Troubleshooting a Forced-Air HVAC System

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The HVAC (heating-ventilation-air conditioning) forced-air systems that heat and cool most homes can seem somewhat mysterious because their components are largely hidden inside the walls or tucked into a utility room, basement or attic. Because the system is largely self-governing, a homeowner may go months or years without any problems at all. Over time, out of sight becomes out of mind — until a problem arises.

If a problem does occur, the immediate impulse is to call a service repair person. Often, this is exactly the right response since some problems can be handled only by a skilled repair technician. However, there are also a surprising number of problems that can be addressed by simple DIY fixes that any homeowner — no matter their skill level — can try. Before calling a repair service, take a few minutes to carefully diagnose the problem and consider one of the easy fixes that may save you hundreds of dollars.

Basics of an HVAC System

You will have an easier time troubleshooting HVAC problems if you know just a little about its operation. The typical forced-air HVAC system contains several components that interact with one another, any one of which can be the source of some of the common problems:

  • The heating and cooling units​. At the heart of each forced-air HVAC system is a furnace unit that heats air and an air conditioning unit that cools air. These units operate independently but share the same blower and ductwork system that distribute air through the house. The air conditioner — consisting of a condenser and compressor unit located outside the home and an evaporator coil mounted indoors above the furnace — is normally powered by electricity. The furnace unit can be powered by electricity or more often by natural gas or another fuel.

  • The ductwork system​ consists of metal channels that carry the heated or cooled air from the furnace plenum outward through the house. There are two types of ducts: the air-delivery ducts that carry air outward to registers in the room and the air-return ducts that recirculate room air back to the furnace unit. A filter is incorporated into the return ductwork system to trap dust, pollen and other particulates out of the air being recirculated.

  • The blower​ is an electrically powered motor that spins a circular "squirrel cage" fan to circulate air around the furnace heat exchanger or the air conditioner's evaporator coils and onward through the system's ductwork.

  • Vent pipes​ are found on furnaces that burn natural gas or another fuel. These vent pipes deliver fresh air into the furnace's burner chamber for combustion and also vent exhaust fumes to the outdoors.

  • A low-voltage thermostat system​ monitors room temperature and sends electronic signals to the furnace or air conditioning unit to turn the system on and off as needed.

Common HVAC Problems

Any of these components can be the source of problems with a forced-air HVAC system. Simple, easy-to-fix HVAC problems often have some of the same symptoms as complex, expensive problems, so there's often no way to know immediately what you're facing, but by ruling out the easy DIY solutions, you'll soon know if it's necessary to call a service technician.

System Doesn't Turn On at All

It's fairly common for a furnace or air conditioning unit to simply stop running entirely. Though this is disconcerting and possibly dangerous if it happens during severely cold or hot weather, the situation is often correctable with simple DIY remedies. Normally, if a system doesn't turn on at all, it is because there is no electricity flowing to operate the blower and other components or because no low-voltage thermostat signal is reaching the furnace/AC units.

Try These DIY Fixes:

  • Check the circuit breaker.​ If a circuit breaker has tripped, the furnace or AC will be without power. Simply flipping the breaker back to the "on" position will get it running again. Normally, the breaker will be found in the home's main service panel, but AC units may have a secondary breaker located in a subpanel or in a switch box near the outdoor compressor unit.

  • Check the furnace shutoff switch.​ Furnaces usually have a simple on/off wall switch used to turn the unit off when repair or inspection is needed. If this switch has been accidentally turned off (a common occurrence in homes where kids play in utility areas), then simply turning this switch back to the "on" position will restore the system to operation.

  • The furnace's pilot light may be out,​ or its electronic ignition component may be malfunctioning. When this happens, you may hear the furnace click as it tries to ignite when the thermostat sends a signal, but the burners don't ignite, and the blower doesn't move any air. It is usually a fairly easy matter to relight a standing pilot light or to reset the electronic ignition on a newer furnace. Sometimes, simply cleaning these parts will return the furnace to proper operation.

  • Check the operation of the thermostat.​ Easy fixes can range from simply replacing the backup battery in the thermostat to reconnecting loose wire connections in the thermostat to replacing an old, outdated thermostat. Any of these fixes is an easy DIY project. This could be a good time to install a new, energy-efficient programmable thermostat.

When to Call a Pro:

If these simple fixes don't work, then call your HVAC service technician. It's possible that you are looking at a major wiring problem or an issue with a component that only a pro is able to diagnose and fix. Sometimes, this is a fairly inexpensive fix, such as cleaning or replacing the hot surface ignitors. These parts are quite fragile and generally shouldn't be handled by homeowners.

System Has Insufficient Airflow

If your furnace or AC seems to be operating correctly but the flow of air from the registers seems meek, the problem likely lies in some form of obstruction in the ductwork system.

Try These DIY Fixes:

  • Make sure the air registers are fully open.​ Surprisingly often, the problem is simply that someone has tampered with the vent register levers, often children.

  • Check the ductwork dampers​ to make sure they are properly set. Your HVAC ducts likely have adjustable damper "gates" used to control the volume of air flowing through them. These are helpful for balancing the flow of air in the system, but they sometimes get out of adjustment. Opening them will improve airflow to the registers.

  • Replace the furnace filter.​ A badly clogged air filter can greatly reduce the flow of air through all the ducts and registers.

  • Seal leaky ducts​. If your ductwork seams or joints aren't tightly sealed, enough air may be leaking to reduce flow to the rooms. Seal leaking seams and joints with foil HVAC tape (not regular duct tape).

  • Have the ducts and registers cleaned.​ Although it's rare for dust alone to actually reduce airflow, it's been known to happen. While it's possible to do this work yourself, duct cleaning from a professional service isn't very expensive, and it's much easier than doing it yourself. In homes with pets, the problem sometimes lies right at the registers, with pet hair clogging the register cover.

When to Call a Pro:

If none of the DIY fixes has worked, it's possible the problem is with the furnace itself. The solutions offered by a professional can be as simple as correcting a problem with the ductwork or as complicated as installing a new, larger furnace. If you have done some major remodeling that has involved adding new ductwork to serve new living space, it's possible the furnace is not large enough to adequately serve the increased demand.

Heated Air Is Not Warm Enough

If your heating system takes a very long time to bring the room temperature up to the requested thermostat setting, it's possible the burners aren't operating efficiently.

Try These DIY Fixes:

  • Remove the access panel on the furnace and inspect the flames​ while the burner is on. Ideally, the flames should be blue in color with a steady flame. If the flames are yellowish or flickering, the burners may be dirty, requiring cleaning.

  • Shut off the furnace,​ allow the burners to cool completely and then use a vacuum cleaner and soft brush to remove any soot or buildup on the burner jets or in the combustion chamber. Also, lift off the lower access panel and vacuum out the blower chamber. While you're at it, also replace the furnace filter.

When to Call a Pro​:

If simple cleaning is not enough, call an HVAC technician, who can clean and adjust the burners with great precision. Be prepared that the technician's evaluation may determine that the furnace has reached the end of its effective life span. About 15 years is the expected life span of a good furnace.

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Air Conditioned Air Is Not Cool Enough

If the air conditioner is blowing air from the duct registers in sufficient volume but the air is not properly cooled, the problem is probably with the AC compressor and condenser unit — the metal unit with a fan that normally sits outside your home.

Try These DIY Fixes:

  • Check thermostat settings.​ If the air conditioner has accidentally been set to "fan," the system will simply circulate indoor air without engaging the air conditioner or furnace. Make sure the thermostat is set to "auto" or "cool."

  • Clean the metal fins​ on the AC condenser unit. This will require that you shut off the unit, remove the outer housing and then use a soft brush and vacuum cleaner to carefully clean the metal fins to remove dust, leaves and other debris. Your air conditioner cools the air by radiating heat collected from the cooling coils that run through the indoor evaporator unit. If the metal fins on the outdoor condenser unit are dirty, the heat can't be effectively dissipated. Cleaning the condenser should be an annual maintenance task.

  • Make sure that air flows freely​ around the condenser unit. Most often, shrubs and other landscape plantings become overgrown and block the flow of air around the condenser. If you use some kind of protective fencing around your AC unit, make sure it is set back far enough to allow air to flow freely.

When to Call a Pro:

If you're satisfied that the system components are clean but the AC unit is still spitting out warm air, it's possible your system has a refrigerant leak that only a professional technician can address. It's also possible there is a problem with the compressor or another component that can only be fixed by a technician. Finally, it's also possible the AC unit has reached the end of its effective life span and needs to be replaced. A typical AC unit lasts 12 to 15 years before it needs to be replaced.

System Rapidly Cycles On and Off

If your furnace or air conditioner unit cycles on and off rapidly — sometimes each cycle takes only a few seconds — the solution can be remarkably simple, or it can be a very expensive repair.

Try These DIY Fixes:

  • Change the filter.​ Surprisingly often, a simple clogged filter is causing heat buildup in the system. This triggers the limit switch in the furnace, causing it to shut down as a safety measure. Simply replacing the system filter will return it to proper operation.

  • Clean the blower wheel.​ If dust has built up on the blower wheel, it may not be circulating air fast enough to keep the temperature sensors in the furnace from overheating and shutting down the furnace. At the same time you change the filter, take a few minutes to brush and vacuum the blower wheel. It may be easiest to unbolt the blower and motor entirely in order to completely clean and vacuum these parts.

  • Check the air supply vents​. If enough air registers are blocked or closed, the furnace can't circulate air fast enough to keep the sensors from overheating. Make sure the registers are open and they aren't blocked by furniture or other objects.

  • Check the thermostat.​ A faulty thermostat can also cause the system to cycle on and off rapidly. Sometimes, this is a matter of old, low-voltage wiring running from the thermostat to the furnace. This problem can also happen if the thermostat is positioned near a window or where direct sunlight falls on it, which can give it misleading temperature readings.

  • Check furnace vent pipes for obstructions​. Older furnaces often have an exhaust flue running up through the roof, while modern energy-efficient furnaces usually have one or two PVC vent pipes running out through a sidewall. If any of these pipes becomes obstructed, it can cause the furnace to overheat and rapidly cycle on and off. Clearing obstructions away from the pipes, like ice or drifted snow, often returns the furnace to proper operation.

When to Call a Pro:

If the DIY fixes haven't worked, there is a good chance you have a faulty limit switch or flame sensor. While a skilled DIYer may be able to diagnose and replace these components, for most people, this is a job for a professional. There are also other mechanical problems that can cause this symptom, but they too are best handled by a pro.

Fan Is Too Noisy

The air circulating through the HVAC system is driven by a squirrel-cage-type blower wheel driven by an electric inducer motor. While it's normal to hear the blower fan when an HVAC system is operating, the noise should be a steady background hum, not a rattling, clanking or squealing noise. If your system begins to make unusual sounds, it needs attention. The blower fan and motor are usually reached through a lower access panel on the furnace below the access panel for the burners.

Try These DIY Fixes:

  • Check the fan belt for wear and tight fit.​ A loose or worn fan belt can cause a squealing or squeaking sound when the fan motor kicks in. This is a project for a moderately skilled DIYer, but it certainly can be done. The fan belts may not be accessible on newer high-efficiency furnaces.

  • Lubricate the blower and motor.​ Older furnaces may have motor and blower shaft bearings that can be lubricated by removing the caps and applying a drop or two of light machine oil. This is often not possible with newer furnaces that have blower motors with sealed bearings.

  • Adjust or tighten the blower frame.​ If the squirrel-cage blower has loosened in its frame, it may cause rattling. Fixing it can sometimes be as easy as readjusting and tightening the frame bolts.

When to Call a Pro:

If there are no obvious problems to fix, it's time to call a technician. The motor itself may be wearing out, requiring replacement by a professional. A skilled DIYer may be able to replace a fan motor.

references

Bryan Trandem is an avid home improvement DIYer and trained Master Gardener. He has been writing and editing books and articles on gardening, home improvement, woodworking, and home decor for more than 30 years. He lives in Minneapolis, MN.

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