Low Heat

Low Heat

RUNS TOO MUCH & LOW HEAT – The gas furnace seems to run too long or the furnace kicks on too often. 

  • May be the result of a periodic safety shutdown that causes the furnace to stop burning during a heating cycle. 
  • This is often accompanied by a higher-than-normal gas bill and inadequate heating in extreme weather.

POSSIBLE PROBLEM & DESCRIPTION

Clogged Air Filter

  • A dirty or clogged air filter will prevent the furnace from delivering the proper amount of heated air to the space and will often cause the furnace to cycle on and off on its over-temperature device. 
  • A dirty air filter will commonly not cause noticeable problems during mild weather conditions, but may cause insufficient heating during very cold conditions.
  • A clogged air filter will generally be identified by a weak flow of air from the registers that is very warm or hot.
  • If these symptoms are identified, try removing or changing the filter to see if the conditions improve.

Restricted Air Flow

  • Similar to a clogged air filter, restricted airflow will also prevent the furnace from delivering the proper amount of heated air to the space and will often cause the furnace to cycle on and off on its over-temperature device. 
  • Restricted airflow will commonly not cause noticeable problems during mild weather conditions,  but may cause insufficient heating during very cold conditions.
  • Restricted airflow will generally be identified by air from the registers that is very warm or hot.
  • Common causes of restricted airflow are dirty or clogged air conditioner coils, too many closed or covered registers, or bad / under-sized ductwork.

Open Return Air Ducts 

  • The blower on a gas furnace should pull air from the conditioned space (through the return air duct system), heat it, then deliver it back to the space (through the supply air duct system).
  • Open return air ductwork, in a crawlspace or attic will drastically reduce the furnace's ability to heat the space.
  • Instead of pulling air from the space, open return air ducts will draw-in cold air that must be reheated.
  • A test during cold weather conditions to see if your return duct system is drawing-in cold air:
    • 1) At the thermostat, turn the system switch "Off" and the fan switch "On".  
    • 2) Check the temperature of the air coming out of your supply registers.  In ideal conditions, the air temperature should be the same as the temperature in the conditioned space.  If the temperature is much lower than the conditioned space temperature, this would indicate openings in the return duct system.

Open Supply Air Ducts 

  • The blower on a gas furnace should pull air from the conditioned space (through the return air duct system), heat it, then deliver it back to the space (through the supply air duct system).
  • Open supply air ductwork will also drastically reduce the furnace's ability to heat the space.
  • Instead of delivering heated air back to the space, open supply air ducts can waste heat by blowing treated air into the crawlspace or attic.
  • Supply air registers that do not blow air when the blower is on, or registers that have cool air coming out of them with the blower off may indicate open supply ducts. 

Blower Door Open 

  • Do not allow your furnace to operate with the blower door open.
  • The open blower door may pull-in cool air or prevent proper air circulation.

Undersized Furnace 

  • Gas furnace heating output is rated in BTU (British Thermal Unit).
  • A gas furnace must be sized with enough BTUs to adequately heat your home in cold weather conditions.
  • An undersized furnace may perform adequately in mild weather, but struggle to keep-up as the outdoor temperature drops.
  • Most furnaces have a rated temperature rise between 45 - 75 deg F.  That would be the difference between the air entering the furnace and the air leaving the furnace.
  • If the temperature rise is correct but the furnace struggles to keep up in cold weather, this may indicate an undersized furnace.

Blocked Vent Pipe

  • A blocked vent or intake pipe on a gas furnace should produce a pressure switch fault, but if only partially restricted, a furnace may run normally in mild weather conditions and only have issues during cold weather and long run cycles.
  • Common causes of a blocked vent pipe are external restrictions (such as material placed against the pipe outlets), internal restrictions (such as leaves, birds or bird nests inside the pipe), or water build-up caused by an inability to drain water from the vent (improperly sloped or clogged drain).

Blocked Drain Line 

  • 90% or higher efficient gas furnaces produce water when operating.  This water must be drained out of and away from the furnace.  If the drain line becomes clogged, the water will build-up in the furnace and eventually cause a pressure switch fault. 
  • A slow drain may show no symptoms during mild weather conditions and only cause a problem during periods of long run-cycles.
  • A common cause of blocked furnace drains during very cold weather is freezing.

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