A.O. Smith DB/DW-720-1810 Automatic Reset High Temperature Limit Control, Thermal Balancer

AUTOMATIC RESET

HIGH TEMPERATURE LIMIT CONTROL

CAUTION

LIMIT CONTROLS ARE NOT TO BE USED AS THERMOSTATS.

ALL MODELS - This limit is a safety device wired in series with the ignition system. Set the limit control dial to a minimum of 10°F (6°C) above the maximum designed system temperature. If the boiler outlet water temperature should exceed the high limit setting, the main gas valves will close but the circulator will continue to operate. Maximum adjustable setting is 240°F (116°C) for boilers and 200°F (93°C) for hot water supply heaters. The differential is adjustable from 5°F (3°C) to 45°F.

SINGLE STAGE AND MODULATING MODELS - Are equipped with the limit like the one shown in fig. 4.

Gas valves either close or open to full gas input, depending upon heat requirement.

DUAL STAGE MODELS ONLY - HIGH/LOW OPERATOR

These boilers are factory equipped with a (combination) temperature controller to provide high and low firing rates for smooth operation. (fig. 5).

The temperature dials should be adjusted to have the HI dial set as a limit to be 10°F (6°C) above desired boiler temperature. These boilers are shipped with the HI dial set to 200°F (93°C) with a 15°F (8°C) differential, the LO dial set to 160°F (71°C) with a 10°F (6°C) differential.

The heater operation with above set points will be as follows:

1)When heater temperature rises to LO thermostat setting the main gas valve will close its second stage operator to drop input to approximately 50% of total input.

2)System conditions will determine if temperature rise at LO thermostat sensor location will continue to rise until shut-off by high limit or drop causing heater to go back to full input.

Other system design temperatures will require resetting of the dial. Total span of temperatures (HI dial cut-off point to LO dial cut-on point) should not be less than temperature drop of the system.

MODULATING MODELS ONLY

These boilers are factory equipped with a modulating gas valve that automatically controls the outlet water temperature by modulating or proportioning the rate of gas thus maintaining a desired constant outlet water temperature regardless of varying flow rates.

Set temperature controller for desired outlet water temperature. (fig. 6).

Adjust the set point for the desired operating temperature by turning the set point adjusting screw on the front of the case until the desired value is reached on the set point indicating dial. The set point is calibrated to be at the center of the proportioning range, (see the manufacturers instructions for complete information on testing and calibrating this control.)

THERMAL BALANCER

This device serves as a pump shutdown delay switch to balance the boiler water temperature to system temperature before the pump stops. Overshooting of boiler temperature is prevented and stack loss after shutdown is minimized.

SAFETY FLOW SWITCH

The safety flow switch is a safety device which must be installed at the water outlet of the unit to prevent main burner operation in the event of inadequate water flow through the boiler.

This switch may be mounted in a horizontal pipe line or a vertical pipe line with upward water flow. Do not install the switch where the water flow is downward.

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