10 Insights for Implementing VRF in Cold Climates

Variable Refrigerant Flow (VRF) systems have been in existence since 1982, yet engineers in the U.S. have been slow to embrace these electric, refrigerant-based heating and cooling systems. The cause? There is a pervasive misconception that heat pumps function inefficiently at lower temperatures. This tarnished reputation stems from the original heat pumps equipped with single-speed compressors and inefficient fans.

However, advances in technology have transformed this situation. Contemporary VRF systems boast variable-speed compressors, adjusting their compressor and fan operation based on demand. When implemented correctly with the following insights, these VRF systems can function with an efficiency that's more than twice that of electric resistance heating and natural gas furnaces, even at temperatures below 0 °F.

Understand capacity deratings and consider local conditions.

Despite the progress in VRF technology, appropriate implementation requires understanding its constraints. As outdoor temperatures plunge, both efficiency and heating capacity diminish. This is because there is less available heat to extract from the environment and channel into the conditioned space, which increases the workload of the compressor.

Consider the example chart below, which presents performance data from a VRF manufacturer. Observe how the coefficient of performance (COP) and capacity decrease as the outdoor air temperature drops. Still, the COP remains high even at temperatures below 0 °F. Understanding the capacity deratings reveals the actual capacity under design day conditions. It is crucial that the design engineer consults a manufacturer’s rep to account for deratings from elevation, air temperature, and refrigerant pipe rise/run lengths in the design.

Specify Flash Injection Technology.

To counteract COP decline in severe cold, VRF outdoor heat pumps can incorporate a Flash Injection Circuit. This system operates by injecting lower-temperature refrigerant into the compressor, enabling it to function at higher speeds without heat build-up. This technique allows VRF systems to deliver 100% of the heating capacity at temperatures below 0 °F.

Size equipment based on heating requirements.

One option in climates with temperatures below 0 °F is to oversize the VRF system. This often results in surplus cooling capacity, but this is rarely an issue due to the system's superior part-load performance. Cooling oversizing should be restricted so the outdoor heat pump doesn't operate below a 10% part-load ratio, as this can lead to efficiency loss due to oil return, parasitic power, and compressor cycling.

Elevate outdoor heat pump to shield from snow drift.

Snowfall or drift can obstruct the VRF outdoor heat pump, decreasing performance or causing failure. A helpful guideline is to mount the outdoor heat pump at least 12” above the ground or the highest anticipated snow depth. VRF outdoor heat pumps can be raised on roof curbs, pre-engineered stands, or a custom stand. An open bottom stand can help minimize snow accumulation against the unit's base. Installing wind baffles and strategically positioning outdoor heat pumps can help avoid excessive snow accumulation and wind blasts, ensuring optimal performance during the heating season. In certain applications, it may be necessary to install wind and rain barriers. Icy rain can freeze fan blades, and wind blasts can spin outdoor heat pump fan blades in reverse, potentially causing damage.

Install a drain pan heater.

In extremely cold conditions, frost accumulation on the outdoor heat pump can impair performance and cause damage. VRF manufacturers combat this with a defrost cycle, which temporarily reverses the heat pump cycle to defrost the outdoor heat pump using heat from indoor air. This defrost cycle, typically automated by the VRF system, can result in ice build-up in the drain pan, which can be prevented by installing a drain pan heater.

Add supplemental heating.

Supplemental heating, such as electric resistance, steam, hot water, or gas, can be introduced to compensate for any heating deficiency. Thanks to its superior efficiency, the VRF system typically provides the initial stage of heat. In a retrofit scenario, a secondary system may already be present, which aids in the cost-effectiveness of the new VRF system. Furthermore, supplemental perimeter heat is always advantageous in cold climates due to cold convective currents from windows. It's essential to design the control sequence for VRF with supplemental heating such that the supplemental heating system activates only when necessary, avoiding a reduction in the heating system's efficiency.

Oversize DOAS for heating.

A Dedicated Outdoor Air System (DOAS) is typically installed with VRF systems to provide the mechanical ventilation mandated by code. This system introduces fresh outdoor air into the space and often incorporates energy recovery to utilize energy from the exhaust airstream. Instead of supplying neutral-temperature air, the heating coil can be sized to deliver extra heat to supplement the VRF system. Note that air temperatures 15°F or more above the room temperature can reduce ventilation efficiency as per the ventilation code, increasing the ventilation rate by 25%. In emergencies, a recirculating damper can be installed on the DOAS unit, which can remove the ventilation load component and provide additional heating.

Strategically locate outdoor heat pump to recover waste heat.

If feasible, Air-Source VRF outdoor heat pumps can be installed in the path of a central building exhaust. The building heat, which would otherwise go to waste, would be recovered back into the building, eliminating the concern of a cold climate.

The outdoor heat pumps can also be installed indoors, in a mechanical room, when the discharge air is ducted outside; however, installing the system in a mechanical room can present challenges in retrofit applications as VRF heat pumps typically have low-static pressure propeller fans that would require additional large ducts, dampers, and louvers. Another downside of installing the system indoors is that the system would also require heat to be generated from another source, which would make the system more energy-intensive.

Avoid aggressive setback strategies on extremely cold days.

On extremely cold days, having an aggressive setback control strategy should be avoided. With the reduced heating capacities, the system may struggle to reach the occupied setpoint temperature. Having a higher temperature set for unoccupied periods will allow the system to recover faster during the building's operating hours.

Consider a water-source VRF system.

Water-source VRF systems come with a significantly higher initial cost due to the requirement of additional equipment and maintenance, such as a water loop with a heating and cooling source (e.g., a boiler and fluid cooler or ground loop) and water treatment. However, water-source VRF systems benefit from a neutral temperature throughout the year, allowing the system to operate at its maximum efficiency. To provide a quantitative analysis of the most cost-effective system, design engineers should conduct an energy model and perform a life cycle cost analysis.

Author Bio: Dan Nguyen, PE, is a Mechanical Engineer at Millig Design Build.