Full Output Breakdown
| Parameter | Value | Notes |
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🏭 Same Output — Fuel Comparison
How much of each fuel would deliver the same heat output at the same AFUE, at typical Canadian energy prices.
| Fuel | Input Needed | Unit Cost | Cost / Hour |
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How to Use the Boiler Output Calculator
Choose the fuel your boiler burns. Natural gas is the most common in Canadian cities. Propane is typical for rural properties. No. 2 fuel oil is still common in Atlantic Canada and Quebec. The fuel type sets the higher heating value used in the background energy calculation.
Pick the AFUE that matches your boiler's CSA certification rating. ENERGY STAR gas boilers in Canada require at least 90% AFUE. Condensing boilers typically range from 90–98%. Use the custom option if you know the exact AFUE from the boiler's product data sheet.
Find the input rating on the boiler nameplate — not the output. In Canada, CSA-certified boilers list both. Enter in kW for metric or BTU/h for imperial. If you only have the output rating, use the boiler efficiency calculator to back-calculate the input.
At full fire, the PLF is 1.0. If you want to know the output when a modulating boiler is firing at 30% of rated input, enter 0.30. This is useful for checking whether a condensing boiler can maintain condensing mode at part-load return temperatures.
The results show net output delivered to the system and the flue losses that escape up the stack. The fuel comparison table shows what the same output would cost per hour using alternative fuels at current Canadian prices, which helps evaluate fuel-switching decisions.
Understanding Boiler Output, AFUE, and Flue Losses
Every boiler nameplate in Canada lists two numbers: the input rate and the output rate. Most homeowners and even some contractors focus only on one, which leads to sizing mistakes and efficiency miscalculations. This calculator makes both numbers visible and shows exactly where the energy goes.
Input vs. Output: Where the Energy Goes
A boiler's input rating is how fast it burns fuel. A natural gas boiler with a 30 kW input consumes 30 kW worth of gas when firing. Not all of that heat ends up in your hydronic system. Some exits through the flue as sensible heat in hot exhaust gases, and in non-condensing boilers, a significant amount leaves as latent heat in water vapour. The output rating is what remains after those losses — the heat that actually gets into the water.
What AFUE Actually Measures
AFUE, the Annual Fuel Utilization Efficiency, accounts for flue losses plus standby losses over a full heating season under standardised test conditions. An 80% AFUE boiler delivers 80 cents of heat for every dollar of fuel consumed. The 20% that's lost goes up the flue and through jacket losses when the boiler is in standby. The ENERGY STAR threshold for gas boilers in Canada is 90% AFUE, and the best condensing units now reach 98%. Use our boiler efficiency calculator to compare AFUE across units you're considering.
Condensing Boilers and Return Water Temperature
A condensing boiler earns its extra efficiency only when the return water temperature is low enough for flue gases to condense. The condensation point for natural gas combustion products is around 55°C. When return water is colder than this, the boiler recovers the latent heat from water vapour in the exhaust, adding 10–15% extra output beyond what a standard boiler would get from the same fuel input. When return temperatures climb above 55°C — due to oversizing, high thermostat setpoints, or wrong hydronic design — condensing stops and the boiler performs like an 80–85% unit despite the higher purchase price.
Part-Load Operation and Modulation
Boilers rarely fire at full rate. On a mild Canadian autumn day, a properly sized boiler might fire at 30–40% of its rated input. A fixed-output boiler short-cycles at this load: it fires briefly, satisfies the thermostat, shuts off, and restarts minutes later. Each startup wastes energy. A modulating boiler throttles down to match the actual heat demand, running continuously at low output and maintaining condensing-mode return temperatures consistently. The mod-con boiler calculator analyses part-load efficiency for specific modulating units.
Fuel Comparison at Canadian Energy Prices
The cost to deliver a given amount of heat varies significantly by fuel type. Natural gas is typically the lowest-cost option in Canadian cities with pipeline access, at roughly $0.04–0.07 per kWh of heat delivered at 90% AFUE. Propane runs two to three times higher per unit of heat in most regions. No. 2 fuel oil sits between the two but carries more price volatility and carbon cost. The fuel comparison table in this calculator uses representative Canadian retail prices to put all fuels on a level playing field. Compare to your actual utility bills for the most accurate local comparison.
Boiler Output and Heat Loss Matching
Once you know the actual output your boiler delivers, compare it to your building's design heat loss. If the output at full fire is within 15–25% of the heat loss, you're in the right range. If it's more than 40% above the heat loss, the boiler is oversized. Use the boiler sizing calculator to verify your selection, and the heat load calculator if you don't have a confirmed heat loss figure yet.
Frequently Asked Questions
Input is the rate at which the boiler consumes fuel energy. Output is the heat actually delivered to the hydronic system after combustion and flue losses. The ratio is the AFUE. A boiler rated at 100,000 BTU/h input with 95% AFUE delivers 95,000 BTU/h output to the water. CSA certification in Canada requires both figures on the nameplate. Always compare your building heat loss to the output rating when selecting or verifying a boiler. If you only have the output, use our boiler efficiency calculator to work backwards to the input.
AFUE is measured under standardised test conditions, not at your specific outdoor temperature. In Canadian winters, a condensing boiler's actual efficiency depends heavily on return water temperature. When the return is below about 55°C, the boiler condenses flue gases and recovers latent heat, pushing real-world efficiency above the nameplate AFUE. When return temperatures are high, condensing stops and efficiency drops toward mid-80%. Pairing a condensing boiler with outdoor reset control keeps return temperatures low through most of the heating season.