Came out to a no heat. After performing a diagnostic check. We found that the system was working and power had been restored. The customer said that it must of just happened. We continued to investigate the system and we found that the PVC flue pipe had previously frozen over. This happens because the 90% furnace utilize a secondary heat exchanger to extract maximum heat from combustion gases. This process cools the exhaust gases below 130°F, causing water vapor—a natural byproduct of combustion—to condense into liquid water, which must be drained via a PVC pipe. During extreme cold temperatures this can cause the flue pipe to freeze up and cause a restriction that can shut the system down until it thaws out. We recommended adding a supply cut out in the basement to help to prevent freeze ups at the furnace as well as water supply lines and drain lines.