Gas and Oil Home Heating Systems

forced air gas furnace example

Provided to $uper $aver by- Yale Wolk, Erinson Home Inspections

Gas and oil heating systems use combustion-i.e., they burn a fuel- to produce heat. The heat can then be transferred to air, as is the case with a furnace, and the heated air is then circulated through the house in a system of box-like enclosures called ducts. Or the heat can be transferred to water; in this case, the system is referred to as a boiler. The heated water is then circulated through the house in pipes, which feed into a device in each heated room that spreads the heat, typically radiators or baseboard convectors, or circulates beneath the floor or ceiling as a radiant heat system.

Combustion Can Produce Carbon Monoxide

Exhaust gases from combustion can contain carbon monoxide, an odorless, colorless and poisonous gas. When inhaled, it depletes the oxygen in your blood, and with enough exposure, can kill you. The threat of carbon monoxide is increased when the system is not operating efficiently, such as when it needs cleaning or adjustment, or when there is any blockage in the exhaust system. You best protection is an annual servicing of the system by a qualified heating and cooling system technician.

Exhaust gases are vented out of the house through a special pipe called a flue, which can run through a chimney or may be a seperate metal vent pipe. This exhaust gas system is seperate from the system that conveys warm air throughout your house. Combustion heating appliances and sxhaust flues deteriorate with age. If your gas or oil heating system is 15 years old or more, there could be cracks or pinhole leaks which could allow exhaust gases to escape. If they do, it is possible for them to mix with the air stream going throughout the house. This is how carbon monoxide spreads. As noted, your best safeguard is to have your system checked annually by a qualified technician. If there are any problems, have them repair immediately. If your aging system concerns you, replace it with a modern, high-efficiency model.

Your flue needs attention as well. Sometimes exhaust gases re-enter the house due to blockage in the flute. It can be blocked by a bird nest, or part of the flue lining itself. Get your chimney flue cleaned regularly by a chimney sweep so that block-ages can be discovered before any harm occurs. These are similar to smoke detectors, emitting an alarm if carbon monoxide is detected. Prices range from $30 to $80 depending upon the model that you purchase. Use them just as you would smoke detectors: put one near the furnace or boiler, such as at the top of the basement stairs, and another one outside your bedroom. The price for these devices are small compared to the consequences of a major build-up of carbon monoxide in your home.

Gas and Oil Hot Air Furnaces

Furnaces use either gas or oil to heat air, and then blow the heated air throughout your house. When you turn up the thermostat, burners ignite in a metal enclosure called a heat exchanger (sometimes called a combustion chamber in an oil furnace). The heat exchanger’s job is to absorb the heat from the burner flames and then transfer that heat to the air stream which will enter your rooms. The air stream will not start as soon as you turn the thermostat up, because the heat exchanger isn’t hot enough. There is a built-in delay so that you won’t have cold air blowing through your house.

When the heat exchanger does get hot enough, a large fan (blower) starts. The blower forces air around the heat exchanger where the air is warmed. It is then blown through box-like passageways called ductwork. The heated air is introduced to individual rooms by small opening in the floor or walls, called registers. These typically have adjustment controls on them so that you can open and close the air flow in specific locations. There may be damper controls on the large ducts leading from the air supply side of your furnace:metal levers, which move in a limited arc, marked “closed” at one of the arc and “open” at the other. The lever controls a metal plate inside the ductwork, mounted in the air stream. When the control is in the “open” position, the plate is parallel to the air stream, allowing it to flow freely. When “closed”, the plate moves perpendicular to the air stream, blocking the flow. These damper controls help you to direct and control the air flow in different sections of the house.

Most furnaces recycle air from the house for combustion and for the supply that provides heat to your rooms. There are air returns located at strategic points in your home, typically on each floor. Generally they are large open grilles placed in walls, though in older homes they may be small, the same size as the air supply registers. Some new gas furnances do bring outside air directly into the furnace for combustion: the 90+ units, (indicating an efficiency level of 90% or more). These furnances use plastic pipe for drawing outside air for combustion, and for exhausting waste gases from combustion. Even though they draw outside air for combustion, house air is still re-circulated through then to be warmed and sent to the rooms again.

Burners. Gas furnaces typically have multiple burners, generally 3 to 6 of them. Heating capability is measured in British Thermal Units/Hour, or BTU/H, and as a rule-of-thumb, each burner has a heating capacity of about 20,000 BTU/H. Oil furnaces, on the other hand, have a single burner, generally mounted at the bottom of the unit. It is often referred to as a gun burner because it “shoots” the flame into the combustion chamber. The most advanced burners on oil furnaces incorporate an air intake and mixing system that spins, much like the blades of a jet turbine. This boosts the temperature and the efficiency of the system, and pouts contemporary oil furnaces on par with high efficiency gas furnaces.

Furnace Filters. Return air picks up dust and dirt in your home-regardless of how well you keep your home clean-and needs to be cleaned before it recycles through the system again. That’s the job of the filter. The filter not only keeps the operating parts of your furnace clean, but it also cleans the air you breathe. The most commonly used filters are the slide-in, disposable filters which you can get at your local hardware store. Generally they are inexpensive, particularly on sale. Some disposable filters contain more filtering material, or they have had a special process applied which results in an electrostatic charge, and will cost more. Other filters are washable. These are made of plastic or metal mesh, and can be cleaned with a hose or by putting them under a faucet.

The top-of-the-line filter is an electronic or electrostatic filter. These use an electrical charge to ionize particles of dust, dirt and pollen, which then stick to a special screen. The electronic filter is a special installation, and expensive ($600-$800), but is extremely efficient at removing pollutants from the air; most will remove over 90%. These too should be cleaned regularly, so they are designed with sections that slide out for cleaning. Whatever type of filter you may choose, remember that a clean filter is key to proper furnace operation, and it is key to maintaining health as well, so change or wash your filter every 30 to 60 days, regardless of the type.

Maintaining Your Furnace

Maintenance is the key to dependable and efficient service from your furnace, and to maintaining safety. Both gas and oil furnaces should be serviced annually. A gas furnace is subject to rust, which can be caused by excess water vapor in the exhaust. When natural gas is burned, water vapor is one of the exhaust byproducts. In today’s higher efficiency furnaces, a poorly adjusted and poorly serviced gas furnace may be subject to excessive water vapor build-up in the exhaust, and some of it can simply come back down the flue into the heat exchanger. Rust can start to build up, weakening the heat exchanger walls. the result can be cracks or pinholes in the heat exchanger that can allow the exhaust gas to mix with the house air you are breathing. Carbon Monoxide in the exhaust can enter your home that way. Annual maintenance is a major step toward preventing rust build-up and the problems it causes.

Oil furnaces need annual tune-ups. All of the operating parts should be cleaned, and the air/fuel oil ratio coming into the burner must be properly calibrated. Otherwise, the oil furnace can start to produce soot (a black, powdery residue) due to inefficient combustion. This soot travels out the exhaust and can settle on your home’s exterior. When it rains, the water can combine with soot to stain and streak your roof and home’s exterior. Proper annual maintenance can prevent this. An oil furnace has an oil filter in the supply line between the oil tank and the burner, put there to filter out impurities that may be in the line. This filter needs to be changed each year as part of your annual tune up.

Hot Water Boilers

Homes heated by hot water use a boiler which feeds heated water through pipes to radiators, baseboard convectors, or pipes which run under the floor or in the ceiling in systems referred to as radiant heat. When you turn up the thermostat, burners ignite below the tank (ater jacket) to heat the water inside. The burners ignite below the tank (water jacket) to heat the water inside. The burners may use either gas or oil for fuel. When the water reaches a pre-set temperature, a circulating pump comes on, which forces the water through the system. The hot water circulates through the house, and then returns to the boiler in a seperate piping system.

Hot water heating uses a low-pressure system. Pressure is measured in pounds per square inch (psi), and when the system is cold, the pressure should read approximately 15 psi. When the water is heated, the pressure should not measure above 30 psi. Should it exceed that, safety devices will automatically trip and the system will shut down.

Expansion Tanks. The hot water boiler is 100% full of water. When the burners come on, the water heats and expands. The system uses an expansion tank to receive a portion of the expanded water so that the system will stabilize pressure and then circulate water through the house. In older systems, the expansion tank is an elongated cylinder, filled with air. As the water expands, it pushes its way into the tank against the air cushion is pushing back against it. That keeps proper operating pressure in the system. Then when the system shuts off, the water cools and the cushion of air pushes the cooled water back to the boiler, refilling it.

Newer hot water systems use a different style of expansion tank, a squat, round cylinder-often red-often called a compression tank or an extrol tank. These are located just above the boiler itself, sometimes on the side. Rather than using air as a buffer, they contain a thin, flexible, diaphragm membrane. This membrane stretches as the expanded hot water pushes against it, then pushes water back as the water cools.

Older expansion tanks can become waterlogged: the water absorbed the air in the tank and there is no cushion. They can fill completely with water. This acts to reduce the amount of water available to the boiler and the remainder of the system, and pressure in the system can rise above normal. The system will then shut down as safety devices are activated. The pressure relief valve will activate and the pipe on that valve will discharge water. If this occurs, the remedy is to drain the expansion tank. To do this, follow these steps.

First turn off power to the system at your electrical service panel; look for the circuit breaker of fuse on that circuit. Then turn the expansion tank shut-off valve to the “off” position. Under the expansion tank you will find a faucet-like drain valve. Attach a garden hose to it, and open the drain. Next look for a small air-release valve on the tank itself. Use a wrench and open it. The tank should now drain the water inside. When the tankhas drained, reverse the steps. Close the air release valve, then close the drain valve on the expansion tank. Open the shut-off valve part way. At that point you should hear water running back into the expansion tank. When the sound stops, open the shut-off valve the remainder of the way. Check the pressure gauge, and when it reaches 15 psi (pounds per square inch), turn the power to the system back on. One final step: when the system warms up, you will have to bleed air from the radiators or convectors. Start at the top floor and work down.

Water-loggin should not occur with a membrane compression tank. If it should occur, your tank probably has a ruptured diaphragm inside. In this case, the entire tank will have to be replaced. Shut the system down and call your service person.

Safety Devices. A hot water system has numerous safety devices that automatically trigger in case of a malfunction. There is a pressure reducing valve which reduces the pressure of the water coming into the system to the desired 15 psi level. There is a pressure relief valve which will trip if the pressure in the system exceeds 30 psi, releasing excess-pressure water down an attached pipe. (This is similar to the TP valve on your water heater.) There is a burner aquastat- somewhat similar to a thermostat- that monitors the temperature of the water in the tank and shuts off the burners if the temperature gets too high. And there is also a pump aquastat, which is, strictly speaking, not a safety device, but rather it automatically controls the operation of the circulating pump. When the temperature of the water reaches the pre-set point, the aquastat turns on the pump, and allows it to run until the water temperature drops to a second pre-set level. Each of these controls help to make the hot water heating system a dependable and enjoyable source of heat.

Steam Heating Systems

Hot water systems are replacing steam systems wherever possible. The two systems are similar in most respects, but with two significant differences. A steam system works by gravity: the hot moist air rises through the house withough being forced by a circulating pump, whereas most hot water systems use a circulator to boost the hot water flow throughout. The other distinction is that steam boilers are not 100% full of water like those in the hot water system. A steam boiler will have 50-75% of its volume occupied by water, and the remainder occupied by air. The combination of water-usually with minerals in it-and air leads to a build-up of two waste products: scale and sludge in the boiler and piping. These must be cleaned out on a regular basis if the system is to function. It is this cleaning requirement that has led many homeowners with steam systems to convert to hot water systems when possible.

To learn more about how maintaining your home and heating system can save money, visit  www.jan-leasure.com