Came on 02/16/26 and tested basement system. Furnace will intermittently come and off within seconds. Not steadily heating. At 1st we diagnosed the pressure switch and found that pressure switch was intermittently loosing power. We replaced the pressure switch, but system kept acting the exact same way. When we disconnect the flu pipe, system heats up properly with no fault. So we know problem was coming from that flu pipe. We run a scope and did not see anything clogging flu pipe. We vacuumed and put pressure in the flu pipe to see if anything was on the way. We noticed that there are more than 3 elbows in the flu pipe and flu pipe is more than 30 feet with a 2” pipe. Per manufacturer, if there more than 2 elbows in flu pipe, and pipe is over 20 feet, the flu pipe should be run with 3” pvc pipe. Additionally, there is no fresh air run to the system. Furnace is in a tight spot. Consequently, the flu pipe needs to be upsized and fresh air vent should implemented. We need to send a