There are cases where some areas of the building require space heating, while others require air conditioning. This can be resolved with independent heating and cooling equipment for each room, but installation can become very expensive with this approach. Instead, MEP engineers recommend a centralized HVAC system that is capable of simultaneous heating and cooling.
Some types of HVAC equipment have a heating mode and a cooling mode, but cannot provide both outputs simultaneously. Ductless heat pumps are one example, designed to operate as air conditioners during the summer and as heaters during the winter.
Centralized HVAC systems in large buildings typically have separate heating and cooling equipment; for example, tall buildings often have a chiller and a boiler. However, if hydronic piping and air ducts are shared, heating and cooling cannot be provided at the same time.
Identify the best heating and cooling configuration for your building.
This article provides an overview of three common HVAC configurations that allow simultaneous heating and cooling:
- Four-tube hydronic systems with chiller and boiler
- Water source heat pumps with common water circuit
- Variable Refrigerant Flow (VRF) Systems
When do buildings need simultaneous heating and cooling?
Depending on the equipment found in a building or its physical layout, there may be heating and cooling loads present at the same time. Consider the following examples:
Data centers produce large amounts of heat and air conditioning is typically required year-round, even during winter. However, office spaces in the same building require space heating to provide an adequate temperature for occupants.
Restaurants and other buildings with commercial kitchens may require space cooling even during the winter to remove heat released by food and cooking equipment.
Some building layouts lead to simultaneous heating and cooling needs. For example, areas close to walls may require space heating during the winter, while rooms closer to the center of the building may require space cooling.
Four-pipe HVAC systems
Air handlers in hydronic HVAC installations typically have a supply tube and a return tube. As a result, the system can circulate hot water from a boiler or cold water from a refrigerator, but not both. In other words, the entire building is heated or cooled.
A four-pipe installation has a similar layout with a chiller, boiler, and air handlers. However, there is one important difference: there are two supply pipes and two return pipes for each air handler, providing a choice between hot water and chilled water. In this case, the chiller and boiler can operate simultaneously, as there is an independent water circuit for each unit.
Another advantage of four-pipe systems is that they allow dehumidification without additional equipment:
- An air handler may use a higher cooling output to condense moisture from the air.
- The overcooling of the dehumidification process is compensated by the heating coil.
As you would expect, the main disadvantage of a four-pipe installation is the price, since the system uses twice as much hydronic piping. This is the only way to have hot and cold water available at each fan coil.
Water Source Heat Pumps
As mentioned previously, electric heat pumps can switch between heating and cooling mode. If a building is equipped with a water source heat pump for each area, simultaneous heating and cooling is possible with just one supply pipe and one return pipe.
- Heat pumps in heating mode extract heat from the water in the hydronic piping.
- Heat pumps in cooling mode remove heat and release it through the hydronic piping loop.
In other words, water source heat pumps are simply exchanging heat between areas of the building that need cooling and areas that need heating. Excess heat can be released to the outside with the cooling tower when cooling loads are greater, and a boiler can be used when heating loads are greater. Alternatively, the entire system can be connected to a larger, high-efficiency heat pump to provide the balance between heating and cooling.
Variable Refrigerant Flow Systems
Variable refrigerant flow systems have very high efficiency, utilizing refrigerant lines rather than hydronic piping. VRF systems utilize outdoor units with heating and cooling functions, connected to indoor fan coils via refrigerant lines.
There are two main VRF configurations that can provide simultaneous heating and cooling for separate areas of the building:
- A supply and return line to each fan coil, with a main branch selector that supplies superheated or subcooled refrigerant as needed.
- Three lines for each fan coil, where two correspond to heating and cooling, and the third is a common return. In this case there is an individual selector on each indoor unit.
Two-pipe VRF systems are more flexible since the refrigerant lines from all indoor units meet at the branch selector, which acts as a common hub. The system can be easily expanded by adding more lines to the selector. Three-pipe VRF systems are slightly more energy efficient, but are more difficult to expand because existing refrigerant lines must be modified.
Final Observations
HVAC engineering is characterized by a wide range of options for heating and cooling buildings, even when both outputs are required simultaneously. As with any other engineering decision, all options have strengths and limitations, and the best system configuration is determined by specific project conditions. Professional HVAC engineers can analyze your building's heating and cooling needs and design an ideal solution.
