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As the days get shorter and temperatures turn milder, most HVAC systems are taking a well-earned vacation. For a short while, a cool breeze through open windows will be enough to keep us comfortable in our homes. But soon, homeowners will switch their thermostats from cooling mode to heating mode, and some of them will unfortunately discover their HVAC system does not heat. It's time to pull out those extra blankets and make the call to a service professional sooner than expected.

On the commercial side of things, business owners and managers should be setting up their semi-annual HVAC maintenance that is essential to keeping business running as usual. Freezing employees or equipment doesn't bode well for productivity.

The Importance of Maintenance

Regular maintenance is a requirement for the survival of all HVAC systems. There are several components your service professional should check during a maintenance inspection.

In general these should include cleaning the outdoor and indoor coil, replacing air filters, checking all electrical connections for corrosion and tightness, checking refrigerant levels (when applicable), clearing or cleaning the condensate drain line, and confirming proper operation. There are many other checklist items that are specific to each type of HVAC system.

Common Winter Failures

Heat pumps, furnaces, boilers, chillers and rooftop units (RTUs) have unique components that need to be inspected on a yearly basis prior to the start of the winter season to ensure proper operation during the coldest months. If these items are not cleaned and inspected, and any issues corrected, there is increased risk of premature failures and water losses caused by freezing temperatures.

Heat Pumps

Elevated electric use (and consequently, an unusually high electric bill) can occur because the outdoor heat pump has failed (likely the compressor) and the building is being heated with the auxiliary heat strips. The auxiliary heat strips should only be energized during the defrost cycle or if the outdoor temperature is extremely low. A warning sign that the outdoor heat pump has failed is the auxiliary heat indicator will be displayed on the thermostat.

Basic Residential Furnaces

The most common failures for a basic residential furnace are related to dirty air filters, dirty flame sensors, and worn out ignitors.


Boilers have similar components to furnaces. Common failures for boilers include no or low water pressure, failed flow switch, no gas to the building, dirty burners, dirty flame sensor, bad ignitor, the thermostat was turned off, or faulty controls.


Chillers use refrigeration to cool a fluid. If untreated water is used in a chiller system, it will fail prematurely. The service professional should regularly monitor the water levels and quality of the water to ensure it meets the manufacturer's specifications. Many chillers operate during cold weather conditions and are subject to freezing. Most chiller systems have air handlers that allow a percentage of outside air into the building for "fresh air". If the chiller protection fails, it is likely that outside air will be the cause of a ruptured coil. Some chillers are drained and winterized, but these can still be subject to failure if not done properly.


RTUs are commercial heating and cooling units. These units may be cooling only or have a furnace unit combined. Commercial buildings can require cooling demands during mild and even cold conditions. For these units to properly function in this cold environment they must be outfitted with special controls. Aside from compressor failures, the most common component failures in the winter season are low ambient controls, crankcase heaters, and heat exchangers.

One thing is certain, there is never a good time for HVAC systems to fail; it could be argued, however, that the frigid dead of winter is one of the worst times. This is one reason fall maintenance is so important. A service professional can provide a complete maintenance inspection and recommendations based on the condition of the equipment. Understanding common winter failures can provide a starting point to help determine whether component repair or unit replacement is warranted.

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DONAN, a family owned company founded in 1947, provides full-service Forensic Engineering and Fire Investigation Services. For over 60 years, Donan Engineering has been providing unbiased answers to their client's complicated questions. Their team of licensed and experienced forensic experts provides investigative services in the fields of engineering investigations, fire and explosion origin & cause, roofing investigations, and component testing. The diversity of their multidisciplinary staff allows them to provide not only a complete service, but a complete service whose conclusions can be successfully and completely supported.

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