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Posted by HVACGod on February 16, 2008, 8:30 am
>
>> Sorry in advance for the length of this post, but I wanted to try to be
>> as specific as possible on this problem.
>>
>> I have two 16+ year old Armstrong natural gas furnaces (model number in
>> subject line above) for my residence. Both identical spark ignition,
>> 125,000 BTU 5-burner units.
>>
>> I had a soot build up problem with the lower level unit (the one that is
>> used most frequently) that caused the flame on the left-most burner to
>> slightly roll out from the heat exchanger chamber, so I called a
>> reputable local service company to come out and clean both of the units.
>>
>> They removed the flue and front panel and then thoroughly cleaned the
>> sooted up heat exchanger and flue vent. (They found the heat exchange
>> cavity by the leftmost burner completely blocked with soot). They also
>> removed and cleaned all the burners too and then re-assembled everything.
>> In this process, they also discovered that the air mixture controls for
>> the 5 burners were shut down completely and opened them up (all the way
>> to max settings) to let the burners "breathe" properly. The flame tip
>> color turned a much deeper blue and the dark blue "inner" flame mantles
>> appeared to go down to about 3/4" long. The good news here was that this
>> stopped the orange tipped flame from rolling out of the combustion
>> chamber and resulted in a noticeably hotter flame. (I've also been
>> monitoring CO in the area around the furnaces and it has gone from 20+
>> PPM when the furnace was running to 0 PPM now.)
>>
>> Unfortunately, changing the air mixture seems to have introduced a new
>> problem. Now during extended runs of this furnace (for example when
>> coming up to temperature in the AM after a setback period the previous
>> night), the flue roll out switch trips shutting down the system. When
>> this happens it requires a manual reset of the roll out switch button.
>> If the furnace is simply "cycling" to maintain temperature, then it does
>> not seem to trip and everything is fine. The drafting on the furnace
>> seems to be good so I suspected that it was just burning too hot for the
>> switch.
>>
>> Thinking this was possibly a weak/old/failing roll out switch, I swapped
>> the switch with the one in my upstairs furnace, but this did not fix the
>> problem and it still tripped just the same (and the upstairs unit which
>> now has the switch from the downstairs unit did not trip). At this
>> point, I have tried backing off the air mixture to about 50% from being
>> wide open on all 5 burners, but this too seems to have had no affect.
>>
>> The only thing that I've found that does seem to prevent this switch from
>> tripping is to manually feather the gas shut off valve to the point where
>> I can observe a noticeable reduction in the flame height on the 5
>> burners.
>>
>> I'd appreciate thoughts on what is happening here. I do not believe it
>> is an issue with a blocked flue since both furnaces were thoroughly
>> cleaned and share the same flue stack/chimney and the upstairs unit does
>> not have this same problem. I suspect it may be that the air/fuel
>> mixture is still not correct or possibly that the gas pressure from the
>> gas valve to the burner orifices on the lower level furnace may be set
>> too high. (I am not sure how these are set.) My service company is
>> stumped and after being charged for two trips, I am not anxious to have
>> them come back till I understand what is happening here.
>>
>> Thoughts and ideas on what I can look at or try are very much
>> appreciated. I know these units are only about 65% efficient and getting
>> old, but replacement at this time is not an option since they are
>> otherwise still running well!
>>
>> Bob
>
> Replacement time is *ALWAYS* an option. From my personal experience with
> my customers, its not unusual for their utility bills to drop as much as
> 60% or more with a new system. Changing to a correctly sized 80+ furnace
> will make a huge difference, and even more so for a 90+ furnace.
>
> You might have to replace it sooner than you anticipated anyway... One or
> 2 cells sooting up, is a bad thing.. I would suggest that the heat
> exchanger be inspected for cracks... bet ya find some. Armstrong's paper
> thin, clamshell, heat exchangers(especially the older ones) are not noted
> for their longevity. When the cracks are found, then your not gonna have
> much of a choice in the matter other than spend the money and replace it
> or be cold.
>
Noon is absolutely correct - odds are better than not that there is a crack
in the exchanger(s). We carried Armstrong as a secondary for years until
issues with their heat exchangers outweighed their willingness to correct
them. Just like Janitrol/Goodman/Amana - if I see an Armstrong still in
service any time I'm in the field - irrespective of the season I dig into
the exchanger. If the cells have not already been replaced, a compromise is
found in 3 of 5 pieces of equipment, I would guess.
Aside for that 16 year old equipment is costing you more to operate than it
would be to replace with upgraded equipment in the big picture.
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