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I have high efficiency natural gas furnace. In the picture you see intake and exhaust vents on the outside of the house. Specially in colder days temperature reaches below target temperature, thermostat is saying Heat On but heat doesn't turn on. I have to go out of the house and blow little bit of air into exhaust vent. It turns on. Any permanent fix? Also, I found that there is a debris inside it, I pulled out what I could. Couldn't clean completely.Furnace exhaust on the right and intake on left

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A high-efficiency furnace incorporates pressure sensors in its combustion air path to measure the airflow. These sensors are delicate because they must be sensitive. The pressures they monitor and detect are small -- about what it takes to inflate a toy balloon.

When a pressure sensor begins to fail or act strange in any way, the furnace controller errs on the side of caution and shuts down. Lighting the burner without adequate airflow is very dangerous.

You can not tell if one of these sensors is working properly just by looking at it. Sometimes, if a sensor is almost working, you can tap on it or blow into it to make your furnace start. In that case you know you must replace the sensor soon.

In your case you must begin by thoroughly cleaning the debris from both intake and exhaust vents. Check the entire airflow path for any obstructions.

If the sensors are connected to the airflow path by thin pipes or flexible hoses, remove, clean, and replace these. If the sensors are mounted directly on the vent pipe, you can't do much more than make sure the sensing surface -- inside the vent pipe -- is clean.

If, after you've unblocked and cleaned everything, you can still only get your furnace to light by blowing into the exhaust vent, then you need to call the repairman.

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  • Thank you so much for such detailed explanation.
    – HKA
    Jan 24, 2022 at 15:18

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