Efficient Residential Heating Systems Begin With Proper Size Furnace For Building Shell

The 2006-07 winter season is the first since 2001-02 in which the cost for heating the American home is expected to decline from that of the previous winter, reducing the national average household fuel expenditure from last winter’s $948 to a projected $873 this season, according to the “Short-Term Energy Outlook” report released in January by the Energy Information Administration (EIA). Although the report is favorable from the national perspective, cold weather heating costs in the Northeast comprise a substantial percentage of the average homeowner’s annual budget. That, plus the fluctuation of fuel costs, makes the installation of an energy-efficient residential heating system of paramount concern when building a new home or undertaking renovations to an existing structure. Jeffrey Gephart of Vermont Energy Star Homes says the type of heating system depends on a number of factors including the building’s thermal boundary or shell, the systems available, the fuel choices available, and the homeowner’s interest and capability. He refers to the home as a system itself. “We want to look at that structure holistically and assist people to make the best possible choice for that system and for them. How big is the home? What is the shape? How much glass is there? What type of insulation does it have? Who is the builder? Where are the thermal boundaries in the building? Are they constructed properly? Are they properly defined?” “It is best to have the shell well-insulated for several reasons,” says Matt Sharpe, senior project manager for Efficiency Vermont. “The shell will be there a lot longer than the heating system. The other impact is that you can afford a smaller, less expensive system when you have a lower load. You reduce needs through [the building’s] efficiency and then choose a heating system with a lower load that matches. That’s the first strategy to look at.” Gephart adds, “First and foremost, make the thermal boundary all it can be. Put as much money there as you can. Once the shell is built, you’re done spending money on it. You do keep spending money on the care and feeding of the heating system. The exterior portion of the thermal boundary has to shed snow and ice away from the building and protect it from outside temperatures, then be insulated to keep heat in. The internal air barrier needs to be aligned and in contact with insulation. Once there is a good-quality structure and good-quality thermal boundary, we look at ways that we can make it effi cient to limit the amount of heat put into it. We look at the design heat load for that building then, based on a discussion with our enrollee, assist them to select the appropriate equipment for the load on that building.” According to Sharpe and Gephart, not getting the right size system has been an issue. “If a system is too large, it will short-cycle—satisfy the heat load and then shut off,” says Gephart. “Fuel usage is similar to the difference between city driving and highway driving. It’s more efficient to use less fuel over a longer period of time.” Short cycling can shorten the life of a heating system. Vince Comegno, sales and service representative at Avonda Air Systems in South Burlington, says he always recommends that homeowners have a load calculation done. “Don’t assume that what you currently have is the correct size. Oversized is as bad as undersized. Oversize creates short cycling that uses more fuel when it starts up. Oversizing shortens the life of the equipment. Short cycles create stress through expansion and contraction on the heat exchanger, eventually leading to premature failure. Fuel source is a major consideration in selecting a heating system. “Electricity is the most expensive Btu (British thermal unit) on the market,” says Gephart. “The State discourages electric heating systems, particularly resistance heating such as electric baseboard or furnace heat. A lot of power has to go over the line. If we heat with electricity, then utilities have to buy more electricity at the times when that electricity is most expensive—when it’s coldest. Peak-power purchases equal high demand, equal higher price. It is costly to all of us, whether we use it or not.” People often select the type of fuel their heating system uses based upon the cost per unit. “Each have different amounts of Btu content,” Sharpe points out. Although electricity averaged $.14 per kilowatt-hour in January, its $/MMBtu (one million Btu), adjusted for equipment efficiency, was $41.02, while natural gas at $1.54 per unit adjusted to $16.21 per MMBtu. Oil translates to $22.70 per MMBtu from $2.51 per gallon. It is interesting to note that of the 8.1 million people using oil to heat their home, approximately 78 percent (6.3 million households) reside in the Northeast. “These are the highest efficiencies expected if you buy the highest efficient equipment on the market,” he adds. “People sometimes have to read between the lines of marketing reality. Marketing plays to our desires. Don’t buy just because a product’s marketing plays it a certain way. Look at the facts; there’s a lot of resources out there.” About 85 percent of Vermonters use hydronic heating systems consisting of a boiler with baseboard or radiant in-fl oor or staple-up types of heat distribution, says Gephart. “When they’re using a boiler-fi red or radiant distribution type of system, we usually also try to heat the hot water with the boiler and use a side-arm storage tank. Because most Vermont homes are not designed for air conditioning, that’s more of the type we see.” Although more European style on-demand heat and hot water systems are being used in this area, they are still in the vast minority. The basic baseboard hot water system is up to 45 percent more efficient than a hot air system, says Comegno. “The upfront cost is more to install a hot water system, but the efficiency is greater. Boilers can be oil or gas. With oil boilers, the highest efficiency is around 86 to 87 percent. They’re achieving up to 93 percent with gas in what they call a condensing boiler. There are different types of boilers. For oil, it is cast iron or steel. The average life on a steel boiler is 15 to 20 years; for cast iron it is 30 or beyond. Gas has cast iron, some steel, and the new option is aluminum in high efficiency. The ones with 90 percent or above are aluminum. The high-efficiency models can come in wall or fl oor mount. The real high efficiency boiler we like to recommend is a modulating boiler.” The modulating boiler has an outdoor reset built into its controls, which allows it to adjust the firing rate according to the outdoor temperature while maintaining the indoor climate. “In-floor radiant is up to 40 percent more efficient than hot water baseboard heat,” says Comegno. “It depends upon the system, as well as the way it is installed, the building construction, how well it is insulated, and the type of floor. The advantage of radiant heat has to do with thermal physics. Heat wants to flow to cold—hot air rises, not heat. If you’re walking across a cold floor and your feet are warm, the cold floor is extracting heat from your body. Therefore, if the floor is warm, heat is not being extracted from your body and your feet are warmer. If your feet are warm, the rest of you is warm. The stratification itself is probably half a degree to a degree different from the floor to six feet with the radiant; with baseboard, the difference can be as much as two degrees from the floor to a six-foot level. That’s even higher with warm air as hot air wants to rise.” When combining central air and heat, it’s less expensive and more cost effective to add central air to a hot air system and the new furnaces on the market are more efficient, Comegno says. “They’ve all become condensing furnaces measuring in the 90 percent and up efficiency range by the AFUE (annual fuel usage efficiency) rating given on this equipment by the U.S. Department of Energy. “Furnaces have physically gotten smaller in size. By making the furnace smaller, they can now heat the air and dissipate it into the living space more quickly,” Comegno says. The most efficient hot air furnace has a twostage gas valve. “When the furnace first calls for heat, it will attempt to heat the space at a lower Btu output. If the interior has not reached sufficient temperature within a certain period of time, or if the furnace has an outdoor air sensor to know what the outdoor temperature is, it will go into a higher stage Btu output. It won’t run on high heat all the time, and that’s where you get the energy savings. Traditionally you would see as much as a four- to six-degree temperature swing in the living space. The two-stage technology has reduced the cycling and temperature swings giving more constant temperature and more constant comfort throughout the living space. They were smart in achieving two goals: efficiency and comfort.”