If the project goal is a carbon neutral building, an HVAC system should be included and/or sized for a project only after all other features of the building have been optimized. Design the building form, mass, and openings to take full advantage of natural ventilation and daylighting opportunities. Use electric lighting only in spaces where the natural light is insufficient and during hours without sun. If possible, make use of ceiling fans--with an air movement of about 150 fpm (1.7 mph), a person experiences the same level of comfort with the air temperature 3-5°F higher than a space without air movement. Incorporate thermal mass and evaporative cooling if appropriate for the weather and building use, and only use compressor cooling as a back-up.
Underfloor Air Distribution Systems:
This is not a system type that is available in the DOE-2 simulation engine used by Green Building Studio. Green Building Studio assumes a very low static pressure to obtain a conservative estimate of actual energy savings. To better simulate this type of HVAC system, it is recommended that more detailed energy analysis be performed using eQUEST or EnergyPlus. The appropriate eQUEST or EnergyPlus file can be obtained from the Export and Download Data Files page. For guidance on accepted modeling workarounds for Underfloor Air Distribution systems, review the Energy Design Resources Guidelines.
Typical HVAC Central Plant or Built-up Systems:
Use chilled water (CHW) and hot water (HW) instead of refrigerant to move heating and cooling to central air handlers which contain the fans, economizers, cooling and heating coils and dampers.
Chillers supply the chilled water.
Cooling towers which supply condenser water (CW) to the chiller are used instead of the air condensers on found on most Direct Expansion (DX) unitary equipment.
Boilers typically supply the HW (can be steam from district heating).
Typically contain CHW, HW and CW loops, along with piping, pumps and controllers.
Typical HVAC efficiency metrics:
EER (energy efficiency ratio) -- This is a measure of how efficiently a cooling system operates. EER is most commonly applied to window units and unitary air conditioners and heat pumps. The EER is the ratio of Btu/hr. of cooling at 95°F divided by the watts used at 95°F.
SEER (seasonal energy efficiency ratio) -- This measures how efficiently a smaller residential air conditioner or heat pump operates over an entire cooling season, as opposed to a single outdoor temperature. As with EER, a higher SEER reflects a more efficient cooling system. SEER is the ratio of the total amount of cooling Btu's the system provides over the entire season divided by the total number of watt-hours it consumes.
HSPF (heating seasonal performance factor) -- This is the measurement of how efficiently heat pumps operate in heating mode over an entire heating season. The higher the HSPF, the more efficient the system. HSPF is calculated by dividing the total number of Btu's of heat produced over the heating season by the total number of watt-hours of electricity required to produce that heat.
AFUE (annual fuel utilization efficiency) -- This measures how efficiently a gas furnace or boiler operates. AFUE is the percentage of energy consumed by the system that is converted to useful heat. For example, a 90% AFUE means that for every Btu of gas used the system provides 0.9 Btu of heat. The higher the AFUE, the more efficient the system.
kW/ton -- This is most often used to determine chiller efficiency and measures the energy input in kW over the tons of cooling provided. The lower the ratio, the more efficient the chiller. COP (coefficient of performance) indicates how efficiently a heating or cooling system (a heat pump in heating mode and a chiller for cooling) operates at a single outdoor temperature condition. It measures the useful output in Btu/h, divided by the electric energy input in watts. Higher COP values indicate a more efficient system.