Oil-burning furnaces and boilers have a different venting system than gas appliances. Oil burners are forced draft and operate at higher temperatures than gas burners. Oil burners draw in combustion air through a fan, and the air mixes with oil droplets under pressure. There is some excess air, but like a gas furnace, not enough to ensure good draft. We need to add draft air.
The products of burning natural gas are primarily water and carbon dioxide. The combustion products from oil also are water and carbon dioxide, but they may also contain more particulate (pure carbon as smoke), nitrogen oxides, sulfur oxides, carbon monoxide and toxic gases.
Oil appliances do not use a draft hood like conventional gas burners. There would be a risk of combustion products entering the home through an open draft hood.
Oil burners operate under different conditions (for example, outdoor temperature, chimney temperature, start-up versus steady state), yet the goal is constant draft to ensure that exhaust flows up the chimney. The goal is a constant draft that allows flue gases to move at an appropriate rate. Excess draft results in faster-moving gases, higher temperature and wasted heat. We need something to regulate the draft and maintain efficient operation.
The barometric damper or draft regulator is a metal device built into the appliance exhaust flue (vent connector), typically 12 to 18 inches from the furnace or boiler. It ensures an adequate supply of draft air for the chimney. Its job is similar to the draft hood on a gas furnace, up to a point.
Oil burners, which are forced draft (the fan pushes air into the combustion chamber), require a single-acting barometric damper. This means that the damper swings only inward to allow draft air into the chimney. The damper won’t let the high temperature, sooty exhaust products back into the house if it is adjusted properly. Although the pressure is positive at the burner, the pressure in the vent connector can be at or below atmospheric pressure.
During normal operation, it is slightly below atmospheric pressure. That’s why we need to allow room air into the vent to balance the pressure, control the draft rate and allow the exhaust to flow up the chimney.
The damper pivots on its horizontal axis to allow room air into the flue when the flue pressure is lower than the room air pressure. This air also cools the exhaust and decreases the draft effect on the fire. The damper is weighted, and the adjustment of this weight is critical to the operation of the damper.
When the flue pressure is greater than the room air pressure, the damper swings closed to prevent flue gases getting into the living space.
Common problems with barometric dampers (draft regulators) include the following:
• Rusting • Misadjusted damper
• Inoperative damper • Spillage
• Missing damper
Rusting. The exhaust flue and damper are in a hostile environment and can rust. There could be perforations in the damper or the damper housing. Rust can obstruct the movement of the damper. A damper that is stuck, whether closed, open or partly open, can affect draft, efficiency and safety.
If the damper is stuck closed, the draft may be excessive, wasting heat. The chimney can also overheat, causing a fire. The draft also may be inadequate, leading to incomplete combustion, soot and puff back at the burner.
If stuck open, heat loss from house air going up the chimney will increase heating costs. A damper stuck open also could allow exhaust products, including carbon monoxide, into the house. This is a potentially lethal condition.
Rust holes also may allow excess heat loss up the chimney or dangerous combustion products into the house.
Look at the condition of the damper inside and outside with a flashlight. Water leaking, condensing or both onto the damper should be eliminated.
Check the movement of the damper with the furnace off by gently pushing the damper open with a screwdriver. (Note: If the appliance has been operating, this will be hot! Do not touch the damper.) The damper should swing back to the closed position.
If it does not move freely, check inside the flue at the damper for obstructions such as soot or rust.
When the furnace starts up, the damper usually swings open and remains slightly open while the burner is on. You can check for room air going into the flue with your hand or a tissue. You should never feel hot exhaust gases coming out of the flue. Remember not to touch the damper or flue.
Inoperative damper. This could be the result of the following:
• mechanical damage
• loose hinges
• poor fit
The implications are the same as those of rusting. Check the movement of the damper by gently pushing it open with a screwdriver. The damper should swing back to the closed position. If it is stiff, check inside the flue at the damper for obstructions such as foreign debris, soot or rust.
Missing damper. The damper may never have been installed or it may have been removed. The implication of a missing damper is no draft air. Exhaust products may not move properly up the chimney because they are not being supplemented with sufficient volumes of draft air. They may also move too quickly up the chimney. This affects combustion, possibly producing carbon monoxide and reducing efficiency.
Follow the exhaust flue from the furnace to the chimney. The damper usually is installed on the flue itself. Some dampers are installed at the base of the chimney and double as clean-outs. These are more prone to obstruction from debris in the chimney and rusting.
Some new, energy-efficient burners do not use a draft damper. These units, sometimes called high-static burners, have a powerful fan, capable of overcoming any atmospheric draft conditions. These units usually are labeled as not requiring a barometric damper. In situations where you cannot see a damper, and if you are unsure whether one is needed, recommend that a specialist check it out. Alternatively, note the make, model and serial numbers, and call the manufacturer or check the installation manual (often available online).
Misadjusted damper. The counterweight on the damper may be missing or improperly set. The damper may remain open when the burner is not in use, increasing heating costs, or the damper may not open sufficiently, reducing the volume of air available for draft purposes. This results in inefficient operation.
The damper should be completely closed if the burner is off. Check the damper position when inspecting the flue. Check the position again while the furnace is operating. The damper should swing open. It often opens wider on startup, then partially closes. The damper may swing slightly as exhaust products pass through the flue. The damper should close when the burner shuts off.
Spillage. Spillage may occur at the barometric damper if it is a double-acting damper, if the damper is stuck open or if the chimney flue is restricted in some way. Spillage also can occur if part of the damper is installed in the wrong place, is missing or is rusted out, creating a passage for combustion gases to escape. Spillage of combustion gases into the basement is a life-threatening condition.
With the burner off, look for any obvious holes in the damper itself. Look at the positioning of the damper in the exhaust flue. It should not be installed on an elbow, where combustion gases would bounce off the face of the damper. It should be installed so that the face of the damper is parallel to the flow of gases, and it must be in the same room as the burner.
Ensure that the damper is closed when the burner is off. With the burner operating, place the palm of your hand at the face of the damper, but don’t touch it! You should be able to feel the cold draft air moving across your hand into the damper opening. Any hot or wet gases moving across your hand out of the damper or black soot particles deposited on your hand mean that the damper is spilling.
In this article, we have looked at one component of one type of heating system. We strongly suggest that you recommend inspection by a specialist for any combustion appliance when people first take possession of a home, and we encourage homeowners to set up an annual service agreement to ensure that their appliance is maintained for safety, efficiency and longevity.
Unless you have special training and knowledge, approach the inspection of combustion appliances from a generalist perspective. We have talked about some of the common defects you can see with barometric dampers, but home inspectors do not use tools such as draft gauges to evaluate these or other devices. We are like the family doctor—a general practitioner, not a specialist.
For more information on inspecting heating systems, you can access the ASHI@HOME Heating modules. All 10 ASHI@HOME courses qualify for ASHI Continuing Education hours. Contact ASHI Headquarters at 847-759-2820, or Carson Dunlop at 800-268-7070.