LARGER VIEW of an octopus furnaceHow to Diagnose & Repair No Heat - Loss of Heat for Warm Air Furnaces

  • DIAGNOSE & FIX HEATING PROBLEMS-FURNACE - CONTENTS: No heat service procedures for warm air furnaces: how to diagnose loss of heat and how to get the furnace working again; when do you need to call a heating service company? What to check first. How to diagnose loss of heat in a building with warm air heating equipment. Troubleshooting heating system furnace, burner, controls, or heat distribution problems. How to inspect & repair hot air heating systems - Furnaces. Duct system inspection, defects, repairs.
  • POST a QUESTION or READ FAQs about how to fix a warm air heating furnace that is not working - how to diagnose the loss of heat in a building when the heat source is an oil or gas fired furnace.

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Warm air heating furnace troubleshooting:

How to diagnose & fix hot air furnace heating system problems. This article series answers most questions about hot air or warm air heating system troubleshooting, inspection, diagnosis, and repairs.

This article explains the diagnosis and correction of no heat conditions for warm air or hot air furnace and air duct heating systems. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

We explain how to recognize and fix common heating system operating or safety defects and how to save money on home heating costs.

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How to Diagnose Loss of Heat with Hot Air Furnace Systems

Wam air furnace and duct system sketch (C) Carson Dunlop AssocThis article explains how to diagnose and fix warm air or hot air furnace heating systems that are not working. Carson Dunlop Associates' sketch (left) shows the main layout of a basic warm air heating system.

[Click to enlarge any image]

If you don't know whether your heat is provided by a furnace (hot air) or boiler (hot water), or whether your fuel is oil, gas, or electric, and whether your heat is hot water, steam, or warm air,

If your heating system uses water or hydronic or hot water or radiator or baseboard hot water heating systems, instead of the article found here you should

Note: some hybrid or mixed heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator) - described

How to Diagnose Loss of Heat with a Warm Air or Furnace Heating System

Heaing system emergency switch (C) Daniel Friedman

  • If the furnace oil burner or gas burner is not turning on at all check these points first:
    • Electrical power to circuits powering the heater (or air conditioner or heat pump) may be turned off: at the main electrical panel and sub panels, check the circuit breaker or fuse supplying power to the unit; check the service switch at the furnace or air handler, and if the system is air conditioning or a heat pump, check the service switch outside at the compressor/condenser unit to be sure it is "on".

      Don't forget to check for loose or corroded electrical connections. A bad connection at power or at a control connector can cause total loss of heat or intermittent or on-off heater operation.
    • Check Furnace Electrical Switches and Live Power: Is the furnace turned on? Check the electrical service switch at the furnace as well as others that may be present in the home. Is there electric power to all of the heating system components such as an oil burner at the furnace and power to the blower fan?

      Has someone left the inspection or furnace access door open on the furnace? (Modern furnaces have a switch which forces the furnace to turn off if the inspection door is opened - to avoid injury to someone working on the blower assembly.)
    • Oil Burner safety reset switch is off: the heating or air conditioning system may have been turned off on reset - see our "off on reset" and other advice
      at  CAD CELL RELAY SWITCH (modern equipment) and

      also  STACK RELAY SWITCH (older oil fired furnaces and boilers).

      Cad cell control or stack relay buzzing or jamming trouble: if you hear or have ever heard buzzing from any heating system control that includes a relay switch, watch out for a control cover that is in contact with a relay switch in the unit. If the cover is lightly touching the relay switch the control may simply buzz when the relay is trying to move.

      But a too-tight control cover can also prevent a relay from operating, leading to loss of heat. Simply pulling off the cover to let the relay move freely can immediately discover and "fix" this problem. Details and an explanation are

      Where are all the heating system reset buttons? If you are looking for the main reset button on oil fired heating equipment you'll want to see:

      on older oil fired furnaces, Stack Relay Switch on older oil fired boilers and furnaces.

      For gas fired furnaces

      and SPILL SWITCH, FLUE GAS DETECTOR (gas fired equipment),

      At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.
    • Check the heating fuel supply for the furnace: if it is a gas furnace, is the gas turned on? If your gas is from bottled or LP gas, is the tank empty? Check that no one has shut the gas supply valves.

      If it is an oil fired heating furnace, check the gauge on the oil tank. Are you out of oil?
      See OIL TANK GAUGES for details

      An oil or gas fired heating furnace may go off on safety reset even if there is a good fuel supply. And at an oil furnace, even if there is plenty of heating oil in the tank if the oil filter has become blocked the system will try to start but won't run - this will require a service call.

      Check for a broken or slipping coupling (or a frozen coupling) between the oil burner motor and the fuel unit. In this case the burner starts in response to a call for heat, but there will be no ignition whatsoever because no oil is being delivered to the nozzle assembly.

      Check these additional causes of no oil flow to the oil burner: an oil safety valve that has been "closed" (someone turned it the wrong way), low oil or no oil in the heating oil tank (the tank may not be completely empty but oil may be below the oil pickup line if the oil lines are exiting at the top of the tank), a clogged oil filter or fuel unit strainer, a clogged oil burner nozzle, a air leak in the heating oil supply piping, or a crimp or clog or restriction in the oil piping.

      If you recently ran out of oil and then got a delivery, and if your system uses a single oil line between tank and burner, the line or fuel unit may be air bound and needs to be bled. [8]
  • Once you have assured that you have heating fuel and that electrical power is turned on to the heating unit check these controls:
    • Heating Thermostat is in the "off" position or is set to a temperature that does not call for heating (or cooling) See FAN AUTO ON THERMOSTAT SWITCH for details about what turns the blower fan on and off.

      Condensate tray overflow switch: If the fan blower unit combines heating and air conditioning, an air conditioning condensate pan float switch could be in the "shut off" position due to water in the condensate overflow pan or because the switch has been damaged or moved.

      Fan limit switch may be damaged or inoperable.
      See FAN LIMIT SWITCH the flue gas spillage switch: If your furnace is gas-fired, a flue gas spillage switch may have shut the system down after sensing possibly dangerous flue gases (that can contain fatal carbon monoxide).

      See SPILL SWITCH, FLUE GAS DETECTOR to check the flue gas spillage sensor found at gas fired heating equipment draft hoods or near gas burners
    • Stack relay switch may be installed on the flue connector of an oil fired furnace and may be in the "safety - off" position. To identify and reset this control see STACK RELAY SWITCH
    • Loose electrical connections: Here is an example:

      Then the furnace draft fan began to pulse several times upon starting. After making sure the control module showed a steady green, I asked a friend and he suggested that I wiggle all the spade connectors to be sure there is good contact between the copper and aluminum. It seems that there only needs to be a little corrosion to cause problems. That seems to have done the trick. . - M.F. 12/24/2014

  • No heat or not enough heat or not enough warm air volume from the Furnace: is any air coming out of the supply registers? Is it warm or cool?

    No warm air comes out: If no air is coming out of supply registers, is this true throughout the building (the blower is blocked or not running) or just in some areas (some registers or duct sections are closed or blocked)?

    Heating air comes out but it is too weak or slow: check for a dirty clogged air filter
    see AIR FILTERS for HVAC SYSTEMS , a dirty-clogged blower fan, blocked, disconnected ductwork.
  • Check the warm air heat thermostats: is the thermostat(s) turned up above ambient air temperature in the occupied space? Is the thermostat set to "heat" position (if a heating/cooling/off switch is present on the thermostat). See the discussion of thermostat switches
  • Check furnace blower operation: Does the furnace cycle on, the oil burner or gas burner runs, but the blower never comes on? If so the fan limit switch will turn off the oil burner when the heating supply plenum temperature limit is reached. Normally the blower fan comes on when the fan limit switch senses that supply plenum temperature has reached the "cut on" point, and the moving air keeps the plenum from getting much hotter. This sounds like a problem with the blower fan assembly.

    Check the blower compartment. If the blower motor is running and the system uses a fan belt to drive the blower fan, perhaps the belt is loose or broken. If the blower fan motor just won't start on its own but the fan will start and continue to run if given a manual "spin" , also see FAN MOTOR START CAPACITORS.

    An open blower compartment door or bad blower compartment door switch will also keep the blower from running.

    Watch out:
    working in the blower compartment is dangerous - if power is on you can lose a finger if the fan starts! Modern air handlers have a safety switch in the blower compartment door that should turn the blower off when you open the door.
  • On gas fired heaters check for flue gas spillage at the heater or for a bad flue gas spillage switch: If your furnace is gas-fired, a flue gas spillage switch may have shut the system down after sensing possibly dangerous flue gases (that can contain fatal carbon monoxide).

  • On oil fired boilers or furnaces check for a bad automatic draft damper switch. If the draft damper does not open in response to a thermostat's call for heat the heating appliance will not start.
  • Not enough warm air volume from the furnace and duct system: check for a dirty air filter at the return register or furnace. Check for blocked, crimped, leaky, or disconnected air ducts.


Details of Hot Air Heat Furnace Controls and Switches

For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems


Definition of Water to Air Heat Exchanger Heating Systems

Some heating systems combine both hot water and hot air to heat a building, such as water to air systems which use a heating boiler (oil, gas, or electric) to heat water which circulates through (and inside of) a heat exchanger (that looks like a car radiator).

For a detailed guide to inspecting, diagnosing, maintaining & repairing mixed, hybrid, water to air heating systems

Dual hot water and warm air heating systems: as we explain
at HEATING SYSTEM TYPES, some buildings are heated by a combination of separate hot water systems (circulating hot water through radiating devices like baseboards or radiators in some areas) and hot air systems (circulating warm air through ductwork into the occupied space in other areas).

These buildings will have both a hot air furnace and a completely separate hot water heating boiler installed. In this case these are completely separate heating systems and usually each serves different building areas.


Continue reading at FAN LIMIT SWITCH TROUBLESHOOTING or select a topic from the More Reading links shown below.


Or see FURNACE DIAGNOSTIC CHECKS, GAS for a gas furnace homeowner checklist

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DIAGNOSE & FIX FURNACE PROBLEMS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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