Boiler Short Cycling Energy Cost Calculator

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Short cycling: A boiler starting and stopping more often than needed during a heat call. Some control cycling can be normal.

Cycling loss: The estimated percent of fuel input lost because of cycling. It is a planning assumption, not AFUE or a combustion-test result.

Variable fuel rate: The fuel cost per therm, gallon, or MMBtu that changes with fuel use. Fixed customer charges are not included.

Therm: A unit used on many natural-gas bills. This calculator keeps fuel quantities in the unit selected for the boiler fuel.

MMBtu: One million BTU, an energy quantity often used on commercial fuel records. It is not a fuel-use rate such as BTU per hour.

Mistake: Entering total gas use when the meter also serves a water heater, dryer, or stove in Annual heating fuel use.
Fix: Estimate and enter the boiler heating portion when possible.

Mistake: Including a flat customer charge in Variable fuel rate.
Fix: Use only charges that change when fuel use changes, such as commodity and usage-based delivery charges.

Mistake: Treating Current cycling loss as the boiler's AFUE rating.
Fix: Enter only the estimated share of fuel input lost from cycling, using a technician estimate or a clearly marked planning assumption.

Mistake: Setting Improved cycling loss higher than Current cycling loss while expecting savings.
Fix: Lower Improved cycling loss or raise Current cycling loss so the after-improvement loss is no greater than the current loss.

Mistake: Using a propane price in Variable fuel rate after choosing natural gas for Boiler fuel.
Fix: Match both Annual heating fuel use and Variable fuel rate to the selected fuel unit.

• This is a planning estimate based on the entered cycling-loss percentages. It does not measure combustion efficiency, AFUE, burner condition, or actual cycle behavior.
• The calculation holds the estimated useful annual fuel requirement constant before and after the improvement. Weather, thermostat settings, building use, and hot-water demand can change actual fuel use.
• Only usage-dependent fuel charges are included. Fixed utility charges, taxes that do not change with use, diagnostic fees, repairs, maintenance, and replacement costs are excluded.
• Annual heating fuel use can overstate the boiler heating load when the same fuel also serves water heating, cooking, or other equipment.
• The estimate does not diagnose safety, venting, combustion, controls, high-limit faults, or equipment sizing. Use qualified service personnel for those issues.

Calculation method

The calculator starts with your current annual fuel use and treats the entered current cycling loss as the share of that fuel input lost from cycling. It first estimates the fuel requirement without the current cycling loss, then applies the improved loss assumption to that same requirement. DOE guidance identifies minimizing boiler short-cycling losses as an operating-efficiency practice. [1]

current_loss_fraction = Current cycling loss / 100

improved_loss_fraction = Improved cycling loss / 100

useful_fuel_requirement = Annual heating fuel use * (1 - current_loss_fraction)

projected_annual_fuel_use = useful_fuel_requirement / (1 - improved_loss_fraction)

annual_fuel_saved = Annual heating fuel use - projected_annual_fuel_use

annual_fuel_cost_savings = annual_fuel_saved * Variable fuel rate

current_extra_fuel = Annual heating fuel use * current_loss_fraction

current_extra_cost = current_extra_fuel * Variable fuel rate

Worked example

With 1,000 therms per year, a $1.50 per therm variable rate, a 10% current cycling loss, and a 3% improved cycling loss, the useful fuel requirement is 900 therms. The projected use is 927.84 therms, so estimated fuel saved is 72.16 therms per year and estimated annual fuel-cost savings is $108.25.

Calculation choices

Current annual variable fuel cost equals annual heating fuel use multiplied by the variable fuel rate. Projected annual variable fuel cost equals projected annual fuel use multiplied by that same rate. Current extra fuel and current extra fuel cost compare the entered current cycling loss with a zero-cycling-loss reference. The calculator blocks an improved cycling loss greater than the current cycling loss because that would describe a worse, rather than improved, condition.