Understanding The Low Firing Rate Baffle

818-1-2This bulletin will discuss the low firing rate baffle: what it is, what it does, and when it is needed. The Beckett low firing rate baffle is a small metal plate. It is about 3-3/4″ x 1-3/4″ in size, and has a length-wise bend in the middle. (Figure 1.)

The baffle is installed over the back wall of the blower wheel scroll, which is visible when the transformer is swung back. This wall is also the front of the wiring box. (Figure 2.)

What does it do?

Some burners, such as the Beckett Model AFG, are designed to fire a range up to 3.00 GPH. Therefore, the air supply must be large enough to support these higher firing rates. But, these burners are also designed to fire at lower firing rates. When this is done, the air supply must be reduced in order for the burner to light and run. The primary methods of reducing air are the inlet air shutter, the static plate, and the combustion head.

818-3The low firing rate baffle is also used to partially restrict the flow of combustion air to the combustion head. When the baffle is placed in the stream of air going from the blower wheel to the head, it increases the load on the blower wheel. This reduces the CFM and increases the operating pressure. This is like a dam in a rushing river. On the upstream side, energy is hard to control. On the downstream side, it is easier to control as the water is metered through the dam. (See Figure 3.)

The same principle applies with the low firing rate baffle. At the lower firing rates, the combustion head has an easier time controlling the reduced air supply. The result is better starts and running characteristics. Flame stability at start-up is improved due to higher blower operating pressure which tends to dampen flame pulsations. It also means less sensitive air shutter adjustments.

(IAS = Inlet Air Shut-off)

Head Firing Rate, GPH W/O IAS W/IAS
.50 X X
F0 .65 X X
.75 X X
.85 X
F3 .90
.85 X X
F6 .90 X X
L1 .50 – .85 X X
V1 .75 – 1.00 X X

Figure 4

Another advantage is that the inlet air controls must now be opened more to overcome the added resistance of the baffle. This means that the openings are less prone to restriction due to build-up of dust, lint or pet hair.

When do you need it?

As the name indicates, the baffle is used at the low firing rates for the Beckett Model AFG burners. The range is from 0.50 to 0.90 GPH, depending on the combustion head used and if the Air Inlet Shut-off is used. Figure 4 shows when the low firing rate baffle should be used. The “X” on the chart indicates baffle needed.

For your convenience, all Beckett “B” spec AFG burners have the low firing rate baffle included in the carton. Where necessary, the baffle is already installed on burners used with OEM products.