Gas Boilers: General Information
New Standard- and High-Efficiency Gas Boilers
Over the last 20 years, a new generation of higher-efficiency gas furnaces and boilers has come to market. An essential difference in the design of these units is how 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.
Non-Condensing Gas Boilers
Residential gas boilers sold in Canada today are required to have an AFUE rating of at least 80 percent. ENERGY STAR qualified boilers must have an AFUE rating of at least 85 percent. The following are some ways manufacturers have improved efficiency levels:
- Elimination of continuous pilot lights. Most boilers on the market today use some form of intermittent ignition device, usually electronic ignition.
- Improved insulation levels. Because boilers store more heat internally than warm air furnaces do, they are subject to greater heat losses, both out through their casing (sides) and up the chimney when they are not being fired. To reduce heat lost from casings, new boilers have much better insulation to keep the boiler water hot.
- Better draft control methods to reduce flue losses. Many boilers use draft hoods. The draft hood is located downstream of the boiler proper. It draws household air into the gas vent along with the flue gases. This stabilizes the airflow through the appliance, isolating the burner from outside pressure fluctuations. But it also continuously draws heat from the boiler and warm household air up the chimney. A vent damper is now usually installed downstream of the draft hood to close off the exhaust when the burner is not operating. When the gas burner turns off, the damper is closed automatically after a short period; before the burner lights again, the damper opens.
Other boilers that use aspirating gas burners have eliminated the need for a draft hood entirely by using a powered exhaust system, usually incorporating an induced draft fan. With no dilution air, high resistance to spillage during the on cycle, and minimal flow up the stack during the off cycle, these units tend to give superior performance to those using draft hoods and vent dampers.
Today, many gas boilers have replaced the naturally aspirating gas burner with a power burner. These use a fan on the burner to improve the combustion process and ensure the development and maintenance of an adequate draft. These burners, similar to ones used in advanced oil-fired equipment, tend to have a high-pressure restriction or even close off the combustion air passage when the burner is not operating. This minimizes off-cycle heat losses without requiring a flue damper. Such units minimize dilution air, or have sealed combustion, and have performance characteristics similar to or better than the aspirating burner with a powered exhaust system.
Condensing Gas Boilers
Condensing gas boilers employ either an aspirating burner with an induced draft fan, or a power burner, similar to the units described previously. However, they have an additional heat exchanger made of corrosion resistant materials (usually stainless steel) that extracts latent heat remaining in the combustion by-products by condensing the combustion products before they are exhausted. A chimney is not needed, reducing the cost of installation. 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.
A condensing boiler can have an AFUE rating of 90 percent or higher. But in practice, condensing boilers in hydronic (hot water) heating systems can have difficulty achieving this efficiency. For the condensing boiler's heat exchanger to extract all the potential latent heat effectively, the system has to run with the lowest possible return water temperatures, preferably not exceeding 45–50°C (113–122°F). Unfortunately, most radiator systems are designed to operate at significantly higher return water temperatures, which makes it difficult for the flue gas to condense. If the return water temperature is too high, actual operating efficiency may be only slightly higher than that of the better models of non-condensing boilers.
For a condensing boiler to achieve its potential, the heating system must be designed to return water to the boiler below the temperature of the condensing flue gas. Residential applications that normally operate at sufficiently low return water temperatures include
- radiant floor heating
- pool water heating
For radiator systems, it may be possible to lower the return water temperature with techniques such as
- using an outdoor reset controller as discussed in Chapter 2 to lower the supply water temperature in the "shoulder heating seasons" (late spring and early fall) to get efficiencies up during these periods, although this method is not effective in the peak heating season
- using radiator systems that have sufficient heat exchange surface to operate effectively at lower temperatures
- using the return water to preheat service water (as shown in Figure 11 on page 40) for combined space and water heating systems
For a condensing boiler to operate efficiently, a total systems approach to design is required.
Combined Space, Water Heating and Ventilating Systems
One way to potentially maximize efficiency and reduce costs is to integrate space and water heating in a single appliance.
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 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.
An integrated, high-efficiency 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). This type of system may entail a lower overall capital cost than individual appliances; it eliminates the need for multiple exhaust systems; and it maximizes efficient operation.
In practice, condensing gas-fired boilers in hydronic heating systems can have difficulty condensing 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 11.
Figure 11 Schematic of high-efficiency combined space and water heating system
Standard-efficiency gas-fired combined systems also exist, but their overall efficiency potential is lower than for condensing units. A standard-efficiency boiler coupled with an external storage tank is another efficient combined system.
Early "combo" systems used a conventional natural draft water heater and a fan coil to supply heat to the distribution air. These units suffer from low efficiency and limited life and have been supplanted by the optimized systems described above.
Condensation ProblemsIn the house More 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 may 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 In the chimney Condensation in the chimney is another possible problem. The lower flue temperature achieved by the improved efficiency of today's heating equipment has created the possibility of another problem – damage caused by condensation inside a chimney, particularly a masonry chimney located on an outside wall, where it is chilled by exposure to 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 problems. Another cause of condensation is that the new, more efficient furnaces need smaller chimneys than the 200-mm2 (8-sq.-in.) flue tile that has been standard for many years. Combustion gases, already cooled by the improved heat exchangers in the furnace, rise slowly in the cold, oversized flue and 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. Some solutions to these problems are described in Chapter 7. |
Source: National Resources Canada (NRCan) - Office of Energy Efficiency