Forced hot water boilers are special purpose water heaters. While furnaces carry heat in a warm air system, boiler systems distribute the heat in hot water, which gives out heat as it passes through radiators or other devices in rooms throughout your house. The cooled water then returns to the boiler to be reheated. Hot water systems are often called hydronic systems. Residential boilers generally use natural gas or heating oil for fuel.

Instead of a fan and duct system, a boiler uses a pump to circulate hot water through pipes to radiators. Some hot water systems circulate water through plastic tubing in the floor, a system called radiant floor heating. Important boiler controls include thermostats, aquastats, and valves that regulate circulation and water temperature.

Although it costs more, it is generally much easier to install “zone” thermostats and controls for individual rooms with a hydronic system than with forced air. Some controls are standard features in new boilers, while others can be added on to save energy.

As with furnaces, condensing gas-fired boilers are relatively common, and significantly more efficient than non-condensing boilers.

Every boiler unit comes with an AFUE (annual fuel utilization efficiency) rating. The higher the rating, the more efficient a boiler will be. The models that are the most energy efficient tend to have the highest upfront cost attached. Models with a rating of 90 or higher will reduce home heating costs by 30 percent or more overnight when replacing an older unit. High efficiency units have an AFUE of 90 percent or higher. These are increasingly becoming available only as condensing boilers. They trap much of the heat waste and use it to preheat the water, which ensures that the system runs on less energy. A condensing model often loses only about 2 to 3 percent of its heat when all is said and done. However, the trapped heat doesn’t count towards the AFUE.

With good maintenance and care, boilers can last a very long time. There are gas units still working after 40 years. The typical boiler has a life expectancy of 15 years. Has yours been working fine for 20 years or more? As long as it’s still functioning at comfortable temperatures, you may not need to replace it.

A yearly inspection is recommended by most professionals in the industry, and that’s not just to have repeat business. A crack in the boiler’s heat exchanger can allow poisonous gases to leak into your home. If you haven’t had it inspected, you should do so immediately. If multiple issues are found during an inspection, you should think about replacing your system.