You’ll generally see the most variety in natural gas boilers, since these are the most popular models. They may fall into one or more of the following categories.
Conventional, System, and Combination Boilers
Conventional or standard boilers are large and time-consuming to install, but they’re the best type for larger homes. Standard boilers have a boiler unit and two tanks: one for hot water and one for cold water yet to be heated. On the other hand, system boilers have only a boiler unit and one hot water storage tank. They represent a good balance between efficiency and performance for average- to large-sized homes.
Combination or combi boilers have no tank and only heat water as you need it. They combine the functions of a boiler and a water heater, though the boiler water and domestic water flow through separate pipes. They’re small and cost-effective, but they can struggle to provide hot water to more than one faucet or appliance at once. They’re often used in small houses or apartments.
Hot Water vs. Steam Boilers
Boilers can be further divided into steam and hot water boilers. The latter heats water up to temperatures between 140 degrees Fahrenheit and 180 degrees Fahrenheit, then pumps it through the home’s radiators, baseboard units, or radiant heat systems. Hot water boilers offer greater energy efficiency and control over temperature. In contrast, a steam boiler actually takes water past the boiling point and sends the steam through the home heating system without needing a pump. These produce more heat but take more energy to run.
Condensing vs. Non-Condensing Boilers
Finally, boilers can be condensing or non-condensing. Traditional boilers are non-condensing, which means they waste a bit of heat by venting exhaust fumes. Condensing boilers capture this exhaust and use it for extra heating, making this type of boiler far more efficient and expensive.
Standard vs. High-Efficiency Boilers
Heating efficiency is measured by an annual fuel utilization efficiency (AFUE) rating, which is the percent of fuel energy that gets turned into usable heat. New boilers must have an efficiency rating of 80% or above, but older homes may still have boilers with 55%–75% efficiency. Boilers marked as high-efficiency have AFUE ratings of 90% or higher.
Electric boilers can have nearly 100% efficiency, and the best gas and propane models can reach 98.5%. Oil boilers top out around 95%, and wood boilers generally reach 90% efficiency. The most efficient boilers are usually condensing combi boilers. High-efficiency models tend to cost a bit more up-front, though you’ll save more in heating costs over time.
Sealed vs. Non-Sealed Boilers
Older boilers may have non-sealed systems. This means they take air from indoors and vent exhaust through your home’s flue—a duct for smoke and waste gases—which requires a special chimney liner. This can be dangerous since there’s a chance exhaust gases like carbon monoxide may accidentally leak indoors. They’re also less efficient since they take in already heated air and waste heat in the form of exhaust. It’s safer and more economical to choose a sealed boiler, which takes in air from outside and re-uses exhaust heat.