VRF vs Chiller for an Office: Load, Floor Plate and Running Cost

What the decision turns on

Not brand and not tonnage. The choice between VRF and chilled water is decided by how big the load is, how it varies through the day, how much plant space exists, and who will maintain the system in year seven.

An office that fills at 9 am and empties at 6 pm is a different thermal problem from one where three floors out of ten are occupied on a Saturday.

Side by side

Part load is the real argument for VRF

Most offices spend most of their hours well below design load. Weather is mild for much of the year, floors are partly occupied, and meeting rooms are empty more often than full.

VRF handles that well. Compressors modulate, only the zones in use draw cooling, and there is no large central plant that has to run to serve a small demand.

The counter is that VRF's advantage narrows as the building gets bigger. Long refrigerant runs, large numbers of indoor units and the practicalities of servicing distributed equipment all push toward centralisation at scale — which is the chilled water case.

The item that is missed most often

Fresh air is not included in either system.

Both VRF and chilled water systems, as usually priced, cool and recirculate room air. Outdoor air has to be introduced and treated by a separate unit, sized to the ventilation rate the design adopts — commonly referenced to ASHRAE 62.1, with the Indian framework in NBC 2016, Part 8.

This omission is responsible for a large share of "the AC is working but the office feels stale" complaints. It should be a named line in the budget from the first estimate.

Efficiency compliance

Equipment efficiency and system controls for commercial buildings in India are addressed by ECBC 2017. Either system can be designed to comply; neither complies by virtue of its type. What matters is the specified equipment efficiency, the control strategy and how the design is actually implemented — so ask for the compliance basis alongside the equipment schedule.

Mistakes that cost real money

• Selecting the system before a load calculation exists. Tonnage chosen from a rate per square foot is a guess, and both oversizing and undersizing are expensive in different ways.
• Forgetting the fresh air unit. The single most common gap in an HVAC budget.
• Not reserving plant space early. Chilled water needs a plant room and riser space from the first architectural layout, not after the floor plate is fixed.
• Ignoring out-of-hours use. If a few rooms run at night or at weekends, design for it — otherwise a large plant runs for a tiny load, every week, for the life of the building.
• Treating maintenance as someone else's problem. Distributed equipment in ceilings across ten floors is a different commitment from one plant room, and it should be evaluated before selection, not after handover.

Standards referenced

Energy performance to ECBC 2017; building services provisions in NBC 2016, Part 8; ventilation rates commonly referenced to ASHRAE 62.1. System selection, tonnage and compliance must be established by a load calculation and by the project's MEP consultant for the specific building.