Furnace Repair: 20 Troubleshoot Items You Can Check Before Calling For Service
As your furnace ages, it endures wear & tear. Over time, components may start to fail. Here are things you can do to determine what may be wrong with your furnace. Many of these problems are easy fixes you can do, and what you need to do is described below.
Standard-Efficiency (80%) Gas Furnace
Complete HVAC System with Air Conditioner, Furnace and Ductwork shown
FURNACE TROUBLESHOOT ITEMS
FURNACE WON’T RUN: Furnace Shut-Off Switch Is Turned Off
A gas furnace has a Furnace Shut-Off Switch. It looks like a light switch as is typically attached to the furnace (though may be on a nearby wall). Be sure it’s turned on. It is not unheard of that you might bump this switch while changing a filter, or a child may have turned it off to see what would happen.
FURNACE WON’T RUN / HEAT: Check The Gas Shut Off (from the house to furnace)
The is a gas shut-off to stop gas flow into the furnace. It’s located quite near the furnace. If the button’s ridge or handle is the same direction as the gas line, it’s on. If the button’s ridge or handle is not the same direction as the gas line, it’s turn off.
Gas to furnace is ON (handle lines up with pipe).
Gas To Furnace Is OFF (handle is perpendicular to pipe)
In the first photo the red handle is the same direction as the gas line = Gas Is ON. In the second photo, the red handle is not the same as the direction of the gas line = Gas Is OFF. Be sure Gas Shut-Off is the same direction as the gas line.
FURNACE WON’T RUN: Check The Furnace Door To Be Sure It’s Correctly In Place
If the furnace door is not correctly placed, it may shift a bit over time and no longer press on the safety-switch inside the furnace. If the door is positioned right, you should not see gaps between it and the furnace-body. Remove door and replace (positioned correctly).
Note: The photo (above) of the Furnace Shut-Off Switch shows the furnace door is not correctly positioned (you can see the gap at the top right corner).
FURNACE WON’T RUN: Thermostat Not Working
Your home thermostat receives a small low-voltage supply of electricity from the furnace. It communicates to the furnace with this electrical supply. Inside the furnace is a transformer which converts home electricity to low-voltage for the thermostat. If the transformer fails, or the fuse protecting it blows, your thermostat can’t communicate with the furnance
Most Programmable Thermostats are battery operated. The thermostat may be working correctly (because it’s running on batteries) but the thermostat can’t communicate with the furnace due to the transformer problem.
Furnace transformers look different, and are located in a different location, depending on Furnace Brand and Model.
If your furnace shuts down completely, the transformer may have failed. Or, the thermostat itself has failed. You will need and HVAC Technician to determine what’s wrong and replace what has failed.
FURNACE WON’T RUN: IGNITER:
Only a few old furnaces that use a Pilot Light are still in service. Years ago, the Pilot Light was replaced with an Electric Igniter.
The Electric Igniter:
- Eliminates fuel being wasted by the Pilot Light
- Eliminates heat generated inside the furnace by the Pilot Light. In summer, your A/C has to cool this heat.
- Eliminates Pilot Lights blowing out on windy days
At The Beginning Of Each Furnace Heat-Cycle:
- A small fan starts running to remove exhaust gases
- The Igniter heats up until it’s glowing orange
- The Burners Ignite
- The Igniter Shuts off
- The Blower begins running to move heat to the rooms
Over the years, the metal in the igniter cracks as a result of expansion and contraction of the metal each time the igniter cycles.
If your furnace completely stopped working, this is one of the most likely causes. If you are able to see the furnace burners, you are able to see the orange glow (If you cannot see the burners, you have a High-Efficiency Furnace — their Burners-Chamber is not visible). Turn the thermostat down below the home’s current inside temperature, then 5 degrees over that temp to start the furnace. If you do not see the orange glow, it’s likely the Igniter has failed.
FURNACE IGNITER GLOWS, BUT NOTHING MORE HAPPENS: Gas Control Valve
The Gas Valve (above) Has A Blue Button. NOTE: Gas Valves Look Different depending on Furnace Brand
Furnace Gas Valves are very reliable, but can fail as the furnace ages. If you can see the orange glow of the Igniter, but the burners don’t light, they are not getting gas. As a Safety Feature, the Gas Valve is designed to shut the gas off completely if anything inside the Valve fails. Don’t Mess With Gas! Call an HVAC Technician to repair your furnace.
FURNACE SHUTS OFF BEFORE IT SHOULD: CLOGGED FURNACE FILTER
If your furnace runs, but only for a short time, then shuts off (before the house temperature reaches what the thermostat is set at). You may have a clogged filter. If a filter becomes clogged the furnace overheats and shuts itself off early to protect itself. Filters must be checked each month. If the filter looks dirty, it is, and needs replaced.
If your furnace is “short-cycling” — and not heating the house enough, check your filter before you call for service. High efficiency air filters clog faster, and may not look “dirty enough to be replaced” — but they are. Their filter-media as smaller holes (that’s why the filter is more efficient), These holes clog easier and faster.
Cracked Furnace Heat-Exchanger: When your furnace cycles, the burners heat the Heat-Exchanger. The furnace blower turns on and moves indoor-air over the Heat-Exchanger to heat the home.
With each furnace cycle, the Heat-Exchanger heats up and expands. After the cycle, it cools down and contracts. This expansion and contraction has the potential to crack the Heat-Exchanger as the furnace ages. The older the furnace is, the higher this potential.
IMPORTANT TIP: A furnace’s Heat-Exchanger can fail at any time, allowing Carbon Monoxide (Co2) into the home’s living space. Co2 is a colorless and odorless gas which can make you ill in lower concentrations and kill you in high concentrations.
It’s quite possible a Furnace Exhaust Vent can become blocked or disconnected, allowing Carbon Monoxide into your home.
Furnace Venting blocked by bird’s nest.
Furnace Vent Pipe that has become disconnected. This can easily happen if someone working in the attic bumps the exhaust venting.
AL’s TIP: Protect those living in your home with Carbon Monoxide Detectors. They provide 24/7 protection by monitoring the air for Co2. Better detectors have a digital readout which tells you the Co2 concentration (if any) at any given time.
There needs to be 1 Co2 Dectector on each level, 1 near the furnace (if inside the living space) and 1 in each bedroom (if the door is kept closed at night).
Note: Cooking with gas appliances produces a small amount of Co2, so your detector may show a very low concentration when these appliances are in use.
TIP: Co2 Detectors are worn out at 10 years, and must be replaced for reliable Co2 detection.
Carbon Monoxide Detector with digital readout for current Co2 Concentrations
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