There is a broad range of mechanical engineering and design options available for meeting the heating and cooling needs of residential buildings. Normally, these systems differ in terms of the medium used to deliver their heating or cooling output.
- Water Piping: Water source heat pumps, radiators using chillers, cooling towers, and boilers.
- Air Ducts: Packaged rooftop units, packaged ceiling hung units
- Refrigerant Lines: Split AC systems, VRF systems
- Direct: Used by window-type air conditioners and PTAC units, which operatedirectly between indoor and outdoor locations, without ducts.
This article will provide an overview of some of the most common mechanical design options used for indoor residential spaces, as well as the strengths and limitations of each type of system:
Detailing Mechanical Engineering and Design Options
Mechanical Design Option #1: Four-Pipe System with Chiller, Cooling Tower, and Boiler
This mechanical design gets its name from the fact that it has two separate water circuits, one carrying hot water and another carrying chilled water, each with a supply and return pipe. The basic operating principle of four-pipe systems is the following:
- Cooling is accomplished by a chiller and cooling tower: A chilled water circuit is used to remove heat from indoor spaces, and the cooling tower is used to reject it outdoors. If the chiller’s compressor comes equipped with a variable-speed drive, this system can offer a very high efficiency in cooling mode.
- Heating efficiency is determined by the type of boiler. In general, gas-fired boilerstend to be more cost-effective than those running on oil or electrical resistance heating.
- Fan-coil units are equipped with both hot and cold water coils, granting them the flexibility of either mode of operation.
The main advantage of four-pipe systems is their ability to use both modes of operation simultaneously and independently. This can be especially useful if heating and cooling needs are different across building zones, and especially in apartment buildings and multi-family dwellings where preferences and schedules normally vary by occupant. Of course, a four-pipe system is an expensive mechanical engineering system to install due to the presence of three separate water circuits: two for the distribution of cold and hot water, and a third one used by the chiller to reject heat through the cooling tower.
Mechanical Design Option #2: Water-Source Heat Pumps with Cooling Tower and Boiler
A heat pump can be described in simple terms as a reversible air conditioner: it can deliver indoor cooling through the refrigeration cycle, but can also operate in heating mode with a much higher efficiency than most types of boilers, especially electrical resistance boilers.
Due to their reversible operation, water-source heat pumps offer great flexibility in residential buildings. Individual units can be set to operate in different modes, and in combined heating and cooling applications the overall system can be extremely efficient:
- Heat pumps in cooling mode extract heat from indoor spaces and release it into a common water circuit.
- Then, heat pumps in heating mode can extract the heat now carried by the water, and release it indoors, as required.
The fact that heat pumps share the same water circuit means that the cooling tower and boiler only have to balance system loads, rather than meeting them fully:
- If the cooling load is greater than the heating load, the cooling tower only has to reject the heat difference, not the total heat removed from all spaces.
- The same logic applies if the heating load is higher than the cooling load: the boiler only has to make up for the difference, not the full heating load.
- If the heating and cooling loads happen to balance each other out, both the cooling tower and boiler can remain off.
A four-pipe system lacks these capabilities: the chiller must assume the full cooling load while the boiler provides the full heating load – all the heat absorbed in the chilled water loop is rejected by the cooling tower, and can’t be used for space heating purposes because water circuits are independent.
HVAC systems based on water-source heat pumps are extremely efficient, although expensive due to the fact that every zone must be equipped with an individual heat pump, in addition to having a common water circuit, a cooling tower, and a boiler.
Mechanical Design Option #3: VRF System with Rooftop Condensers & Gas Boiler
VRF stands for variable refrigerant flow, and VRF systems get their name from the fact that refrigerant is used to transport heat instead of water:
- One or more remotely located condensers provide a flow of refrigerant for multiple indoor fan-coils, and a variable speed drive is used to regulate flow according to load. The units can also provide their own heating.
- For supplementary heating, a gas-fired boiler with perimeter radiation can be added to the system.
- Two-pipe VRF systems require all fan-coils to operate in the same mode, but with three-pipe systems, it is possible to provide simultaneous heating and cooling for different areas of the building.
Other than operational flexibility, an advantage of this mechanical design option is their ease of installation: refrigerant lines are more compact than water piping and air ducts. These systems still have a relatively small market share in the USA, but are very common in Japan, where they were developed, and Europe. According to ASHRAE, VRF systems tend to have a comparable cost to that of chiller-based systems, potentially higher if the technology must be imported.
The modular nature of VRF systems is another strong point in favor of this technology. If there will be a building expansion, it is possible to expand the system by simply adding a new condenser and the corresponding indoor evaporators.
Mechanical Design Option #4: PTAC Units with Electric Resistance Heating
Packaged terminal air conditioning units (PTAC) are compact systems, very similar to old window-type air conditioners: the system is self-contained and does not require refrigerant lines, water piping, or air ducts, greatly reducing the installed cost. Some PTAC units are equipped with a resistance heater, allowing them to operate in both heating and cooling modes.
PTAC units offer the advantage of being self-contained and independent from each other. This gives them an advantage in projects that will be built in several stages, for example, apartment buildings, since it is possible to expand HVAC capacity as needed without having a common system on which all units depend.
The main limitation of this mechanical system is that they tend to be outclassed by other systems in terms of efficiency, especially when in heating mode. Resistance heating offers a coefficient of performance of 1.0, which means they must draw one watt of electricity per each watt of heating; on the other hand, heat pumps typically operate with a COP of 2.5 or more, or even above 4.0 if a high-efficiency heat pump is selected.
There is a broad range of heating and cooling technologies available for residential buildings, and also a high degree of flexibility in how the overall system can be configured. No system can be considered superior to the rest under all circumstances – every project offers unique conditions that favor some technologies over others.
What kind of mechanical engineering design has worked the best for you? Let us know by commenting below.