Green Building Design Guide in Construction: HVAC

The following are design guides for energy conservation when designing a green building or a building that wants to achieve LEED certification.

Cooling Design Strategies

When design for energy efficiency, its not only selecting energy efficient equipment that will give you the most savings.  One has to look at the air conditioning system as a whole for the best way to lower your building cooling energy.  The most important invention for air conditioning systems is the variable frequency drive (VFD).  For a capacity that justifies it, always use a variable air volume system for air distribution with VFD air handlers, variable volume flow for chilled water and condenser water pumps, and variable frequency drive chillers.  Use chilled water instead of direct expansions systems when viable and remember that water cooled condenser and water cooled chillers are always more energy efficient than air cooled ones.  Use air conditioning units with high EER values and chillers with low kW/ton values.

Heating Design Strategies

For most climates in North America, office buildings are usually heated with air distribution systems that use variable air volume (VAV) boxes with re-heat coils.  These systems cool the air down to 55 °F for cooling and then heat up the air to 95 °F for heating in the zones that require it.  The reason is that during the winter you will often find buildings that require heating for perimeter areas but at the same time require cooling for interior zones.  The reheat coils use hot water for heating that supplied from a boiler.  Use gas-fired, condensing boilers with AFUE ratings of 90% or better.  Reheat coils can be electric as well, but hot water is preferable as you the energy cost of using electric coils is much greater than the energy cost of using hot water.  Other heating options include dual-duct systems, which are more energy efficient for heating than VAV with re-heat coils.  But these require large spaces above the ceiling for the larger networks of ducts to fit.  When using a furnace for heating, select a high efficiency furnace with an AFUE rating of 90% or above.

Demand Control Ventilation

Use carbon dioxide sensors in rooms that are densely occupied.  Densely occupied rooms are rooms with a human density of 25 people per 1,000 square feet.  These rooms are usually conference rooms, class rooms, some retail stores, bars/nightclubs or anywhere that has a high density of people.  Densely occupied places are designed to have relatively large amount of outdoor air to satisfy its ventilation requirement.  A carbon dioxide sensor can sense when a room is unoccupied and sends a signal to the outdoor air damper in the air handler to reduce the amount of outdoor air going in so that only the required amount of fresh air enters the air handler for the people currently occupying a room.  If the carbon dioxide sensor was not in the room, the air handler would always supply the maximum amount of outdoor air or fresh air for that room resulting in the air handler having to cool more outside air that it should be, and thus wasting more energy than necessary.

Heat Recovery

In buildings where the weather is hot and humid most of the year or when the building uses 100% outdoor air, a heat recovery system is essential.  A heat recovery system transfers the energy from the air that is being exhausted out of the building into the fresh air that is going into the building.

Domestic Hot Water

For heat domestic hot water, use water heaters with thermal efficiencies of 90% or above.

With these design guides, you can achieve a successful HVAC design that will help the building attain LEED certification, and considerably lower the heating and cooling energy in the building.

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