Strategies to Reduce HVAC Use in Warehouses
Warehouses represent a vast underestimated opportunity for improved operations, lower operational costs, reduced climate impact and a more sustainable building stock through energy management. Overall, heating, ventilation and air conditioning (HVAC) accounts for roughly 30 percent of total energy cost in a warehouse. Warehouses are often only partially heated to prevent freezing, and they are rarely cooled—unless of course it’s refrigerated or freezer warehouses. Occupancy requirements may be intermittent. Here are several key ideas you can employ to optimize your HVAC systems.
Efficient Furnaces and Air Conditioners
According to ASHRAE, the best choices for warehouses are high-efficiency condensing furnaces and unit heaters (92+ efficiency) and high-efficiency air conditioners (EER 11.5+ for larger units, SEER 14+ for smaller units). To adjust for outdoor furnaces’ compromised efficiency, consider locating a condensing furnace inside the building, such as suspended from the ceiling, to accommodate the most efficient HVAC equipment.
A best practice is to vary space conditioning temperatures and ventilation rates in accordance with occupancy patterns, building activities and the needs of the stored goods. Use programmable thermostats with time clocks, setbacks and demand control ventilation to reduce energy requirements of HVAC equipment. Installing internet thermostats on heaters will allow for monitoring and control of multiple HVAC units that sometimes get left in the heating position even during the summer. Another smart strategy: Divide the building into thermal zones with separate controls based on space function.
In many warehouse applications, especially in loading areas, it is costly and inefficient to maintain temperatures of 60° to 70°F (16° to 21°C) for employees’ comfort. For these situations, consider radiant heaters to heat the building occupants instead of the air. Radiant heating allows space temperatures to be kept lower than with forced air convection furnaces. Radiant heating can be successful in maintaining higher surface temperatures, even while air temperatures drop. Radiant systems maintain higher floor temperatures and reduce thermal stratification more effectively than simple all-air systems. Higher floor and interior surface temperatures lead to higher maintained internal mass temperatures, facilitating air temperature recovery after temporary cold blasts, such as opening dock doors. However, radiant systems may need to be supplemented by air systems to maintain appropriate ventilation rates, pressure balance between the building and outside and overall desired air temperature.
Air heating systems for high bay spaces in cold climates often cause excessive temperatures near the ceiling in order to achieve desired air temperatures near the floor, resulting in excessive roof heat losses, increased heating loads and excessive energy usage. Unit heaters in high bay spaces often result in significant stratification. Ceiling mounted fans or high velocity diffusers can move hot air to the floor level during the heating season, reducing the range of temperatures over the height of the space. Similarly, air rotation technologies reduce stratification.
Operable clerestory windows or skylights allow hot air to escape during warmer weather.
Recovering cooling or heating, such as exhaust, is essential for trimming costs and saving energy. Outside air can be expensive, while reusing air comes with a much lower price tag. Capture exhaust heat from mechanical equipment to bolster space heating. Exhaust air energy recovery can be provided through a separate energy recovery ventilator (ERV) that conditions the outdoor air before it enters the air-conditioning or heat pump unit, an ERV that attaches to an air conditioning or heat pump unit, or an air conditioning or heat pump unit with the ERV built into it.
Cool Roof Technologies
A warehouse roof can be one of the biggest sources of energy inefficiency—they are notorious for soaking up solar heat. While absorbed heat might be advantageous for some facilities in the winter, the downside of that heat gain is that it often means wasted money overall.
Cool roof systems have two critical properties: a high solar reflectance, which means they absorb less energy from the sun, and a high thermal emittance, which means they radiate a large percentage of the energy they do absorb back to the sky. This is a good option to consider when designing an addition or a new facility.
Transpired Solar Walls
Transpired solar walls are simple, but high-impact, energy sources available for certain commercial building applications. Outside air passes through a south-facing, perforated, solar collector wall and is pre-heated 30° to 55°F on sunny days before entering the building’s ventilation system. By pre-heating cold outdoor air with solar energy, transpired wall collectors remove a substantial load from a building’s conventional heating system, reducing maintenance, improving air quality and saving energy and money. To reduce capital costs, the system may even qualify for a 10 percent federal investment tax credit. This is another good option when designing an addition or a new facility.