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As a furnace cleaning company, we sometimes get calls for issues that would be more appropriately referred to a furnace repair company. Sometimes customers with furnace troubles attempt the process of elimination to reach a diagnosis, eliminating the least-expensive potential fix first (cleaning). Though there is some minor overlap between cleaning and repair, if your furnace is not working, your first call should be to a repair company.
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With that said, furnace cleaning can help minimize the need for furnace repair, and on occasion, the recommended repair is simply to clean a specific malfunctioning (read: dirty) part. Additionally, furnace cleaning technicians are typically equipped to replace minor parts, such as flame sensors and hot surface igniters

Your Furnace's Sequence of Operation

To understand the relationship between furnace cleaning and the unit's smooth functioning, it helps to understand a furnace's "firing order," formally known as its sequence of operation. This is the series of steps a gas furnace runs through before generating heat. In a nutshell, once heat is summoned by the thermostat, the draft inducer motor begins whirring. The air pressure switch then monitors the air flow being generated by the inducer to make sure it is sufficient, and if it approves, a connection is made between the inducer and the hot surface igniter. The igniter then glows and the gas valve opens, creating a flame. The flame sensor, for its part, then asks, is there a flame? If the answer is yes, the process remains uninterrupted and heat is generated. If the answer is no, the system will shut down, in order to prevent the release of raw gas into the atmosphere with no flame to consume it. This process continues until the thermostat reaches the desired temperature.

Any interruption to this sequence of operation will effectively halt the furnace, and there are plenty of factors that can contribute to an interruption. If the draft inducer is compromised, for example, the pressure switch will not communicate to the igniter that everything is A-okay, and the igniter will not spark. Dirt and debris can play a major role in these interruptions. A furnace flue clogged with debris would prevent the inducer from creating enough draft to close the pressure switch, stopping the process before it even gets started. A worn or dirty flame sensor incapable of detecting a flame would result in the gas valve shutting off. You get the idea.

The Primary Goal of Furnace Cleaning

Despite furnace cleaning sometimes resolving what would seem to be repair issues, its chief aim is maintenance—helping to keep the system in working order by improving air flow through the removal of system-clogging debris. As explained above, a single dirty component can stop the heating process in its tracks. Maintaining system cleanliness will minimize disruptions to your home's heating.

An additional aim of furnace cleaning, and HVAC system cleaning in general, is the improvement of indoor air quality: by removing debris and contaminants from your furnace and other system components, you reduce the number of pollutants circulating throughout the system only to be expelled into your living space. That same debris that clogs up HVAC system parts and hinders air flow doesn't do your lungs any favors either.

Overlap Between Cleaning and Repair

If we refer back to our sequence of operation and the issues that can disrupt it, we have to consider that sometimes the problem is as simple as a single faulty part. Though the lifespan of a furnace is roughly 15-20 years, the lifespan of some individual parts is much shorter. The igniter and flame sensor, for example, have lifespans of only about 5-7 years, so it stands to reason you'll need to replace these before you replace the furnace itself. Because these parts are fairly easy to install and small enough to carry a sufficient stock, many furnace cleaning technicians will replace these, with the customer's approval, if they observe much wear on the part. The replacement of these components, in addition to capacitors and contactors, are about the extent to which furnace cleaning technicians will engage in parts replacement.

One of the biggest (and most dangerous) cleaning-related repair issues is when the furnace shuts down due to a clogged furnace flue. The flue, of course, is the channel or pipe through which harmful gases produced by the combustion of your furnace are exhausted to the outside. If the flue becomes clogged with animal nests, leaves, or other debris, harmful carbon monoxide can back up into your living space and put your family at risk. Furnace flues without caps are particularly susceptible to this hazard. Our technicians are frequently called to the homes of customers whose furnaces have been red-tagged and shut down by a repair company due to a blocked flue.

A surprisingly common contributor to furnace overheating and subsequent shut-down is a clogged air filter. If the filter is choked with dirt, air flow from the return duct into the furnace is restricted and can cause the high-limit switch to detect too much warmth and shut off the burners. A blanket of dust over the evaporator coil can have a similar effect, restricting air flow from the heat exchanger to the supply duct and causing a backdraft. Because the entirety of the HVAC system is dependent upon proper air flow, any hindrance to this flow in the form of dirt or debris can wreak havoc and lead to repair issues.  ​

When to Call it When

If your furnace is not working, or not working well, your first call should be to a repair company. It may end up that the diagnosis involves the evaporator coil needing to be cleaned, or that the furnace flue is dangerously clogged, both of which you'd likely be referred to an HVAC cleaning company for. However, the diagnosis should come from a licensed HVAC repair technician. Sometimes similar symptoms can have different causes, so to rule out any other issues, a proper evaluation should be performed. 

If you are looking for a cleaning of your furnace or furnace flue as part of routine maintenance on your system, or if you are looking to improve your indoor air quality by minimizing pollutants present within, call a furnace cleaning company. During the course of their service they'll make mention of anything they see amiss and refer you to a repair company if appropriate.

Wanna dig deeper? Download our free tipsheet: 5 Questions to Ask Before Hiring a Furnace Cleaning Company. 

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Many thanks to our technician Roy S for lending his expertise to this article.