The New High-Efficiency Furnaces

3. The New High-Efficiency Furnaces

Ever since the fuel price scare of the early 1970s, the industry has been working to improve the efficiency of furnaces and boilers. The introduction of improved burners with flame-retention heads was the first major step in boosting the efficiency of conventional oil-fired heating equipment. The high-static burner, which has recently come onto the market, has further enhanced the efficiency capability of heating systems. Now manufacturers have produced a new “mid-efficiency ”class of oil furnace designed to make the most of the superior performance of these new burners. Other companies have developed condensing furnaces that cool the combustion gases enough to recover the heat that is normally lost in the form of water vapour. New technologies are now allowing appliances to efficiently integrate two different functions, such as space and water heating, simultaneously.

Recently, a number of new oil-fired systems that can be vented directly out the side wall of the house, thereby eliminating the need for a chimney, have been approved in Canada.

Mid-Efficiency Furnaces

In addition to an improved high-static oil burner, a non-condensing, mid-efficiency furnace (Figure 9) features an improved low-mass combustion chamber (usually ceramic fibre) and passes the hot combustion gases through a superior heat exchanger that enables the circulating house air to extract more heat. The need for a barometric damper and the large requirement for exhaust gas dilution by house air have been eliminated in the most efficient designs.

The mid-efficiency furnace must keep the exiting gases above a certain temperature to prevent water vapour in the flue gas from condensing inside the furnace or venting system, where it can cause corrosion and other serious problems. The exit temperature of the combustion gases can be as low as 150°C (302 °F).

Some of the new oil-fired equipment can be vented directly though the side wall of the house without the need for a chimney.

One sidewall-venting type uses the forced raft of a high-static burner to expel combustion gases. Others go further and use sealed combustion with a high-static burner.

Another sidewall-venting system uses an induced draft fan. This fan is normally located downstream of the furnace at the inside wall of the house and pulls the gases from the furnace out of the house through a small exhaust vent. Some of these sidewall systems require dilution air from the house or have a long run time after the burner shuts off in order to purge the furnace system of any combustion gases. Both tend to reduce efficiency.

Figure 9 Mid-efficiency oil furnace

Benefits of a good mid-efficiency furnace are much lower combustion and dilution air requirements, as well as more power to exhaust the combustion products (an advantage in newer, tighter housing), a safety shut-off in case of raft problems, and a more effective venting system.

Mid-efficiency furnaces may have a seasonal efficiency of 83 to 89 percent and may use 28 to 33 percent less fuel than an old conventional furnace producing the same amount of heat.

Many heating industry experts believe that new technology furnaces, available in small enough sizes to fit the needs of even super-insulated houses, will be the next major development in oil heating. This can come about in two ways – either by eveloping alternative oil burner technology or by integrating the functions of different home energy requirements, such as space and water heating systems.

Condensing Oil Furnaces

Heat is carried away not only in the high temperature of the flue gases, but also in the water vapour they contain. The water vapour that is produced when natural gas is burned holds a substantial amount of latent heat – about 11 percent of all the energy in the fuel. Oil, on the other hand, produces only half the water vapour of gas; thus, oil has much less energy tied up in the form of latent heat.

A condensing furnace uses an extra heat exchanger made of stainless steel to extract more heat from the combustion gases before they leave the furnace, dropping the exit temperature to between 40°C and 50°C (104°F and 122°F). This results in water vapour from the flue gas condensing inside the heat exchanger and releasing its latent heat to the house air circulating through the furnace. At this point, the combustion gases are so cool that they require only a narrow plastic vent pipe that goes out the side wall of the house, instead of up the chimney. The condensate runs to a drain outlet.

With oil containing only half the hydrogen of natural gas, the potential for efficiency improvements by condensing the flue gas is much lower for oil than for natural gas – the dew point is lower, so the furnace has to work harder to condense less. Also, with higher sulphur levels, the condensate is corrosive, so that any condensing heat exchanger for oil must be even more corrosion-resistant. The fact that oil combustion also produces a certain amount of soot, which can concentrate the acidic condensate as “acid smut ” at certain points on the heat exchange surface, makes things even more difficult. This type of furnace is only marginally more efficient than a well-designed mid-efficiency furnace.

Some condensing oil systems use two vents: the conventional chimney and a plastic pipe out the side wall of the house –both open at the same time. With this type of system, there is a strong possibility of the flue gases bypassing the water-spray condensing system and going straight up the chimney. The barometric damper is also retained, so that the additional dilution air increases the overall heat loss from the house while lowering the dew point even further; this makes it even harder to condense the flue gases. The net efficiency from such a system is less than that for a mid-efficiency furnace.

For all these reasons, a condensing oil-fired furnace is a questionable choice, at best.

In-House Condensation Problems

More efficient heating systems, combined with better draft-proofing and insulation, can result in less air infiltration, which, in turn, may lead to excess moisture 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 these are 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, for example, by ensuring that your clothes dryer vents to the outside, using lids on pots when cooking, and keeping showers short. You should consider installing 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 may not be necessary to have a humidifier in a more airtight house. Finally, as a last resort, you should talk to a contractor about installing a heat recovery ventilator (HRV) that will increase the ventilation in your house and decrease humidity without wasting energy.

In-Chimney Condensation Problems

In-chimney condensation is another possible problem. The lower flue temperature achieved by the improved efficiency of today ’s heating equipment has created the possibility of damage caused by flue gas condensation inside a masonry chimney, particularly one located on the outside wall where it is chilled by exposure to the outside air. Look for a white, powdery efflorescence on the outside of the chimney, spalling or flaking of the bricks, crumbling mortar joints, wet patches on inside walls behind the chimney, pieces of tile at the bottom of the chimney, and water running out of the cleanout door or around the bottom of the chimney behind the furnace. The most common cause of all of these problems is condensation inside a cold chimney. Water vapour is produced when oil or natural gas is burned, but humid house air drawn into the chimney also contributes to the problem.

Another cause of condensation is that the new, more efficient furnaces need smaller chimneys than the 200 mm x 200 mm (8 in. x 8 in. ) flue tile that has been standard for many years. Because of this, the combustion gases, already cooled by the improved heat exchanger in the furnace, rise slowly in the cold, oversized flue where they are sometimes cooled to the dew point of the water vapour they contain. The resulting condensation can then leak into the bricks and cause structural or water damage. If this is caught in time, there are simple remedies. Use of a double-walled stainless steel flue pipe from the furnace to the chimney or a stainless steel chimney liner or both can usually stop this. Other solutions to these problems are described in Chapter 6.

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Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency