HVAC Heat Exchangers: Types and their roles

What is a Heat exchanger?

A heat exchanger is a device that transfers heat between two or more fluids without mixing them. It is used in heating, cooling, and energy recovery applications across industries.

How Heat Exchangers Work

Heat exchangers work based on the principle of thermal conduction, where heat moves from a hotter substance to a cooler one.

In heating, a heat exchanger absorbs heat from a source (gas, electricity, or water) and transfers it to the indoor air.
In cooling, it removes heat from indoor air and transfers it outside.

Heat Exchangers in HVAC :

In HVAC systems, heat exchangers transfer thermal energy between air, water, or refrigerants to regulate indoor temperatures efficiently.

They are used in heating (furnaces, boilers), cooling (air conditioners, chillers), and ventilation (energy recovery ventilators). Common types include air-to-air, water-to-air, and refrigerant-based heat exchangers.

Key Roles of Heat Exchangers in HVAC:

🔥 Heating → Transfers heat from a boiler, furnace, or heat pump to warm air or water.
❄️ Cooling → Removes heat from indoor air and releases it outside.
💨 Ventilation & Air Quality → Recovers heat from exhaust air to improve efficiency.
🌍 Energy Efficiency → Reduces energy consumption by recycling heat.

Types of Heat Exchangers in HVAC

Here’s a table differentiating the common types of heat exchangers used in HVAC systems:

Type of Heat Exchanger	Primary Function	Working Mediums	Common Applications	Advantages	Disadvantages
Air-to-Air Heat Exchanger	Transfers heat between two air streams without mixing them	Air-to-Air	Energy Recovery Ventilators (ERVs), Heat Recovery Ventilators (HRVs)	Improves efficiency, reduces heating/cooling load	Limited to ventilation applications
Water-to-Air Heat Exchanger	Transfers heat between water (or glycol) and air	Water-to-Air	Hydronic heating, fan coil units, geothermal systems	High efficiency, compact design	Requires water circulation system
Refrigerant-Based Heat Exchanger (Evaporator & Condenser Coils)	Transfers heat between refrigerant and air or water	Refrigerant-to-Air / Refrigerant-to-Water	Air Conditioners, Heat Pumps, Chillers	High efficiency, essential for refrigeration & cooling	Requires refrigerant management
Water-to-Water Heat Exchanger	Transfers heat between two water loops without mixing them	Water-to-Water	Chillers, Boilers, District Heating/Cooling	Prevents contamination, efficient heat transfer	Requires pumps and piping system
Plate Heat Exchanger	Uses metal plates to transfer heat between two fluids	Air-to-Air, Water-to-Water	Boilers, Chillers, Heat Pumps, HVAC energy recovery	Compact, efficient, easy maintenance	Higher pressure drop, requires cleaning
Shell-and-Tube Heat Exchanger	Uses tubes inside a shell for heat exchange	Water-to-Water, Refrigerant-to-Water	Chillers, Boilers, Industrial HVAC	Handles high pressures, durable	Bulky, higher installation cost

1. Air-to-Air Heat Exchangers

Used in ventilation systems to recover energy from exhaust air and improve efficiency.

Plate Heat Exchanger → Uses metal plates to transfer heat between two airflows.
Rotary Heat Exchanger → Uses a rotating wheel to transfer heat and humidity between incoming and outgoing air.
✅ Energy-efficient, improves indoor air quality.

2. Air-to-Water or Water-to-Air Heat Exchangers

Used in hydronic heating and cooling systems.

Water-to-Air Heat Exchanger → Transfers heat from hot water (from a boiler) to air (for forced-air heating).
Air-to-Water Heat Exchanger → Used in chillers to remove heat from air and transfer it to water.
✅ Found in radiant heating, geothermal systems, and fan coil units.

**Air-to-Water or Water-to-Air Heat Exchangers**

3. Refrigerant-Based Heat Exchangers

Used in heat pumps and air conditioners to transfer heat between indoor and outdoor environments.

Evaporator Coil → Absorbs heat from indoor air, cooling it.
Condenser Coil → Releases absorbed heat outdoors.
✅ Essential for cooling systems like AC units and heat pumps.

4. Water-to-Water Heat Exchanger

This type of heat exchanger transfers heat between two separate water loops without mixing them. It is commonly used in chillers, boilers, and district heating/cooling systems. These systems help prevent contamination while maintaining efficient heat transfer. However, they require pumps and piping systems for operation.

5. Plate Heat Exchanger

A plate heat exchanger uses thin metal plates to transfer heat between two fluids efficiently. It is widely used in boilers, chillers, heat pumps, and HVAC energy recovery. Due to its compact size and high efficiency, it is preferred in modern HVAC systems. However, it has a higher pressure drop and requires periodic cleaning to maintain efficiency.

6. Shell-and-Tube Heat Exchanger

This type consists of a bundle of tubes enclosed in a shell, allowing heat exchange between two fluids. It is commonly found in industrial HVAC, chillers, and boilers, where it handles high pressure and temperature applications. While durable and efficient, it is bulky and requires a higher installation cost compared to other types.