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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7535
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Lennox SLP98UHV furnace pressure switch giving error codes.

Resolved Question:

I installed a new Lennox SLP98UHV furnace in our home. However, the air temperature in the basement was close to freezing. I should have warmed up the ambient air to between the recommended 16 C and 27 C. The furnace ran for a couple of hours, but I think was overrun with condensate and it shut down. It seems I can hear ice/water in the cold end header box or thereabouts when I try to restart it. I am attempting to thaw it out and am getting close. It runs for a few minutes, then shuts off and reads an error code of E227 - Low pressure switch open during trial for ignition or run mode. And error code E228 - Unable to perform successful pressure switch calibration. Does it sound like the condensate system froze, would that result in the listed error code readings? Can I just continue to thaw with hair dryer and warm the basement with heaters to solve my issue? Does the pressure switch now need to be changed or is the remaining bit of water causing the issue?

Submitted: 5 years ago.
Category: HVAC
Expert:  Phil replied 5 years ago.
Phil :

You are correct in your diagnosis and cure. It's just that there is still ice in it. The pressure switch is open because the combustion blower is not able to pull the slight vacuum required due to its exhaust tube blocked with ice.

Also, hair driers are slow; hotter driers can damage the plastic vent parts. Some manufacturers recommend using hot water over the frozen parts. Others recommend low wattage electric heater pipe wrap **RATED FOR USE WITH PLASTIC PIPE** over the full run of the vent pipe and insulating over it with fire rated fiberglass pipe insulation - there are local codes on that. It will be best to check with your local city plumbing inspector before you proceed.

If the vent piping is short you may want to consider boxing it in.

Let me know how this looks on your end, we can go from there.

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