Highest Efficiency Boilers (Gas Propane & Oil) [930 Units Studied]

The most efficient boiler fired by natural gas or propane is 97% AFUE meaning that it transfers 97% of the heat produced into the water being heated.

By contrast, a high efficiency oil boiler can be expected to be 90% efficient at most.

Most Efficient Boiler Study

We concluded a study of more than 900 of the most efficient boiler products currently available. The data is summarized in this table.

Boiler TypesHigh Efficiency Range
Natural Gas Boiler90% to 97%
Propane Boiler90% to 97%
Oil Boiler87% to 90%

What is a Condensing Boiler

A condensing boiler is one that extracts heat from the super-hot exhaust gasses before it escapes out the vent. This is often referred to as flue gas recirculation within the industry.

Common terms for the process are “reclaiming” and “recovering” the heat before it is lost. These are the highest efficiency boilers (gas, propane and oil).

The result of condensing technology is that more of the heat produced is used to heat the water. With less heat loss, efficiency can be significantly increased. That’s obvious.

Did you know that replacing a non-condensing boiler with a condensing system that utilizes an outdoor temperature sensor can reduce energy costs by up to 30%, according to the Energy Information Administration.

High Efficiency Gas Boiler Study – Natural Gas (NG)

High Efficiency Natural Gas Boiler

The Energy Information Agency’s 2020 study shows that 47% of all US homes heat water with natural gas, the same percentage as electricity. Those two energy sources, as a result, account for 94% of water heating.

Residential natural gas boilers that use condensing technology deliver efficiency ratings up to 97%, though most brands’ most energy efficient boilers are 92% to 96% AFUE models.

Where are natural gas boilers most common?

Everywhere natural gas is available. Almost all large metropolitan areas and big cities outside the Northeast have natural gas infrastructure.

The states with the highest percentage of natural gas boilers are Utah (80%), California (75%), Colorado (69%), New Mexico (69%), New Jersey (68%), Illinois (68%), Nevada (67%) and Michigan (64%).

Rural areas without gas lines rely on propane, mostly, or oil, mainly in the Northeast.

What are the high efficiency gas boiler brands?

Energy Kinetics has a clear focus on being extremely energy efficient. It tops the natural gas niche with a 97% efficient unit.

From there, here are major brands that offer efficiency levels of 95% or higher: AO Smith, Bryant, Biasi, Baxi, Bradford White, Bosch, Burnham, CAMUS, Crown, Dunkirk, HTP, IBC, Ideal, Kenmore, Lennox, Lochinvar, Navien, New Yorker, NTI, Peerless, PurePro, Rheem, Rinnai, Slant/Fin, Solaia, Triangle Tube, Utica Boilers, Velocity Boiler Works, Veissmann, Weil-McLain and Williamson-Thermoflo.

High Efficiency Propane Boiler Study – Liquid Propane (LP)

High Efficiency Propane Boiler

Just 3% of all homes in the US use propane to fuel their home heater system. Not surprisingly, states with the highest percentages of their population being rural have more propane users.

Where are propane boilers most common?

The states where more than 10% of homes use propane are Vermont, New Mexico, Delaware, Maine and New Hampshire.

What are the high efficiency propane boiler brands?

The brands that make natural gas boilers also make propane versions of the same boilers. They are listed in the section above on natural gas boiler brands. Bosch, Burnham, CAMUS, Slant/Fin and IBC are among the manufacturers that make the most high efficiency propane boiler models.

High Efficiency Oil Boiler Study

High Efficiency Oil Boiler

The US DOE Energy Star program lists nearly 200 residential oil boilers that meet its standards.

The minimum efficiency requirement is 87% AFUE, or annualized fuel utilization efficiency.

The most efficient oil boilers are 91% AFUE efficient. That’s quite good, considering older oil boilers had efficiency ratings below 75%. In fact, the US Department of Energy says that most older boilers have efficiency ratings of 56% to 70%.

Where are oil boilers most common?

In the Northeast – Maine (34% of homes), New Hampshire (22%), Vermont (22%), Connecticut (21%), Massachusetts (15%). And in Alaska (14%)

What are the high efficiency oil boiler brands?

Energy Kinetics (up to 91%), Peerless (90%), Crown Boiler (88%) and New Yorker (88%). Brands making 87% Energy Star certified energy efficient boilers are Burnham, Dunkirk, PurePro, Solaia, Force, Buderus, Pensotti, Rand & Reardon, Slant/Fin, Utica, Veissmann, Williamson-Thermoflo and Weil-McLain.

Combi Boilers

Combi Boiler

You will increase your energy efficiency by installing a combi boiler when adding a new boiler or replacing an old one. This is because you will get the benefits offered by two separate functions – space heating and water heating in one appliance. The result is a reduction in energy use and cost.

The market for combi boilers is growing rapidly, according to respected industry source Contractor.com. According to the report, 31% of boilers installed in North America are combi boilers.

What is a combi boiler?

A combi boiler is one that heats water for heating your home and for supplying your hot water needs.

Note – The water heated in the boiler isn’t pumped through your hot water supply pipes. Instead, a pipe runs through your hot water tankless, and water heated by the boiler runs through the pipe to heat your hot water, called domestic hot water.

About 42% of boiler models are combi boilers, according to Energy Star.

The highest efficiency combi boilers have an AFUE rating of 90% to 97%.

Top brands that make energy efficient combi boilers are those you’d expect: Bosch, Bradford White, Burnham, Crown, Dunkirk, HTP, Lochinvar, Peerless, Rinnai, Slant/Fin, Utica and Veissmann.

Energy Efficient Boiler Cost

Quality, energy efficient boilers are not cheap. Here are retail boiler costs and the installed cost of a high efficiency boiler when hiring a contractor for the work. Installed cost includes all supplies, hooking up the boiler and testing it. A new thermostat is also part of most installations.  

High Efficiency Boiler TypeRetail CostInstalled Cost
Natural Gas Boiler$1,800 to $6,800$3,700 to $10,600
Propane Boiler$1,950 to $7,350$3,850 to $10,800
Oil Boiler$2,150 to $6,150$4,000 to $9,600

Cost factors include boiler fuel type, as you can see, plus the size of the boiler from 40,000 BTU output to 299,000 BTU output, the upper limit for residential boilers. 

Pros and Cons of Energy Efficient Boilers

There are trade-offs for choosing one of the most efficient boiler models.


Higher efficiency: Condensing boilers are more than 90% efficient – up to 95% AFUE for most brands, though you will find quite a few 96% AFUE high efficiency gas boiler models. Standard-efficiency boilers have AFUE ratings in the low to mid 80s.

Low installation cost: Most energy efficient boilers can be vented through a side wall. That’s a cheaper way to install the vent system than going through the roof.


Higher cost: Many brands make a high efficiency gas boiler, for example, and a standard efficiency model that is nearly identical except that it lacks condensing technology. The cost of the energy efficient units are typically $250 to $400 more than the standard-efficiency units.

Potential corrosion issues: The condensate from a condensing boiler is highly acidic. It is drained away, often into a neutralizer. But the potential for corrosion remains, if the condensate contacts metals that are not corrosion resistant. As a result, the average durability of high efficiency boilers is lower.

Are the Most Efficient Boiler Systems Worth the Money?

Yes, in most homes, the upgrade is worth the cost.  For the extra $250 to $400, you will reduce energy use by up to 16% compared to a new standard efficiency model. In 2019, Energy Star calculated lifetime energy cost savings for energy efficient boilers to be between $780 and $1,404. 

HomeGuide provides an accurate operating cost for a boiler: $800 to $2,000 per year based on how heavily the system is used. So, 16% of that operating cost represents $128 to $320 per year. You will recover the extra cost of a high efficiency gas boiler or high efficiency propane boiler in two to five years in most homes.


Energy efficient boilers in use today are lasting an average of 20-30 years. US Boiler reports boilers lasting up to 50 years when properly maintained and with normal repairs.

To get the best durability from a high efficiency boiler, have it cleaned and tuned every 2-5 years based on use and the condition of the water coming into your home. Talk with a local boiler service contractor to see what the normal frequency of service is for your area.


Most units have a warranty of at least 5 years, and some models are covered for up to 10 years.

Some manufacturers boast longer warranties, up to Lifetime, but these warranties are usually heavily prorated, so that beyond 7-10 years depending on the warranty, your percentage of warranty coverage will drop significantly.  

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.
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