This system will be either gravity or forced. You can determine which by whether there is a fan. In some of the old “octopus”-type furnaces, the fan might be difficult to locate. It might be on top of the unit 51⁄2 feet off the ground, or there might be fans in return ducts.
After the burner has fired, wait until the fan begins to operate. It should begin before the burner is shut down by the high-temperature limit control. If it begins after the burners are shut off, the fan controller is either faulty or out of adjustment. Also, if there is a canvas connection between the furnace and the main supply duct, check its condition. Torn or open sections should be patched. The canvas connection is used to isolate the supply ducts from the vibrations and sounds that develop within the furnace.
Be advised that with some older systems, the fabric connector may contain asbestos. Since warm air is constantly flowing across the fabric when the heating system is operating, the presence of asbestos would be considered a potential health hazard. See “Asbestos” in chapter 20.
When the fan is operating, is there excessive noise or vibration? There should be none. If there is a power humidifier, it is usually wired so that it will operate only when the fan is running. Turn the humidifier on by turning the humidity control up. If there are portholes in the unit, see if the fan or drum is rotating. If there are no portholes, listen to hear if the motor goes on and shuts off when the humidity control is lowered. Very often the humidifier is in need of a tune-up. This is usually indicated by excessive mineral deposits around the humidifier and the duct to which it is attached.
When the system is operational, check the flow and temperature of the air discharging from the various heat-supply registers. Put your hand in front of the register. A weak flow can be the result of air leakage in the distribution ducts, a dirty air filter, a loose fan belt, or the need for rebalancing. When the temperature of the discharging air in one room is lower than in other rooms, it is probably caused by a heat loss in the branch duct leading to that room. The return register can be checked by placing a tissue on the grille. Because the return duct draws air in, the tissue should stay in place without falling down.
The item of main concern is the heat exchanger. When this portion of the heating system breaks down, the furnace or its heat exchanger must be replaced. When the furnace is aging or excessive corrosion is noted, the condition of the heat exchanger should be suspect. Also, if the flames in a gas-fired furnace are unstable and appear to be dancing, this often indicates a crack in the heat exchanger. It is recommended that prior to the contract closing, the furnace and associated equipment be tuned by a competent service organization. At that time, the serviceman can check the condition of the heat exchanger. Checking the heat exchanger usually requires the use of sprays, propane torches, and mirrors or disassembling a portion of the furnace and should be performed by a professional.
If the furnace is the high-efficiency condensing type, check the unit and associated piping for corrosion. Corrosion is a major concern in these furnaces because of the acidic nature of the condensate. Sulfur in the fuel oil or natural gas can combine with the flue-gas condensate to form sulfuric acid. Also, when indoor air is used for combustion, if the air contains chlorides, the chlorides can combine with the water vapor in the flue gas to form a corrosive condensate containing hydrochloric acid. Chlorides in the air can come from laundry bleach, household cleaners, even chlorinated tap water. To counter these problems, most condensing furnaces use outdoor air for combustion. They also generally use a plastic pipe for the exhaust stack and condensate drain.
Check the condensate drainpipe for blockage and loose sections. The condensate pipe should discharge into a floor drain that is connected to the house sewer. If the floor drain is not lower than the condensate drain or there is no floor drain, a condensate pump should be used. It is not good practice to have the condensate discharge into a hole in the floor slab.
The air supply should also be checked for blockage. If the supply pipe is run through the roof, the exposed end should be shaped like an inverted U with the open end facing down. This keeps the rain out, and since the supply and exhaust pipes are generally near each other, it prevents exhaust gases from being drawn into the supply pipe.
With a condensing furnace, the exhaust vent should not be combined with the exhaust from another appliance such as a water heater. The exhaust gases can be vented through a side wall, the roof, or a masonry chimney. However, if connected to a chimney, the plastic pipe should run up to the top of the chimney where the exhaust gas can vent to the atmosphere. If the exhaust pipe terminates at the base of the chimney, the corrosive condensate that forms will attack the masonry.
If the exhaust pipe has horizontal sections, see if any low spots or sagging sections could trap condensate and thereby block the exhaust gas from venting. Horizontal sections should have a slight pitch back to the furnace. This is especially important if the section is near the termination by a side wall or attic. If the exhaust pipe is sloped away from the furnace, the condensing water vapor in the exhaust gas could freeze and eventually block the opening in the pipe.
Condensing furnaces are noisier than conventional furnaces. They have a higher airflow, which is needed to absorb the additional heat resulting from the added heat exchanger. The higher airflow produces more sound and vibration. There is also a draft-inducing blower that is needed to overcome the increased resistance to the flowing exhaust gases. Although the operation of a condensing furnace is noisier than that of a conventional furnace, it is generally not considered annoying. If you find it bothersome, record that fact on your worksheet. The system is in need of a tune-up and adjustment by a competent heating contractor.
Condensation of water vapor in the exhaust gases also occurs in a pulse-combustion furnace.
Consequently, the overall concerns of exhaust-gas venting and condensate removal, as discussed previously for condensing furnaces, apply. Because of the nature of the combustion process in a pulse furnace, the sound of sixty to seventy tiny explosions per second can be heard outdoors at the side-wall terminations of the air-supply and exhaust-vent piping. In those cases, where the sound is considered objectionable, mufflers can be installed on the supply and exhaust pipes to reduce the sound level.