You could replace almost every older appliance and gadget in your home with a more efficient model, and that would shrink your carbon footprint and utility bills. However, the question for most homeowners is payback time: How many years will it take to return the purchase price of your shiny new Energy Star refrigerator or lights?

A boiler may offer one of the shortest payback periods. There is no rule of thumb for replacing a boiler based on energy bill totals. Climates vary widely, and many other factors affect heating bills. You may have an efficient boiler and poor insulation, driving up energy costs. Having said that, the single most important number on a boiler might be the date. Here are the criteria to consider when looking to replace a boiler.

Read the Label

A boiler label or owners' manual should note the annual fuel utilization efficiency rating. Look for an AFUE rating above 85 percent - the minimum number that qualifies for the Energy Star label. A 15-year-old boiler is likely below 60% efficiency. It may also be dangerous. Newer boilers use sealed combustion methods that draw in oxygen from the outside air and improve the air quality in the boiler room.

Measure the Boiler

When assessing your old boiler or purchasing a new boiler, make sure it is the right size. An oversized boiler will waste energy, and it may be noisy. An undersized boiler will work too hard without adequately heating the home, and parts will wear out faster. There is no simple rule for assessing the appropriate boiler size for a given size of home. Hire an experienced HVAC contractor to review your floor plan, ceiling heights, heat loss and heat distribution system to calculate the appropriate size boiler for you.

Electric Efficiency

In addition, watch out for a gas pilot light, which is inefficient and should be replaced. Electric ignition systems save energy by not leaving a pilot light burning all the time. They are standard on newer boilers and required on Energy Star-approved models.

Check the Thermostat

Also check for an outdated thermostat connected to your boiler. Look for an aquastat, which controls the water temperature of the boiler instead of just responding to the ambient temperature in the home. A modulating aquastat or outdoor reset takes technology one step further. It can adjust the boiler water temperature based on the outside temperature.

Photo credit: Lennox

Updated March 22, 2018.