers play a vital role in ensuring VRF success. Wayhomestudio | Freepikcom
Cramm identifies two main culprits: improper installation and insufficient maintenance.
Maintenance challenges
Indoor unit filters must be changed regularly. In buildings with dozens of indoor fan coil units, this can strain maintenance staff – especially if resources are already stretched thin. Neglecting filter changes restricts airflow, which can lead to compressor damage.
Outdoor condenser coils, typically microfin types, require cleaning at least twice a year. “Microfin condensers trap dirt and debris much more readily than older fin-tube designs. If they’re not hosed down seasonally, airflow drops, and the system can overheat and fail,” Cramm explains.
Since VRF heat pumps operate year-round, unlike DX cooling systems that run only in summer, the number of operating hours and the risk of contamination are much higher. If in-house maintenance isn’t feasible, Cramm advises contracting qualified service providers to ensure proper upkeep.
Installation best practices
The most critical installation step is purging refrigerant piping with an inert gas (nitrogen or argon) during brazing. “This is essential. It prevents slag formation and ensures clean, solid joints,” stresses Cramm.
All brazers must be trained and certified by the VRF manufacturer. Even experienced pipefitters can introduce tiny amounts of debris without proper training – debris that can later damage compressors.
Another crucial point: refrigerant charge must be recalculated if the field layout changes. Charge quantity depends on pipe length and volume. “If installers modify routing but don’t update the refrigerant charge, the system can become overcharged. Overcharging is a leading cause of premature compressor failure,” Cramm warns.
Additional field considerations
Suction line side-stream filters with fine mesh elements and isolation valves should be installed during startup to capture residual slag from brazing. Once filtered, these can be valved off.
Engineers must ensure designs comply with ASHRAE Standard 15 and the International Mechanical Code, which restrict refrigerant piping in egress paths or corridors. If piping must cross such areas, it must be enclosed or protected to prevent exposure in case of leaks.
Engineering and design responsibilities
From the design phase onward, engineers play a vital role in ensuring VRF success. One common design misstep, according to Cramm, is relying on VRF units to handle ventilation air, especially in humid climates.
She notes: “VRF fan coils are not well-suited to remove latent load. Always decouple ventilation air by using a Dedicated Outdoor Air System (DOAS). This allows proper dehumidification and supplies dry air directly to occupied spaces.”
ASHRAE’s publication on DOAS provides detailed calculation guidance for such systems. When combined with VRF, this approach turns VRF units into sensible-only cooling devices, simplifying operation and eliminating condensate management issues.
