Furnace Maintenance Heats Up

October 30, 2000
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Most heating contractors have a standard schedule of maintenance items to be checked when their service technicians are performing a preventative maintenance service. But it is often the little, overlooked details that can be the key to preventing problems and saving you — and your customers — time and money.

Among those items I’d like to talk about are those that involve safety issues, are not commonly known, or those that can be beneficial to service techs with some further explanation.

Because of limited space and the importance of the combustion system, I have left out the electrical portion of the natural draft furnace.

Cleaning Up Soot

When a furnace is being cleaned, it can be tempting to only clean those furnaces that appear dirty.

One of the most common indicators of a dirty furnace is soot accumulation on the vestibule and in the heat exchanger, flue diverter, and flue pipe. Few technicians want to see this, because it entails tedious and messy work to clean up properly.

However, many times this could have been avoided during previous maintenance checks by cleaning the furnace before it began to look dirty. Some of the things that cause a furnace to soot can be easily prevented. Here are some tips.

  • Burners: Linear-type burners start getting dirty long before the problem can be spotted visually. The primary air entering the burner air inlet and exiting the burner with the natural gas draws dust particles with it. These dust particles begin to form an almost invisible coating of “lint” on the underside of the burner gas outlet. (See Figure 1.)
  • This coating of dust particles slowly builds up and changes the combustion characteristics of the burner until the combustion becomes rich and begins to soot the furnace. Burners developing this problem rarely appear dirty — unless they are critically inspected.

    A solution to this problem is to arbitrarily blow the burners out with nitrogen during maintenance. This eliminates this problem from occurring if done routinely.

  • Pilot assemblies: On occasion, you may run across a pilot that appears to be burning too rich (too yellow). This can be caused by debris in the gas orifice or in the primary air inlet of the pilot assembly. Pilots that have primary air inlets are known as aerated pilots. (See Figure 2.)
  • Most pilot assemblies do not have a primary air inlet, but those that do are not easily spotted because the air inlet is often hidden from sight.

    - If you suspect that a pilot is burning too rich, a simple fix is to remove the pilot tubing from the gas valve and blow nitrogen through the entire assembly. If the assembly has a primary air inlet, it will probably be purged if enough pressure is used.

    - If purging with nitrogen does not correct the problem, remove the assembly from the furnace and disassemble it. There may be debris behind the pilot orifice. I have found machines where bugs have built critter condos in the assembly, and close inspection was the only solution.

  • Heat exchangers: Many heat exchangers have baffles in them to slow down the combustion products, which enables more heat removal before they exit the furnace. These are not easily seen.
  • – If they become sooted up, they can cause drafting problems that can easily be overlooked when cleaning the furnace. They are inserted in the exchanger outlet and must be accessed by removing the draft diverter. (See Figure 3.) These must be removed to properly clean a heat exchanger.

  • Flue caps: When inspecting a furnace, ensure that the flue cap is in place. If it is missing, the flue will not draft properly, and the furnace can soot up.
  • – If a furnace is sooted, make sure the flue cap is not clogged with soot. A clogged flue cap will make the furnace soot up again in no time. If you go to the trouble of cleaning a sooted furnace and don’t clean the flue cap, you have wasted your time and your customer’s money.

    NEXT WEEK: Maintenance of the combustion air system, and a word on heat exchangers.

    Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and application training for service technicians. He can be reached at 602-943-2517.

    Publication date: 10/30/2000

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    Armstrong 100K BTU upflow furnace

    everett palmer
    December 24, 2008
    I've been up all night trying to figure out why my furnace flame keeps rolling out, taking out my rollout switch and putting the furnace into lockout. Internet info indicates that it is soot blockage of the exhaust path. However, it is a forced draft (induced) exhaust and does not seem to be blocked. Could this be caused by an incorrectly adjusted pressure regulator at the (propane) tank or the gas valve? The flame, although blue, starts up fine and then kind of "sags" only on one or two of the four burners. Then all the burners will shut off entirely.

    furnace failure

    Clyde Z
    January 9, 2012
    Your problem is that your burner tubes need cleaning. The reason why your furnace is locking out is a safety matter. FLAME SENSOR not sensing the flame so it locks out the furnace. So, I believe you need to clean your burner tubes (thats where the actual flames come out). Could be easy repair or cleaning, it all depends on how your burn tubes are connected to the manifold. Good Luck



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