Central Heating Systems
High-efficiency boiler with domestic hot water tank.
Boiler and furnace efficiency is often measured or compared by annual fuel utilization efficiency (AFUE). AFUE measures how much of the fuel that goes into the system gets turned into heat in the system. The typical Maine home has a boiler with an AFUE of 75% and many new systems can achieve efficiency ratings of 90% or higher. This efficiency difference can significantly cut your fuel bill but Efficiency Maine recommends considering improvements in the energy efficiency of your home such as air sealing and insulating.
This can also be a time to address other heating distribution improvements including sealing and insulating ducts and pipes, particularly in basements and crawl spaces, and adding distribution controls (more on these below). As a result of improvements to the building envelope and distribution system, you may be able to install a smaller heating system than the one currently in place, further reducing the costs of installation and on-going heating.
Efficiency Maine provides $300 rebates for high-efficiency boilers or furnaces that are ENERGY STAR rated, do not have tankless coils for domestic water heating, either condense combustion gases or have advanced heat purge controls, and can either modulate heat output or are installed with fully insulated duct work or heating lines. More information on installation criteria can be found here. A heating contractor can help you pick a model that fits your home and meets Efficiency Maine’s incentive requirements. A high-efficiency system will typically have most of the following features or capabilities:
- Condensing unit: A condensing unit extracts heat from the water vapor in combustion gases, getting more “bang for your buck” from the exhaust before it goes up the chimney. Condensing systems can achieve burner efficiency as high as 98%.
- Modulating burner: The best boilers and furnaces have the ability to heat at different rates depending on the heating demand of the home. A simple ‘on/off’ boiler may be operating inefficiently if it is “short cycling” or turning on and off frequently. Heating systems operate most efficiently when they are running for a long time and at constant temperatures (think of warming up your car on a cold morning). Reducing short cycling through modulation is a significant efficiency improvement.
- Sealed Combustion: Sealed combustion systems bring outside air directly into the unit and typically vent exhaust gases directly to the outside. Sealed systems are typically more efficient than those that rely on air from the basement for combustion and venting. They also reduce the risk of backdrafts and carbon monoxide leaks.
- Insulated Distribution Systems: Insulated duct work and hydronic heating pipes get heat to the part of the house where it is needed and minimize losses to areas where heat is not intended. This improves your systems efficiency, reduces run time, and saves on energy costs.
- Heat-purge control: This type of control extracts any usable heat from the boiler when it is starting up or shutting down and delivers it to a water heating tank or area of your house instead of letting it go up the chimney.
- Cold-start control: This type of control allows the boiler to cool down when there is no call for heat or domestic hot water. This is especially helpful during shoulder or summer months when there are few calls for heat.
Distribution Systems
Example of an outdoor temperature reset control.
Boilers distribute heat through either a hot water or steam hydronic distribution system. Furnaces distribute heat through forced hot air. Many high-efficiency boilers or furnace replacements are paired with efficiency upgrades to the distribution system including pre-and post-purge controls, modulating aquastats, limit switches or adding distribution zones paired with programmable thermostats.
- Post-purge or time-delay relay controls: A circulator pump upgrade allows the distribution water to circulate before or after the burner fires, which allows the system to create only the amount of heat needed.
- Modulating aquastat (outdoor temperature reset): Aquastats control water temperature in boilers and other hydronic heating systems. A modulating aquastat will vary the temperature that the water is heated to based on the outdoor temperature to save energy during the shoulder seasons of fall and spring.
- Limit switch: On furnaces, this switch allows for air ciculation based on the temperature of the air in the ductwork. Warm air can be delivered independent of whether the furnace burner is on or off.
- Separate zones: Separating your home into smaller heating zones can allow you to customize heating to space usage patterns. Programmable thermostats can make heating these zones even more efficient by timing heat delivery to occupancy.
A heating contractor can advise you about which distribution controls would improve the efficiency of your home’s distribution system. The Department of Energy has additional descriptions of distribution system efficiency upgrades here.
Take Action
Find a Heating Contractor: Click to find a heating contractor in your area who can advise on heating upgrades and accessing Efficiency Maine incentives and Energy Loans.
Get up to $3,000 in Incentives: Homeowners are eligible for up to $3,000 in incentives through the Home Energy Savings Program. Pair your heating project with air sealing and insulation to get the maximum incentive and increase your ongoing energy savings. Find out more about incentives and how to finance your heating system replacement here.
Compare Heating Options: Are you considering a new heating system or a different fuel but you’re not sure where to start? The Compare Heating Options Tool can help estimate the heating costs of a new system.
Summit Natural Gas Customers may be eligible for additional rebates. Find out more about these incentives here.
Unitil Natural Gas, Maine Natural Gas, and Bangor Natural Gas Customers may be eligible for additional rebates. Find out more about these incentives here.