HVAC and Aquatics Glossary

HVAC and Aquatics professionals hardly ever interact with each other...except for natatoriums. Because the two fields are so different, they have many of their own terms, acronyms and buzzwords that can sometimes lose people from the other side. Fortunately, Chloramine Consulting is fluent in both languages, and we created this handy glossary.

Scroll down to read in alphabetical order, or click the first letter of your term to search faster.



Absolute Humidity - (n) The amount of water vapor (or moisture) in the air at a given temperature.

Airflow - (n) A term describing the movement of air. Can be applied to both velocity (see FPM), air volume (CFM) and direction.

Air Conditioner - (n) A system designed to cool air and reduce humidity. Air conditioners are great for just about any indoor application (or vehicle), but are not appropriate for indoor swimming pools. See PDU.

ASHRAE - (acronym) ASHRAE stands for American Society of Heating, Refrigerating and Air-Conditioning Engineers. It is a global society of engineers and trade professionals who come together to set industry standards and best practices. ASHRAE publishes technical bulletins, journals, and a design handbook full of great information. For indoor air quality, ASHRAE 62.1 is where you will find their recommended guidelines and standards.


Breathing Zone - (n) The area just above the water where swimmers breathe. It is the lowest air in a natatorium, and also where the highest concentration of chloramines and other DBPs form what we call the chloramine bubble.


CFM - (acronym) CFM stands for Cubic Feet per Minute. CFM is the standard unit of measurement of the volume of air flow. CFM can be calculated several ways, the easiest is by knowing the airflow velocity (see FPM), and multiplying it by the cross sectional area of the duct or plenum it's traveling through.

Chemical Controller - (n) An electronic panel used to control various automated systems in a swimming pool pump room. Usually a chemical controller receives inputs from probes (such as pH sensors and ORP sensors) and sends outputs to feeders (such as chlorinators and feed pumps for liquids).

Chloramines - (n) Disinfection Byproducts (DBPs) of chlorine oxidizing nitrogen compounds and non-living organics in water. Chloramines are used generically to mean all DBPs, though there are hundreds of different types that are not actually chloramines. There are only three types of chloramines: Monochloramine, Dichloramine, and Trichloramine. They form when chlorine combines with ammonia and begins replacing hydrogen atoms with chlorides. Chloramines are heavier than oxygen, and build above the pool and wet deck, creating what is called the chloramine bubble.

Chloramine Bubble - (n) A term used to describe airborne chloramine vapor that hovers in the breathing zone of swimmers. It forms because chloramines are heavier than oxygen. See source capture, Evacuator, Hyper-Dissolved Oxygen and breathing zone.

Chlorinator - (n) A device used to dispense chlorine into a water system. Usually calcium hypochlorite (cal hypo). Some consider feed pumps for liquid chlorine that are controlled by a chemical controller to also be chlorinators.

Chlorine - (n) The most common primary disinfectant and residual sanitizer used in swimming pools. Chlorine comes in many forms, but the most common are liquid chlorine (sodium hypochlorite), cal hypo (calcium hypochlorite), trichlor (trichloro-s-triazinetrione), and dichlor (sodium dichloro-s-triazinetrione). See combined chlorine and chloramines.

Combined Chlorine - (n) Chlorine that has combined with nitrogen compounds such as ammonia or urea. Combined chlorine is measured in Cl2 units instead of N units like ammonia is, which can be misleading. Combined chlorine = Total Chlorine - Free Chlorine. It is the most common way to measure the amount of chloramines in water.

Condensation - (n) The process of water vapor changing phase from gas to liquid. Usually, in an indoor swimming pool this occurs on metal surfaces or within the PDU's condenser coil(s) as a part of the moisture removal (dehumidification) process.


Dew Point - (n) The temperature where water vapor begins to condense back into liquid form. In swimming pools, this commonly occurs on exterior windows that are not properly "washed" with conditioned supply air, because the glass can be colder than the dew point, causing condensation.

Dichloramine - (n) Waterborne chloramine that is created after a monochloramine is exposed to enough Hypochlorous Acid (HOCl). Dichloramine's formula is NHCl2.

Disinfection Byproducts (DBPs) - (n) Various byproducts created when chlorine oxidizes nitrogen compounds and/or non-living organics in water. DBPs are often referred to as chloramines generically, even though there are hundreds of types of DBPs that are not chloramines. They can also be referred to as chloro-organic compounds. Other common DBPs in swimming pools include trihalomethane gasses such as chloroform. There are too many to name.

Duct - (n) A hollow conduit for airflow to travel through. In HVAC systems, there are supply ducts, return ducts, and sometimes exhaust ducts. Ducts come in many shapes and sizes, such as rectangular, circular, oval, etc. They are also available in flexible (flex) duct and rigid. The type of duct used depends on whether it is negatively pressured (return, or suction side) or positively pressured (supply side).


EA - (acronym) EA stands for Exhaust Air, which is air discarded from circulation. To keep a natatorium slightly negatively pressured, the air system must exhaust slightly more than outside air (OA) being drawn into circulation, usually by a factor of 15%. In that example:  OA x 1.15 = EA.

Efflorescence - (n) A change on a masonry surface due to moisture pushing through a cementitious material; either encrusting with salt crystals or calcification. In swimming pools it usually happens opposite of a pool wall, where water presses through the concrete shell. On natatorium buildings themselves, vapor pressure can push moisture outward from the indoor pool, which causes efflorescence where there is a lack of a vapor barrier.

Energy Recovery - (n) A process in which a pool dehumidifier or other system reclaims energy that would otherwise be lost. In natatoriums, this normally means the PDU reclaiming heat (with a reheat coil, or something similar) and using that energy to supplement heating in the natatorium or in the swimming pool water itself.

Exhaust Air - (n) Air that is discarded from a room, never to be brought back into circulation. Traditional natatoriums usually exhaust from the ceiling or from the return air path in the PDU itself. The new standard of care is becoming source capture exhaust, which targets airborne chloramines. Exhaust air is NOT the same as Return Air, though they are often confused with each other.


FPM - (acronym) FPM stands for Feet Per Minute. FPM is the standard unit of measurement of the velocity of airflow. Multiplying the FPM by the cross sectional area of the duct/plenum allows us to figure out the volume of airflow, which is measured in Cubic Feet per Minute (CFM).


GPM - (acronym) GPM stands for Gallons Per Minute. GPM is the standard unit of measurement of the volume of water flowing through a specific plumbing system or vessel.


Humidity - (n) The amount of moisture or water vapor in the air. Also see Absolute Humidity and Relative Humidity (RH).

Hyper-Dissolved Oxygen (HDO) - (n) Purified, concentrated oxygen that is dissolved in water, that provides benefits to water chemistry. As a side benefit, HDO introduces pure oxygen into the swimmers breathing zone, which helps displace and dilute the chloramine bubble.

Hypochlorous Acid - (n) The strong, killing form of chlorine in water. Formula for hypochlorous acid is HOCl. HOCl is in equilibrium with the far weaker form of chlorine in water, the Hypochlorite Ion (OCl-). The lower the pH, the higher the percentage of HOCl, and therefore, the stronger the chlorine, and usually that translates to a higher ORP, which is a good thing for sanitization, disinfection and oxidation in swimming pools. HOCl combines with nitrogen compounds like urea and ammonia to create combined chlorine, which includes chloramines and other disinfection byproducts (DBPs).










Mechanical Contractor (n) A licensed installer of mechanical systems, duct and other equipment. Most states require a mechanical contractor and stamped drawings for a building permit.

Mechanical System - (n) In the context of indoor swimming pools and commercial buildings, the mechanical system includes any component of the HVAC system: duct, PDU, fans, controllers, etc.

MEP - (acronym) MEP stands for a Mechanical, Electrical & Plumbing engineer or engineering firm. MEPs create drawings and specifications, and eventually construction documents that get stamped for submittal in the permitting process. Sometimes engineering firms do not have all three of these trades in one house, but the term MEP still refers to any of the three types of engineers. 

Monochloramine - (n) Waterborne chloramine that is created when ammonia (NH3) is exposed to enough Hypochlorous Acid (HOCl). Monochloramine's formula is NH2Cl.


Natatorium - (n) A building or room containing an indoor swimming pool.

Nitrogen Compounds - (n) Various molecules based on nitrogen (N). The common nitrogen compounds we see in swimming pools are forms of ammonia (NH3), which is usually derived from Urea (CH4N2O).

Nitrogen Trichloride - (n) The formal name for Trichloramine (NCl3).


OA - (acronym) OA stands for Outside [or Outdoor] Air. Conditioned rooms need a certain amount of fresh air (OA) to be brought into circulation. Natatoriums are no exception. To keep the natatorium slightly negatively pressured, the air system must exhaust slightly more than the OA, usually by a factor of 15%. In that example:  OA x 1.15 = EA.

ORP - (acronym) ORP stands for Oxidation Reduction Potential. ORP tells us how well chlorine is able to perform. Most states allow a minimum of 650 mV. Anything over 800 mV is excellent. ORP is measured using electronic probes, and it tells us the speed of electron transfer in water (also called conductivity) as measured in millivolts (mV). The speed of electron transfer tells us the potential for oxidation and reduction reactions to occur, which gives us a gauge of how well chlorine is performing as an oxidizer and disinfectant.

ORP Probe - (n) An electronic device submerged in water that measures the conductivity of water in millivolts (mV). This allows a chemical controller the ability to display ORP in real-time.


Pascals (Pa) - (n) The standard unit of measurement for pressure. See vapor pressure.

Pool Dehumidification Unit (PDU) - (n) A PDU is a dehumidifier built specifically for an indoor swimming pool. It is different from an air conditioner in many ways, but primarily a PDU is built to remove moisture and deliver a comfortable natatorium environment with an RH between 50-55%. PDUs are also built to withstand the corrosive environment of a natatorium for longer than a standard air conditioner.

Plenum - (n) A box that collects air that connects to an HVAC system directly or via duct. This is different than a duct, in that a duct does not normally collect air directly.




Relative Humidity (RH) - (n) Relative Humidity is the amount of water vapor (or moisture) in the air, relative to how much the air could possibly hold at a given temperature. Also see Absolute Humidity.

Return Air - (n) Air from a room that is pulled into the air handling system for reconditioning and circulation. Normally in swimming pools, return air is captured through large vents (or grilles) built into a wall. All the room's air volume should be drawn into the return, typically at a rate of 4-6 times per hour. Also see Supply Air and Exhaust Air.


Source Capture - (v) Collecting a specific type of air through a plenum or duct located in a specific place. In indoor swimming pools, the targeted air is contaminated with airborne chloramines and other DBPs, which tends to stay low in the breathing zone, forming the chloramine bubble. Source capture, through use of the Paddock Evacuator® captures the contaminated air as close to its source as possible, and exhausts it from the room.

Static Pressure - (n) The resistence to airflow in an air system, including ducts, plenums, and mechanical equipment like PDUs and fans. It is essentially the amount of pressure a fan has to push or pull against to move air throughout the duct system. Static pressure is measured in inches (or centimetres) of water column

Supply Air - (n) Conditioned air that is pushed into a space from an HVAC system, like a PDU. In natatoriums, the supply air is critical to controlling air quality, as it dictates the movement and direction of airflow in the space. Also see Return Air and Exhaust Air.


Trichloramine - (n) Waterborne chloramine that goes off-gasses and becomes airborne. Trichloramine is created after dichloramine is exposed to enough Hypochlorous Acid (HOCl). Trichloramine's formula is NCl3. It is formally known as Nitrogen Trichloride.


Urea - (n) A nitrogen-based substance that comes from human waste, such as sweat and [especially] urine. Urea is not easily oxidized, and takes excessive amounts of chlorine to remove from water. It is a primary source of disinfection byproducts (DBPs) like chloramines, trihalomethanes, and more. Urea's formula is (CH4N2O).


Vapor Barrier - (n) A moisture-proof membrane used in walls and ceilings to prevent the transfer of moisture. Most buildings use an exterior vapor barrier to prevent moisture from coming into the building. Natatoriums, however, need an interior vapor barrier to prevent moisture from pressing out of the building, which causes issues like efflorescence. Also referred to as "vapor paper". See Vapor Pressure.

Vapor Pressure -(n) The equilibrium pressure (measured in Pascals [Pa]) of a vapor above its liquid or solid form in a closed container or building. In other words, Vapor pressure is the pressure of the evaporated liquid above the liquid itself. In a natatorium, its the pressure of moisture in the air that evaporated from the swimming pool, and is contained in the natatorium room. 


Water Column (WC) - (n) A unit of measurement for static pressure (in inches or centimetres). It represents how much pressure it takes to lift water in a cylinder. Nowadays, this can be measured digitally with a tool called a manometer. 0.5" WC is a fairly typical amount of static pressure in commercial HVAC. It can also be expressed as 0.5 in.wc.

Water Vapor - (n) Evaporated water in the air. Also called humidity or moisture.