Desa DNV40PB Inspecting Burner Flame, Normal Operating Sequence, To Turn OFF GAS To Furnace

OWNER’S MANUAL

OPERATING

FURNACE

(Continued)

TO TURN OFF GAS

TO FURNACE

1.Set thermostat to lowest setting

2.Turn equipment gas valve clockwise

to OFF position. Do not force equipment gas valve.

3.Unplug furnace if service is to be per- formed.

To Shut Down Furnace for an Extended Period of Time

1.Turn equipment gas valve clockwise

to OFF position. Do not force equipment gas valve.

2.Set thermostat on OFF.

CAUTION: Do not try to adjust heating levels by using the equip- ment shutoff valve.

THERMOSTAT

CONTROL

OPERATION

The thermostat responds to the room tem- perature. When the room temperature drops below the thermostat setting, the thermostat turns the furnace burner and blower on. When the room temperature goes above the thermostat setting, the thermostat turns the furnace burner off. The blower will con- tinue to blow about two to three minutes after the burner goes out. The thermostat keeps the room at a constant temperature.

INSPECTING BURNER FLAME

To observe burner flame, remove furnace front cover. Remove the two screws at the lower edges of front cover. Lift front cover away. Locate viewing window on front of combustion chamber. You can see burner flame through this window.

Correct Burner Flame: Blue flame with well-defined burner ports.

Incorrect Burner Flame: Yellow or lazy flame without well-defined burner ports.

If burner flame is incorrect, shut off furnace and contact a qualified service person.

Correct Flame	Incorrect Flame
Pattern	Pattern

Figure 19 - Burner Flame Patterns

NORMAL

OPERATING

SEQUENCE

1.When the room temperature drops below the thermostat setting, the thermostat calls for heat. A 24-volt circuit is completed through the time delay relay coil. After three to five seconds, the contacts in the relay close. The blowers come on.

2.One motor drives both the combustion air blower and the circulating room air blower. The combustion air blower draws air from outside through the in- take air vent. This air discharges into the combustion chamber. The combus- tion air blower then forces the combus- tion products out of the exhaust tube(s). The Combustion air blower is sealed so that air does not pass between it and the circulating room air blower.

3.When the blower motor reaches 75% of normal r.p.m. (three to five seconds), air from the blower turns on the

microswitch. When the microswitch activates, the electrical circuit is com- pleted through the limit switch to the control module. The glo bar begins to glow. In about 15 seconds, the glo bar becomes hot enough to ignite the burner.

4.Another 24-volt circuit is completed through the module to the main gas valve. The valve opens. Gas passes through the valve to the main burner. The gas is ignited by the glo bar.

5.After the main burner ignites, the flame detector senses the presence of flame. This opens the circuit to the glo bar. The glo bar shuts down.

6.The circulating room air blower pulls return air in from the living area. This air is heated as it flows past the com- bustion chamber. The heated air is then blown out into the room.

7.When the room temperature rises above the thermostat setting, the circuit to the valve opens and the burner shuts off.

8.The blower continues to run about two to three minutes until the time delay relay coil cools. When the coil cools, the circuit to the motor opens. The blower shuts off.

LIMIT SWITCH

WARNING: Do not operate furnace without a properly work- ing limit switch.

The limit switch will turn off the gas to the main burner if the furnace becomes too hot. When this happens, the main burner will go out and the blower will continue to run for two to three minutes. This usually happens when circulating air is partially or com- pletely blocked.

Occasional cycling of the limit switch is not bad. This indicates safe operation. Limit switch cycling will most likely occur on warm days. If this happens too often or for a long period of time, the circulating air system should be thoroughly cleaned by a qualified service person.

If the circulating air system is cleaned and the limit switch continues to shut down the furnace, the limit switch is defective. A defective limit switch cannot be repaired. It must be replaced. For safety reasons, never operate the furnace without a limit switch.

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