Replacement Gas Boilers


Unlike furnaces that heat air, boilers heat water, providing either hot water or steam for heating. Steam is distributed via pipes to steam radiators, and hot water can be distributed via baseboard radiators or radiant floor systems.

If your boiler is old or has not been well maintained, you should consider replacing it with a new high-efficiency boiler. High-efficiency boilers are much less expensive to operate, and will save you money all winter long.

Efficiency Ratings


The Annual Fuel Utilization Efficiency (AFUE) rating indicates what percentage of the energy in the fuel is converted into usable heat. An AFUE of 80% means that 80% of the energy in the fuel becomes heat for the home and the other 20% escapes up the chimney and elsewhere. Many older boilers have AFUE ratings of only 55-65%. If your boiler has not been well maintained, then its efficiency may be even lower.

The current minimum AFUE for a new fossil-fueled hot water boiler is 80%, and the minimum rating for a gas-fired steam boiler is 75%. New high-efficiency boilers can have AFUE ratings of 90% or higher.

Energy Efficient Improvements


New high-efficiency boilers incorporate one or more of the following improvements to increase their AFUE.

  • Power burners mechanically mix gas and air and inject the mixture into the combustion chamber. A small motor drives a blower that draws in combustion air from the outside. Since these burners closely control the air/fuel mixture, they are very efficient.
  • Electronic spark ignition ignites the pilot light only when heat is demanded. When the device is energized, an electrode creates a spark that ignites the pilot. The pilot then ignites the burner. Eliminating the constantly burning pilot can save several dollars each month.
  • Hot surface ignition eliminates the pilot light altogether. The igniter is energized electrically and quickly attains a very high temperature. When the gas valve opens, the gas is ignited by contact with the hot surface igniter.
  • Vent dampers prevent heat from escaping up the chimney when the main burner is off. Because any air vented up the chimney or flue must be replaced by outside air, vent dampers can also help reduce infiltration.
  • A condensing boiler incorporates a larger heat exchanger or a secondary heat exchanger that condenses the water vapor produced in the combustion process and recaptures the heat from this condensation. So much heat is extracted that exhaust from this type of boiler is cooler and can be vented horizontally, eliminating the need for a chimney.

Retrofitting Gas Boilers


If your existing boiler is still in good condition, you may be able to retrofit it to increase its efficiency. Typical improvements include:

  • Modulating aquastat (hot water boilers only)
    An aquastat is a thermostat that regulates the temperature of a hot water boiler. Normally, the aquastat keeps water in the boiler around 160-180F. In milder weather, the water really doesn't need to be that hot. A modulating aquastat, also called an outdoor reset, senses outdoor temperatures and adjusts the hot water temperature accordingly, saving up to 10% on fuel costs.
  • Time-Delay Relay (hot water boilers only)
    A time delay relay helps extract the most heat out of your circulating hot water before actually turning on the boiler. When the thermostat calls for heat, the relay turns on the pump(s) that circulate the hot water through the system. After a preset time interval, the boiler turns on to maintain the temperature of the hot water. A time delay relay costs about $100 and can cut your fuel costs by up to 10%.
  • Indirect water heater
    When installing a replacement boiler, consider replacing your existing storage water heater with a well-insulated indirect tank that connects to the boiler.

How to Proceed


Contact your heating service company to do a thorough evaluation for you. They should be able to estimate how efficiently your existing boiler is performing, its useful life, and whether replacing it with a new, properly sized, high-efficiency unit makes sense.

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