2. Basic Heating Equipment for Gas-Fired Systems
2. Basic Heating Equipment for Gas-Fired Systems
Most natural-gas-fired heating systems today are either forced-air or hydronic (hot water) systems as noted in Chapter 1. This chapter discusses the equipment that make up these two distinct systems.
Equipment for Forced-Air Systems
CONVENTIONAL GAS FURNACE
An old conventional natural-gas-fired, forced-air heating system is shown in Figure 2. This system consists of a furnace with a naturally aspirating gas burner. Older units were equipped with a standing (continuously lit) pilot light; the newer ones feature electric ignition. The combustion gases pass through the furnace, where they pass heat across a heat exchanger and are exhausted to the outside through a flue pipe and vent. A draft hood serves to isolate the burner from outside pressure fluctuations at the vent exit by pulling varying quantities of heated house air into the exhaust as required. A circulating fan passes cooled house air from the return ducts over the furnace heat exchanger, where the air is warmed up and passed into the ductwork that distributes the heated air around the house.
Figure 2 A conventional gas-fired, warm-air furnace
Notice that there are two entirely separate air movement paths: the combustion path supplies air to the burner and to the draft hood and carries hot combustion gases through the burner, heat exchanger and flue pipe to the vent and out of the house; the heat distribution and cold air return path circulates and heats the air inside the house.
Conventional gas furnaces have a seasonal efficiency of about 60 percent. Although the majority of Canadian homes have gas furnaces similar to this type, such equipment does not meet the new seasonal efficiency standards and is no longer sold in Canada. Today, new furnaces must meet minimum energy efficiency requirements as set out in the Regulations of Canada's Energy Efficiency Act. The minimum seasonal efficiency, or AFUE, as of 1995 is 78 percent (see Chapter 3).
The other common type of gas-fired system is an oil-fired furnace that has been converted to natural gas, usually with either a power burner or a power-assisted burner. This type of unit has a fan with a burner to assist in the combustion process and in the development and maintenance of an adequate draft. The dilution device is a double-acting barometric damper, rather than a draft hood, but it performs a similar function.
Oil furnaces that have been converted are generally more efficient than conventional gas furnaces, with seasonal efficiencies in the range of 63 to 68 percent; however, they are not nearly as efficient as the new types of standard and high-efficiency gas furnaces.
GAS FURNACE WITH AUTOMATIC VENT DAMPER
Figure 3 Vent-dampered gas furnace
A vent-dampered gas furnace has a vent damper in the flue exhaust, downstream of both the furnace heat exchanger and the draft dilution device (Figure 3). A thermostat controls the damper: when the gas burner turns off, the damper is closed automatically after a period; when the thermostat signals to start the furnace, the damper opens before the burner ignites. By closing off the vent during much of the off cycle, the damper prevents some of the warm household air from being drawn up the chimney and lost to the outdoors. These furnaces usually have an electric or electronic ignition. Fuel savings are generally in the range of 3 to 10 percent, compared with a conventional furnace. However, some of the savings can be lost if a conventional gas-fired water heater (see Chapter 8) is also connected to the same chimney. The water heater is still vented and is burdened by an increased draft, augmenting the heat lost through the water heater. The vent-dampered gas furnace does not meet the now-applicable minimum standards for energy efficiency.
MAXIMIZING EFFECTIVENESS IN FORCED-AIR HEATING SYSTEMS
The performance of an existing forced-air heating system can be improved by adjusting the furnace fan and getting the heat where you want it.
Adjusting the Furnace Fan
Heat output from a warm air system can often be increased by adjusting the controls that turn the fan on and off automatically. Fan controls are usually located in a metal box, often mounted on the front of the furnace, near the top. Inside the box is a temperature dial with three pointers. (To remove the cover, you must either squeeze it or remove metal screws.) The lowest setting is the fan "off" pointer; the next one is the fan "on" setting (Figure 4). The third and highest pointer is the safety limit control that shuts the burner off if the furnace gets too hot. This safety limit is normally set at the factory. Do not adjust this safety limit setting.
Figure 4 Circulating fan control
The "on-off" fan control pointers have usually been set for an "on" temperature of 66°C (151°F) and an "off" temperature of 49°C (120°F). To increase the amount of heat distributed by the furnace, most heating experts now recommend changing the setting to an "on" temperature of 49°C (120°F) and an "off" temperature of 32°C (90°F). These changes will cause the fan to come on sooner after the burner starts up and to stay on longer after the burner shuts down. This allows the circulating air to extract more heat from the furnace so that less heat is lost up the chimney or through the vent.
The fan control dial is spring-mounted, so it must be held firmly with one hand while you adjust the pointer with the other. Make sure the "auto/manual" switch is set to "auto" after replacing the cover of the metal box. If you feel uncomfortable or unsure of what to do to modify these settings, ask your furnace serviceperson to make the setting changes for you during the next service call.
These modified temperature settings may result in slightly lower air temperatures coming from the room registers at the beginning and end of the furnace cycle. If the cooler air at either end of the cycle makes you feel uncomfortable, try raising either the fan "on" setting to 54°C (130°F) or the fan "off" setting to 38°C (100°F), or try both, whichever is appropriate.
A two-speed fan will allow you to get more heat out of the furnace while providing for continuous air circulation and more even temperatures throughout the house when the furnace is off; however, your electricity bill may increase significantly.
Some of the new high-efficiency furnaces use a more efficient, variable speed, high-efficiency, brushless DC motor to run the circulating fan. For extended or continuous fan operation, such a unit can save a significant amount on your electricity bill while making the delivery of heat more even and comfortable.
Getting the Heat Where You Want It
Uneven heat distribution is sometimes a problem, which often results in the inability to heat some rooms in the house, such as upstairs bedrooms. This can be due to warm air leaking out through joints in the heating ducts or to heat loss from ductwork passing through the basement or, even worse, through unheated areas such as a crawl space, attic or garage.
Sealing all joints in the ductwork with a special water-based duct mastic (sealant) will reduce or eliminate warm air leaks. Look in the Yellow Pages™ under "Furnaces – Heating" or "Furnaces – Supplies and Parts." (High-temperature duct tape may work, although it tends to degrade or permit air leakage over time.)
When the circulating fan is running, the house heat loss can significantly increase if leaky ducts are located in an exterior wall, an attic or a crawl space, allowing the heated air to escape. This is one more good reason to ensure that all ducts are well sealed.
Ducts passing through an unheated area such as a crawl space or an attic should first be sealed, then wrapped with batt or duct insulation. Do the same for long duct runs in the basement. As a minimum, it is recommended that the warm air plenum and at least the first three metres (10 feet) of warm air ducting be insulated. Better still, insulate all the warm air ducts you can access. Use batts of insulation with foil backing, or enclose the insulated ducts in the joist space. If your basement is presently heated by the heat loss from the ducts, it may be necessary to have additional registers installed in the basement after you insulate. This will help to ensure that the heat will go only where you want it, when you want it, without being lost along the way.
Rooms on upper floors or far from the furnace are sometimes difficult to heat because of the duct losses previously described and because of friction and other resistance to airflow (such as right-angle bends) in the ductwork. This can sometimes be corrected by slightly modifying the
ductwork after the ducts have been sealed and insulated, and by balancing the airflow in the supply ducts (Figure 5) to redirect the flow of air from the warmer areas to cooler rooms.
Figure 5 Balancing damper in the supply duct
In some forced-air distribution systems, balancing dampers may be located in the secondary warm air ducts, close to where they branch off from the rectangular main heating duct. Often the dampers can be identified by a small lever on the outside of the duct (Figure 5). The position of this lever (or sometimes a slot in the end of the damper shaft) indicates the angle of the unseen damper inside the duct. If there are no such dampers, you will have to use the ones in the floor registers.
Start by closing the dampers in the ducts that supply heat to the warmest rooms (even if completely closed, they will probably still supply some heat to these rooms). Wait a few days to see what effect this has on the overall heat balance, then make further adjustments as necessary. Such adjustments may slightly reduce the total airflow through the furnace, but this will be balanced to some extent by a slight increase in the temperature of the delivered air.
It may be more practical to hire a service technician experienced in heat balancing to do the job. If you make too large a reduction in the airflow, you could cause an undesirable rise in the temperature of the air inside the furnace plenum. It is a good idea to have this temperature rise checked by your furnace serviceperson.
Most houses have been designed with inadequate cold air returns. The result is that there is not enough airflow through the furnace. Putting additional cold air returns in living areas, particularly in bedrooms, can improve air circulation and heating system efficiency while improving comfort and air quality in the house.
| Some years ago, it was mistakenly thought that one way to get around the problem of inadequate cold air return was to open up the cold air return ductwork or plenum in the basement area near the furnace or even to take off the furnace access panel near the air filter. This is dangerous. The depressurization caused by the circulating fan can actually disrupt the combustion and result in spillage or backdrafting of combustion products. These combustion products can then be circulated through the house instead of going up the chimney. In certain cases, this can cause carbon monoxide poisoning. |
For heat distribution problems that cannot be corrected by damper adjustments and other duct modifications, have a qualified serviceperson do a complete and proper balancing of your distribution system.
Programmable ThermostatsThe easiest way to save heating dollars is to lower the temperature setting on your house thermostat, when possible. As a general rule, you will save 2 percent on your heating bill for every 1°C (2°F) you turn down the thermostat overnight. Programmable thermostats have mechanical or electronic timers that allow you to preset household temperatures for specific periods of the day and night. In a typical application, you could program the thermostat to reduce the temperature an hour before you go to bed and to increase it before you get up in the morning. You could also program it to reduce the temperature for any period during the day when the house is unoccupied and to restore the temperature shortly before you return. For example, you could have the temperature set at 17°C (63°F) when you are sleeping or not at home and at 20°C (68°F) when you are awake and at home. Experiment with the unit until you find the most comfortable and economical routine for you and your family. ENERGY STAR qualified programmable thermostats The thermostat will include instructions for the installer to adjust the cycle to suit your heating/cooling equipment. It will come pre-programmed with recommended temperature settings, but you may readily change them to suit your comfort and daily schedule. Many offer additional features that allow you to
When used properly, ENERGY STAR labelled thermostats can save you 10 to 15 percent on your heating bills. Zone control thermostats Improved thermostats |
Equipment for Hydronic (Hot Water) Systems
DESIGN AND OPERATION
A hydronic heating system uses hot water to distribute heat around the house and has three basic components:
- a boiler to heat the water
- heating units in most rooms, usually baseboards or radiators, which are often located on an outside wall
- a pump to circulate the water from the boiler to the radiators and back through a piping system
A natural-gas-fired boiler uses the same type of burner (either naturally aspirating or power) as a natural-gas-fired forced-air furnace, but a boiler is generally smaller. There is only one air path, which goes to the boiler; this is split between the burner and the dilution device, either a draft hood or a double-acting barometric damper (in the case of a power burner). A boiler does not need the fan and filter housing that makes up a large portion of a forced-air furnace.
Most boilers require a circulating pump to push heated water through the pipes and the radiator system (Figure 6). The seasonal efficiency of conventional boiler systems is similar to that of conventional furnaces, which is around 60 percent. Today, new boilers must meet minimum energy efficiency requirements as set out in the Regulations of Canada's Energy Efficiency Act. The minimum seasonal efficiency, or AFUE, as of 1999 is 80 percent (see Chapter 3).
Figure 6 Schematic of a hydronic (hot water) heating system
MAXIMIZING EFFECTIVENESS
The performance of hydronic heating systems can be improved in several ways.
Improving Heat Distribution
Old-fashioned gravity heating systems that circulate water by natural convection are less efficient than systems with a circulating pump. Slow heat circulation may cause house temperatures to fluctuate noticeably between firing cycles. It can also take a long time to restore the house temperature after a nighttime thermostat set-back. In addition, a gravity system cannot circulate hot water to radiators or baseboard heaters in basement living areas, where they would be below the level of the boiler. All of these problems can be overcome by adding a circulating pump and replacing the open expansion tank with a sealed and pressurized expansion tank near the boiler. If you have a gravity system, discuss the possibility of upgrading it with your plumbing and heating contractor.
Balancing the Heat
Balancing the heat delivered to different areas of the house is as important with hydronic heating as it is with a forced-air system. Radiators are often fitted with a simple manual valve that can be used to control the amount of water flowing through them. Such valves can be used to vary the heat delivered to different rooms of the house in the same way that balancing dampers are used in a forced-air system.
One device that can vary the heat output automatically is a thermostatic radiator valve (Figure 7), which can be set to control the temperature in any room. This valve, however, will not work on radiators or baseboard heaters installed on what is called a "series loop" system. In such a system, the water must pass through all the radiators, one after the other, on its way back to the boiler. If there is more than one loop in the system, some balancing of the heat output can be achieved by adjusting the valves that control the water flow through each loop. The heat output of baseboard units can also be controlled to some extent by regulating the built-in damper, which operates much like the damper in a warm air register.
Figure 7 Thermostatic radiator valve
Outdoor Reset
Most hydronic heating systems have the boiler temperature set for 82°C (180°F). A device that has reduced energy consumption in many hydronic heating installations is an outdoor reset controller, which controls the circulating water temperature in relation to the outside air temperature. As it gets warmer outside, the boiler water temperature is reduced. However, some boilers can be subject to thermal shock or corrosion if the return water temperature is too cold. Before applying one of these devices to your system, consult your plumbing and heating contractor to ensure that your boiler can handle it, and that the distribution system will perform effectively at the lower temperature.
Chimney LinersThe combustion of natural gas sends a great deal of water vapour up the chimney. If the chimney is too cool, the vapour will condense; the alternate freezing and thawing of the water, as well as the acidic corrosion from the condensate, can seriously damage masonry chimneys. This problem is particularly serious with outside chimneys, which are much cooler and exposed more to the elements. If your gas heating system is vented through an existing masonry chimney, you can usually avoid these condensation problems by inserting an approved metal liner, either a double-walled B-vent or a single-walled, stainless steel Underwriters' Laboratories of Canada (ULC) liner. Approved liners reduce the size of the flue so that the chimney will match the requirements of the gas-fired appliances being vented. The reduced diameter of the flue allows gases to go up the vent faster with less chance of cooling down. At the same time, the inside surface of the metal liner is warmed more quickly by the flue gases escaping the chimney, reducing the likelihood of condensation. Metal liners should be used with natural gas furnaces and are a requirement in many provinces/territories. Contact your local utility or provincial/territorial authority for specific advice. |
CONVERTING OIL FURNACES TO GAS
If you are presently heating with oil, you may be able to convert your existing furnace or boiler to gas. This involves replacing the oil burner with a gas conversion burner and modifying the venting system.
Not all types of oil furnaces can be converted. Also, a conversion is practical only if the equipment is in good enough condition to have a reasonable life expectancy after conversion. Oil furnaces converted to gas have low seasonal efficiencies in the range of 63 to 68 percent.
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Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency