The U.S. Correctional system is vast and deeply entrenched. With over 1.9 million incarcerated individuals housed across more than 5,500 correctional facilities (this includes federal and state prisons, local jails, and detention centers), it operates at a scale unmatched anywhere in the world. These institutions never stop running. They provide housing, meals, medical care, laundry, and more, all day, every day.
That nonstop operation burns through an enormous amount of fuel, and much of the resulting heat goes straight out the exhaust. From laundry rooms to kitchens to boiler stacks, valuable energy is lost as waste heat. But it doesn’t have to be. Air-to-water (ATW) heat recovery systems provide an innovative way to capture that heat and put it to work, reducing fuel use, utility costs, and environmental impact.
Air-to-water heat recovery systems move thermal energy from hot air (usually exhaust air) into water. It’s like preheating a pot of water using the steam from another one that's already boiling. The key pieces of an ATW heat recovery system typically include:
• Heat exchanger: The main unit that pulls heat from air and transfers it to water.
• Fan and ductwork: To move hot air across the exchanger.
• Piping: Carries the warmed water to where it’s needed.
Economizers, especially rooftop air-side models, can often be installed with minimal disruption to existing infrastructure. Because they’re designed to integrate with current HVAC systems, rooftop economizers can typically reuse the building’s existing ductwork. This keeps installation costs down and avoids the need for extensive system overhauls or prolonged downtime.
Correctional facilities are among the most energy-demanding buildings in the public sector. They operate continuously, supporting critical infrastructure for lighting, heating, ventilation, kitchen operations, sanitation, and laundry. This 24/7 baseline means energy demand is constant and substantial.
Data from the Commercial Buildings Energy Consumption Survey (CBECS) indicates that prisons have a median site Energy Use Intensity (EUI) of 69.9 kBtu/ft²/year, surpassing many other public building types. This high EUI reflects the large energy requirements for daily operations, particularly for thermal loads such as space heating and water heating. These are typically served by large, fossil-fuel-powered boilers and water heaters.
This fossil fuel dependency is baked into the system. Most prisons rely on centralized, gas-fired mechanical infrastructure, much of it aging and inefficient. As researcher Patrick Greiner notes, the rise in U.S. incarceration rates has fueled a proportional increase in energy-intensive infrastructure, embedding correctional systems deeper into industrial fossil fuel consumption
Financially, this entrenchment is a burden. The Prison Policy Initiative estimates that utilities cost around $795 per incarcerated person per year, adding up to $1.8 billion annually across the entire U.S. prison system. A typical state-run correctional institution may spend $30,000 to $40,000 per month on energy.
Much of this energy is wasted. Boilers, dryers, and kitchen systems generate high volumes of recoverable heat, yet most of it is discharged unused. Facilities are paying to produce heat they don’t use, then paying again to generate more for water heating or sanitation.
Air-to-water waste heat recovery offers a clear path to improve this balance. By capturing and redirecting waste heat into hot water systems, correctional facilities can reduce fuel consumption, lower emissions, and enhance overall energy efficiency without disrupting their core operations. 4 Key Recovery Opportunities in Correctional Facilities
Most institutions rely on commercial-grade washing machines and gas-fired or electric tumble dryers, often consolidated in central laundry plants. Dryers, in particular, produce large volumes of hot exhaust air – typically between 140°F and 180°F – that is vented outside through ductwork.
An air-to-water heat recovery system can be installed within the dryer exhaust duct. The preheated water is then directed to:
• Washing machine inlet lines to reduce the load on water heaters
• Storage tanks, where it can be used later during peak washing cycles
• Boiler preheat loops (in facilities with integrated thermal networks)
Correctional kitchens run multiple large appliances, including ovens, steam kettles, dishwashers, griddles, and fryers. This heat is extracted through overhead Type I exhaust hoods connected to HVAC or dedicated ventilation systems. Most of it exits the building without being used.
Modern recovery setups use hood-mounted or rooftop heat exchangers to extract heat from the outgoing air. These are paired with insulated piping that delivers the captured energy to:
• Hot water loops used for dishwashing or handwashing
• Preheat tanks feeding kitchen-specific water heaters
• HVAC return air systems to redue heating loads during winter months
Facilities with centralized air-cooled chillers, walk-in freezers, or large-scale refrigeration systems for cold food storage can recover compressor discharge heat or reject heat from condenser coils. The compressor waste heat from those systems is stable and predictable (90°F to 130°F), offering a consistent source for localized water preheating for nearby dishwashers or hand sinks.
Boilers are the largest and most consistently recoverable source of waste heat. They provide space heating, domestic hot water, and steam for sanitation throughout a facility. They are typically gas-fired firetube or watertube boilers, and they vent flue gases through vertical stacks at temperatures exceeding 300°F, sometimes as high as 500°F, depending on operating pressure and boiler load.
Installing an economizer, a type of counterflow heat exchanger, on the flue gas outlet allows this residual heat to be recovered. Economizers typically use finned-tube coils to transfer energy from the exhaust gas into the boiler feedwater line, raising its temperature before it enters the boiler shell. This delivers several advantages:
• Reduces the boiler’s fuel consumption, since the burner no longer needs to raise feedwater from ambient temperature
• Stabilizes boiler operation by improving thermal efficiency and reducing stack temperature loss
• Increases condensate return temperature, which can lower overall corrosion risk inside the boiler.
Many correctional facilities support in-house production through prison industries programs, manufacturing textiles, license plates, furniture, and more. These workshops often contain machinery like sewing machines, presses, dryers, ovens, welding stations, and compressed air systems, that generate localized, concentrated heat during use.
Compared to laundry or boiler rooms, the total recoverable energy from these spaces is relatively low. The heat output tends to be more diffuse and intermittent. That said, there may be niche cases where targeted recovery systems make sense.
For example, exhaust ventilation above heat-intensive zones, such as press machines, could be routed through small-scale air-to-water exchangers. Recovered heat could then support localized water heating needs like handwashing stations, janitorial sinks, or laundry preheat loops if plumbing is nearby.
Correctional facilities weren’t designed with energy efficiency in mind, but that’s exactly what makes them such high-potential candidates for waste heat recovery. The infrastructure is already in place. The systems run constantly. And the waste heat is there, just waiting to be reused.
ATW waste heat recovery is a practical upgrade, and with solutions like the ENERVEX PowerVex RHX, it’s easier and more reliable than ever. The RHX system eliminates the biggest concern that typically comes with waste heat recovery: downtime. It’s a fully integrated, rooftop-mounted unit that installs quickly, often with less than 30 minutes of interruption, and includes built-in bypass dampers to keep systems running even during maintenance. With just a few utility connections, facilities can start reclaiming heat without overhauling their infrastructure or disrupting operations. That means real savings, reduced emissions, and no trade-offs.
Ready to turn waste into value? Visit enervex.com to learn how the PowerVex RHX can help your facility cut costs and capture what you're already paying for.