Boiler Energy Savings Calculator

How it works

Your house needs a certain amount of delivered heat each winter. The weather sets it, your thermostat sets it, and how leaky the house is sets it.

The boiler does not. What the boiler decides is how much fuel it takes to deliver that heat, and AFUE, the annual fuel utilization efficiency, is the share of each unit of fuel that ends up warming your rooms rather than going up the flue.

Because the heat you need is fixed and the fuel you buy is what flexes, the saving from a new boiler 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 boiler you have now
• AFUE_new — the rating of the boiler you are pricing

The subtraction trap is worth spelling out, because almost everyone falls into it. Going from 72% to 95% is a gap of 23 percentage points, so it is tempting to call it a 23% saving.

It is not. The saving is measured against your old bill, and it works out to 23 divided by 95, or 24.2%.

Those two only agree 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, and the $7,500 install takes about 17.2 years to earn it back.

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

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.

Why the saving is a ratio, not a subtraction

Walk it through in units and the ratio stops being a technicality. Picture the heat your house needs over a winter as a fixed quantity that has to arrive no matter what.

At 72% AFUE, every 100 units of fuel you buy delivers 72 units of it. To deliver that same heat at 95%, you need 72 ÷ 95 of the fuel you buy today, which is 75.8% of it.

Your bill lands at 75.8% of what it is now, and the 24.2% left over is the saving.

At these ratings the gap between the honest answer and the naive one is small enough that nobody notices, but it widens fast as the old rating drops. An upgrade from 60% to 80% is a 20 point gap and a 25% saving.

From 55% to 90% is a 35 point gap and a 38.9% saving. The rule underneath is that the saving is the point gap divided by the new rating, so the worse your current boiler, the more the real number beats the one you would get by subtracting.

Efficiency ratings simply do not add and subtract the way percentages feel like they should.

What AFUE covers, and what it misses

AFUE is the only honest way to compare two boilers, and that is worth defending. It comes from a standard test, so every manufacturer’s number means the same thing, and crucially it is a seasonal figure: it folds in the cycling, the standby losses, and the heat that drifts up the flue between burns.

A peak, steady-state efficiency figure always flatters a boiler, because no boiler in a real house runs flat out for a season. AFUE is the number on the yellow label for exactly this reason, and a salesperson quoting anything else is quoting the wrong thing.

What it does not do is promise you a bill. It says nothing about pipes running through an unheated basement, giving away heat before it reaches a radiator.

It says nothing about oversizing, and an oversized boiler short-cycles, which drags real performance below the label. Two boilers with identical AFUE can post different bills in different houses.

Treat it as a fair comparison between units, not a forecast for yours. Worth noting in your favor: a twenty-year-old boiler rarely still hits the rating it shipped with, so the real saving often runs a little above what this shows.

Above 90 percent, condensing changes the job

There is a hard threshold in the middle of that new-boiler slider, and it explains why quotes jump. To get past roughly 90% AFUE, a boiler has to pull so much heat out of its own exhaust that the water vapor in it condenses back into liquid.

That released heat is where the extra efficiency comes from. It is real, and it comes with baggage.

The exhaust leaves cool, so it will not rise up a masonry chimney the way hot flue gas does, which means new plastic venting out through a wall, and often abandoning or relining the chimney that served the old unit. The condensate itself is mildly acidic and has to reach a drain, sometimes via a neutralizer and a small pump if the drain is not conveniently placed.

None of that is exotic, but it is a day of work and materials that a like-for-like non-condensing swap simply does not involve, and it is a large part of why the installed price steps up rather than sliding up.

So price both. Put a good non-condensing quote and a condensing quote through this calculator separately.

The higher rating does not automatically win once the install difference is on the table.