How to troubleshoot HVAC systems | Pro Construction Guide
How to troubleshoot HVAC systems

How to troubleshoot HVAC systems

Many complaints about poorly operating HVAC systems can be remedied with simple repairs.

Many typical complaints about poorly operating HVAC systems can be remedied with a little investigation and simple, straightforward repairs.

To troubleshoot HVAC systems today, you’ll probably need expensive high-tech tools and diagnostic equipment as they are more efficient and sophisticated than ever.

But many typical complaints about poorly operating HVAC systems can be remedied with a little investigation and simple, straightforward repairs. Follow these tips to troubleshoot HVAC systems.

Problem: The air conditioner is running, but it isn’t cooling the space.


This is probably the number one summertime complaint about HVAC systems, and while most homeowners will have already checked the most likely causes before calling in a repairman, you should check again.

Make sure the thermostat is on and turned down below room temperature, and if there are separate indoor and outdoor units, make sure both are on. Check all breakers in the house and reset buttons on the units. If the unit has a condensate pump, check to make sure it’s working. Some pumps have a safety switch that shuts off the outdoor unit if the pump fails.

If both units seem to be running fine and still won’t cool the space, make sure all windows in the space are closed and supply and return vents are open and unobstructed. Check the filter and clean or replace it, if necessary. Make sure there are no ducts leaking air.

If all those conditions are met, you’ve narrowed it down to a few possible problems:

  • Low or restricted refrigerant. Recharge the coolant or repair the refrigerant lines.
  • A faulty metering device. Replace it.
  • Bad reversing or compressor valves. Replace them.

Problem: The outdoor unit won’t start.


This is a typical problem with air conditioners and heat pumps, and some of the causes can be serious and expensive to fix. The simple potential causes include an improperly set thermostat, emergency shutoff or circuit breakers that have been tripped, and an outdoor disconnect that has been turned off.

If the unit has a condensate pump, it could be unplugged or the safety switch might be open. The unit may also be “locked out,” meaning a safety device is preventing it from normal operation. Re-set that by turning power off at the breaker for one minute, then turning it back on and waiting 10 minutes for the unit to power up.

Beyond those simple fixes, it could mean one of the safety devices is open. Check any low pressure, low temperature and high-pressure, high temperature safety devices to see if they’ve been tripped.

If they have, you’ll have to diagnose the thermostat, contactor, time-delay relay, thermostat cable and control module to determine which one is faulty and needs replacement.

Problem: Air flow at the vents is weak.


Of all the typical problems with HVAC systems, this is the one most likely to be corrected by a simple fix. Most of the time, poor air flow is due to a dirty air filter. Today’s high efficiency filters are so effective at filtering out fine dust and allergens that when they clog they can cause serious air flow restrictions.

Dirty, blocked or closed ducts are another common cause. Make sure all vents are open and unobstructed, dampers are open, and there is no dirt or debris blocking any of the ducts. You may need to insert a camera into the ductwork to ensure there are no blockages.

While checking for blockages, make sure the ductwork is correctly sized and installed. If it isn’t, that could also cause problems.

The more serious fixes for poor airflow are typically found in the blower, fan or coil. Faulty blower motors should be replaced, although sometimes a simple cleaning of the blower wheel will fix it. The indoor coil might also be faulty, freezing up and restricting air flow. If it is, replace it, but first see if a dirty coil needs cleaning to get it going again.

Check that fan speed isn’t set too low. Reset it at the proper speed if needed. Other issues that could reduce air flow by adversely affecting fan speed are a loose, worn fan belt and incorrect pulley sizes. Check and repair if needed.

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