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A Furnace Clean-and-Check Primer

Aug. 1, 2004
Clean and checks ensure that your customers' furnaces perform as safely and efficiently as possible,

by Glenn Taylor

It's that time of year again. Days are shorter, leaves are changing color, and the air is crisp. This can only mean one thing: It's time to perform furnace clean and checks on your customers' systems.

Clean and checks ensure that your customers' furnaces perform as safely and efficiently as possible, and prevent the no heat calls that can come on a Sunday afternoon in January. This service should also be promoted at a time that will help balance your work load at the end of the cooling season.

A Thorough Inspection
When performing a clean and check, begin by talking with customers and asking if they have any issues regarding their system:

  • Are there any abnormal noises?
  • How are their utility bills? Remind them of the forecasted increase in fuel costs and how their heating bills are going to increase this year. This type of information may prevent a customer education call later in the heating season. Youre also opening the door to talk about upgrading their equipment.
  • What about allergies?
  • Are they comfortable? Are there any hot or cold areas?

It's also important to explain what your inspection and cleaning process involves and why you do it. Discuss how having their heating and cooling system serviced twice a year will improve the efficiency, reliability, and longevity of their equipments life . This sets you up to talk about a maintenance contract.

After talking with the customer:

  1. Check filters. Replace if needed.
  2. Check burners. If they are cast iron, use a wire brush and compressed air to clean the rust that can occur during the cooling season.
  3. If the furnace uses a hot surface igniter to light the burner, watch it heat up. If it has a hot spot, its color will be a brighter orange color, or may even be white at this point. If so, the hot surface igniter is cracked and should be replaced.
  4. Make sure the burners light smoothly. Adjust burner air shutters if needed to obtain complete combustion of the fuel. This ensures safe, efficient operation.
  5. Check the temperature rise to ensure that its within the limits listed by the manufacturer. The maximum temperature rise can be found on the data plate.
  6. On newer furnaces, which prove the flame-by-flame rectification principle, measure the flame current. Use a meter that can accurately measure the micro amps of current.

In addition, measure the flame current to ensure its within acceptable limits. If its too low, there may be a problem with the furnace electronic control or flame sensor rod. The burner might also be rusted and not properly grounded.

If the flame current is low, put a jumper wire on a screwdriver and connect the other end of the jumper wire to the furnace burner frame, which is grounded.

Next, place the blade of the screwdriver into the flame between the burner and the flame sensor rod. If the flame current doesnt go up, the fault lies with the electronic control. If the flame current goes up, the furnace electronic control is okay and the fault is with the flame sensing rod or burner ground.

Remove the jumper wire from the screwdriver and ground the burner with the jumper wire. If the flame current stays high, theres a problem with the burner ground. If the flame current doesnt stay high, the flame rod is at fault. At some point, clean the flame rod with sandpaper. If the rod is burned badly, replace it.

  1. Check vent motor performance. On newer systems, particularly those 80%+ AFUE models, the vent blower wheel is made of galvanized sheet metal and can become corroded. Cooling combustion acids cause corrosion.

Use a manometer or slope gauge to measure the performance of the vent motor assembly. This reading will show if the vent motor is moving sufficient air through the furnace. Most manufacturers print this operating pressure on the furnaces pressure switch (or switches, if its a two-stage furnace).

  1. Inspect the vent system. If the galvanized vent system is corroded, the products of combustion are condensing in the vent system, which causes damage.

The amount of excess vented air in high-efficiency furnaces is far less than the amount from old standingpilot furnace models. Theres also no warm air going through high-efficiency furnaces during the cycle, as it once did in the older models.

Therefore, the vent system must be replaced with one that meets the national fuel gas code.

  1. Check fresh air ventilation — Ensure the system is correctly set up for the proper amount of fresh air ventilation, if required. If not done correctly, your customers could have high utility bills, cold air complaints, and a premature heat exchanger failure because of the excessive amount of cold, outdoor air coming into the system.

If the heat exchanger temperature doesnt heat up above the dewpoint temperature of the products combusted, condensation will form and cause the heat exchanger to fail prematurely.

  1. Check the heat exchanger for cracks, corrosion, or sooting. Sooting indicates improper burning of the furnaces fuel. This could be caused by improper manifold gas pressure, a wrong-sized oil nozzle, incorrect primary air shutter adjustment, and the incorrect amount of combustion air supplied to the furnace or the furnace's vent system.
  2. Check out the humidifier and electronic air cleaner for proper operation. Pre-filters should be replaced or cleaned as needed, along with the cleaning of the filter's collector cells.

If there is a bypass humidifier, and the homeowner is complaining that the house isnt staying warm enough in cold weather, take a look at the setup of the humidifier.

The bypass humidifier may be circulating too much air from the supply side of the furnace back into the return side, which will cause the furnace's limit to trip. Set up this way, the system will be cycling on the furnace limit, not the indoor thermostat.

To correct this problem, reduce the amount of air being bypassed from the supply ductwork through the humidifier back into the return ductwork.

Glenn Taylor is manager of technical training for Trane Residential Systems. He can be reached at 903/581-3042.