VRF Enabled Makeup Air – Decarbonization Solution
As developers and engineers look for ways to provide improved energy efficiency with their eye on decarbonization, heat pumps are getting a lot of attention. Both air and water source heat pump technologies deliver exceptional value when addressing these two requirements. For space conditioning, engineers are getting quite comfortable designing with these technologies, widely incorporating variable refrigerant flow (VRF) and geothermal systems, but how do we address 100% outside air units?
In the multi-unit residential market, where 100% makeup air units (MUA) are required for corridor ventilation, it presents several challenges. For now, we’re going to look at MUA only, and disregard energy recovery options as I will look at those in a later post.
So, you need 100% outside air and you want to leave the gas line at the street! The quick solution would be a 100% electric MUA. Another approach is a packaged air source heat pump (ASHP) AHU with supplementary heat. These units need 100% redundant heat backup since the ASHP is locked out at ambient temperatures below 10°F. To avoid these limitations, a more elegant solution is to employ an LG VRF ASHP as the primary source with electric pre-heat. This solution, we’ll refer to as VRF enabled MUA, takes advantage of the low ambient (-22oF) heating performance of LG VRF equipment.
The VRF enabled MUA is manufactured with an electric pre-heat coil upstream of the ASHP coil. In heating mode, this coil maintains the air entering the ASHP coil greater than 41oF. The ASHP provides any additional heat required to deliver neutral conditioned air the building.
On a recent project, we designed a 5,000-cfm MUA for corridor MUA. The chart below shows a comparison of energy input during heating season for an all-electric solution vs. the VRF enabled MUA. This chart takes into consideration the degradation of ASHP coefficient of performance (COP, which is the ratio of energy produced to the energy input required) with lower temperatures. Specifically, the performance data for the 20-TR LG VRF ASHP was used. Clearly, the VRF enabled MUA requires significantly less power input (25-75% reduction) through the entire outdoor temperature range.
The VRF enabled MUA is a viable solution for both new and retrofit applications. The solution is scalable and can address any size air handler. Although, not specifically addressed in this post, integrating VRF with energy recovery equipment makes the analysis even more favourable for VRF as it eliminates the requirement for electric preheat.