Reheat in HVAC is the process of warming air back up after it has been cooled below the dew point to remove moisture. Air conditioning systems dehumidify by chilling air until water condenses out of it, but this leaves the supply air too cold for comfort. Reheat adds just enough warmth to bring that dried air to a comfortable temperature before it enters the room.
Why Cooling Alone Creates a Problem
A standard air conditioner removes humidity as a side effect of cooling. When warm, moist air passes over a cold coil, the coil’s surface temperature drops below the air’s dew point, and water condenses out. That’s how your system dehumidifies. The catch is that the air leaving the coil can be uncomfortably cold, sometimes in the low 50s°F, even when the space doesn’t need that much cooling.
This mismatch shows up most often in mild, humid weather. On a 75°F day with high humidity, you don’t need much cooling, but you still need significant moisture removal. Without reheat, the system either overcools the space or cycles off before it removes enough moisture, leaving the room clammy. Reheat solves this by separating the two jobs: the cooling coil handles humidity, and the reheat coil brings the temperature back to where you want it.
How the Reheat Process Works
The sequence is straightforward. First, an air handling unit cools supply air down to or near the dew point. At this temperature, moisture condenses on the coil and drains away. The air is now dry but too cold. A reheat coil, located downstream of the cooling coil, then warms the air to the target supply temperature before it reaches the occupied space.
The energy cost of this two-step process is real. Research has shown that the energy needed to cool and then reheat air is roughly 1.65 times the energy required to simply cool the air to the desired supply temperature without dehumidification. You’re essentially paying to remove heat, then paying again to add some back. That tradeoff is why engineers put significant effort into choosing the right reheat method and minimizing how much reheating is needed.
Types of Reheat Systems
Electric Resistance Reheat
Electric reheat coils convert electrical energy directly into heat, much like a space heater. They’re simple to install because they don’t require boilers, piping, or pumps, and they have fewer moving parts, which means less maintenance. The downside is operating cost. Electric resistance heating is the most expensive way to reheat air. One state energy standard estimates that the source energy usage for electric reheat is roughly three times higher than for hydronic reheat.
Hot Water (Hydronic) Reheat
Hydronic systems use a boiler to heat water, which then circulates through coils in the ductwork. Installation costs are higher because the system needs boilers, piping, and pumps, and the equipment takes up more space. But the payoff comes in operating efficiency. Studies comparing the two methods found that hydronic reheat uses about 75% less energy than electric reheat during normal cooling operation, and the total air handling unit energy drops by roughly 24% to 33% depending on the operating mode. Hydronic systems can also integrate with solar thermal panels, further reducing energy costs in buildings that have them.
Hot Gas Reheat
Hot gas reheat is the most energy-efficient approach because it recycles heat the system is already producing. Here’s how it works: the refrigerant leaving the compressor is a high-temperature gas that normally flows to the condenser to dump its heat outdoors. In a hot gas reheat system, a three-way valve diverts a portion of that hot refrigerant to a reheat coil in the supply duct instead. The diverted gas warms the dehumidified air, then returns to the refrigerant circuit through a check valve that prevents backflow. Since you’re reusing waste heat from the cooling process rather than generating new heat, there’s no additional energy input for the reheat portion.
Wrap-Around Heat Pipes
A completely passive option uses heat pipes that loop around the cooling coil. The evaporator section of the heat pipe sits upstream, precooling the incoming air and absorbing heat. That absorbed heat transfers through the pipe to the condenser section downstream, where it warms the air after the cooling coil has dehumidified it. Heat pipes have no moving parts and require no additional energy. They reduce the load on the cooling coil (because air arrives precooled) while simultaneously providing free reheat on the other side.
Where Reheat Is Required
Reheat is optional in many comfort-cooling applications, but it becomes mandatory in spaces with strict humidity requirements. Operating rooms, for example, must maintain relative humidity between 20% and 60% while holding temperatures between 68°F and 75°F, all while delivering a minimum of 20 air changes per hour. That combination of high airflow, tight humidity limits, and precise temperature control makes reheat unavoidable. The cooling coil has to run cold enough to keep humidity in range, and reheat brings the air temperature up to the surgical team’s comfort requirements.
Laboratories, cleanrooms, and pharmaceutical manufacturing spaces face similar constraints. Any environment where humidity must stay within a narrow band regardless of outdoor conditions or internal heat loads will typically rely on reheat to decouple temperature control from moisture removal.
Energy Code Restrictions
Because reheat is inherently energy-intensive (you’re simultaneously cooling and heating), building energy codes limit its use. ASHRAE Standard 90.1, the most widely adopted commercial building energy code in the U.S., restricts simultaneous heating and cooling in its Section 6.5.2. Constant-volume reheat zones are essentially prohibited unless the system meets specific exceptions, and even then, the volume of air being reheated must be minimized.
In practice, this means most modern commercial systems use variable air volume (VAV) with reheat rather than constant volume. A VAV system first reduces airflow to the minimum required for ventilation or code compliance before activating the reheat coil. This way, you’re only reheating the smallest volume of air possible. Hospitals receive some allowance under the exceptions, but even hospital zones must demonstrate that the minimum air volume being reheated doesn’t exceed what applicable codes and accreditation standards require.
Choosing the Right Approach
The best reheat strategy depends on building size, available infrastructure, and how critical humidity control is. For small commercial spaces or retrofits where simplicity matters and reheat runs infrequently, electric resistance coils keep installation costs low. For larger buildings with existing boiler plants, hydronic reheat delivers significantly lower operating costs over the life of the system. Hot gas reheat makes sense in applications where dehumidification runs continuously, like indoor pools, supermarkets, or humid climates, because the “free” heat from the refrigerant cycle compounds into major savings over time.
Wrap-around heat pipes are worth considering in new construction where the ductwork can be designed around them. They’re particularly effective in hot, humid climates where the cooling coil is already working hard to remove moisture. Since they reduce both the cooling load and the reheat load simultaneously with zero energy input, they can pay for themselves quickly in the right application.