Part 1 of 3 in the Building HVAC Plants Series. Next article: VRF Heat Pumps
Key Takeaways
- Boiler and Chiller systems: Remain the go-to method for heating and cooling large commercial buildings, with a Gas-Fired Boiler supplying heat and a Flooded Chiller providing cooling
- Gas-Fired Boilers: Heat water to about 180°F through natural gas combustion and heat exchange. Electric Boilers offer a cleaner option but cost more to run
- Flooded Water-Cooled Chillers: Use a refrigeration cycle to cool water to around 44°F with proven reliability and large capacity, though heat pump tech is gaining ground
- Seasonal changeover: Required when switching between cooling and heating modes, making this setup common in office buildings, schools, condos, and hotels
How Commercial Buildings Heat and Cool with Boilers and Chillers
The classic commercial HVAC setup pairs a Gas-Fired Boiler for heat with a Flooded Chiller for cooling. This is Part 1 in a three-article series on building HVAC plant systems. The next two articles cover VRF Heat Pumps and Air-Source Heat Pump Chillers.
These articles focus on large commercial buildings with a “plant” style setup. The plant heats and cools a hydronic system that serves the whole building. Think office towers, schools, condo buildings, and hotels. We won’t cover smaller gear like rooftop units or air quality systems like ERVs and DOAS.
The boiler-and-chiller combo is the most proven approach in commercial HVAC. The industry has decades of experience with it. But the push for decarbonization is changing the landscape. Many buildings now retrofit or modify their boiler and chiller systems through HVAC retrofits and commercial system upgrades.
How Gas-Fired Boilers Heat Commercial Buildings
A boiler heats water, usually to about 180°F. Glycol can also be used. The process starts with burning natural gas in the burner. The heat transfers to the water through a heat exchanger. Then the combustion byproducts vent up the exhaust stack and exit safely outdoors.
Boilers heat building water fast and reliably. They produce much higher water temps than heat pump chillers or VRF systems. The downside is natural gas. It’s not the cleanest option and adds to the building’s carbon footprint.
Note: This article covers “Water Boilers” that create hot water for heat and domestic hot water. That’s different from “Steam Boilers,” which boil water into steam for process applications.
Electric Boiler as a Gas-Free Option
Some buildings use an Electric Boiler instead. Rather than burning gas, electricity powers a resistance heater that warms the water the same way. Two benefits: no natural gas use and no exhaust venting needed. There are no combustion byproducts to deal with.
The downside? Operating cost. As straight electric devices, they cost more to run than gas-fired boilers. That gap widens during peak energy pricing.
How Boiler Piping Works in a Building Plant
The Veris diagram above shows how a boiler adds heat to a building’s water system. Two boilers run in parallel, connected to the “Hot Water Source” piping. Heated water flows out to the building’s heat exchangers for space heating and domestic hot water.
Once the water gives up its heat, it returns to the boilers through the “Return Water” piping. On this system, pumps sit on the return side for each boiler. Other setups place pumps differently. We’ll cover those variations later in this series.
Note: One key component not shown here is a Buffer Tank. Buffer tanks add thermal mass to the water loop. This makes heating and cooling more stable, especially in low-volume systems.
How Flooded Chillers Cool Commercial Buildings
Chillers use a refrigeration system to cool water down to about 44°F for the building’s AC system. The chiller’s evaporator pulls heat from the water (or glycol) through a heat exchanger. That cooled water then circulates through the building. Understanding how refrigerant heat transfer works is key to getting the most out of chiller performance.
What Makes a Chiller “Flooded”?
This article focuses on “Flooded” Water-Cooled Chillers. The term “flooded” means the shell-and-tube evaporator is filled with liquid refrigerant. That gives you the best heat exchange between refrigerant and water. A centrifugal compressor handles compression. The water-cooled condenser dumps heat through a cooling tower loop.
Flooded chillers with water-cooled condensers are a proven workhorse for building AC. They offer big capacity and reliable chilled water production backed by decades of design refinement. They also provide strong capacity control features.
Large flooded chillers have lost some popularity in certain markets. The main knock: they aren’t heat pumps. Heat pump chillers are gaining traction thanks to their energy savings. We’ll cover those in a later article in this series.
Flooded Chiller Piping Layout
The diagram below shows two flooded chillers with evaporators and condensers piped in series. The dark blue piping carries chilled water to heat exchangers. Those exchangers cool the secondary loop’s water for building AC. The light blue piping brings warm return water back through the pump to start the cycle again.
The condensers dump their heat into water sent to a cooling tower via the dark red piping. Once cooled, that water returns to the condensers in the bright red piping to repeat the cycle.
Note: The dark blue decoupling line in the diagram includes a valve in the primary loop. It regulates pressure between the chilled water supply and return. This helps maintain proper flow rates through the heat exchangers as building load changes. Flow rates are critical to how much heat transfers during cooling or heating.
🎩️ Related podcast episode: Jim Fultz of Copeland breaks down Thermal vs Economic Balance for Heat Pump Performance on the HVAC Know It All Podcast.
Boiler and Chiller Combined: Year-Round HVAC Plant Operation
To heat and cool a building year-round, boilers and chillers connect to the same piping system. There are different ways to pipe and run this setup. The diagram below shows one basic approach.
The chiller and boiler share common supply piping. Depending on the mode, the water goes out as either chilled or heated supply to the building. The return piping brings water back to the plant after it has done its job.
What Is Seasonal Changeover in Commercial HVAC?
The key concept for this type of system is seasonal changeover. If the plant runs cooling all summer and into fall, there comes a time when the chiller shuts down and the boiler fires up. This happens at the end of the “shoulder season” when cooling is no longer needed and the building needs heat.
The diagram doesn’t show the related changeover valves. These must open and close to switch the system from cooling to heating and back again. Understanding how building controls and BMS network architecture manage these transitions is important for techs maintaining these systems.
Summary: Boilers and Chillers in Commercial HVAC
The boiler-and-chiller combo is a tried and true way to keep a building’s water at the right temperature. Gas-fired boilers supply heat. Flooded chillers supply cooling. The system works and has for decades.
That said, the industry is shifting. The push to cut building carbon has led many owners to retrofit or remove gas-fired boilers. Electric boiler options remain viable. And large flooded chillers face more competition from heat pump models that can heat and cool. Flooded chillers now come in heat pump versions too, keeping them relevant.
Understanding how inverter technology and variable speed compressors improve system efficiency helps techs stay current with modern alternatives.
Understanding building HVAC plants is both interesting and practical for anyone in the HVAC industry. When techs can see how equipment fits into the whole building system, they gain real ability to apply what they know in the field.
