As noted in Chapter 1, most oil-fired heating systems today are either forced-air or hydronic systems. This chapter discusses the equipment that makes up these two distinct systems.
A schematic drawing of basic oil-fired, forced-air heating system is shown in Figure 1. This system consists of burner fed by heating oil, usually from a storage tank within the house, firing into a combustion chamber in the furnace. The combustion gases pass through the furnace where they give up heat across a heat exchanger; they re then exhausted to the outside through a flue pipe and chimney. A barometric damper, acting as a valve in the flue pipe, isolates the burner from changes in pressure at the chimney exit by pulling varying quantities of heated room air into the exhaust. A circulating fan passes cool house air from the cold air return ducts over the furnace heat exchanger, where it warmed up, and then passes it into the hot air ducts, which distribute the heated air throughout the house.
Notice that there are two entirely separate air movement paths. The first, the combustion path, supplies air to the burner and follows the hot combustion gases through the heat exchanger and flue pipe to the chimney and out of the house; it also includes air drawn through the barometric damper. The other path circulates and heats the air within the house.
In many houses, the quantity of air drawn through the barometric damper is much greater than the quantity required for combustion and can represent as much as 10 to 15 percent of the total heat loss in the house. Thus, anything that reduces this airflow, without compromising the performance of the furnace, will lead to increased fuel savings and efficiency.
Figure 1 Oil-fired forced-air furnace
Some newer furnaces might have an optional direct connection for outside air for combustion (sealed combustion) , instead of using indoor air. However, care must be taken if this approach is followed. On a cold winter ’s day, if the air is not warmed somewhat before it reaches the burner, it could cool the fuel oil and cause start-up problems.
A similar problem occurs when the oil is stored in tank outside the house, rather than in the heated space. When the outside temperature gets very cold, the oil in the tank cools down as well. The oil can become very viscous (thick) , and you may not be able to get the oil from the tank to the burner – hence, no heat. Even if it does finally get to the burner, it may be so thick that your oil burner cannot atomize it properly, and you will get poor combustion. If you have an outside tank, consider some form of heating either from the tank or the line, and install a smaller holding tank within the house to help prevent these problems. Even better, consider bringing the tank inside the house.
There are several ways to improve the performance of an existing forced-air heating system.
Adjusting the Furnace Fan
Heat output from a forced-air system can often be increased by adjusting the controls that turn the fan on and off automatically. The fan controls are usually located in a metal box, often on the front of the furnace, near the top. Inside the box (to remove the cover, you must either squeeze it or remove some metal screws) is a temperature dial with three pointers (Figure 2) – the lowest setting is the fan OFF pointer; the next one is the fan ON setting. 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 the safety limit setting.
Figure 2 Circulating fan control
The ON/OFF fan control pointers are usually 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 taken from the furnace, most heating experts now recommend changing the ON temperature to 49 °C (120 °F) and the OFF temperature to 32 °C (90 °F). These changes make the fan come on sooner after the burner starts up and stay on longer after it shuts down, allowing the circulating air to extract more heat from the furnace and losing less heat 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 changes may result in slightly lower air temperatures from the registers at the beginning and end of the furnace cycle. If the cooler air at either end of the furnace cycle makes you feel uncomfortable, try raising the fan ON setting to 54°C (129°F) , the fan OFF setting to 38°C (100°F), or try both, whatever is appropriate.
A two-speed fan will allow you to get even more heat out of the furnace while providing for continuous air circulation and more even temperatures throughout the house while the furnace is off. However, this will increase your electricity bill.
Some of the new high-efficiency furnaces use a more efficient, variable-speed, direct-drive commutating motor to run the circulating fan. The speed of the fan depends on the heat demand. 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 results in the inability to heat some rooms in the house, such as upstairs bedrooms. This can be due to the leakage of warm air 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, an attic or a garage. When the circulating fan is running, the house heat loss rate 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 all the ducts are well sealed.
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.
Ducts passing through an unheated area such as a crawl space or an attic should 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 install additional registers there after you insulate the ducts. This will ensure that the heat will be going 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 heat losses described previously and also because of pressure losses from friction and other restrictions to airflow (such as right-angle bends) in the ductwork. This can sometimes be corrected by light modifications to the ductwork after the ducts have been sealed and insulated and by balancing the dampers in the supply ducts (Figure 3) to redirect the airflow from the warmer areas to cooler rooms.
Figure 3 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 they can be identified by a small lever on the outside of the duct as shown in Figure 3. 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 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.
However, you should be careful. It may be more practical to have a trained service technician do the job. If you reduce the airflow too much, you could cause an undesirable rise in the temperature 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, which result in not enough airflow through the furnace. Putting additional cold air returns in the living area, particularly in the bedrooms, can improve air circulation and heating system efficiency while improving comfort and air quality in the house as well.
Several years ago, it was mistakenly thought that one way to get around the problem of inadequate cold air returns was to open up the cold air return ductwork or plenum in the basement area near the furnace, or even 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 back drafting of combustion products. These combustion products can then be circulated around the house instead of going up the chimney. In certain cases, this can have catastrophic results and can cause carbon monoxide poisoning.
For stubborn 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.
Automatic Setback Thermostats
The easiest way to save heating dollars is to lower the temperature setting on your house thermostat when possible. An automatic setback thermostat will adjust your home’s temperature automatically. These thermostats have a mechanical or electronic timer that allows you to preset household temperatures for specific periods of the day and night. As a general rule, you will save 2 percent on your heating bill for every 1°C you turn down the thermostat overnight.
You could program the thermostat to reduce the temperature a short while before you go to bed and to raise it again 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. A good guide is to 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 home.
Experiment with the unit until you find the most comfortable and economical routine for you and your family.
If you have a hot water (hydronic) system, you can also reduce energy use through zone control. In this system, thermostat-controlled valves on each radiator may permit the control of individual room temperatures. A plumbing and heating contractor can provide more information about zone control and can install all required equipment when the heating system is installed. Zone controls are also available for forced-air heating systems, usually with dampers in main duct passages driven by separate thermostats in different areas of the house.
A hydronic heating system uses hot water to distribute the heat around the house and has the following 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
Figure 4 Oil-fired boiler
An oil-fired boiler (Figure 4) uses the same type of burner as an oil-fired, forced-air furnace, although a boiler is often somewhat smaller and heavier. There is no circulating fan and filter housing as with a forced-air system. Instead, most boilers require a circulating pump to push heat around the house through the pipes and the radiator system, as shown in Figure 5. The seasonal efficiency of old conventional hydronic systems is similar to that of conventional forced-air systems, which is around 60 percent.
Figure 5 Hydronic heating system
There are several ways to improve the performance of a hydronic heating system.
Improving Heat Distribution
Old-fashioned gravity systems that circulate water or steam by natural convection are much 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 setback. In addition, a gravity system cannot circulate hot water to radiators or baseboard heaters in basement living areas, where they are 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 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 6) that can be set to control the temperature in any room. However, this will not work on radiators or baseboard heaters installed in what is called a “series loop ”system. In this particular 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 6 Thermostatic radiator valve
Conventional hydronic systems have the boiler water temperature set at 82°C (180°F). A device that can reduce energy consumption in many hydronic heating systems is a controller, which controls the circulating water temperature in relation to the outside air. As it gets warmer outside, the boiler water temperature is reduced. Care must be taken not to reduce the temperature too much or corrosion could result.
Homeowners can improve the efficiency of their heating systems by investing in one or more of the improvements described in this section.
Downsizing heat output and improving combustion are two ways to upgrade your oil-fired heating system. Following are descriptions of these approaches.
Most residential oil furnaces or boilers manufactured before the late 1970s were installed with a cast-iron head burner. If your furnace or boiler still has the original burner, it has a relatively low seasonal efficiency, of about 60 percent. If you don ’t know what type of burner you have, ask your serviceperson.
There are four basic reasons for this low seasonal efficiency: an inefficient conventional burner, an inefficient furnace, air dilution or an oversized system.
Most heating systems in older homes are greatly oversized. Moreover, homeowners have often added insulation, caulking, and weatherstripping, and made other improvements to reduce heat loss and cut fuel consumption. As a result, the old systems are even more oversized.
We know that an automobile gets much better fuel economy when cruising on the highway than when continually starting, accelerating and decelerating in the city. Like an automobile, most oil furnaces perform best when running at their steady-state condition, with the maximum stable flue gas temperature. But the burner must for run between 7 and 20 minutes to reach this point, and some oversized units may never run long enough to get there, even on the coldest days.
Ideally, the oil burner would run continuously when the outside temperature is at the lowest temperature expected for your area, or what is called the “design temperature. ” At that point, the furnace would be operating close to the steady-state efficiency for which it was rated. A running time of 45 to 50 minutes per hour at your local coldest design temperature is a practical goal. Discuss this particular concern with your serviceperson.
An oil heating system can be downsized simply by replacing the existing oil burner nozzle with a smaller sized one. Nozzles are rated in U. S. gallons per hour with typical sizes of 1.1, 1.0, 0.85, 0.75, 0.65, 0 60 and 0.50.
With conventional cast-iron head burners, you should be careful not to reduce the firing rate too much or incomplete combustion will result along with reduced furnace efficiency. If you have such a burner, you should consider reducing the nozzle only one size. In any case, with a conventional burner, the size should not be reduced below the minimum firing rate given on the manufacturer’s rating plate.
A large number of conventional oil furnaces have been retrofitted with flame-retention head burners, which has improved their seasonal efficiency. With a flame-retention head burner (see Figure 7) , the nozzle size can be reduced significantly, as good combustion performance can be maintained; the lower limit on the firing rate is governed by the flue gas temperature leaving the furnace. In general, you should maintain a furnace exit temperature above 204°C (400°F) if you have an outside chimney and 177°C (350°F) if you have a chimney inside the house. The proper nozzle size for your house and heating needs can be determined by your serviceperson.
There are a number of relatively straightforward things that can be done to improve combustion performance and the efficiency of an existing oil-fired furnace or boiler.
Install a Flame-Retention Head Burner
The performance of an oil-fired heating system depends to a large extent on how well air and fuel oil are mixed in the burner, a function performed by the atomizing spray nozzle that mixes air and fuel into a combustible mist.
Figure 7 Flame-retention head burner
Compared with the old cast-iron head burner, flame-retention head burners do a much better job of mixing air and fuel. This reduces the amount of excess air required for good combustion. The result is a tighter, hotter flame for the same amount of fuel (see Figure 8, page 32).
A flame-retention head burner can increase the seasonal efficiency of an old oil-fired furnace (rated at an AFUE of 60 percent) by about 15 percent. Because of the increased flame temperature and higher efficiency, it is recommended that the nozzle be reduced at least one size and that a ceramic fibre combustion chamber be used.
Flame-retention heads are now almost standard on new furnaces and can also be added to most older furnaces. Contact a reputable oil service firm or fuel supplier to discuss a flame-retention head for your system.
New advanced flame-retention head burners with high-static pressure can run at even lower excess air levels. The increase in efficiency can be close to 20 percent. At the same time, the burner is powerful and can overcome pressure fluctuations generated at the vent termination, producing a stable flame even under adverse weather conditions. The pressure drop across the burner head also stops heated house air from flowing through the burner, combustion chamber and furnace and out the chimney during the furnace off-cycle. Finally, the high-static pressure makes the burner almost completely independent of depressurization within the house, a good quality to have in a tight home. It can also run as a sealed combustion unit. Because of its many advantages, it is recommended that any new furnace you buy be equipped with this type of burner.
If you have made changes to increase the efficiency of your existing system, either by replacing your burner with a flame-retention head or high-static burner or by replacing your furnace or boiler, you should have someone take a closer look at your chimney. If it is a masonry chimney located on the outside wall of your house, it may be too big for the amount of gases that will now be going through it. The flue gases can cool down and condense in the chimney, causing deterioration over time. The installation of a stainless steel, double-walled flue pipe from the furnace to the chimney can help. If the problem persists, consider adding a stainless steel liner to the chimney. Aside from preventing condensation problems, these changes can improve the chimney draft and the overall performance of your heating system.
An oil-fired heating system wastes heat if incomplete combustion causes a heavy layer of soot to form on the inside of the heat exchanger. This is reduced by the use of a flame-retention head burner, although a significant amount of soot can still be produced at the beginning and end of each firing cycle, which can also cause oil smells in the house.
Soot formation and the associated odours can be dramatically reduced and even eliminated by having a delayed-action solenoid valve installed on the burner between the oil pump and the burner nozzle (see Figure 7, page 30).
Figure 8 Flame patterns with different burner heads
Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency