How to read a boiler efficiency rating and what it actually means for your facility.

The three efficiency ratings you will see

Most commercial boiler specifications reference one or more of three efficiency measurements. They are not interchangeable, and a boiler that looks more efficient than a competitor on one metric may look different when the same metric is used for both.

Combustion efficiency measures how completely the boiler is burning its fuel. It is calculated from the flue gas temperature and the oxygen content of the exhaust, and it reflects how much of the fuel’s energy content is being released during combustion versus lost up the stack as unburned fuel or excess heat. A combustion efficiency of 85% means 15% of the fuel’s energy is leaving the building through the flue. This number is relatively easy to measure and is often the figure used in field tune-up reports, but it does not account for other heat losses in the boiler system.

Thermal efficiency is a broader measurement that accounts for additional heat losses beyond the flue, including radiation and convection losses from the boiler jacket. It is the ratio of heat actually transferred to the water or steam versus the total heat input from the fuel. Thermal efficiency is lower than combustion efficiency for the same boiler because it captures more of the ways heat can escape without doing useful work. AHRI Standard BTS-2000 governs thermal efficiency testing for commercial boilers, and an AHRI-certified thermal efficiency rating is the most apples-to-apples comparison available when evaluating competing products.

Fuel-to-steam or fuel-to-water efficiency, sometimes called overall efficiency or seasonal efficiency, attempts to measure the boiler’s real-world performance across a range of operating conditions rather than at a single design point. This is the number that most closely reflects what a facility will actually see on its fuel bill over a heating season, but it is also the hardest to standardize because it depends on the specific load profile, the system design, and the operating conditions at the facility. Two boilers with identical AHRI thermal efficiency ratings may perform differently in terms of seasonal fuel consumption depending on how their modulation characteristics match the building’s load curve.

Why the test conditions matter

An efficiency rating is only meaningful in the context of the conditions under which it was measured. A condensing boiler rated at 94% AHRI thermal efficiency achieves that rating at a specific entering water temperature that allows the boiler to condense water vapor from the flue gases and recover the latent heat. If the same boiler is installed in a system where the return water temperature is consistently high, the boiler may not condense effectively and the actual operating efficiency will be lower than the nameplate rating suggests.

This is not a flaw in the rating system. It is a reminder that the efficiency a facility actually achieves depends on how well the boiler’s operating requirements match the system it is installed in. A condensing boiler in a low-temperature radiant system will perform closer to its rated efficiency than the same boiler in a high-temperature baseboard system. The rated efficiency tells you what the boiler can achieve. The system design determines whether it actually achieves it.

The same principle applies to turndown ratio and part-load efficiency. A boiler with a 10:1 turndown ratio can modulate from 100% to 10% of its rated input, maintaining efficiency across a wide range of load conditions. A boiler with a 4:1 turndown ratio cycles on and off more frequently at part load, and each startup and shutdown cycle involves losses that do not show up in the single-point efficiency rating. For a building that spends most of its heating hours at 30% to 50% of design load, which describes most commercial buildings for most of the heating season, the part-load efficiency behavior matters more than the peak efficiency rating.

What condensing actually means

Condensing efficiency is worth understanding specifically because it is the efficiency claim that appears most frequently in commercial boiler marketing and is most often misunderstood.

Natural gas combustion produces water vapor as a byproduct. In a conventional boiler, that water vapor exits through the flue as a gas, carrying latent heat with it that is lost to the atmosphere. A condensing boiler recovers that latent heat by cooling the flue gases enough that the water vapor condenses back into liquid. The condensation releases the latent heat, which is transferred to the water in the system instead of escaping up the stack.

The efficiency gain from condensation is real and significant. The difference between a conventional boiler operating at 82% efficiency and a condensing boiler operating at 94% efficiency represents roughly 12 cents of every dollar spent on fuel. Over the course of a heating season in a large commercial facility that distinction adds up to a meaningful number.
The condensation only occurs when the flue gas temperature drops below the dew point, which requires the returning water temperature to be low enough to cool the flue gases to that threshold. For natural gas, that threshold is approximately 130 degrees Fahrenheit return water temperature. Above that temperature the boiler operates in non-condensing mode and the efficiency advantage narrows. System design that keeps return water temperatures low is what allows a condensing boiler to deliver on its rated efficiency over time.

How to use efficiency ratings in a specification decision

When evaluating competing boilers, the most useful comparison is AHRI-certified thermal efficiency at comparable test conditions. If one manufacturer’s rating is AHRI-certified and another’s is not, the comparison is not reliable. If both ratings are AHRI-certified, compare them at the same entering water temperature to ensure the condensing boiler advantage is being measured under conditions where condensation actually occurs.

Beyond the peak efficiency number, ask about the turndown ratio and the part-load efficiency characteristics. A boiler that achieves 94% at full load but has limited turndown will spend significant time cycling in a typical commercial building. A boiler that achieves 92% at full load but maintains near that efficiency across a 10:1 turndown range may deliver better seasonal fuel performance in a building with variable load.

Finally, confirm that the system design supports the boiler’s efficiency requirements. A condensing boiler in a system that cannot achieve low return water temperatures is a condensing boiler that is not condensing. The efficiency you pay for on the specification sheet needs to be achievable in the actual operating conditions of the facility.

GP Energy Products works with engineers and facility managers across Pennsylvania, New Jersey, Delaware, and Maryland on commercial and industrial boiler selection. When a boiler replacement project comes to us, efficiency ratings are part of a broader system conversation that includes the distribution system design, the load profile, and the operating conditions that will determine what the boiler actually delivers. Reach out before the specification is finalized and we will help make sure the efficiency rating you are comparing means what you think it means.

References
1. AHRI Standard BTS-2000. Commercial Boilers. Governs thermal efficiency testing and certification for commercial boilers. ahrinet.org
2. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Boilers. Covers combustion efficiency, thermal efficiency, and part-load performance for commercial boilers. ashrae.org
3. American Boiler Manufacturers Association. Boiler Efficiency Guide. Covers efficiency measurement methods and their application to commercial boiler selection. abma.memberclicks.net
4. U.S. Department of Energy. Improving Steam System Performance. Covers boiler efficiency in steam applications including fuel-to-steam efficiency measurement. energy.gov