The most common mistake in burner selection is looking only at the boiler's kW rating and picking an "equivalent" burner. The right selection requires assessing capacity together with combustion chamber back pressure — a choice made without both ends up either failing to reach rated output or over-stressing the combustion chamber.
What is back pressure, and why is it decisive?
Back pressure is the resistance a burner has to overcome to push the fuel–air mixture into the combustion chamber. That resistance comes from the combustion chamber's geometry, the number and length of the flue passes, and the boiler type. Two boilers with the identical kW rating can have completely different back pressure values.
A burner that is a good match on capacity (kW) alone will never reach its rated output if it isn't powerful enough to overcome the back pressure — no matter how strong the fan is, back pressure is the limiting factor.
How to read the operating diagram
Burner manufacturers publish a capacity/back pressure operating diagram for every model. It has two axes:
- Horizontal axis: burner capacity (kW)
- Vertical axis: back pressure (mbar)
The diagram shows a shaded operating envelope. The correct selection is one where your boiler's actual capacity and back pressure values fall inside that envelope. If the values fall outside it, that burner is not suitable for that boiler — even if the capacity alone looks sufficient.
Factors to weigh in a capacity calculation
| Factor | Why it matters |
|---|---|
| Fuel type and supply pressure | On gas burners, network pressure determines gas train and regulator sizing; insufficient pressure means capacity is never reached |
| Back pressure | As above, often the real limit on reaching rated output |
| Modulation requirement | Where load varies widely, a modulating burner reduces on/off cycling and saves fuel — see burner types |
| Emission class | Class 2, Class 3 or Low-NOx, depending on the legislation applying to the site |
| Altitude correction | Air density falls with altitude; burner capacity must be re-calculated using the manufacturer's correction factor |
| Sound level | Residential and hotel plant rooms may require a sound attenuation hood |
| Service and spare part availability | Directly affects downtime cost when a fault occurs |
What goes wrong with an incorrect selection
- Failure to reach capacity: heating stays inadequate, the system is forced to run at full output constantly.
- Excessive noise and vibration: the burner works harder than it should to overcome back pressure.
- Sooting and a darkening flame: the fuel–air ratio can't be balanced on a poorly matched burner — see why is my burner sooting.
- Premature wear: components under constant strain — electrode, nozzle, fan motor — wear out earlier than expected.
Why a site survey can't be skipped
A boiler's back pressure often can't be read directly off a nameplate or brochure; in most cases it requires either a site measurement or the boiler manufacturer's technical documentation. A selection made by phone or from a photo alone, on the logic of "another brand's burner at the same kW", is no substitute for a site survey.
Frequently Asked Questions
What is the most common mistake in burner selection?
Looking only at the boiler's kW rating and choosing an "equivalent" burner. A selection made without accounting for the boiler's back pressure can either fail to reach rated output or over-stress the combustion chamber.
What is combustion chamber back pressure?
It is the resistance a burner has to overcome to push the fuel–air mixture into the combustion chamber, determined by chamber geometry, flue pass configuration and boiler type.
Can the same burner be used on two boilers with the same kW rating?
Not always. Two boilers with identical kW ratings can have different back pressure values, so the same burner may not perform equally well on both.
Does altitude affect burner capacity?
Yes. Air density falls with altitude, which affects the amount of air available for combustion. At high-altitude sites, burner capacity must be re-calculated using the manufacturer's stated correction factor. ---
About Beray Enerji
Beray Enerji is an Istanbul-based burner service company operating across Türkiye, with primary coverage in the Marmara, Thrace and Aegean regions. We provide capacity calculation, back pressure assessment, burner selection and commissioning across all makes and models, backed by a technical team with 30 years of field experience. For an assessment of your plant room, contact us on +90 (000) 000 00 00 or at info@berayenerji.com.
Related services: Burner maintenance · Burner repair service · Contact
References
- EN 676 — Automatic forced draught burners for gaseous fuels
- EN 267 — Automatic forced draught burners for liquid fuels
- Burner manufacturer capacity/back pressure diagrams