The moisture condensates on the inside of the exhaust pipe when the moist air meets a cold surface.
Under very cold conditions usually outdoor temperatures below 5 deg Fahrenheit the condensation can cause ice to buildup inside the exhaust and cause blockage. The u-tube manometer will be zero even though the fan can be heard running.
This is a temporary condition that will clear itself on its own. In most cases temperatures for two or more days in the 20’s will correct the problem.
There is currently no industry-recognized way to prevent this from happening and the increased radon exposure during this time is minimal considering that the system reduced the radon in the soil prior to the occurrence.
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