On heat demand (first or second stage) the switch senses that the combustion air inducer is operating. It closes a cir- cuit to the furnace control when pressure inside the cold end header box decreases to a certain set point.

Set points vary depending on unit size. See tables 8, 9 and

10.The pressure sensed by the switch is negative. If the air intake vent pipe or outlet vent pipe becomes obstructed during operation, the switch senses a change of negative pressure and opens the circuit to the furnace control and gas valve. A bleed port on the switch allows relatively dry air in the vestibule to purge switch tubing, to prevent con- densate build up.

The switch is factory set and is not field adjustable. It is a safety shut-down control in the furnace and must not be by− passed for any reason. If switch is closed or by−passed, the control will not initiate ignition at start up.

DUAL COMBUSTION AIR PROVE SWITCH

 

G61MP−090, −110 & −135

first stage

negative

pressure

 

second stage

positive

 

 

pressure

FIGURE 9

To troubleshoot the prove switches, temporarily jumper them. The unit will not fire with the switches jumpered. Therefore, the prove switches must be bypassed after the combustion air in- ducer is activated. This will determine if the prove switches and furnace are operating properly. However, this may not in- dicate if the sealed combustion system is operating properly.

Checks of pressure differential can aid in troubleshooting. When measuring the pressure differential, readings should be taken at the prove switch. Lack of differential usually indicates problems in the intake or exhaust piping, but may indicate problems in the heat exchanger, condensing coil, head- er boxes, combustion inducer or other components.

Measuring pressure differential

The differential pressure is the difference in pressure mea- sured across the cold end header box orifice.

1 − Remove thermostat demand and allow unit to cycle off.

2 − Install a tee in the negative (−) line and a tee in the positive

(+)line running from one of the prove switches to the cold end header box.

3 − Install a manometer with hose from the negative (−) side of the manometer to the tee installed in the nega- tive (−) line and with hose from the positive (+) side of the manometer to the tee in the positive (+) line.

NOTE − Both sides of the cold end header box are negative. However the (+) port reads less negative pressure than the

(−) port.

CAI & COLD END HEADER BOX ASSEMBLY

(−045 & 070 SHOWN)

 

Install tee’s in the

 

negative line and

 

positive line then

 

connect hoses to

 

manometer.

_ +

+ _

cold end header box

prove switches orifice size

FIGURE 10

4 − Operate unit and observe manometer reading. Read- ings will change as heat exchanger warms.

a. Take one reading immediately after start-up.

b. Take a second reading after unit has reached steady state (approximately 5 minutes). This will be the pres- sure differential.

The pressure differential should be greater than those listed in table 8, 9 or 10.

5 − Remove thermostat demand and allow to cycle off.

6 − Remove manometer and tee’s. Reinstall combustion air sensing hoses to the prove switch.

7 − Repeat steps 1 through 6 for the other prove switch.

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Lennox International Inc G61MP Dual Combustion AIR Prove Switch, First stage Negative Pressure Second stage Positive
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G61MP specifications

Lennox International Inc. has long been a leader in HVAC solutions, and the G61MP gas furnace exemplifies the company's commitment to innovation and energy efficiency. This high-efficiency gas furnace operates with an Annual Fuel Utilization Efficiency (AFUE) of up to 96%, allowing homeowners to save significantly on energy costs while maintaining a comfortable indoor environment.

One of the primary features of the G61MP is its modulating gas valve. This technology enables the furnace to operate at multiple levels of heating output, ensuring that it can adjust its performance according to the home’s heating demands. By modulating the heating output, the G61MP maximizes efficiency and minimizes energy waste, providing a consistent temperature and improved comfort.

The G61MP also incorporates advanced two-stage heating, which enhances its performance by reducing temperature fluctuations in the home. This feature is especially beneficial during transitional weather periods when heating needs may vary widely. With its ability to operate on a lower stage most of the time, the furnace can produce quieter operation and reduced energy consumption.

In terms of design, the G61MP includes a sealed combustion chamber that draws air from outside the home for combustion, enhancing safety and efficiency. This design feature eliminates the need for indoor air, improving indoor air quality. The furnace is also equipped with a high-efficiency ECM blower motor that not only boosts comfort but operates quietly and uses less electricity compared to traditional motors.

Further enhancing its efficiency, the G61MP utilizes a variable-speed blower, which allows for precise control of airflow throughout the home. This results in better humidity control and improved air distribution, providing an overall more comfortable environment.

The Lennox G61MP is compatible with the iComfort S30 smart thermostat, allowing homeowners to control their heating remotely, adjust settings based on their schedules, and access energy usage data. This connectivity transforms the G61MP into an integral part of a smart home ecosystem, facilitating energy conservation and comfort optimization.

Lastly, the G61MP is designed with durability in mind. Its robust construction and high-quality materials ensure reliable performance and lengthy service life. Homeowners can feel assured that they are investing in a heating solution that will meet their needs for many winters to come. In summary, the Lennox G61MP gas furnace stands out with its high efficiency, advanced technology, and user-friendly features, making it a top choice for homeowners seeking exceptional heating performance.
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