Carrier Infinity 58MVb Code 13
They may be right. It may be a design issue. The ventor (combustion air inducer, goes by several different names) fan needs to suck against a minimum feet of venting on the older models (MVPs) and I worked on LOTS of them. Burner won't light/burn properly without a very specific air to fuel mixture. This resistance equals a certain airflow thru the heat exchanger which equals a certain amount of combustion air (Bernoullis principle), P1V1=P2V2 etc. Not enough combustion air and you can get a rollout. The low pressure switch and pressure switches are SUPPOSED to protect you against that but if they are out of calibration or the setting (design problem) is wrong then they won't shut you down. Adding some pipe will force the ventor to suck harder. It is an ECM motor in that ventor and may need more of a resistance to calibrate itself (ECMs are finicky that way). They design furnaces but there is NO way they can anticipate every install scenario with negative pressures in houses etc etc and that can affect the burner. After 32 yrs of seeing Lennox having revisions (in the model/series #) and every other brand, it takes at least 2-3 yrs B4 they get feedback from the field and tweak everything to get rid of nuisance tripping etc. Then they issue TSBs, technical service bulletins which only the serious techs bother to read. The rest just parts change their way thru life (another story). LOL
"Cut it twice and it is still too short".
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Last edited by yuri; 02-12-2011 at 09:02 AM.