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.
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.
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.
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.
– 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.
– 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