Two boiler quotations can show different efficiency percentages without describing different real-world performance. Before ranking them, check the fuel heating-value basis, the test conditions and the equipment included in the calculation. Keep these details beside the percentage in your comparison sheet.
What do net and gross mean?
Lower heating value (LHV, or net calorific value) excludes the latent heat associated with water vapour formed during combustion. Higher heating value (HHV, or gross calorific value) includes it. For the same useful output and fuel quantity, an LHV-based efficiency is numerically higher because its energy-input denominator is smaller.
A condensing arrangement can recover some of that heat, so an LHV-based figure may exceed 100% without creating energy. That does not mean every boiler or operating condition can deliver that result. The heating-value basis and actual heat-recovery conditions must accompany the number.
A worked comparison with explicit assumptions
Assume, only for illustration, a fuel with LHV of 36 MJ per unit and HHV of 40 MJ per unit. If a quoted operating point delivers 34.2 MJ of useful heat for one unit of fuel, its efficiency is 34.2 / 36 = 95% on LHV, or 34.2 / 40 = 85.5% on HHV. Those two percentages describe the same output and input quantity. These are hypothetical round numbers, not the properties of your gas or a product guarantee.
Ask for the fuel analysis or agreed heating value used in your quotation. Do not apply the illustrative conversion factor to another fuel. Also check whether reported gas volumes use the same reference conditions before comparing consumption figures.
Identify the measurement boundary
For steam, useful heat depends on steam flow and conditions and on the feedwater entering the system. For a hot water system, the water flow and temperature change are relevant. Ask whether the quoted performance includes an economizer or condenser and whether auxiliaries, blowdown and distribution losses are inside or outside the boundary.
A combustion result, a boiler thermal-efficiency test and an annual plant energy bill answer different questions. Request the exact definition rather than treating them as interchangeable. The Spirax Sarco boiler efficiency reference explains heating values and the heat balance in more detail.
What to request from each supplier
- Efficiency basis: LHV or HHV, fuel specification and reference conditions.
- Useful output, operating pressure and temperatures at the quoted point.
- Load percentage and additional points representing your normal operation.
- Included heat-recovery equipment and stated exclusions.
- Test method, measurement boundary and agreed acceptance procedure.
- Auxiliary electricity, standby assumptions and water-treatment costs shown separately.
The decision should reflect your operating schedule and total supplied package. Use the steam boiler quotation checklist to request comparable offers, or review hot water boiler selection when water temperatures drive the application. Contact us with the duty and fuel data to clarify the performance basis for a proposed model.
