An example of typical multi-zone ductwork. Each duct in excess of 5 in. dia should include burglar bars when protecting sleeve walls.

Can you remember that miserable night you spent in a hotel when the heat or A/C wasn’t working right - too hot or too cold, or just not working at all? Or the circulation fan that made so much noise you couldn’t sleep the night before a big meeting? Chances are the next day you were not in a good mood.

Every HVAC professional knows well that a poorly performing, unbalanced, or noisy air conditioning system is virtually the first thing occupants in a new or remodeled facility will complain about. Inadequate conditions in your home, office, or travel lodging can be intolerable. Imagine, then, the impact similar inadequacies might have on a “captive” population that has limited resources for complaining when it is disgruntled … such as jail or prison inmates.


“Ordinary,” or anything that even approaches “standard” or “the usual” is nowhere close to meeting the many design standards in a corrections environment. Security is the top design priority for every aspect of a corrections facility. Keeping everyone safe - whether facility staff member, inmate, outside maintenance personnel, or visitor - dictates how the facility is designed and built, right down to every floor, wall, door, window, roof, system, and fixture detail imaginable. Nothing is too small to be overlooked. Any weakness represents dangerous opportunity for residents who attempt to exploit it.

An effective corrections HVAC system must operate reliably to help keep a potentially hostile population “cool,” with design and construction that does not offer inmates physical access to the various components. Reliability is, of course, a major goal, but not just to keep the users in a good mood. While inmates may be sentenced to five or 10 years, many of the operations staff serve longer terms. A hot, stuffy environment can add to staff and inmate anxiety. Anxious or angry inmates are more difficult to control. With the difficulty in today’s labor market of finding and retaining qualified staff, a reliable HVAC system can be an instrumental tool in retaining staff.

Access by inmates equals targets of opportunity for serious mischief by disassembling or vandalizing components to make weapons or create functional problems. Reliability also means improved lifecycle costs and reduced hands-on maintenance. Equipment maintenance takes much longer inside a secure perimeter in a corrections facility. Tool control plus check-in/check-out procedures can double or triple the time (and cost) required for maintenance and repairs.

For this tour of air distribution design considerations for the corrections environment, we will look at air-handling alternatives for “living” areas - individual cells, adjacent dayrooms, as well as showers and auxiliary spaces.

Final design choice hinges on variables such as region of the country and its ambient climatic conditions, population densities within the facility, and economic factors, such as the owner’s budget. 

Multi-zone AHUs provide positive ventilation and zone control. Most moving parts can be located in a secure mechanical room.


Five types of air-handling systems - multi-zone, VAV, constant air volume, heat pumps, and evaporative cooling - represent the primary menu choices, with the final decision based primarily on ambient conditions and owner preferences (which often equates to budget) but with one special caveat: in prisons and jails theoverridingconsideration on system equipment and configuration will always be security.

Are critical components exposed to inmates? Will maintenance personnel have to access equipment through inmate areas or work inside a populated area? While individual safety is always a priority, opportunities for inmates to have any kind of interface with maintenance staff and access or exposure to maintenance tools also represent a serious concern. Eliminating and/or avoiding these exploitable opportunities tops the list of design/construction criteria in the corrections environment.

With those thoughts in mind, let’s look more closely at the options.


Where different areas (zones) within a residential structure generate differing heat loads, ventilation needs will vary, too, dictating that a system deliver variable temperature air to zones to maintain appropriate occupied space standards. The multi-zone approach yields constant positive ventilation with good individual zone control. Zone differentials and exposure loads will vary.

Northern exposures have little to no direct sunlight, reducing the cooling demand in summer but increasing somewhat the winter heating demand. These loads, however, can be managed in part by building materials’ R factors. In today’s corrections construction, residential units are typically solid, precast concrete modules whose sheer mass helps moderate extreme differences. Dayrooms are often controlled on zones separate from cells. However, if the staff bars inmate access to cells during dayroom hours, economics may dictate a zone shared between dayrooms and cells.

More significant zoning considerations occur in multi-purpose rooms, staff offices, or control rooms. Multi-purpose rooms are usually adjacent to dayrooms and are subject to varying people loads. This also occurs with staff offices. Where supervision is from a secure control room, this space will nearly always require a separate zone or a separate HVAC unit.

Each multi-zone AHU's ductwork has a dedicated heating/cooling coil. But more zones means paying more. The price tag for selective/individual/isolated control is higher than other options, typically making this approach the most expensive on the menu. If holding down project construction costs is a major client factor, multi-zone will not be the first choice, at least in the short term.

On the other hand, the multi-zone’s fewer moving parts and long-term reliability usually yield a good life cycle cost, saving money over the long haul. From a security perspective, virtually all the multi-zone system’s parts requiring service or maintenance are typically located outside the secure perimeter in an isolated mechanical room with no inmate access.

An additional security safeguard for the multi-zone choice is that with all air-handling elements safely in the mechanical room - except vent openings - there is nothing consequential outside that area, making this system highly tamper-resistant. By the same token, if the equipment is outside the secure perimeter with no inmate access, maintenance is accomplished more easily and safely.

Most multi-zone units today are triple-decks, with a chilled water cooling coil, a hot water heating coil, and a bypass section serving each zone. Triple-deck units are more energy efficient than older, two-deck versions because the triple-decks are generally set up to prevent simultaneous heating and cooling. This energy saving feature is a tradeoff for limited humidity removal when there is a low cooling load and high humidity.

Separate central air-handling spaces and multi-zone units in particular will generally boost footprint and building costs. So, again, if saving construction money is a priority, the multi-zone system would not be a “first choice.”


The VAV approach tends generally to be more energy efficient than the multi-zone approach because the fan motor load varies with the cooling load. Temperature, humidity, and CO2 sensors and controls in each zone allow both automatic opening and closing of dampers to maintain outside air ventilation and variations in heating/cooling requirements in the area served.

While VAV satisfies multipurpose rooms and staff office needs, it can be difficult to apply for dayrooms and cells because most codes require so much outside air that it is difficult to vary airflow significantly.

Although the VAV AHUs are typically located safely out of an inmate’s reach on a rooftop or in a secured mechanical room, locating VAV control and heating boxes where maintenance personnel have minimum/no contact with residents can be challenging.

For line-of-sight purposes, these low-level housing units have no air-handling equipment on the roof.


As with most residences, the constant volume solution treats the entire area served as a single zone, moving air at a constant rate and temperature into the conditioned space. These systems can work well in moderate climates in well-insulated buildings with lots of building mass to help smooth temperature fluctuations. CV systems are frequently used in open bay dormitories but are not as efficient for situations where there are a variety of multipurpose rooms with varying occupancy loads.

Sometimes a hybrid VAV/CV system is used with a VAV box serving the multipurpose room with the remainder of the housing unit served with constant volume. These systems usually do not take advantage of varying fan speed energy savings, but instead “ride the fan curve.” This hybrid system holds construction costs down, and allows some temperature control flexibility in multipurpose rooms.

Simple CV systems are considered the most secure approach because air handlers can be easily located outside the secure perimeter, and they have a bare minimum of moving parts and adjustments.

Owners on a budget and those endorsing the KISS (keep it simple, stupid) principle often choose CV systems. DX cooling is largely used in conjunction with rooftop CV systems, providing the simplest system available with the lowest construction cost. Of course, chilled water can save cooling energy compared to DX systems, but much of the energy savings are offset by the complexity of locating pumps, valves, and chillers outside the secure perimeter.

A CV variation for corrections housing is an air or water cooled heat pump. Rooftop packaged air cooled heat pumps usually have the lowest initial cost of all the system types. However, these systems’ applications are limited to regions that do not experience significant ambient humidity or cold winters. Most of the readily available packaged systems do not provide adequate dehumidification when handling the large volumes of outside air required for dayroom/cell ventilation. Air cooled heat pumps also lose efficiency when outdoor temperatures drop and need backup heat from electric resistance coils or gas burners.

Geothermal water cooled heat pumps do not share the winter inefficiencies of air cooled heat pumps. These units provide very good life-cycle costs in areas without high ambient humidity and appropriate geothermal conditions. However, the humidity limitations of air and water cooled packaged heat pumps are somewhat similar. At this time, a wide variety of packaged geothermal equipment is not available. If you choose a geothermal heat pump for corrections housing, you will be an early adopter, and can expect the attendant fame and fortune (or their opposite).


Evaporative cooling is used for housing unit ventilation and cooling by a number of jurisdictions in the western states. These systems can provide temperature relief where low ambient humidity is able to take advantage of the psychrometrics. Evaporative cooling is worthy of fuller discussion in another article.


Heat recovery variations can be added to any of the systems described above. Because of the large volumes of outside air, hot gas bypass or other heat recovery methods can reduce energy consumption while adding complexity to construction and maintenance.

Hot gas bypass reheat is one of the simplest HR systems for corrections use. It has proven to be relatively effective in providing some level of humidity control while still being economical. As with VAV systems, there can be security compromises if reheat boxes are located where they must be maintained from inside the secure perimeter.

HR ventilator systems condition incoming supply air with exhaust air without mixing the two airstreams, transferring heat from one stream to the other according to season. The exchange occurs as the separate supply and return ducts pass their air through the heat exchange core. The core captures up to 85% of exhaust stream energy to condition the filtered supply air. HR ventilation systems are particularly challenging to locate outside the secure perimeter. Heat exchange cores have to be located relatively close to the conditioned areas, with distribution preferred in short, straight duct runs.

In the corrections environment, equipment installation is generally above the conditioned zones in an area secure from inmate access. Close coordination is required in the early design phase to allow maintenance from outside the secure perimeter without adding more square feet and construction dollars to the housing unit.

HR ventilators can be a good approach for conserving energy in “tight” facilities, where low velocity fans maintain a relatively low air exchange rate across a heat recovery medium. Balanced systems exhaust stale cell/dayroom air with one fan, while a separate fan unit draws in fresh supply air in equal portions. HR ventilation systems are particularly effective for creating a healthier inside environment by removing accumulated moisture and/or pollutants in the atmosphere that were generated by the general population, showers and lavatories, etc. Further, this system works well in northern climates that do not have extreme seasonal temperature variations.


As we have indicated already in this discussion, there are a number of options for locating air-handling and conditioning equipment. While cost is always an item of concern, it will rarely trump worries about security. To keep costs in line, designers and contractors are constantly seeking more cost-effective ways of meeting or exceeding code requirements while maintaining a safe and secure environment for corrections staff and inmates.

Typical locations for air handling equipment, from most secure to least secure, are:
  • Mechanical room accessible from outside the secure perimeter
  • Rooftop
  • Outside ground-mounted
  • Mechanical room accessible from inside the secure perimeter
  • Suspended above secure ceiling
  • Suspended above non-secure ceiling
  • Suspended and exposed
  Locations are selected based on the system needs, the “character” of the population, cost, and ease of maintenance. The preferable locations will always be those where the inmate population itself cannot gain access to the equipment and where authorized personnel can access the equipment for maintenance without exposure to the inmate population.


Prisons, jails, and other similar detention facilities have all of the normal needs for conditioned air, plus, as we have shown, their own special considerations. Further complicating the design situation is the need, in some cases, for automatic and manual smoke removal. When gas is used to help regain control of a housing unit, staff will sometimes manually use smoke or gas removal fans to manage the length of time that gas is retained.

With the threat of insurrection always present, corrections authorities need to be able to control a restless and dangerous population, while protecting the lives of their charges and employees at the same time. There is little opportunity for design experimentation or error, making HVAC design and construction of correctional facilities always a challenging task.