FAN Timer Board and Limit Control

main electrical panel; however, accessory equipment such as electronic air cleaners and humidifiers may be included on the furnace circuit. Although a suitably located circuit breaker can be used as a service switch, a separate service switch is advisable. The service switch is necessary if reaching the circuit breaker involves becoming close to the furnace, or if the furnace is located between the circuit breaker and the means of entry to the furnace room. The furnace switch (service switch) should be clearly marked, installed in an easily accessible area between the furnace and furnace room entry, and be located in such a manner to reduce the likelihood that it would be mistaken as a light switch or similar device.

The power requirements for all models: 120 VAC, 1 ∅, 60 Hz., 12A.

Accessories requiring 120 VAC power sources such as electronic air cleaners and humidifier transformers may be powered from the ST9103 EFT. Do not use the direct drive motor connections as a power source, since there is a high risk of damaging the accessories by exposure to high voltage from the auto- generating windings of the direct drive motor.

Thermostat wiring connections and air conditioning contactor low voltage connections are shown in the wiring

diagrams. Some micro-electronic thermostats require additional controls and wiring. Refer to the thermostat manufacturer's instructions.

The thermostat should be located approximately 5 feet above the floor, on an inside wall where there is good natural air circulation, and where the thermostat will be exposed to average room temperatures. Avoid locations where the thermostat will be exposed to cold drafts, heat from nearby lamps and appliances, exposure to sunlight, heat from inside wall stacks, etc.

Normal heat anticipator setting: 0.1 A. For more precise adjustment, the heat anticipator may be adjusted to the amperage draw of the heating control circuit as measured between the "R" and "W" terminals of the thermostat. To reduce the risk of damaging the heat anticipator, do not measure circuit without first removing one of the two wires first. To determine the heating circuit amperage draw:

1.Disconnect one of the “R” or “W” wires from the thermostat terminal.

2.Connect an ammeter between the wire and the thermostat terminal to which it was attached.

3.Note the amperage reading when the heating contacts are closed. (System switch must be on “HEAT” if so equipped.

4.Re-connect the thermostat wire. If the thermostat is serving a combination heating and air conditioning system, pay particular attention to polarity.

5.When the thermostat is reconnected and re-plumbed, adjust the heat anticipator setting to match the observed amperage reading.

The Electronic Fan Timer integrates control of all burner and circulator fan operations. This control is the central wiring point for most of the electrical components in the furnace. The Honeywell ST9103 has a fixed fan delay on time of 30 seconds after the burner ignites. The United Technologies 1158- 120 has an adjustable fan on time that is set by selecting the dipswitch combination displayed in Table 6. This fan on delay can be set at 30, 60, 90 or 120 seconds. This provides a delay between the burner ignition and blower start-up to eliminate excessive flow of cold air when the blower comes on. The Honeywell ST9103 has an adjustable fan off time of 60, 90, 120 and 150 seconds that is set by selecting a dipswitch combination on the control board displayed in Table 5. Similarly the United Technologies 1158-120have an adjustable fan off time of 2, 3, 4 or 6 minutes displayed in Table 6. The fan off delay time starts when the burner motor is de-energized at the end of a call for heat. Blower shutdown is delayed to remove any residual heat from the heat exchanger and improve the annual efficiency of the furnace.

The electronic fan timer board works in conjunction with snap disc limit controls, which perform a safety function, and breaks power to the oil burner primary control, which shuts off the burner if the furnace over-heats. The limit control is thermally operated and automatically resets. The limit control is factory installed, pre-set and is not adjustable.

If a limit control opens, the Honeywell ST9103 will energize the circulating fan. When the limit control closes the burner is re-energized and the heating cycle begins again.

If the limit control opens with the United Technologies 1158-120electronic fan control, the circulating fan will be energized as well. When the limit closes, the control initiates a two minute delay. When this delay is finished, the fan off timer will begin. At the end of the fan off time cycle the burner will be energized, initiating a normal burner cycle.

TABLE 5

Honeywell ST9103

	Dip Switch					Blower Off Delay
		Position				Time

	1		2
	On			On		60 seconds

	On			Off		90 seconds
	Off			On		120 seconds

	Off			Off		150 seconds

					TABLE 6
		United Technologies 1158-120

	Dip Switch					Blower Delay
		Position				Times
1		2		3	4	On	Off
						Seconds	Minutes
Off		Off				30
On		Off				60

Off		On				90

On		On				120

				Off	Off		2

				On	Off		3

				Off	On		4

				On	On		6

Note: It is advisable not to set the fan on delay time for a time period longer than 90 seconds at highest input. Longer fan on delay times may result in nuisance limit trips.

CIRCULATING AIR BLOWER

All WML-C AND MPL-B furnace models are equipped with a direct drive blower system. Direct drive blower speed adjustments are not normally required in properly sized extended plenum duct systems. The motor RPM and air CFM delivery will vary automatically to accommodate conditions within the usual range of external static pressures typical of residential duct systems. Under-sized duct systems may require a higher blower speed to obtain a reasonable system temperature rise. Some older duct systems were not designed to provide static pressure. They typically feature special reducing fittings at each branch run and lack block ends on the

8	30318 R4 9/9/2005

MPL-B, WML-C specifications

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