Automatic Air Vent. This is an air valve that automatically releases air from the system without any manual intervention.


The blackened surface in a collector that absorbs the solar radiation and converts it to heat energy.


The ratio of solar energy absorbed by a surface relative to the solar energy striking it. Solar collectors have high absorptive coatings.

Active System

A solar water heating system that requires external mechanical power e.g. pumps, collectors to move the collected heat.

Air System

Solar domestic hot water systems employing air-type collectors are available. Hot air generated by these collectors is fan forced through an air-to-liquid heat exchanger with the potable water being pumped through the liquid section of the exchanger. The heated water is then circulated through the storage tank in a similar fashion to the liquid collector system. Air does not need to be protected from freezing or boiling, is non-corrosive, and is free. However, air ducts and air handling units require greater space than piping, and air leaks are difficult to detect.


The angular distance from the horizon to the sun.

Ambient Temperature

The temperature of the surrounding air.


Area on the solar panel that collects energy. When solar collectors are tested the energby collected is measured against the apertura area. When comparing collectors a reference area is important – that is, the surface area from whish the collectors characteristic values are drawn. In the collector test methods according to EN 12975 the reference area is equal to either the apertura area or the absorber area.


Abbreviation for the American Society of Heating and Air-Conditioning Engineers.

Auxiliary Heat

The extra heat provided by a conventional heating system for periods of cloudiness or intense cold when a solar heating system cannot provide enough.


The angular distance between true south and the point on the horizon directly below the sun.


British Thermal Unit (BTU)

The quantity of heat needed to raise the temperature of one pound of water one degree Fahrenheit.



The quantity of heat needed to raise the temperature of one gram of water one degree Celsius.

Closed-Loop System

A solar system where the transfer fluid does not mix with the potable water and uses a heat exchanger to heat the household water by conduction and/or convection.

Coefficient of Heat Transmission

The rate of heat loss in BTU per hour through a square foot wall or other building surface when the difference between indoor and outdoor air temperatures is one degree Fahrenheit.

Collector (Solar)

A device that collects solar radiation and converts it to heat. A solar hot water collector is usually an insulated box, with a tempered, low iron glass cover. Solar pool collectors usually do not have insulation or a cover plate.

Collector Efficiency

The ratio of usable heat energy extracted from a collector to the solar energy striking the cover.

Concentrating Collector

A device which concentrates the sun’s rays on an absorber surface which is significantly smaller than the overall collector area.


The rate of heat flow (in BTUs per hour) through an object when a 1° F. Temperature difference is maintained between the sides of the object.


The flow of heat due to temperature variations within a material.


A measure of the ability of a material to permit conduction of heat flow through it.


This is the brain of the system. The controller monitors the temperatura in the collectors and in the bottom and top of the storage cylinder. When it is warmer in the collectors tan the bottom of the storage cylinder the controller turns on the solar pump and pumps the heat round from the collectors to your storage cylinder, where it is stored ready for you to use.


The motion of fluid such as gas or liquid by which heat.

Cover Plate

A sheet of glass or transparent plastic placed above the absorber in a flat plate collector.


Degree Day - DD

A unit that represents a 1 degree F. deviation from some fixed reference point (usually 65°F.) in the mean daily outdoor temperature. The colder it gets, the more DD available. Degree-day figures quantify how hot or cold the weather has been as a single index number for the region and month (or week). They allow you to account properly for the effect of weather on energy consumption.

Design Heat Load

The total heat loss from a house under the most severe winter conditions likely to occur.

Design Temperature

The temperature close to the lowest expected for a location, used to determine the design heat load.


Domestic Hot Water.

Diffuse Radiation

Indirect sunlight that is scattered from air molecules, dust and water vapor.

Direct Radiation

Solar radiation that comes straight from the sun, casting shadows on a clear day.

Drain Down System

Potable water is circulated from the storage tank through the collector loop. Freeze protection is provided by solenoid valves opening and dumping the water at a preset low temperature. Collectors and piping must be pitched so that the system can drain down, and must be assembled carefully to withstand 100 psi city water line pressures. Pressure reducing valves are recommended when city water pressure is greater than the working pressure of the system.

Drain Back System

The solar heat transfer fluid automatically drains into a tank by gravity. Drain back systems are available in one or two tank configurations. A heat exchanger is necessary, because the city inlet pressure would prevent draining. The heat transfer fluid in the collector loop may be distilled or city water if the loop plumbing is copper. If the plumbing is threaded galvanized pipe, inhibitors may be added to prevent corrosion. Most inhibitors are non-potable and require a double wall heat exchanger. The pump used must be sized to overcome static head.



A measure of the propensity of a material to emit thermal radiation.

Expansion Vessel

A tank in a heating system that provides space for the physical expansion of water or a water solution. The expansion vessel is designed to take and termal expansion and contraction that happen in the solar circuit.


Flat Plate Collector

A solar collection device in which sunlight is converted into heat on a plane surface without the aid of reflecting surfaces to concentrate the rays. It is the most efficient type of collector for use with temperatures between the freezing and boiling points of water and up to about 350 degrees F when used with air as the working medium. Flat plate collectors are normally used with the flat surface facing south and tilted to an angle appropriate to the intended use.

Forced Convection

The transfer of heat by the flow of fluids (such as air or water) driven by fans, blowers or pumps.


Galvanic Corrosion

A condition caused as a result of a conducting liquid making contact with two different metal which are not properly isolated physically and/or electrically.


A column or cartridge containing an active metal which will be sacrificed to protect some other metal in the system against galvanic corrosion.

Glaubers Salt

Sodium sulfate a eutectic salt that melts at 90°F. and absorbs about 104 Btu per pound as it does so.


Is a solar anti-freeze that flows through the pipes on the solar circuit. It is designed to protect down to -28 degrees C. It can also withstand very high temperaturas that occur in solar termal systems without degrading.

Gravity Convection

The natural movement of heat that occurs when a warm fluid rises and a cool fluid sinks under the influence of gravity.



The pipe that runs across the edge of an array of solar collectors, gathering or distributing the heat transfer fluid from, or to the risers in the individual collectors. This insures that equal flow rates and pressure are maintained.

Heat Capacity

A property of a material denoting its ability to absorb heat.

Heat Exchanger

A device, such as a coiled copper tube immersed in a tank of water, that is used to transfer heat from one fluid to another through a separating wall.

Heat Storage

A device or medium that absorbs collected solar heat and stores it for use during periods of inclement or cold weather.

Heat Storage Capacity

The amount of heat that can be stored by a material.

Heating Season

The period from early fall to late spring (in the northern hemisphere) during which additional heat is needed to keep a house comfortable for its occupants.

Heat Pump

A mechanical device that transfers heat from one medium to another, thereby cooling the first and warming the second.

Heat Sink

A medium or container to which heat flows.

Heat Source

A medium or container from which heat flows.


Heat Transfer Fluid used in closed loop systems e.g. glycol/water mixtures or pure water.

Hybrid Solar Energy System

A system that uses both active and passive methods in its operation.



Integrate Collector and Storage for solar water heater.

Indirect System

The solar circuit is called an indirect system and refers to the heat exchanger inside your storage cylinder. It is the most common form of solar water heating system. The storage cylinder is manufactured with two heat exchangers inside. The highest one is what the hot water from the boiler flows through (the boiler will heat 70% of the cylinder) and the lower heat exchanger is what the solar system glycol flows throgh (the solar system will heat 100% of the cylinder). The heat exchanger allows sater from the boiler and glycol from the solar circuit to pass through, but not mix with the water in the storage cylinder itself. Thus, in effect, it really consist of three systems that appear to join at the storage cylinder.

Infrared Radiation

Electromagnetic radiation from the sun that has wavelengths slightly longer than visible light.


The total amount of solar radiation direct, diffused and reflected-striking a surface exposed to the sky.


A material with high resistance (R-value) to heat flow.



Kilowatt-hour (KWH)

A unit of energy equal to 1 kilowatt (1,000 watts) of electric power being used for 1 hour. Equivalent to 3,413 BTU’s of thermal energy or 1 1/3 hp of mechanical energy.



A measure of solar radiation; equal to one calorie per square centimeter.


Murphy’s Law

Anything that can go wrong will go wrong at the worst possible way when you least expect it.



North American Board of Certified Energy Practitioners

NOAA – National Oceanic & Atmospheric Administration Worldwide Weather Data

Consult for lowest local freezes etc. on record, solar radiation data & local weather data.

Night Time Cooling

The cooling of a building or heat storage device by the radiation of excess heat into the night sky.


One-Tank Closed-Loop System

A conventional DHW tank, usually electrically heated, is converted to a solar DHW storage tank by installing an external heat exchanger coil. The lower electrical element is removed, leaving the uppermost of the usual two elements to provide auxiliary water heating and to achieve good stratification (layering of hotter water over progressively colder water).

Open Loop System

Some part of the System is open to the atmosphere, or system contains fresh or changeable water.


Passive System

A solar heating or cooling system that uses no external mechanical power e.g. pump or controller, to move the collected solar heat.

Percentage of Possible Sunshine

The percentage of daytime hours during which there is enough direct solar radiation to cast a shadow.

Potable Water

The household water that comes out of your bathroom and kitchen sinks.

Pressure Gauge

Informs the user about the current pressure in “bar” in the solar circuit. The black hand will point to the systems pressure. This gauge will normally read 1-1.5 bar. It can read as high as 6 bar. It should never read 0.


Pressure Relief Valve. This is a safety valve that will only open up and reléase liquid if the pressure in the system becomes to great and reaches 6 bar.


The solar pum pis usually located in your airing cupboard or in the loft space. It is a different pump to your central heating pump. The solar pump will only work when there is heat to be transferred from the collectors to the cylinder.


An instrument for measuring solar radiation.



Radiant Panels

Panels with integral passages for the flow of warm fluids, either air or liquids. Heat from the fluid is conducted through the metal and transferred to the rooms by thermal radiation.


The flow of energy through open space via electromagnetic waves, such as visible light.

Reflected Radiation

Sunlight that is reflected from snow, water or surrounding trees, terrain or buildings onto a surface exposed to the sky.


A liquid such as Freon that is use in cooling devices to absorb heat from surrounding air or liquids as it evaporates.

Resistance, or R Value

The tendency of a material to retard the flow of heat.


The application of a solar heating or cooling system to an existing building.


The flow channels or pipes that distribute the heat transfer liquid across the face of an absorber.


Scale Inhibitor

Fitted inline to the storage cylinder. This inhibits scale formation in the cylinder increasing life expectancy and efficiency of the product.

Seasonal Efficiency

The ratio, over an entire heating season, of solar energy collected and used as opposed to the solar energy striking the collector.

Selective Coating or Surface

A surface that absorbs radiation of one wavelength (for example, sunlight) but emits little radiation of another wavelength (for example, infrared); used as a coating for absorber plates.

Shading Coefficient

The ratio of the solar heat gain through a specific glazing system to the total solar heat gain through a single layer of clear double-strength glass.

Solar Constant

The average intensity of solar radiation reaching the earth outside the atmosphere. 1 solar constant is 1,000 watts/sq. meter or 1.432 Langleys/min.

SEIA - Solar Energy Industry Association

The national trade association of solar energy manufacturers, dealers, distributors, contractors, installers, architects, consultants, and marketers who work to expand the use of solar technologies in the global marketplace.

Solar Influx

Solar intensity of all diffuse and direct solar radiation measured at the site.

Solar %

An actual measurement with a BTU or watt-hour meter, or a quick estimate of the solar influx at the site during real time monitoring/testing of the system by the contractor or owner.

Solar Radiation

(Solar Energy) Electromagnetic radiation emitted by the sun.

Specific Heat

The quantity of heat, in BTU, needed to raise the temperature of one pound of a material 1° F.

SRRC - Solar Collection Certification and Rating

In 1980 the Solar Rating and Certification Corporation (SRCC) was incorporated as a non-profit organization whose primary purpose is the development and implementation of certification programs and national rating standards for solar energy equipment.

Standby Heat Loss

Heat lost though storage tank and piping walls, even if no hot water is used.

Storage Cylinder

This is where all the heat collected during the day is kept. The storage cylinder is usually sited where your old cylinder was. Warm water is stored here for you to use when washing or bathing.

Sun Path Diagram (Solar Window)

A circular projection of the sky vault, similar to a map, which can be used to determine solar positions and to calculate shading. This allows you to evaluate shading at a specific location form 8:00 am to 4:00 pm each month of the year.


Temperature vs. Energy

Knowledge of temperature is necessary to knowledge of thermal energy, but is not enough. Both the temperature and quantity of material containing the energy must be known. We could not reasonably expect a match flame at 2,000 degrees to be able to heat a swimming pool, but solar collectors at 90 degrees will do it readily if we use enough of them.

Thermal Capacity

The quantity of heat needed to warm a collector up to its operating temperature.

Thermal Mass or Thermal Inertia

The tendency of a building with large quantities of heavy materials to remain at the same temperature or to fluctuate only very slowly; also the overall heat storage capacity of the building.

Thermal Radiation

Electromagnetic radiation emitted by a warm body e.g. the sun or a hot wood stove.

Thermistor Sensing

Device which changes its electrical resistance according to temperature. Used in the control system to generate input data on collector and storage temperatures.


The process that makes water circulate automatically from a warm collector or heat exchanger to a cooler storage tank above it. (See Gravity Convection).

Tilt Angle

The angle that a flat plate collector surface forms with the horizontal plane.


Thermostatic Mixing Valve. This valve located in you airing cupboard near the storage cylinder will mix cold water with the hot water coming out of the cylinder. This will reduce the risk of scalding and allow the solar to store temperature of 85 degrees C in the cylinder. You can change the temperature of the hot water coming out of you cylinder by rotating the knob on top of the TMV.


Ultraviolet Radiation

Electromagnetic radiation with wavelengths slightly shorter than visible light.







Glossary Thermal Solar Energy (pdf)