A New Generation of Boilers Is Super Efficient
Bird's eye view
Boilers burn gas, oil, or wood in a chamber surrounded by water
Boilers usually contain an arrangement of tubes to maximize heat exchange between combustion gases and boiler water. Operation of the burner is controlled by an aquastat that monitors the temperature of the boiler water. A circulator controlled by a thermostat pulls hot water from the boiler and distributes it to various rooms or to an indirect water heaterWater heater that draws heat from a boiler used for space heating; a separate zone from the boiler heats potable water in a separate, insulated tank via a water-to-water heat exchanger. See tankless coil..
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Condensing boilers and furnaces are more than 95% efficient
Condensing burners extract latent heat contained in flue gases via a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.; modulating units can regulate the flame to match heat output with demand, like the gas pedal in a car. Both technologies have helped make the best gas-burning appliances far more efficient than standard models — and they can be direct-vented without a masonry chimney.
Condensing oil burners are now available in U.S. markets, but the devices are not without potential shortcomings. According to the Canadian Office of Energy Efficiency, when oil combusts it produces about half the water vapor as combusting natural gas, so there’s less potential for additional heat from flue gases. In addition, the higher sulfur content of fuel oil produces a corrosive condensate, and the soot produced in combustion can exacerbate the problem. In all, these devices are “only marginally more efficient than a well-designed mid-efficiency furnace,” the office says.
A boiler can heat water and space at the same time
Boilers can be a good choice of equipment, offering twice the bang for buck as they can provide hot water both for domestic use and for space heating. This also contributes to resource efficiency because there is only one heating device to buy and maintain, and possibly less floor space needed. High-performance boilers are among the most efficient equipment items in the arsenal of mechanical system choices, offering efficiencies up to 95 percent.
Building codes enforce minimum air quality standards, not ideal air quality. For a healthier home, keep chimneys or exhaust pipes as far from windows and other air intake points as possible. Prevailing winds can complicate these choices, so knowing the site conditions and hiring a knowledgeable installer is important.
Exhaust and combustion air need to be carefully considered
Standard-efficiency boilers typically will exhaust through a conventional chimney, although some use a power vent — a fan-equipped exhaust pipe that exits through a nearby wall instead of the roof.
High-efficiency boilers give you a lot of flexibility when it comes to venting. The lower temperature exhaust from these boilers can often exit through standard PVC drain pipe, through a wall or roof. If you're hoping to use an existing chimney when upgrading to a better boiler, check with the manufacturer or distributor to see if it's compatible. A special liner may be necessary to prevent the cooler, more acidic exhaust from condensing on and damaging a masonry chimney.
Because boilers are often in basements, the easiest termination of a direct vent or power vent tends to be through a wall, close to the ground. If you choose this route, there are several legal, durability and health concerns to consider. Keeping the exhaust pipe above potential snow drifts and away from operable windows helps prevent harmful gasses from entering living spaces — that's why these specifications are mandated by building codes. Proximity to the building itself can be a concern too, because soot and condensation could stain or even damage some materials.
Air intake is an important consideration too. In a home that is well air-sealed, there may not be enough natural infiltration to supply combustion air without compromising indoor air quality. Even in an older home it's a good idea to have a dedicated outdoor air vent to help maintain a good draft. Many high efficiency boilers already have this covered with proprietary combination exhaust/intake pipes.
Expansion tanks and boilers
Boiler requirements are found in Chapter 20 of the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.. Other requirements relating to gas-fueled boilers can be found in Section 2452. Requirements for combustion and make-up air can be found in Section 2407.
Boilers must have shutoff valves on both the supply and return piping (2001.3). Boilers must also have pressure and temperature gauges that indicate the unit’s normal range of operation. Steam boilers must also have a site glass installed so the midpoint indicates the unit’s normal water level (2002.2, 20002.3). Section 2002.5 requires a low-water cutoff to stop combustion when there’s an insufficient supply of water.
All boilers must have expansion tanks. Non-pressurized tanks must be secured so they can carry twice their water-filled weight (2003.1). Pressurized tanks should be able to withstand two and one-half times the maximum system pressure (2003.1.1). Minimum expansion tank capacities are based on system volume and can be found in Table 2003.2.
Illustration: Code Check Plumbing 3rd Edition. click to buy .
Heat and hot water, too
Hot-water boilers can burn a variety of fuels, including #2 fuel oil, natural gas, propane, and biomassOrganic waste that can be converted to usable forms of energy such as heat or electricity, or crops grown specifically for that purpose. such as wood or compressed wood pellets. But they aren't popular in the United States: hot-water heating systems accounted for about 2% of all heating systems installed in new houses in 2007, according to the U.S. Census Bureau.
Unlike hot-air furnaces, boilers can’t be used for air conditioning or humidification, and boilers are typically more expensive. But hot-water distribution systems are clean and usually quiet, and boilers can pull double duty as a source of both space heat and domestic hot water. Hydronic distribution systems are usually more efficient than forced-air duct systems, many of which are plagued by leaks. Sophisticated controls like outdoor resets can help to minimize boiler fuel use.
Government efficiency minimums. Federal regs require that boilers burning fossil fuels have a minimum annual fuel utilization efficiency(AFUE) Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. (AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. ) of 80%. That means 80% of the fuel consumed must go directly to heat and no more than 20% is wasted.
But boilers with much higher efficiencies are on the market, including condensing boilers fired by natural gas with efficiencies of more than 95%. In cold climates, high-efficiency boilers offer significant fuel savings that should justify their higher initial cost.
MORE ABOUT BOILERS
Newer gas boilers are highly efficient
The best are direct-vent modulating-condensing boilers, which increase efficiency by extracting additional heat from the condensation of flue gases. Rather than relying on a simple on-off switch for the burner, these boilers modulate the gas flame depending on demand, another fuel-conserving feature. Sealed combustionCombustion system for space heating or water heating in which outside combustion air is fed directly into the combustion chamber and flue gasses are exhausted directly outside. direct-vent units draw combustion air from the outside the house, cutting the risk of back-drafting, and they can be vented through plastic pipe, eliminating the need for an expensive masonry chimney. Many are small enough to be hung on a wall. Some manufacturers claim seasonal efficiency of 95% or greater.
Oil burners are fairly efficient, but fuel prices may be volatile. With few exceptions, seasonal efficiencies of the best oil burners are lower than for high-efficiency gas boilers, and there are very few residential condensing oil boilers on the market (see Green Building Products). Oil prices are volatile; in mid-2008, fuel oil had climbed to more than $4.30/gal. in some regions, nearly double its cost a year earlier.
Oil-fired boilers can burn biodiesel without modification, as long as the biodiesel can be stored in a relatively warm location. Boilers designed to burn used motor oil can burn used fryer oil directly, avoiding the expense of converting the fryer oil to biodiesel.
- Fine Homebuilding
- John Hartman/Fine Homebuilding
Jan 5, 2011 12:59 PM ET