VRF systems use a series of local space-by-space fan systems, which are fed warm or cold refrigerant to make warm or cold air to condition the space. They are the most popular systems in Asia and Europe but new to the North American market. They have fans and filters at each room to check and maintain but have simpler controls and are quieter than VAV systems. They typically use air for heat rejection and injection. Their use of refrigerant limits them to smaller buildings. Scaling them to larger buildings would require a water loop for heat rejection and injection. They are an excellent choice for retrofit applications in buildings not originally designed with cooling. The small size of the fresh-air ductwork and the refrigerant piping make them very attractive for those applications.
Chilled beams, a de-coupled system, is an evolution of the old induction units from the 1970s redesigned to use lower-pressure air, be quieter and available in several mounting configurations. They were developed specifically for retrofit applications but are now used for new buildings, as well. They operate by inducing room air through a hydronic coil to warm or cool it before supplying the air back into the space. Active beams use nozzles with fresh air causing a pressure differential to induce the air. Passive beams work with natural convective currents and only provide cooling. They are an excellent fit for offices or occupancies with high space equipment loads (laboratories with electronic equipment, for example).
Radiant heating and cooling is the newest de-coupled system, specifically the cooling side. Developed in Europe in the 1990s, it has crossed the pond over the last 10 years with the number of projects in the few hundreds. It uses electromagnetic heat radiation from a warmed or cooled surface to deliver comfort to the occupants rather than the space. The surface can be a concrete slab with embedded piping for water (high mass system) or a ceiling panel with piping attached to it (low mass system). Most retrofit applications typically utilize low mass radiant systems. It allows occupants to feel the same comfort as all air systems but at lower air temperatures in the winter and warmer air temperatures in the summer, thereby improving system efficiency. Radiant systems also have the fewest moving parts and hence the lowest maintenance costs.
Humidity control is key for the chilled beam and radiant systems because they operate at temperatures above the space dew point to prevent condensation. This limits their available capacity and helps drive capacity control by improving building envelope and lighting efficacy. The panels require the least amount of ceiling space, which is ideal for retrofit of old historic buildings. However, radiant systems come at a premium.
Smart retrofit for the 21st century often requires an analysis and use of blended systems. A recent renovation of an old historic building at a university campus used a combination of chilled beams for office areas and VAV systems for classroom spaces. This was done to accommodate space constraints and balance first costs with efficiency needs.
A key barrier to the adoption of these new systems is the unfamiliarity among both the design and construction community, leading to “fear factor” pricing. On a recent retrofit application, we were able to prove out that a radiant system would have cost the same as a VAV system. This was because of the need to break down larger VAV air-handling units and reassemble them
in the field compared to smaller units for the radiant system. Also, monetizing loss of rentable space to equipment should be considered as part of the system comparison.
Twenty-first century buildings demand 21st century systems, which can happen if we commit to performing comprehensive trade-off analyses.
Editor’s Note: The graphics in this article demonstrate the comprehensive analyses that should be completed before choosing an HVAC system for a specific retrofit project and are not an endorsement for any particular system type.