Subject: Bryant Plus 90 FA furnace problem. HELP!

From: [email-masked] (Steve Carlson)
Date: Thu, 19 Oct 1995 16:19:57

Would appreciate any thoughts on what the following problem might be.

I will admit my skill level on forced air furnace installation would be 
considered NOVICE so pardon the lack of good terminology and common sense.  

Last winter I installed a Bryant Plus 90 natural gas furnace in my home.   
According to the manual I needed to vent the exhaust gases and supply fresh 
air (through an intake supply line) from the outside.  The manual (based on 
run length and number of elbows to the outside) sugested using 4" dia PVC 
pipe, although 2-1/2" would just be in spec too.  I had a run length of 
roughly 15 feet.  So I constructed an exhaust and intake system using the 4" 
PVC pipe.  The furnace ran, but seemed to run rough (flame was nice and blue 
though).  Let me explain what I call 'rough'.  It sounded like, and quite 
literally (the compressor fan? I think) pulsed and labored when it reached a 
harmonic resonant state.   When I disconnected the intake line at the furnace 
and let it suck air from the basement the furnace ran very smooth.   As soon 
as I connected the fresh air intake line back up the furnace (compressor?) 
would start to labor and shake.  It acted like the intake line were plugged.  
This was not the case however.  With the intake line connected I notice the 
furnace ran smooth when I gave it a little resistance by partially covering 
the intake.  

I then thought to try the next smaller size PVC pipe to see what would happen. 
I rebuilt the intake and exhaust with 2-1/2" PVC pipe (which the manual said 
was still in spec, but near the limit for my situation)  The furnace ran great 
when I used the 2-1/2" pipe. 

I now have moved the furnace (for other reasons) to a slightly different 
location.  I have a run length of 10 feet (so even more in spec for the 2-1/2" 
pipe), but the rough/pulsating/laboring problem is back.  Again, if I cover 
the intake ever so little the problem goes away.  What gives?  Is there some 
adjustment or setting I'm missing?  I would REALLY appreciate any hints with 
this.

-=Steve=-

__________________________________________________________
Steve M. Carlson                email: [email-masked]
3M IT                               phone: (612)737-1318
3M Center 224-4N-20
P.O. Box 33224
Saint Paul, MN 55133-3224
__________________________________________________________

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From: John Newby <[email-masked]>
Date: 21 Oct 1995 17:04:33 GMT

Steve, I have no specific experience with this furnace, so I'm going 
to comment based only on "typical combustion engineering priciples" in 
the hope that I may give you some helpful clues.

I suspect that the phenomenum you describe is related to a sensitivity 
of the unit to air/fuel ratio.

The resistance to flow of outlet flue and the inlet air duct (if any) 
systems will determine how much combustion air flows through the unit 
(in conjunction with the inlet air and outlet flue gas temperatures).

You described removing the inlet duct, or partially covering the inlet 
end of the pipe - in both those cases you altered the amount of oxygen 
being delivered to the combustion chamber (probably lowering it in 
both cases - obviously so in the second instance, and I suspect also 
in the first by supplying warmer air (less dense) at a lower overall 
differential pressure than when the inlet duct was connected).

It is quite typical for a combustion process to sensitive to the 
amount of O2 supplied - and normally the combustion becomes "harsher" 
with increasing excess of oxygen supplied to the flame.

An interesting experiment would be to get hold of a combustion product 
analyzer and determine the amount of O2 in the exhaust gas when the 
unit is in "quiet" mode.  Then connect up the inlet duct that you need 
and analyze again.  You could then try changing the resistance of the 
inlet duct to bring the combustion product analysis back to the "quiet 
mode" level if it has changed (increase the resistance of there's more 
O2, for instance).  If it hasn't changed then air flow isn't the 
problem and you've got some kind of harmonic resonance generated by 
the flame that is exacerbated by the inlet ducting shape or volume - 
you could try "tuning" that out by physical changes to the inlet 
ducting run.

There's a major caveat here - you must be sure that the exhaust gas 
analysis falls within the manufacturer's specs.  You must not reduce 
the O2 supply to the point that there is any chance of generation of 
excessive CO in the exhaust gas.  As domestic units must operate 
across a wide range of installation and ambient conditions there's 
usually a fairly wide margin built in, but you must be aware of this 
potentially serious problem - WHICH COULD HAVE LIFE THREATENING 
RESULTS.

One other point comes to mind - another poster has suggested gas 
pressure adjustment - which would, of course adjust the gas flow, 
rather than the air flow.  My suggestion would be that you check that 
the gas flow is at (or below) the manufacturer's specified limits 
before you do anything else.  He should be able to tell you the 
correct setting pressure.

Hope you may find something helpful here.  Please exercise caution in 
any work you do on this problem to ensure that you do not place 
anybody at risk.

John N.