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HVAC Fire Burning Units General

The Old House Web

Parts of this story: Introduction >> Forced warm air heating systems >> Forced hot water (hydronic) heating systems >> Steam heating systems >> Electric resistance heating >>Air conditioning systems

Oil- or gas-fired furnaces and boilers provide heat to the majority of small residential buildings. Such fuel-burning units, whether they are part of a warm air or a hot water system, should be inspected as follows:

Location, clearances, and fire protection. Check that the unit meets local fire safety regulations. No fuel-burning unit should be located directly off sleeping areas or close to combustible materials.

burner label

Record all pertinent information from the manufacturers data plates on HVAC equipment. It will be useful in assessing the equipments capacity later.

Data plate and service records. Locate the data plate on each unit and note its date of manufacture, rated heating capacity in BTUs per hour, fuel requirements, and other operational and safety information. Examine the service records of oil-fired units. These should be attached to the unit or available from the oil distributor or company that last serviced the unit.

Seismic vulnerability. If the building is in seismic zones 3 or 4 (California and portions of Alaska, Arkansas, Hawaii, Idaho, Missouri, Montana, Nevada, Oregon, Utah, Wyoming, and Washington), check fuel burning equipment for the presence of seismic bracing to the structure.

Fuel supply. Gas supply lines should be made of black iron or steel pipe (some jurisdictions allow copper lines with brazed connections). Shutoff valves should be easily accessible and all piping well-supported and protected. Oil tanks should be maintained in accordance with local code or the recommendations of the National Fire Protection Association. All tanks must be vented to the outside and have an outside fill pipe. Buried tanks normally have a 550, 1000, or 1500 gallon (2080, 3785, or 5680 L) capacity; basement tanks are usually restricted to a 275 gallon (1040 L) capacity, with no more than two tanks allowed. Tanks must be located at a minimum of seven feet from the furnace and should be adequately supported and free of interior rust. Outside tanks at grade should have an adequate supporting base. Oil tanks often begin to leak after about 20 years, when the bottom of the tank corrodes from moisture that has condensed inside the tank and settled to the bottom. Feel along the undersides and probe the interiors for such leakage. Look for an oil level gauge and see whether it works. Decide whether the tanks should be replaced. See also the information on buried oil tanks in Section 1.2. Check the oil supply line to the furnace; it should be equipped with a filter and protected from accidental damage and rupture.

Ventilation and access. Make sure the fuel-burning unit has adequate combustion air and is easily accessible for servicing with at least three feet clear on each side of the unit requiring service. Check the local code for requirements. Also check equipment manufacturers guidelines for makeup air, especially where furnaces and boilers are enclosed in a finished basement or closet. A general rule is to provide one inch of free area across the width of the door to the furnace or boiler room or closet for every 1000 BTUH (300 W) of heating. The free area needed should be divided: roughly half at the bottom of the door and half at the top. A grille can also be used in the door.

Condition. Open all access panels and examine the external and internal condition of each unit. On hot air furnaces, look for signs of rust from basement dampness or flooding, and, if an air conditioning evaporator coil is located over the furnace, look for rust caused by condensate overflow. On hot water boilers, look for rust caused by dampness and by leaking water lines and fittings. If possible, check the condition of the interior refractory lining on all oil-fired units.


Look for signs of corrosion around and within oil storage tanks and check the operation of the oil level gauge. Use a dipstick to check for signs of condensation in the tank.

Ignition and combustion. Observe the ignition and combustion process.


Step away from the unit while someone else turns up the thermostat.

Look for a puffback in oil-fired units or flames licking under the cover plate of a gas-fired unit; both indicate potential hazards that must be corrected.

If the unit doesnt light, check the master switch or emergency shutoff to make sure its on, press the reset button, and try again. If it still doesnt light, call a service technician.

Once the unit has been activated, closely observe the combustion process. In oil-fired units, the flame should be clear and clean, and have minimal orange-yellow color. Flame height should be uniform. Gas-fired units should have a flame that is primarily bluish in color. Note whether the flame lifts off the burner head; this indicates that too much air is being introduced into the mixture.

Check gas burners for rust and clogged ports. Soot build up is a sign of inefficient combustion. In oil-fired units, look for soot below the draft regulator, on top of the units housing, and around the burner. The odor of smoke near the unit is another sign of poor combustion.


Consider having a service technician perform a flue gas analysis to determine the units combustion efficiency. This test requires the use of a flue gas analyzer and should be performed in accordance with ASTM D2157, Standard Test Method for Effect of Air Supply on Smoke Density in Flue Gasses from Burning Distillate Fuels.

Venting and draft. Check the smoke pipe between the unit and the chimney. It should have a slight upward pitch with no sags, preferably a minimum of 1/4 inch per foot. Inspect the pipe for corrosion holes, the tightness of its fittings, and the tightness of its connection to the chimney. Check for signs of soot build up in the smoke pipe. Consult local code requirements about the minimum size, required clearance from combustible materials, and number of smoke pipes entering the chimney. Newer, higher efficiency furnaces are not as prone to back drafting because of forced or reduced draft systems. When these systems are used with existing old flues, flues tend to fail early. Check for evidence of rust or leaking in the exhaust flue. Gas-fired units have a draft diverter that is located either on the exhaust stack of a boiler or built into the sheet metal casing of a furnace.


Have a service technician run the furnace or boiler through a complete cycle, then with a match or candle conduct a simple smoke test of the draft at or near the diverter. A draft gauge or CO tester can be used to detect an outward flow of hot exhaust gas; this indicates a hazardous draft problem that must be corrected.

Proper draft is critical to the efficient operation of an oil fired unit. A barometric draft regulator is required above the unit or on the smoke pipe. Inspect for open joints or cracks that allow excess air to enter the combustion chamber or the smoke pipe. All such openings should be sealed. The damper of a barometric draft regulator should be level, free of rust, and not damaged or altered. Improper draft from an oil furnace could cause a build up of carbon monoxide gas in occupied spaces. Have old flues cleaned by a chimney sweep or HVAC service technician. Have a deteriorated flue replaced.


Check the draft regulator by observing its motion when the heating unit is in operation. It should open as the heating unit warms up. The draft regulator is adjusted during the combustion efficiency test.

Operation. The operation of the fuel-burning unit will depend on the type of heating system in which it is used.

draft regulator

This barometric draft regulator should swing freely and open somewhat as the heating unit warms up.

Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.

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