Boiler Energy Savings Calculator

How it works

Your house needs a set amount of heat each winter, measured in British Thermal Units (BTU). Weather, your thermostat, and how leaky the house is decide that number, so your home heating cost follows from it. The boiler does not set it.

What the boiler decides is how much fuel it takes to deliver those BTUs. Thermal efficiency, rated as AFUE, is the share of the energy in each unit of fuel that warms your rooms rather than the flue. So the saving is a ratio between the two ratings, not the gap between them:

• C. Your current annual heating fuel cost.
• AFUE_old. The rating of the unit you have now.
• AFUE_new. The rating of the replacement you are pricing.

That rating covers a whole season of the burner cycling on and off, not one perfect firing. It does not count heat lost from pipes crossing a cold basement, so the saving you actually see can land a little under the number on the label.

Almost everyone subtracts instead. Going from 72% to 95% is a gap of 23 percentage points, so it looks like a 23% saving. It is not, because the saving is measured against your old bill: 23 divided by 95, or 24.2%.

That same 24.2% is also the cut in CO2, since that gas comes straight out of the fuel you no longer burn.

Those two agree only when the new boiler is 100% efficient, and nothing that burns fuel is. On the defaults, that turns the $1,800 fuel bill into $1,364, a saving of $436 a year.

The $7,500 install then takes about 17.2 years to earn back. Cutting a quarter off the bill and still waiting 17 years is what most quotes never show side by side.

✓ Every result is checked against independent reference math. See how we test the calculators →

How to use this calculator

1. Enter your current annual heating fuel cost, taken from a full year of bills rather than one cold month.
2. Enter the AFUE of the unit you have now, read off its energy guide label or nameplate.
3. Enter the AFUE of the replacement you are quoting, plus the installed price for it.
4. Read the fuel savings per year, then the payback in years underneath it.
5. Check the CO2 emissions line for the metric tons avoided, then run a second quote and compare.

A worked example: 72% to 95% AFUE on an $1,800 fuel bill

Say your boiler is an aging unit rated 72% AFUE, you burn $1,800 of gas a year heating the house, and a plumber quotes $7,500 to fit a 95% condensing replacement. The calculator puts the fuel saving at $436 a year.

Notice that is not 23%. The two ratings differ by 23 points, but your bill falls to 72 ÷ 95 of what it is now, which means you save 24.2% of it and $1,800 becomes $1,364. Against a $7,500 install, $436 a year takes about 17.2 years to come back.

That is most of a boiler’s working life, which is the honest reason not to tear out a functioning unit for the fuel saving alone.

The bill is what moves this, not the ratings. Both AFUE numbers are fixed facts that will not change after you buy, but the fuel cost is a market price multiplied by a winter. Leave the boilers alone and raise the bill to $2,600, a colder year or dearer gas, and the same upgrade saves $629 a year and pays back in about 11.9 years instead.

Run it with your coldest recent year and your mildest, and read the payback as that range rather than as a date.

How much CO2 does a new boiler avoid?

Cutting fuel use cuts CO2 emissions by the same share. Burn 24.2% less natural gas and you release 24.2% less CO2, because those emissions come out of the combustion itself.

Turning that share into metric tons depends on what you burn. Per BTU of heat delivered, natural gas puts out the least CO2, fuel oil more, and coal the most.

The U.S. Energy Information Administration publishes a CO2 emission factor for each fuel, measured per unit burned. That factor is what converts a percentage into metric tons.

So the same percentage cut is worth more tons in an oil heated house than a gas one, and more again where coal is burned. Coal heat is rare in homes across the United States now, but the emissions arithmetic does not change.

Even a gas fired unit carries some CO2 on the electricity side, since the pump and controls draw from a grid that still burns coal. That electricity cost is small next to the heating bill, and the emissions behind it are smaller still.

What is a metric ton of CO2 in gallons of gasoline?

A metric ton of CO2 is hard to picture. Nobody has ever weighed a gas, so the figure reads as an abstraction rather than a result you can judge.

That is why some tools restate the CO2 as gallons of gasoline. A gallon is a unit people buy every week, and gasoline burns like any other petroleum product, so the comparison is fair rather than decorative.

The swap uses the same kind of emission factor. Divide the metric tons you avoid by the CO2 released per gallon, and a gallon count falls out.

The U.S. Energy Information Administration lists that factor for motor gasoline, alongside the ones for the fuels burned at home.

Treat the result as a picture, not a trade. Burning less heating oil does not buy free miles, and the emissions from a flue and from a tailpipe are counted in separate places.

How British Thermal Units turn into a fuel bill

Fuel is not billed in British Thermal Units, so the bill does not show that number directly. Gas is sold by the therm or the hundred cubic feet, oil by the gallon, and each of those units holds a fixed amount of heat.

That heat content is what turns a bill back into BTU. A therm is 100,000 British Thermal Units by definition, and the EIA publishes the same figure for a gallon of fuel oil and for a cubic foot of natural gas.

AFUE sits between the BTU you buy and the BTU that reach your rooms. Buy 100,000 British Thermal Units of gas at 72% AFUE and 72,000 warm the house, while the other 28,000 leave through the flue.

That flue loss is why the emission factor and the AFUE rating have to be read together. Emissions follow the BTU you buy, not the BTU you get, so a higher rating cuts the fuel purchased and the CO2 in one step.

Above 90 percent, condensing changes the price

To get past roughly 90% AFUE, a condensing boiler pulls so much heat from its own exhaust that the water vapor turns back to liquid. That recovered energy is the extra thermal efficiency.

The exhaust leaves cool, so it will not rise up a masonry chimney. That means new plastic venting through a wall, plus acidic condensate that has to reach a drain.

Those two jobs are a day of labor and materials a like for like swap avoids, which is why the price steps up rather than sliding up. Ask for the quotes separately, so the cost of the higher rating shows on its own line.

Sizing matters as much as the rating. A load calculation built on ASHRAE design temperatures beats copying the old output, because an oversized replacement short cycles and hands back part of the thermal efficiency you paid for.

What does the payback period leave out?

Payback treats today's fuel cost as fixed. That cost moves every year, and a dear winter shortens the wait while a mild, cheap one stretches it.

Several real items also sit outside the arithmetic:

• Rebates and tax credits. A utility or state program can cut the installed cost before you divide it by the annual saving.
• Repair bills on the old unit. Money you were going to spend anyway is not a cost of upgrading.
• Electricity for the pump and controls. A sealed combustion unit can draw slightly more of it, not less.
• Summer hot water. A combi puts domestic hot water on the same fuel, so part of the saving arrives in July.
• The rest of the house. Insulation and air sealing lower the BTU the burner has to deliver at all.
• Comfort and resale value. Neither converts into dollars in a way worth defending.

A smaller change is often worth running first. A better schedule delivers thermostat savings for almost nothing, and it does not stop you replacing the burner later.