Lochinvar knight fan speed

Hvac tech_2

I have a customer with a lochinvar knight that once a year when the temperature drops below zero it trips on fan speed sometime during the knight. Never any visible ice buildup on the concentric vent , it’s the flush style not the cone type. It’s had a new wiring harness. Any thoughts on ramp settings or outdoor reset not allowing it to ramp up enough when the heat is turned down at night. The vent is close to a inside corner but no overhang and it runs fine all year. It is propane fired
Any thoughts
Thank you

DZoro

Two things, concentric vent, and inside corner. Any possible way to standard 2 pipe it and get out of the corner? Sucks to re pipe but,,,,, it's probably going to always be a headache in extreme conditions, and good chance it is inhaling exhaust during the normal season also.

Hvac tech_2

Forget to sayIt’s propane fired

DZoro

Does the odr read approx. the same as the current outdoor temp?

Hvac tech_2

Within a few degree

DZoro

Hard to see the inside of the intake pipe on that style of vent. But I'm pretty sure you are getting frost inside the pipe causing the fan to attempt a rpm increase and trip out

Steve Minnich

Is there an indirect on this job? If so, are you limiting the fan speed for DHW?

Hvac tech_2

Yes there is a squire indirect but not limiting the fan rpm

Hvac tech_2

As far as the inside corner, it vents to the side of a brick chimney, so it’s a inside corner for a foot then it’s free

DZoro

If intake isn't frosted, can also check for a frozen regulator or insufficient gas line, check incoming pressure during a 100% fire during the cold snap.