I N T R O D U C T I O N

STANDARD CONTACTOR CONTROLLED HEATERS: Instant on/off.

Advantages: Versatility, use of air pressure (or proving) switch, low voltage fan interlock, transformers and applications such as zone control.

Disadvantages: Instant full line loading occurs if a multiple staged unit is turned on all at once. This can occur when stages are controlled by outdoor thermostats and the temperature is below the set points.

QUIET RELAY CONTROLLED HEATERS: Same as standard contactor units except no contactor noise. The use of solid state electronics to control the magnetic coil has reduced the contactor noise of a power contactor to the level of a small enclosed fan relay. [For units with 24V controls only.]

STAGING: The dividing of the Heater controls to operate various portions on demand.

Advantages: Allows partial or full operation as with multi-stage thermostats.

Disadvantages: When used with outdoor thermostats or emergency heat thermostats the entire load may energize instantly.

DELAY BETWEEN STAGES: This is a standard contactor operated unit with delay relay(s) added in the controls circuit between contactors. This is usually used in the higher wattage units involving multiple circuits and elements where it is not economically feasible to install complete sequencing. This option is for units with 24V or 240V controls only.

Advantages: Preventing entire unit from coming on all at once. If the Heater is staged, the first stage is instantly on, there is a short delay between additional stages and heater will shut-down immediately when de-energized. It also overcomes the problem of sequencing at voltages above the limits of the delay relay in the power circuit.

[ If contactor noise is a problem, this may be eliminated by using quiet relays in 24Volt control applications.]

SILENT AND/OR SEQUENCED HEATERS: If this is a requirement there are several variations available that may be tailored to each application. The following information should help you make the proper selection.

SEQUENCED HEATERS (Silent): Silent operation with delay on/off. Requires high voltage interlock of the fan to ensure air flow during delay shut-down.

Advantages: Silent Operation. Incremental loading on start up.

Disadvantages: High voltage interlock must be run in conduit. Temperature over-shoot due to delay off sequence.
[20KW may take as long as four minutes.]

SEQUENCED WITH TWO STAGES: Sequenced units delay on shut-down and require a high voltage fan interlock to maintain fan during that period.

SEQUENCED WITH INSTANT SHUT-DOWN: Instant on/off because all power to the elements and sequencers is routed through contactors.

Advantages: May use low voltage interlock and/or air pressure switch. May use air switch and transformer for zone control. May be multiple staged with incremental loading even when all are turned on at one time.

[If contactor noise is a problem, this may be eliminated by using quiet relays in 24 Volt control applications.]

MERCURY CONTACTOR CONTROLLED HEATERS: A noiseless replacement of the standard contactors generally used. Available with 24, 120, 208/240 & 277V control coils. Direction of airflow MUST be specified.]

Advantages:
Silent Operation
Have a much greater life expectancy
May be staged in all the ways we have discussed with the standard contacts.

Disadvantages:
Higher initial cost.