Burner Fan Motor: Replacing the Combustion Air Fan Motor and Matching Speed

When you replace a burner fan motor, two conditions must be met together: the new motor must cover the fan's shaft power, and its speed must be in the same pole class as the old motor's nameplate speed and very close to it. In combustion air fans the impeller is usually mounted directly on the motor shaft, so motor speed sets the amount of air entering the burner one-to-one; for example the 4 kW, 3000 rpm class DRG-112M-2 runs at 2905 rpm and draws 7.45 A at 400 V. A motor that matches the power but runs at a different speed will not give the same airflow, and the air/fuel ratio will drift.

That is why a motor change on a burner is not just a mechanical part swap. Once the motor is fitted, the combustion setting should be checked by an authorised burner service with a flue gas measurement, and the electrical connection, overload setting and direction-of-rotation check should be done by a qualified electrician. On an appliance with a gas train, flame supervision and a safety chain, skipping these steps is a direct safety risk. You can filter the models that match your nameplate by power, speed and frame among the 120 models on the Buy Electric Motors Online page.

What is a burner fan, and what does the electric motor do in it?

A burner fan is the fan that blows the air needed for combustion into the burner head. "Burner fan" is not the name of an operating principle or a housing type but of an application: the fan inside is usually centrifugal (radial), and because of its compact build a forward-curved, multi-blade impeller is common. Air enters at the impeller eye, passes through the scroll-shaped passage and reaches the air damper and the combustion head.

The fan and the motor are separate parts. The fan consists of the impeller, air passage, damper and inlet. The motor's job is to turn the impeller at a given speed. Two layouts are found on burners:

• Monoblock burner: The fan sits inside the burner housing. The motor is flange-mounted to the housing and the impeller is fitted on the end of the motor shaft (direct drive). The motor stays outside the airflow; only its shaft and flange face look into the fan side.
• Duoblock burner: The fan is a separate unit connected to the burner head by an air duct [3]. Here the motor may sit on a foot-mounted base or be flanged; the drive is direct or via a coupling.

Burner fans run on hot water and steam boilers, thermal oil heaters, furnace and drying lines, asphalt plants and process heaters. The flue-side induced draft fan and the forced draft fan of a boiler are a separate topic; for those see our article on industrial boiler fan motors.

Why does a motor with a different speed upset the combustion setting?

What sets a burner fan apart from other fans is that its airflow is matched to the fuel quantity. When commissioning the burner, the service sets the damper openings for a specific fan speed. By the fan laws, flow changes in direct proportion to speed, pressure with the square of speed and power with the cube of speed (details: affinity laws in pump and fan systems). The examples below assume the old motor's nameplate reads 2850 rpm:

The first row looks small but matters on a burner: about 2% more air at the same damper opening means combustion with excess air; flue loss rises and flame stability may change. In the other direction, a slower motor reduces the air and brings a risk of incomplete combustion, soot and carbon monoxide. So even a small speed difference between two motors with the same number of poles should be corrected by a service adjustment. The second row is a plain mistake: with a 4-pole motor the fan cannot produce the pressure the combustion head needs. Replacing the 60 Hz motor of an imported burner with the same pole count on 50 Hz mains brings the loss in the third row; in that case the fan selection should be reviewed with the burner manufacturer.

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How do you find the motor power for a combustion air fan?

A burner fan motor cannot be tied to one fixed power. The factors are: the airflow set by burner capacity, combustion head and boiler back pressure, impeller diameter and blade type, fan speed, intake air temperature and density, and any coupling or belt loss. The basic relation:

P_shaft = Q · Δp / η_fan (Q in m³/s, Δp in Pa, P in W); motor power = P_shaft / η_drive + margin.

Example assumption: airflow 1700 m³/h, fan pressure 3500 Pa, fan efficiency taken as 0.60, impeller directly on the shaft (no drive loss).

• Q = 1700 / 3600 ≈ 0.472 m³/s
• P_shaft = 0.472 × 3500 / 0.60 ≈ 2.75 kW
• 15% margin for cold air, filter fouling and adjustment: 2.75 × 1.15 ≈ 3.17 kW → 4 kW

A 3 kW motor would run at about 92% load here and could trip in winter when the air is denser. You can run the calculation with your own values in the pump and fan motor power calculator.

Market examples show how wide the range is. In one monoblock burner catalogue, fan motors are listed from 0.75 kW to 45 kW, 3-phase 400 V 50 Hz, with a nominal speed of 2900 rpm [1]. Another manufacturer's industrial burner catalogue lists fan motors from 450 W up to 55 kW at 2800 rpm; the smallest models are 230 V single-phase [2]. The exact value is taken from the burner label and the manufacturer's technical data.

Information to collect before changing the motor

Before removing the failed motor, write down the following:

• Old motor nameplate: kW, rpm, voltage (230/400 V or 400/690 V), current, frequency.
• Burner label: type, capacity range, power stated for the fan motor.
• Mounting: flange type (large B5 or small B14), flange outer diameter, bolt circle, spigot diameter.
• Shaft diameter and length (D×E), key width; how the impeller hub is fixed to the shaft.
• Terminal box position and cable entry direction; does the box still fit when the burner cover is closed.
• Starting method: direct, star-delta or inverter; overload setting in the burner control panel.
• Boiler room ambient temperature and whether the intake air is dusty.

The split of responsibility is clear. Fan selection, the fan curve, impeller and housing design, and the air/fuel setting are the job of the burner manufacturer and the authorised service. The motor supplier's job is to provide a motor that matches the given shaft power, speed, mounting and ambient conditions. If the nameplate is unreadable, you can work from measurements; compare shaft and flange dimensions with the values on the DRG motor technical drawings page.

Speed, poles and mounting: how does the new motor fit the burner?

Speed and pole selection

Burner fans must produce high pressure with a small impeller; the combustion head and boiler back pressure raise the pressure the fan has to deliver. Because the impeller diameter is kept small, the required tip speed can only be reached with high rpm; that is why fan motors in both catalogues are in the 2-pole class [1][2]. In some modern burners the fan is driven by an inverter: speed is reduced at part load and air is set together with the damper. One catalogue gives a maximum speed of 3520–4100 rpm for inverter-driven models [1], which means the motor runs above mains frequency. When replacing the motor on such a burner, the inverter's maximum frequency setting, the motor's mechanical speed limit and the impeller balance must be confirmed with the burner manufacturer's data.

Flange, shaft and connection

On a monoblock burner the motor is flange-mounted to the housing; the exact mounting type is taken from the burner manufacturer's drawing. A motor whose spigot and bolt circle do not match can shift the impeller off-axis even when fitted with an adapter; the impeller rubs on the housing or vibrates. Examples from the B5 flange dimensions in the DRG catalogue: on 90S/90L frames the flange outer diameter is 200 mm, bolt circle 165 mm, spigot 130 mm; on the 112M frame 250 / 215 / 180 mm. The shaft is 24×50 mm on 90L and 28×60 mm on 112M. B14 small flange dimensions are not in the catalogue; for this type, measure the old motor's flange and send the values before ordering.

The impeller sits on the shaft with a key and an end bolt. If the shaft length differs, the impeller moves from its place in the passage and the inlet gap changes. After connecting, run the motor briefly and check the direction of rotation: a centrifugal fan turning backwards still moves some air but does not hold pressure; the air pressure switch locks out the burner or combustion deteriorates. Direction is corrected by swapping two phases; this is work for a qualified electrician. Some small burners use motors built with non-standard shafts or flanges; a standard IEC motor will not fit in that case, and the spare should come from the burner manufacturer.

Matching with DRG motors

DRG Motor is a manufacturer, supplier and seller of electric motors. Store range: aluminium frame 0.25–11 kW, cast iron frame 0.55–355 kW; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V; mounting B3, B5, B14 and B35 with an order note. Small single-phase burner motors are outside this range. The 2-pole models most often needed on burner fans:

The 2.2 and 3 kW models are 230/400 V and are connected in star on a 400 V supply; 4 kW and above are 400/690 V, connected in delta on a 400 V supply and suitable for star-delta starting. For a wider range of fan motors, see the Fan Motors page.

DRG motors for burner fans

• DRG-90L-2 – 2.2 kW – 2895 rpm – 90L aluminium
• DRG-100L-2 – 3 kW – 2895 rpm – 100L aluminium
• DRG-112M-2 – 4 kW – 2905 rpm – 112M aluminium
• DRG-132S1-2 – 5.5 kW – 2930 rpm – 132S aluminium
• DRG-132S2-2 – 7.5 kW – 2930 rpm – 132S aluminium

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Burner fan motor: frequently asked questions

What speed should a burner fan motor have?

It should be in the same pole class as the speed on the old motor's nameplate; most burner fans run with a 2-pole, 3000 rpm class motor. Even within the same class a small speed difference changes the airflow, so the combustion setting should be checked after the change.

Can a more powerful motor replace the burner motor?

One size up can be fitted provided speed and mounting dimensions stay the same; the motor only draws the power the fan demands. However, a larger rating often means a bigger frame and a different flange, so first check that the flange, shaft and terminal box fit under the burner cover.

How many amps does a 4 kW burner fan motor draw?

The DRG-112M-2 draws 7.45 A at 400 V and 7.84 A at 380 V. Starting current is 8.3 times rated (about 62 A at 400 V); take this into account when choosing fuses and contactors.

Does the burner need adjusting after a motor change?

Yes. If the new motor's speed differs from the old one by even a few rpm, the air delivered by the fan changes. An authorised burner service should measure the flue gas, reset the air setting and test the safety chain (air pressure switch, flame supervision).

Send us your burner label and the old motor's nameplate and we will pin down the right DRG model together: phone +90 (542) 666 11 11, e-mail [email protected], contact page. DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate). For all models and current prices: Buy Electric Motors Online.

Sources

1. Oilon – Monoblock burners: Oil, gas and dual fuel – fan motor ratings, 3~400 V 50 Hz, 2900 rpm; maximum speed on inverter-driven models
2. ELCO – Industry Catalogue (burners) – fan motor ratings 450 W–55 kW, 2800 rpm
3. ELCO – EK-DUO / RPD duoblock burners – separate fan unit connected by an air duct