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3. Mid- and High-Efficiency Furnaces and Boilers
Research and development has produced a new generation of higher efficiency gas furnaces and boilers. An essential difference in the design of these units is the way they are vented, eliminating the need for dilution air. The combustion of natural gas produces certain by-products, including water vapour and carbon dioxide. In a conventional gas furnace, such by-products are vented through a chimney, but a considerable amount of heat (both in the combustion products and in heated room air) also escapes through the chimney at the same time. Heat is also lost up the chimney when the furnace is off. The newer designs have been modified to increase energy efficiency by reducing the amount of heated air that escapes during both the on and off cycles and by extracting more of the heat contained in the combustion by-products before they are vented.
Furnaces with these design modifications use much less energy than conventional furnaces, so consider what this means to you in dollars. Refer to the technologies and seasonal efficiencies listed in Table 2, and compare your possible savings with the purchase cost of the equipment. This will help you decide which energy-saving features will give you the most for your heating dollar.
Figure 8 High-efficiency condensing gas furnace
A chimney is not needed and energy efficiency is further increased. Because the flue gas temperature is low, the gases are vented through a PVC or ABS plastic pipe out the side wall of the house. Depending on the combustion and heat exchange design, fuel savings of up to 38 percent relative to a conventional gas furnace can be achieved. Furthermore, polluting emissions released into the environment are also reduced.
A second type of condensing furnace uses a pulse combustion principle. The technology relies on igniting small amounts of gas at frequent intervals; otherwise, it is essentially similar to the previously described condensing furnace.
One word of caution: do not buy a mid-efficiency furnace that is too efficient. You may run into condensation problems in your furnace or venting system. A good rule of thumb is to stay below 82 percent AFUE with this type of unit. If you wish to have a furnace with a higher efficiency, you should consider the condensing furnaces described in the previous section.
Figure 9 Mid-efficiency gas furnace with induced draft fan
Note that if this type of mid-efficiency furnace is installed, local or provincial building codes may require direct combustion air for the furnace.
Contrary to conventional and mid-efficiency furnaces, where efficiency decreases with furnace oversizing, condensing furnaces are actually more efficient when they are oversized and run for shorter periods. Thus, if you are choosing a new condensing furnace, you can get a furnace that is slightly larger than the house heat demand, without suffering an “efficiency penalty.”
Today, many new homes have less than optimally efficient gas furnaces. This is due to a number of factors, including pricing disincentives for builders and the mistaken belief among home buyers that a new home automatically has an energy-efficient furnace and that less efficient furnaces are still a bargain because of today’s relatively low gas prices. There is also the misconception that high-efficiency furnaces are more complex and have higher maintenance costs than mid-efficiency furnaces. For these reasons, the number of inefficient and mid-efficiency furnaces is still far greater than that of condensing appliances.
In provinces where the building code calls for “tighter” homes and the use of heat exchangers, flue gas spillage or backdrafting of combustion appliances caused by exhaust fans (such as kitchen and bathroom fans, clothes dryers, etc.) can be a hazard. Sealed combustion units can be used to help prevent this problem.
In many cases, with new or renovated housing, improvements to the building envelope have reduced the space heating load to the point where it is sometimes difficult to justify the expense of a high-efficiency furnace solely to satisfy the heating load. To take advantage of the efficiency potential of condensing gas-fired systems, it makes sense to combine space heating with other functions, in particular, domestic hot water heating. Domestic hot water loads have remained fairly constant and have even increased over time, making it logical to put more effort into improving the efficiency of the hot water generator. Therefore, it would be natural to combine space and water heating systems.
A high-efficiency integrated space and water condensing gas-fired heating system, using water from municipal mains as the driving mechanism to condense the flue gas, can have efficiencies of over 90 percent for both space and water heating. Space heating can be hydronic or forced air (through a fan coil). The overall capital cost of this system may be lower than for individual appliances, the need for multiple exhaust systems is eliminated, and the efficiency of operation can be maximized.
Condensing gas-fired boilers in hydronic heating systems can have difficulty condensing in practice because the return water temperature is above the dew point of the flue gases. By installing a water-to-water heat exchanger and storage tank for tap hot water upstream of the boiler, the return water temperature can be brought below the dew point, flue gases will condense and the efficiencies will be improved significantly. Such a high-efficiency combined system is shown in Figure 10.
Figure 10 Schematic of high-efficiency combined space and water heating system
Mid-efficiency, gas-fired combined systems also exist, but their overall efficiency potential is lower than for condensing units. A mid-efficiency boiler coupled with an external storage tank is another efficient combined system.
Some combined systems use a gas-fired water heater instead of a boiler as the energy generator and are then coupled with a fan coil to supply heat to the house through a warm air duct system. One promising combination, with a sealed combustion, instantaneous, mid-efficiency water heater as the energy generator, has an external tank and fan coil. An even more efficient system has a condensing water heater as the energy generator, with potential efficiencies near 90 percent. On the other hand, if a conventional, natural draft water heater is used, the resulting combination (“combo”) system will have much lower efficiencies and higher gas bills than if a mid- or high-efficiency water heater is used or when compared to a new gas-fired boiler installation.
High-Efficiency Condensing Gas Furnaces
Condensing gas furnaces are the most energy-efficient furnaces available, with seasonal efficiencies between 89 and 97 percent. Most have burners similar to conventional furnaces, with draft supplied by an induced draft fan. There are additional heat exchange surfaces made of corrosion-resistant materials (usually stainless steel) that extract most of the heat remaining in the combustion by-products before they are exhausted. In this condensing heat exchange section, the combustion gases are cooled to a point where the water vapour condenses, thus releasing additional heat into the home. The condensate is piped to a floor drain.
Mid-Efficiency Gas Furnaces with an Induced Draft Fan
Mid-efficiency gas furnaces mainly use a naturally aspirating burner and do not have a continuously lit pilot light. They are equipped with a powered exhaust, usually with a built-in induced draft fan, and they do not have a draft hood. With more heat exchange, no dilution air, and high resistance to flow during the off cycle, seasonal efficiency is much higher for mid-efficiency gas furnaces than for conventional furnaces, offering energy savings of 23 to 28 percent over conventional gas furnaces. These systems can be vented through a properly sized chimney or out the side wall of the house using high-grade stainless steel. However, there have been problems associated with the use of high-temperature plastic vent pipes with mid-efficiency furnaces. Regulations may forbid the use of certain vent materials in your area. You should discuss all options with your local serviceperson, approvals agency, or gas utility. Due to these venting concerns, the high-efficiency condensing furnace has become the new furnace of choice among Canadians.
Sealed Combustion
Heating costs may be reduced slightly by decreasing the amount of combustion air drawn from inside the house. One way to do this is to use outside air brought in through plastic piping directly to the combustion chamber. This is known as a sealed combustion system and is now applied to many new condensing furnaces both with an induced draft fan and with the pulse combustion types described previously.
Combined Space and Water Heating Systems
One way to potentially maximize efficiency and reduce costs is to integrate space and water heating in a single appliance.
Condensation ProblemsIn the HouseMore efficient heating systems, combined with better draftproofing and insulation, can result in less air infiltration, which, in turn, leads to excess humidity in the house. Heavy condensation on the inside of windows and dampness or mould growth on walls or ceilings are indications of too much moisture. If not corrected, serious structural damage will eventually occur; luckily, indoor condensation problems can be solved. Because most of the indoor humidity arises from regular household activities (such as showering and cooking), your first step should be to reduce the amount of moisture from these sources. You can do this by using lids on pots when cooking, keeping showers short, and ensuring that your dryer vents to the outside. Even better, install exhaust fans in the bathroom and kitchen vented directly to the outside. You should also check the humidifier setting on your furnace if it is equipped with one. In fact, it is often not necessary to have a humidifier in an airtight house. Finally, as a last resort, you should talk to a contractor about installing a heat recovery ventilator (HRV) that will increase your home’s ventilation and decrease humidity without wasting energy.
In the Chimney
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Looking Ahead
Research and development is ongoing in the field of furnace efficiency. Examples of recent developments are described below.Advanced Circulating Fan Control and Motor
An efficient variation on conventional gas furnaces is the dual-capacity, variable-speed furnace with a direct-drive fan driven by an efficient commutating motor. Such a unit can match both the heat output and circulating fan speed to the actual heat requirements, i.e., when the demand for heat is lower, they operate at a lower level, saving energy and improving comfort while considerably reducing energy costs.
Integrated Space, Water, Ventilation and Cooling Systems
A further extension of the integrated concept is to bring heat recovery ventilation control into the operation of space and water heating systems. Prototype units have been run in NRCan’s Advanced Houses Program. Others are under development, and some should be on the market in the near future.
Gas-fired heat pumps are just now reaching the market. This new technology is similar to a typical residential air-source heat pump, with the difference being that the traditional electric motor that powers the compressor is replaced with a gas-fired engine. This unit offers seasonal heating efficiencies ranging from 100 to 150 percent (coefficient of performance of 1.0 to 1.5) while providing air conditioning capability as well. Capital costs for such equipment can be very high.