Energy-performance goals direct HVAC projects
- Introduce energy goals on current projects and know how they may differ from current energy codes or standards.
- Understand standard heating and cooling systems with brief outlines of the systems relative to energy performance and installed cost.
- From the case study (see bottom for link), review a process for evaluating design decisions focusing on HVAC system selection and its impact on other components of the design.
Virtually all new construction projects must meet local or national energy codes and standards, such as the International Energy Conservation Code (IECC) and ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. Design teams have adapted to the enforcement of energy codes by using simple tools for compliance, such as the Department of Energy’s free COMcheck software, or by performing complex energy models to demonstrate performance improvements over code. The most challenging projects for designers, from an energy perspective, are projects where the performance standard of the client far exceeds current code minimums and requires detailed analysis to comply with the client’s energy-performance target.
Currently, most new construction and renovation projects in the United States must meet or exceed either the IECC or ASHRAE 90.1, both of which have various editions that have been adopted by local jurisdictions. One notable exception, California, requires compliance with Title 24, which is essentially a stand-alone energy code written almost entirely from scratch with its own set of rules and forms for compliance. Typically, code compliance is demonstrated via the use of prescriptive forms where all equipment, lighting, envelope, and controls components are listed with their relevant efficiency metrics on a bullet point-based form signed by design professionals.
For heating and cooling systems, typical metrics include boiler efficiency, chiller efficiency, fan energy in the form of peak fan power, and control requirements, such as economizer mode for air handling systems. The COMcheck report does not provide a percentage better than code for heating and cooling systems, but simply a pass/fail relative to code. Virtually any type of HVAC system can meet the required standard if the system efficiency and controls meet the basic requirements. Relative performance of HVAC systems becomes a factor only when a project wants to demonstrate a performance level better than a relevant code—or doesn’t comply with prescriptive requirements of the code.
Once a client requires that a project be a percentage better than code, that typically means energy modeling is required. The energy-modeling methodology is typically based on ASHRAE 90.1, Appendix G, or the Energy Cost Budget Method in the IECC. Either methodology requires that the performance of the building be compared based on the annual energy cost (in dollars), and not energy usage. Both standards require the design team to establish a baseline HVAC system based on a combination of factors including building size, fuel type for heating, and means of heat rejection (air, water, other).
For example, per ASHRAE 90.1-2010, Appendix G, Table G3.1.1A&B, for a nonresidential building greater than 150,000 sq ft using fossil fuel-based heat, the required baseline HVAC system is a "packaged rooftop variable air volume system with reheat." Cooling for the system must be provided by a water-cooled chiller plant, and heat must be provided by fossil fuel-powered boilers. If a selected HVAC system or component of the design uses more energy than the baseline, then the difference in energy performance would have to be accounted for somewhere else, either in the lighting systems or in envelope selection.
A simple example would be selecting an air-cooled chiller instead of a water-cooled chiller. Per ASHRAE 90.1-2010, Table 6.8.1C, the air-cooled chiller would be required to have an energy efficiency ratio (EER) of 9.562 or 1.25 kW/ton. Current air-cooled chillers typically operate in the 1.1 kW/ton range. The water-cooled chiller is required to have an efficiency of 0.62 kW/ton if larger than 300 tons (depending on the chiller compressor technology used), so it is approximately twice as efficient as the air-cooled option.
The design team also could specify a packaged rooftop unit with direct-expansion (DX) cooling with a compressor rejecting heat directly to a condenser fan array and running through a refrigerant-based coil in the air handling equipment. Per ASHRAE 90.1-2010, Table 6.8.1A, a DX cooling system with separate nonelectric heat (non-heat pump) greater than 63 tons would have a required EER of 9.5 or 1.26 kW/ton, which is slightly less efficient than the air-cooled chiller.
Other cooling systems that perform better than a water-cooled chiller, such as a geothermal heat pump, could also be evaluated for reference. Figure 1 gives a general idea for the relative performance of cooling systems. The relative cost of the systems typically follow performance, with more efficient systems often incurring higher capital costs.
When comparing heat-generating systems, the comparison is straightforward as neither energy standard allows fuel switching. The designer cannot compare an electrically heated system to a gas-fired heating system. From an annual energy-cost perspective, a gas-fired heating system will outperform an electric heating system in the clear majority of locations (see Figure 2).
For example, let’s compare two identically sized heating systems, a 300,000 Btu/h electric boiler (99% efficient) and a 300,000 Btu/h non-condensing gas-fired boiler (80% efficient). The electric boiler would use less energy over the course of an hour to heat than the gas boiler: 303,030 Btu (88.7 kWh) versus 375,000 Btu (3.75 therms).
However, given typical national utility rates of $0.11/kWh for electricity and $0.60/therm for natural gas, the electric boiler would cost $9.75 to run for that hour, while the natural gas boiler would cost $2.25 to run. The electric boiler is more efficient; however, the annual energy cost is significantly less for the gas-fired boiler.
Greenhouse gas emissions, specifically equivalent tons of carbon dioxide, have not been addressed in the analysis either, where the emissions generated by the grid electricity exceed the emissions for burning the near-equivalent amount of natural gas for most locations in the United States. Hybrid systems—where the heat is a combination of natural gas-fired and electric heat—are not uncommon; however, the baseline will be based on an all-natural gas-heated system if used, penalizing any electric heat in the proposed design. For that reason, this article will avoid examining hybrid systems and focus on all-electric heating or all-natural gas systems.
Typically, all-electric heating systems are limited to a few options:
- Element-based electric heat.
- Heat pumps in non-extreme climates, including variable refrigerant flow (VRF).
- Electric boiler systems.
- Geothermal heat pump systems.
Electric heating systems are evaluated on how they interact with the cooling systems with regard to efficiency. Systems that can recover heat from the cooling system and use it simultaneously for building heating needs will perform better than systems that can’t. The systems that can recover heat directly include water-source heat pump systems, VRF systems, and geothermal heat pumps systems. From an efficiency perspective, a water-source heat pump and a VRF system can match each other’s performance depending on operating conditions.
A geothermal heat pump system exceeds the performance of the other systems and has a more efficient means of heat recovery both through the commonly connected piping loop and through the ground itself. From a code-baseline perspective, the designer is comparing the design with element-based electric heat (99% efficient), but with no means to directly recover heat. Similar to the cooling options, the electric heating system’s first cost increases as the ability to recover heat is added. The increase in cost is typically associated with additional valves, piping, and controls to accommodate simultaneous heating and cooling on a system and individual heat pump level.
When evaluating fossil fuel-based heating systems, typically natural gas-fired, the primary performance factor is the location of the heating source and the heating medium (steam or water). In terms of locations, the gas-fired heating system can be located directly in the air handing system, as in the case of an indirect-fired gas furnace in a packaged rooftop unit. The gas furnace can have an efficiency of 80% to 90% and benefits from not having to deliver heat via either water or steam to the unit.
However, local reheat at air terminals must be delivered via either a separate steam loop or, more typically, a hot-water loop. The most common natural gas-fired heating systems use either a steam or hot-water boiler. Steam boiler systems typically have an upper efficiency threshold of 84%, not including losses for distributing steam through a building because of steam traps or losses through insulation due to the high operating temperatures.
Hot-water boiler systems may range from 80% to 95% efficient for condensing boiler systems. The decision between picking a condensing versus a non-condensing boiler has more to do with operating temperature of the heating loop than cost. A condensing boiler will achieve no savings over a non-condensing boiler if the return-water temperatures don’t ever drop below 130°F. If the hot-water loop will always run between 180 and 150°F, the additional expense of a condensing boiler may not pay back. From a cost perspective, the heating option with the lowest capital cost is the gas-fired furnace with a small boiler feeding a hot-water loop to variable air volume (VAV) terminals or perimeter heat.
When evaluating the other options, the design is driven more by functional requirements of the building than pure energy efficiency. Many health care clients require a significant amount of steam to be generated for humidification or sterilization processes. Once steam distribution is a reality in a building, it may be more practical to look at a steam boiler system for other uses, such as space heating. For buildings that don’t require steam, a condensing boiler system feeding air handling units (AHUs), air terminals, and perimeter heating has become popular.
Distributing heating and cooling pumping systems have been covered in-depth in other articles. Virtually all new buildings are using variable-speed pumping for hot-, chilled-, and condenser-water distribution. Some HVAC designers are increasing the delta T (the difference between return and supply temperature) to reduce the amount of fluid moved, thereby decreasing pumping power while improving heat transfer and the efficiency of heating and generating equipment.
When looking at air handling or air-distribution options, typically the first decision is whether to go with a dedicated outdoor-air system (DOAS) or a more standard air-distribution system where ventilation requirements and temperature-control requirements are handled by the same air handling system.
A DOAS usually relies on local terminal units to provide the majority of heating and cooling to cover sensible space loads, doing the heavy lifting of providing comfort and reducing the air supplied to a space to cover ventilation and latent loads. The ventilation requirement for most buildings could be as low as 15% to 30% of the air supplied by a typical standard air-distribution system, allowing for smaller ducts, smaller AHUs, and less fan energy. In a DOAS, local heating and cooling would be performed by hydronic fan-coil units, chilled beams, radiant heating/cooling panels, local heat pumps, or VRF cassette-type units.
In a standard air-distribution system, most of the heating and cooling for the building is delivered via AHUs or rooftop units and requires cooling air to be distributed through the building at a uniform temperature, typically between 55 and 65°F. Reheat of the air to deal with perimeter heating spaces with different heat gains could be provided by local heating coils at air terminals (typically VAV terminals).
The DOAS typically is supplying air at load-neutral temperatures (68 to 72°F) and has the ability to lower the temperature to provide more dehumidification or capacity (in the case of an active chilled beam), if necessary. The ability to move less air, supply heating and cooling locally, and increase the discharge-air temperature reduces the energy consumption of the DOAS.
For buildings that require large air-change rates, such as hospitals and labs, it may not be beneficial to pursue a DOAS. This is because the amount of air required to be circulated in the space may exceed the heating and cooling requirements, thus defeating the purpose of the system.
Another benefit of the DOAS is space-related. The largest components of a mechanical system, in terms of real estate, tend to be AHUs. Any method of reducing the size of AHUs reduces the usable space required by the mechanical systems; this also reduces plenum sizes and shaft sizes for duct distribution, thereby reducing first cost. When evaluating a DOAS against a standard distribution system, the cost of real estate must be evaluated because, from a mechanical contractor’s perspective, the DOAS will cost more to implement. An evaluation of a DOAS also should examine maintenance requirements and acoustics performance. Adding terminal units to a DOAS results in higher noise and more maintenance.