We have a Lennox SLP98V high efficiency furnace that got installed about 3 years ago. This weekend it stopped working – no heat – and the thermostat reported alarm number 276 (Watchguard – Calibration Failure)
The error code indicates that the low pressure switch is not properly closing. This switch makes sure that the combustion air inducer is working properly and drawing exhaust gases out of the combustion chamber.
Somewhat of a giveaway for the source of the problem were the gurgling noises that the furnace was producing while the combustion air inducer was running. Apparently the condensate wasn’t properly draining from the exhaust system.
First I checked the condensate pump that the drain pipe runs into but that seemed to work normally and no water was backed up into the drain pipe. Then I noticed the condensate trap on the right side of the furnace and sure enough it has “Clean Out” written on its lower cylinder – see the picture below.
Once I removed the cap (circled in red) a good amount of water and a bunch of combustion particles came out. Apparently the particles had clogged the condensate trap and water had accumulated in the exhaust system. Since cleaning out the trap the furnace is operating fine again and the gurgling noises are gone too.
Moral of the story – as our equipment gets more complicated we really need to study the user manual well and follow the recommended maintenance procedures. Lack of maintenance – for example clogged air filters – has always been a frequent reason for service calls to heating contractors but high efficiency furnaces take this to a new level. Not enough to regularly change the air filter anymore.