Convector Heaters - The Definitive Guide
Offices, hotels, schools, retail stores, sports facilities and other building types typically have rooms that can be efficiently heated by the use of heaters, such as convectors, radiant ceiling panels or infrared radiant heaters.
For these applications, the “appearance” of the heating unit is just as important as its performance. For the right combination of customization and performance, convectors may be an ideal solution for buildings with complex heating needs.
What is a Convector Heater?
Convector heaters – also called convection heaters or simply convectors - are non-fan forced heaters that use natural convection to move the heated air back into the conditioned space – making them much quieter than fan heaters. Without the use of a fan to blow air, the convector is a great choice for minimizing the circulation of dust and pollens, making for a better work environment.
Architects spend countless hours on interior design creating a desired atmosphere in these spaces. Think about how the use of light and the choice of color, furnishings and materials affect the ambiance in a hotel lobby or office boardroom. Consequently, heating units need to “blend in with” or “complement” their surrounding environment.
At the same time, these units must help specifying architects and engineers solve heating problems such as counteracting cold drafty areas near windows, reducing condensation of the glass and aiding in the de-stratification of the air in the space.
With all these factors to consider, there is no one-size-fits-all heating solution for every building. Rather, specifying architects and engineers must solve their distinct heating problems with customizable solutions to ensure optimal cost-efficiency and overall functionality.
Convectors are ideal for use in areas with large spans of windows, such as offices, schools and hotel lobbies. Typically mounted at floor level on exterior walls and under windows, convectors provide an upward movement of air to counteract cold downdrafts and minimize condensation.
Available in a variety of sizes, configurations and colors, convectors also offer design and installation versatility. Architects and engineers can leverage customizable features to develop convectors that fit the custom design specifications of a project as well as solve heating problems without wasting energy or space. Models are available that can be located in a trench, housed in custom enclosures or mounted in a variety of other ways.
Convectors have a wide array of control options from built-in thermostats for controlling individual units to silicon controlled rectifier (SCR) controls that can integrate into Building Management Systems (BMS).
How Does a Convection Heater Work?
Tapping into a natural phenomenon known as “convection,” air inside a convector is heated becoming less dense than the surrounding cool air, enabling it to rise due to buoyancy. As the heated air rises, the cooler air on the floor is drawn into the convector, creating a constant flow. By placing convectors under a window, the heated air rises and blocks the down draft of cold air, creating a heated air curtain.
All convectors contain two components that work together to safely deliver heated air to the space: the element and an over temperature limit.
The element converts electrical energy to heat by passing an electrical current through a specifically designed resistance wire. The elements used in convectors are metal sheathed, and constructed of a spiral wound resistance wire encapsulated in an insulating powder (Magnesium Oxide, MgO) encased in a metal sheath.
Fins are added to the element rod to improve heat transfer by creating a chimney effect, directing air to flow over the element and the larger fin surface to heat the air passing through the unit. Most convection heaters have aluminum fins pressure bonded to the rod. However, heavy-duty and explosion-proof convectors have customized steel fins brazed to the rod to better handle higher demands.
Over-temperature limits are temperature-sensing devices located on or near the element that interrupt the flow of electricity to the element if an abnormally hot condition occurs. In convectors, the over temperature limit device is most commonly activated when an air inlet or discharge opening is blocked by drapery or furniture, causing heat to build up.
Convector Use: When, Where, and How
Considerations for Residential Applications
Since convectors have no moving parts and use natural air flow rather than forced air flow, they are ideally suited to areas where the air movement noise associated with fan-forced heat is undesirable. This includes bedrooms and home offices where convectors can be installed along exterior walls under windows to provide quiet, gentle heat.
However, care must be exercised to insure that there is adequate wall space remaining for placement of furniture and drapery after installation of the convector. In addition, the location of electrical receptacles must be taken into account.
Convectors with electronic hydronic elements have a lower surface temperature than standard convectors, making them good choices for a nursery or child’s bedroom.
For basements, standard convectors should be installed along the above-ground walls to eliminate the cold downdraft, as well as under windows of other walls.
Basements with interior divisions should have a heater and thermostat in each area. In large open basements, several smaller heaters will produce a better heated air distribution than a singular large heater. If the area is only occasionally occupied, portable baseboards may be the better choice. Heating the basement has the additional benefit of warming the floor of the rooms above adding to the main floor comfort level.
Considerations for Commercial Applications
Commercial buildings run the gamut from hospitals and assisted living facilities to schools, hotels and retail stores. To accommodate the very different environments in these facilities, many manufacturers of convectors offer a variety of styles and configurations.
Among them are front- and bottom-inlet convectors, recess-mounted cabinet convectors, sill-line convectors and architectural convectors, which feature design elements that help them blend into or complement most interior spaces.
Like residential applications, the convective heater in commercial applications should be installed on the exterior wall. The aesthetics of front inlet convectors mounted at floor level must be weighed against the ability of cleaning equipment to reach under bottom inlet convectors mounted a few inches above the floor with less damage to the heater. Recess-mounted cabinet or special application sill-line commercial convectors may provide a solution.
In individual office or conference areas, heavy-duty baseboards, draft-barrier convectors or sill-line commercial convectors are the best choice. The decor of the room as well as its heat loss should be used to determine which style best suits the application. If floor to ceiling glass is present, pedestal convectors should be considered.
Large open, multi-person, perimeter offices are ideal applications for heavy-duty baseboards, draft barrier or sill-line commercial convectors and architectural convectors if the window area does not reach the floor. Placing the convector along the entire length of exterior wall eliminates the discomfort of the cold wall effect for people located nearby. As with individual offices, if floor to ceiling glass is present, pedestal convectors should be considered.
The use of convectors in lobbies would be the same as in large open offices above except additional consideration must be given to the fact that people tend to move about more in lobbies. Knowledge of expected traffic patterns is important in heater location, particularly at the end of pedestal runs, if floor to ceiling glass is present and pedestal convectors are considered. Lobbies with multi-story windows and atriums present a unique application for convective heaters.
The amount of heated air necessary to block the downdraft of this large expanse of window, and keep the moisture from forming on the top portion of the window, cannot be generated from floor level convection equipment alone. In these cases, sill or pedestal-mounted convectors installed at floor level, working in conjunction with convectors mounted approximately every 10 to 15 feet (3 to 4.5 m) up the window, will provide sufficient heated air.
Architectural convectors have slots on the bottom of the enclosure for air intake rather than the large holes of most commercial convection equipment. These slots present a finished appearance when viewed from the floor level. Recess or surface mounted cabinet convectors and Slope top convectors are well suited for use in hallways, cafeterias and restrooms because of their heavy-duty construction. Recess mounting is also important in the areas where space is at a premium.
Considerations for Industrial Applications
Factories, warehouses, sports complexes and similar facilities require heaters that can withstand a great deal of abuse but still function properly while requiring little maintenance.
For restrooms, lunchrooms, small to medium workshops and assembly areas with low to medium ceilings, slope top or cabinet-style heavy-duty convectors provide even heating, yet are constructed to withstand normal daily industrial abuse.
Used on exterior walls, the sloped design of slope top convectors prevent them being used as shelves or step stools. Cabinet convectors can be recessed on applications where space is limited and the wall that the heater is recessed into is a non-exterior wall.
In some industrial applications, there is the potential of hazardous gasses being present. Explosion-proof convectors may be better suited for these applications.
Thermostat and Control Considerations
In addition to determining the appropriate type of convector to use in an applications, specifying engineers and contractors also must consider the type of thermostat to use. Thermostats are either integral or remote.
Integral thermostats are factory- or field-mounted on the heater and do not require external control wiring, saving installation costs. However, since integral thermostats are on heaters, mounted at or near floor level, they are best used in areas that are usually not occupied or do not require close temperature control.
Remote thermostats can be located in the area to be heated, requiring the additional expense of wiring between the heater and the thermostat.
Their location within the designed heating space makes them best suited for areas that require higher control accuracy or that are usually occupied.
Do not locate thermostats on exterior walls, in the direct discharge of the heater, above any heat producing devices (coffee stations, copy machines or machinery or too far from the heater.
Convectors can be controlled individually by a built-in thermostat, in groups by a building automation system or any number of options between. When determining the control system consider the required degree of accuracy as well as the designed space parameters. Convector control circuits are either low voltage (24VAC) or line voltage (usually the heater supply voltage).
A common rule of thumb is that electronic or 24VAC mercury bulb thermostats controls are more accurate than standard bi-metal line voltage controls. A location in the center of the heated space is best but be aware of the distance between the heaters and the thermostat. If the thermostat is located too far from the heaters, or at one end of a long narrow room, it will result in over and under heated pockets within the design space.
The primary purpose of a disconnect switch is to completely shut down the heater and provide an additional level of safety from electrical shock and personal injury hazards for the personnel working on the heater.
The disconnect switch opens (disconnects) the sources of electrical power to the unit. The disconnect switch(es) may be located on the heater or at a remote location.
Note: There may be more than one source of electrical power supplied to the heater (i.e. a separate control circuit) so it may be necessary to locate more than one switch to completely disconnect the heater from all electrical power.
Power relays are used to control electrical loads that may be greater than a thermostat rating. Heaters with supply voltage over 277VAC , heaters with amperage ratings in excess of the thermostat rating or heaters where low voltage control is required use power relays to control the supply power to the heater.
In most cases, the power relays used in convective heaters are single pole-single throw devices with contacts rated to 600VAC and holding coil rated from 24 to 277VAC.
The holding coil is normally controlled by a thermostat, building automation system or other control device.
greater than a thermostat rating. But they are usually used when quiet operation and low voltage control is required.
These relays are a combination of a current relay and supply power to 24-volt transformer. There is a time delay of about 45 to 60 seconds between thermostat closure and relay contact closure.
The advantage of transformer relays is their quiet operation and the fact that only one device is required. However, there are two notable disadvantages.
First, more than one relay can be controlled by one thermostat, but since each is energized by the preceding relay, the time delays add from relay to relay. Second, because of the transformers small VA rating, the distance between the transformer rating and the thermostat is limited (maximum recommended distance = 25 ft., 7.6m).
Transformer relays cannot be used on three-phase heaters.
Infinite (SCR) Control
When thermostats or thermostat relay (power or transformer) combinations are used tocontrol convectors, the space temperature is maintained by cycling the heater element full on until the thermostat is satisfied and then full off until the thermostat again calls for heat. This leads to some over and under heating.
For more accurate control, convectors can use SCRs (basically electronic switches) to maintain the space temperature by modulating the element between zero and one hundred percent. This method allows the heater to supply only the amount of heat necessary to keep the space at the temperature selected on the thermostat. SCRs generate a fair amount of heat and are therefore mounted on heat sinks. Because of the size of the heat sinks, they are supplied only in control sections of sill line, pedestal and architectural convectors.
A specific use electronic (remote mount / integral mount) thermostat is normally used to control the SCRs supplied with these heaters. If a standard modulating controller is used for space temperature control, an interface is available.
A control system in its most basic form can contain only one device such as a thermostat, a disconnect switch, a power relay or a transformer relay.
However, most control systems are more complicated because it is often necessary to combine multiple controls into a system to maintain the comfort level of a design area.
Multi-level control systems can be applied to any convective heater but are normally only used with sill line, pedestal and architectural convectors.
The design of the control system starts with the desired results and works backwards to the components necessary and in most cases there will be several combinations of controls that will produce the same results.