DECOUPLING HEATING AND VENTILATION SYSTEMS

As energy use and global warming concerns dictate the tightening of building envelopes and minimizing the use of fossil fuels, and alongside changes to building codes, the topic of building ventilation has been moved into the spotlight. Providing fresh air improves indoor air quality and it is required by code for all current construction projects (both new construction and renovations). Historically, fresh air delivery was commonly combined with the building heating and cooling systems so that a percentage of outdoor air was brought in, mixed with conditioned supply air, and delivered through the same ductwork. These systems would operate continuously to provide code required for fresh air. As building envelopes reach higher levels of thermal performance it allows the building heating and cooling systems to shrink in size and operate less frequently. Yet, through all the changes, fresh air and exhaust airflow requirements are increasing and must be provided any time our buildings are occupied, regardless of the temperature. An emerging solution to this discrepancy is to decouple the heating and cooling system from the ventilation system.

There are several common approaches to decoupling the ventilation system. The best approach is to use a dedicated energy recovery ventilator (ERV) or heat recovery ventilator (HRV) paired with a heating coil. In applications where the indoor environment is especially sensitive to temperature and humidity fluctuations, a cooling coil should also be added to condition incoming fresh air. The HRV or ERV will recover heat from the exhaust air stream and help to partially condition the incoming fresh air. Engaging a heating/cooling coil can work as needed to bring the incoming air to an acceptable temperature and humidity setpoint. This setpoint should be close to room temperature because this air does not need to contribute to heating or cooling the space. An electric duct heating coil is often a cost-effective approach for smaller ERVs with small airflows. Large units may be better served with hot water coils, gas-fired duct furnaces, or DX coil connected to a heat pump. By using a ERV or HRV, both the fresh air and exhaust requirements for a building can be satisfied with a single piece of equipment. With this type of system, it is logical to pull exhaust air from bathrooms (where exhaust is required) and deliver fresh air throughout the space. Delivering fresh at several points widely spread throughout the space is ideal, however it comes at the expense of more ductwork than delivering fresh air at a single central location. Designers should strive to find the appropriate balance for each project between wide distribution of fresh air and simplicity of the system.

In spaces where the exhaust air contains an elevated level of contaminants such a certain manufacturing areas or kitchens with grease laden air, it may be inadvisable or even a code violation to send exhaust air through an ERV or HRV. In other situations, the exhaust system may only need to run intermittently but fresh air must be provided continuously. To handle these situations, a dedicated outdoor air system (DOAS) or 100% outdoor air make-up system should be considered as an approach. The DOAS system should be sized to match the exhaust system and provide the equivalent amount of fresh air. Unlike an ERV which will run continuously, operation of the DOAS system should be linked to the exhaust system controls such that they run simultaneously at similar rates. DOAS systems can take several shapes including fully packaged 100% outdoor air rooftop units and cassette style heat pump fan coils. The theory stays the same: provide room temperature outdoor air so the heating and cooling system does not have to work to condition the outdoor air.

When the incoming fresh air is room temperature, it will not add any heating or cooling load to the space. During the shoulder seasons and moderate outdoor temperatures (40°F to 80°F) a highly insulated and tightly sealed building will experience negligible heat gain or loss through the envelope. During these conditions, the heating and cooling system will have little load and may be able to completely shut off for extended periods of time. When the load is reduced, heating and cooling systems shrink in size and consume less energy and take up less space. Reducing the heating and cooling system size results in a reduction of system gains and losses because blowers, pumps, ducts, and pipes can reduce in size.

Spaces such as kitchens, manufacturing floors, and IT closets with continuous internal heat gains from equipment will have a more continuous cooling load; the benefits of decoupling the ventilation in these areas may be limited because the cooling systems run more continuously. Spaces such as dwelling units, offices, and classrooms which have limited internal heat gains can benefit the most from decoupling the ventilation.

In some cases, decoupling ventilation may allow the total fresh air required to decrease. In a basic ducted system with supply and return at the ceiling level, as is common in office spaces, code defines the “zone air distribution effectiveness” as 0.8 when the system is providing heat. This value suggests the supply air does not mix well in the space and not all fresh air reaches the breathing zone in this configuration. As a result, the code requires the design to provide additional outdoor air (25% more) to account for this poor mixing performance. However, if the supply air is less than 15°F above room temperature, the code permits using a value of 1.0 for the zone air distribution effectiveness, eliminating the need for additional outdoor air. A decoupled ventilation system providing room temperature air can satisfy this requirement.

Before applying the decoupled ventilation design approach, the designer should understand the drawbacks and limitations. The primary drawback is the need for additional mechanical equipment. In some cases, a single piece of mechanical equipment could provide all HVAC functions: heating, cooling, and fresh air. An obvious example of this would be a rooftop unit (RTU) with outdoor air intake or economizer. This system with a single roof top unit would almost certainly be more simple and cheaper to install than separate heating/cooling and ventilation systems. Beyond the added equipment, two separate systems will require more ductwork, diffusers, filters, electrical circuits, control devices, etc. If the decoupled ventilation system requires electric duct heaters to temper incoming fresh air, it is important to consider the inefficiency and expense, and electrical real estate required for electric resistance heating. The principle behind this approach is to allow the heating and cooling system to be reduced in size and run less frequently. However, in some cases there may be benefits or requirements to providing more air movement and air filtration. A large primary heating and cooling system is usually better suited to provide air movement and filtration.

Nate Rogers

Staff Engineer