My furnace ductwork is getting between 200 and 230 degrees Fahrenheit and the vents are around 111 degrees. What would be the cause?

  • 11
    The furnace is working?
    – crip659
    Dec 23, 2022 at 23:14
  • 1
    I've given a generic answer below. If you want specific numbers for your furnace and can't find them on the data plate, I may be able to provide them if you edit in the model number.
    – KMJ
    Dec 23, 2022 at 23:23

3 Answers 3


The furnace should have a name plate or sticker (possibly inside) that indicates the maximum outlet air temperature as well as the expected temperature rise (typically 40-70F). 110F out of the vents themselves certainly sounds high to me.

If air temperature coming out of your furnace exceeds the upper limit of temperature rise given on the heater's data tag, it needs repair. A too-hot furnace heat exchanger can be damaged, might crack, and could leak dangerous combustion gases or carbon monoxide. This might result in a house full of dazed, disoriented, or dead people.

Have you checked the filter? Are you trying to shut off the heat to parts of the house? The necessary repair might be as simple as replacing the filter and making sure vents are open. If the filter incredibly clogged it could be causing too low of air flow, resulting in a high discharge temperature from your furnace. Check the filter, check to make sure return air vents are clear, check to make sure that supply vents are not all shut down with the dampers.

If you have to use the system right now because of the cold snap, be certain your carbon monoxide detectors are working. In fact, make sure they are working no matter what, as they're a critical part of furnace safety.

Once you've checked and eliminated the items above and validated against your furnace data plate, if you're still out of range on temperature it's time to schedule service for your furnace.

  • 4
    +1 for this answer and I will reiterate - check your filter and make sure it is not too restrictive. Don't let the marketing sucker you into putting in a MERV 11 or 13 filter into a system that wasn't designed for that. You're killing your airflow and potentially damaging your heat exchanger and no matter what they say, your residential HVAC system is never going to exchange enough air to trap all the stuff that filter can anyway.
    – Chris O
    Dec 23, 2022 at 23:36
  • 3
    @ChrisO a four or five inch deep MERV 11 can flow really well. I run a MERV 12 five inch thick filter at home, and my insufficient return area is way more of a problem than that filter. Just check the specs if you're going to use one; they have upped the game in tech lately and aren't as bad as they used to be in terms of airflow.
    – KMJ
    Dec 23, 2022 at 23:51
  • 1
    Yeah, I didn't consider thickness. I was mostly referring to the 1" filters most residential furnaces are set up to take. You're right that for a given filter rating, the thicker filters flow more CFM.
    – Chris O
    Dec 24, 2022 at 4:13

I had the belt on a furnace blower break. It sounded like the furnace was working per normal but the motor wasn't driving the fan without the belt.

The result was that the furnace would run constantly as the air wasn't getting to the thermostat efficiently. It would heat the house but just via convection. Since the furnace was on all the time the metal vent components all were very hot to the touch.


Your furnace should have a bonnet control which generally has an over temperature sensor in it as well. Many of these are adjustable. This will limit the temperature output of the furnace. If the safety trips it will shut it down. The burner puts out a constant amount of heat, it is the responsibility of the blower to move enough air to keep it within operating range. Restricting this will cause problems similar to what you have. Simply remove the filters and see what happens. If that solves it your filter is to fine or very dirty. Some older furnaces had adjustable blower speed control, some with the motor others had a variable speed sheave on the motor. Good Luck. Hint when kids go to school in the fall have the furnace checked.

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