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Why have your furnace cleaned?

 


The photos below are of my furnace.  It's been in service for about 15 years.  You know what they say about the mechanic's wife never getting her car repaired? It goes for every trade.  Frankly, I was surprised how bad it was.  Lesson learned: pay attention to the manufacturer's maintenance schedule.

An oil fired furnace typically burns #2 heating oil.  Very similar to diesel fuel, but without the road taxes.  The air and oil are sprayed into the fire chamber under pressure, and ignited with a continuous electric spark.  In MY furnace, as the oil burns, the hot combustion gases move up to six oval slots in the top of the fire chamber, and then move into six long oval tubes, which then end at the collection manifold.  From there the gases leave the furnace through the flue to the chimney.  The blower moves house air from the bottom of the furnace case, around the fire chamber, past the heat exchanger tubes, and then to the house duct work.

The first shot is of the furnace with the AC coil removed.  You can see the heat exchanger tubes.  The fire chamber is a large cylinder below these tubes.  

I have been noticing some oil smell when the furnace first fired up, so I wanted to check for a cracked heat exchanger.  That can happen when it over heats, or just from age.  Metal expands and contracts from heat, and cycles enough times, it may crack from fatigue.  Sure enough, I found four cracks.  They are at the base of the center two heat tubes.  Here is one shot showing what they look like.

Next I wanted to find out why I could tap on the center heat tubes and they sounded hollow, and the outside tubes sounded like they were stuffed with mud. So I pulled the cover off the breech end of the heat exchanger.  (The fire chamber is the cylinder below.)

When I pulled off the breech end of the heat exchanger I found this.

The shot above shows the right hand three tubes.  As you can see, the one in the center of the picture is open, but has junk in the bottom.  The middle one is about half open, but is questionable if it's getting hot gasses through it.  And the one on the far right in the photo is completely clogged.  

When the furnace is running, the gasses from the fire have to go somewhere.  If they all had to go out the two center heat tubes, then this would have over heated those two, and caused the cracks.  In addition, if only two of the heat tubes were carrying the hot gases, you can imagine how inefficient the furnace must have been. 

The bottom line is, when I recommend that you have the furnace checked by a competent service man, this is why.  I didn't do so for 15 years and you can see the end result.  

BTW, I sealed the cracks with furnace cement, and will replace the furnace next spring.  The cement is a temporary fix at best.  I was planning to do so anyway, but now I have a good reason.


 

 

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Last modified: January 02, 2012