Every building presents a unique challenge for the HVAC engineer. When faced with the challenge of designing a system for an existing building, the engineer can do little to alter the structure to reduce the air conditioning load unless a major renovation is done.
The design of a system for a new building provides design flexibility to the HVAC engineer. The building architecture for new and existing buildings must be assessed for orientation to sun exposure and shading. The engineer must also determine how allowances have been made for adequate space within and around the building for placement of the HVAC system components including piping and ductwork.
The engineer must evaluate the heat gains and losses, the maximum actual load – its behavior at partial conditions and the thermal capacity of the building. These are factors that guide the “correct-sizing” of the HVAC system.
BUILDING SHELL – EXTERNAL ELEMENTS
Buildings are thermal containers that either need heat removed or introduced, depending on the changes in season and daily weather conditions. Here are some of the design considerations of the building shell for the HVAC engineer.
- Thin wall vs thick wall – amount of insulation
- Partial vs total glazing, glass tint – size of windows or glass, ie, curtain wall
- Orientation of the building to the sun – direct sun vs shading
- Simple vs complex architecture
- Height and shape of the building
BUILDING – INTERNAL ELEMENTS
The internal heat gain of the building is determined by several factors including; tint of windows or glass, lighting, electronic equipment – computers, printers, etc., number of occupants and seating arrangement of the occupants.
Acoustics – Generally, any noise from an HVAC system is considered annoying. On the other hand, it is sometimes considered “white noise” making the space “quieter” by muting other noises. The design engineer must evaluate the location of the HVAC equipment and system components for sound attenuation. The engineer must also establish the desired sound level within the space relative to the type of occupancy and the desires of the occupants. For example; manufacturing environments tend to tolerate HVAC system noise better than office environments.
Zoning Provides Room-by-Room Comfort Customization
Rather than relying on a single unit to cool or heat an entire building, multi-zoned buildings utilize a network of small, energy efficient units and thermostats to send the right type (warm or cool) and amount of conditioned air to each zone. This allows facility managers and building owners to have room-by-room control.
The number of zones a building can benefit from will depend on its size and layout. In some cases, a single zone can cool or heat an entire floor. In others, a multi-zone system is used to divide the building into several temperature control zones. The greatest benefit of zoning is the level of customization that can be achieved. You can decide to cool or heat (or, both simultaneously) only the zones in use, which not only improves personal comfort, but reduces overall energy consumption and costs. Buildings that benefit from zoned systems include:
- Buildings with two or more stories
- Rooms that get blocked off in the winter or summer because of comfort issues
- Rooms with large windows or high ceilings
- Battles over thermostat control by office mates
- Buildings where traditional ductwork is implausible
ECONOMICS of SYSTEM SELECTION
- For the Buyer, 1) system cost, 2) operating and maintenance costs and, 3) return on investment.
- Investment considerations are the expected longevity of the equipment and auxiliary components. These will determine the write-off lifespan for depreciation tax purposes.
- Costs of Owning are the price of the equipment, materials, labor and services for the installation.
- Operating Costs – generally electricity or gas are the bulk of the operating costs. Also included are maintenance which consists of the service technician’s time, replacement filters, refrigerant and other materials. Repairs to the equipment, throughout its life, is also factored into operating costs.
- Return on Investment – Is an analysis of owning and operating costs to determine the incremental cost with the addition of air conditioning. To a prospective owner, this is an added cost per square foot, per room, per apartment unit, per hospital bed, per clean room, per data storage facility, or per factory worker.
After the building characteristics have been evaluated and the heating and cooling load requirements have been established, the system type can be chosen.
VRF is an energy-efficient method of providing precise comfort control. VRF systems vary their rate of output to meet demand. Traditional systems have a larger plus/minus margin before turning on or off which means occupants experience a greater temperature range and the system uses more energy to bring the temperature back to the set-point. VRF systems are almost always on but are operating in a greatly reduced capacity. This maintains a more comfortable environment and provides tremendous energy savings.
VRF offers a wide variety of applications – everything from spot-cooling or heating a single room using a split-ductless system to a large commercial building with multiple floors and areas that require individual comfort control delivered by a split-zoning system.
DUCTLESS, SPLIT-SYSTEMS (mini splits) have numerous potential applications in commercial, institutional and multi-unit residential buildings. They can also be a good choice where extending or installing distribution ductwork is not feasible.
Split-systems offer small size and flexibility for zoning or heating and cooling individual rooms. Many models can have as many as four indoor air handling units (for four zones or rooms) connected to one outdoor unit. The number depends on how much heating or cooling is required for the building or each zone. Each of the zones will have its own thermostat so you only need to condition that space when it is occupied, saving energy and money.
Ductless mini-split systems are often easier to install than other types of HVAC systems. The connection between the outdoor and indoor units generally requires only a three-inch hole through a wall for the conduit. Also, most manufacturers of this type of system can provide a variety of lengths of connecting conduits. So, if necessary, you can locate the outdoor unit as far away as 50 feet from the indoor air-handling unit.
Since split systems have no ducts, they avoid the energy losses associated with the ductwork of central forced air systems. Duct losses can account for more than 30% of energy consumption for space conditioning.
HEAT PUMP – a Heat Pump is a system that has a refrigeration cycle which can move heat in both directions. Heat Pumps are natural solutions for year-round air conditioning in buildings in moderate climates. All of the systems mentioned in this article are available as heat pump systems which are rapidly becoming the norm.
How does a Heat Pump work? The Heat Pump draws ambient heat from the outside air. Even when it seems cold outside to you and I there is generally enough heat in the outside air for the system to draw from to keep your space warm and comfortable well into fall.
When winter ends and warm temperatures return, the heat pump will reverse its operation. Instead of finding ambient heat in the air, it will pull the heat out of your space and radiate it to the outside. The furnace blower then serves as a variable-speed fan, moving the cooled air produced by the heat pump.
AIR HANDLER – An air handler, or air handling unit is used to circulate air. An air handler designed for outdoor use, typically located on roof tops, is known as a packaged unit (PU) or Roof-Top-Unit (RTU). Air handlers may need to provide heating, cooling, or both to change the supply air temperature, and humidity level depending on the location and application. Such conditioning is provided by heat exchanger coil(s) within the air handling unit. These systems are most commonly used in large commercial applications.
Select Mechanical can help with your HVAC system design and installation. To arrange a consultation, please call our offices at 925-447-1500 or send us a request for an estimate; firstname.lastname@example.org