Adjusting Differential Air Pressure

Hydronic Heating Boilers and Domestic Water Heaters

CLEANING AND MAINTENANCE Continued

The following is a recommended method for setting the differential air pressure (ΔP) for each fan.

LESS	AIR

MORE	AIR

AIR SHUTTER

FIG. 56 Adjusting Air Shutter 399,999 - 750,000 Btu/hr Models

FIG. 57 Loosening Fan Transition Box Screws

FIG. 58 Adjusting Air Shutter 990,000 - 2,070,000 Btu/hr Models

Set Up Procedure

Beside each fan duct is an air pressure switch with a large and a small tube delivering pressure from points inside the unit (see FIG. 59). The pressure in the large tube is the chamber pressure. The pressure in the small tube is the burner pressure. They act together to make the pressure switch. By disconnecting the caps from the tees in the pressure switch hoses and connecting them to either side of a manometer, you can read the differential pressure to the switch.

The “ (+)” connection on the manometer connects to the tee in the tubing from the units front chamber and the “ (-) ” connection on the manometer connects to the tee in the small tubing from the burner.

If the air pressure switch does not make within 60 seconds from the time the fans turn ON, the ignition module will go into a soft lockout period (approximately five minutes in duration) during which time the fans are turned OFF and the module shows the Low Air flash code. If necessary, soft lockout can be circumvented by cycling power to the ON/OFF switch to cycle power to the unit.

Note: If the unit has been firing recently, allow the unit to cool for five minutes with the fans running before beginning the adjustment procedure.

Retain the plastic caps removed from the tees for reinstallation when complete.

Adjustment Procedure

1.Remove the upper front jacket panels from the unit to access the upper chamber.

2.Slightly loosen screws that attach fan transition box to metal base (see FIG. 57).

3.Depending on model, the air shutter(s) may be located either on the side or the rear of the fan duct. Locate the air shutter at the side/rear of the fan duct (see FIG. 56 and 58). Move the air shutter towards the rear or left side of the unit to increase air pressure. Move the air shutter towards the front or right side of the unit to decrease air pressure.

4a. On the 990,000 - 2,070,000 Btu/hr models adjust the air shutter on the left fan until the differential pressure is nominally 1.1 - 1.3 inches water column. Note: The air chamber pressure is 1.2 inches water column for liquefied petroleum (L.P.) and 1.4 inches water column for natural.

4b. The 399,999 - 750,000 Btu/hr models have one fan with a side mounted air shutter. The differential for these models is nominally 1.5 - 1.65 inches water column.

5.Reattach the hoses to the pressure switch and locate the air switch adjacent to the right fan and connect the tubes to either side of your manometer.

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