Reasons Why Your Furnace May Not Be Working In The Middle Of Winter

Why is my furnace not working?

A furnace is a highly engineered product, relying on a certain set of conditions to operate at peak output. There are several reasons why your furnace may not be operating correctly. Some of these may be easily identifiable by the homeowner and others will require consultation with an experienced HVAC contractor, such as Acosta Heating and Air Conditioning. In this article we look at some of the common problems encountered when the furnace is either operating below par or not at all.

Housekeeping – Help your furnace to breathe

To operate at maximum efficiency, a furnace needs to breathe. If you are noticing that the furnace is no longer delivering heat as effectively as it once was, these tips might help you to restore some of the effectiveness. Note that these tips can all be performed without having to touch the furnace, however, we always recommend engaging a trained professional where the following do not resolve your issues.


As the furnace circulates air around your home, it passes this air through a filter or a filter set. This is generally located in your return air duct, right before it connects to the furnace, this filter removes debris from the circulated air in the form of dust, pollen, and pet dander. Not only does this contribute to the indoor air quality in your home, but it also helps the efficiency of the furnace. The filter can quickly become clogged which, depending on the filter, requires a replacement filter or cleaning. (Note – do not operate the system without the filter installed).

A clogged filter can decrease system performance. The furnace blower must work harder, which results in increased energy consumption (and a shorter life for the blower motor!) and as a result, you may not be getting the amount of airflow that you are accustomed to. This decrease in airflow means a reduction in the heat delivered and can even shut the unit down due to the high temperature limit (a furnace safety feature for high air temperatures).

What can I do?

Many manufacturers give a guide as to how often a filter should be cleaned or replaced. This ranges from 90-180 days, where the home has no pets and nobody suffers from allergens to 20 days where the opposites are true, depending on how many pets the home occupies.

If you have any doubt about your system filter, contact us at Acosta Heating and Air Conditioning.

Pressure Switch – Exhaust and Intake Vents.

A furnace requires just the right balance of fuel and air to operate correctly. Correct ventilation is critical for their operation and interrupting or blocking this can lead to decreased performance or even shut down.

A furnace’s pressure switch is an integral safety component of the unit, shutting the unit down where low airflow is detected to prevent a build-up of harmful fumes. The pressure switch can make the difference between your furnace operating, sometimes operating and not at all.

What can I do?

While there are a few causes for pressure switch issues, a common one to look at it is a restriction in the ventilation tubing connected to the unit. Your unit may have one or two tubes (either plastic or steel) connected to outside to allow for outside air to come in and gases to go out. A restriction in these can result in inadequate air flow when the blower operates.

You can inspect the flues outside, for obstructions. Common obstructions are animals (mostly birds!), ice buildup or even just items stored externally that are blocking the vents. Clearing these can sometimes solve your problem.

High efficiency furnaces also need their exhaust tube (through which the gasses exit) to be installed with a fall or slope back towards the furnace. This is to allow for condensate (water that forms in the flue) to drain back to the unit and then to the sewer. If there is no slope or inadequate slope the water that builds up can restrict the air flow and shut off the system via the pressure switch.

Pilot Light

As noted above, a furnace requires just the right balance of fuel and air to operate correctly. This can be seen for older gas models with the pilot light.

The pilot light is a flame, ideally a steady blue flame. On older furnaces you should be able to see the flame via a viewing port on the front of the unit. If the flame is yellow and flickering, then the air/fuel mix is incorrect.

What can I do?

The troubleshooting for the flues noted above may resolve the issue but for the older models, we would also recommend ensuring that the area around the furnace is free of debris, that the unit can easily breathe, and the outside of the unit is clean. If you notice any external damage to the furnace, that might affect how it is sealed.

Common reasons for furnace breakdown and sub-par performance

The following are issues that we encounter commonly that require working inside the furnace housing. We always recommend that a professional is engaged. A DIY job not only voids the warranty but can cause irreparable damage to the furnace and may even result in injury. Please call us at Acosta Heating and Air Conditioning for all your furnace repair, installation, and maintenance needs.

A dirty evaporator coil

This is a common reason that we encounter for restricted air flow in a furnace. The evaporator coil is where your AC system connects in to the air distribution system and is typically located just above or adjacent the furnace, where the ductwork is at it’s largest (plenum). It is prone to accumulating dirt, particularly if the filters are due a change, as the air flow paths in and around the coil are very narrow. A dirty evaporator coil works much like a clogged filter in that it can severely restrict air flow to your home – resulting in the same issues noted in the filter section above, high temperature shut off, low air flow and increased wear on furnace components.

A dirty flame burner sensor

This device senses that the burners are operating and allows the system to start. If you hear your system start up (including the burner) only to switch off within a couple of minutes this may be the cause.

Flooded condensate drain

If a blockage is restricting the flow of condensate from your unit the resulting water can damage the heat exchangers within, leading to severely diminished performance.

Burnt out motors

Can result due to dirty filters/restricted flues and the motors having to work extra hard to overcome the resistance. Similarly, we often see blockages in the fan housing.

Control Board issues

Maybe the furnace is working but not in the way you asked it to! We inspect the board for signs of damage.


There can be several fuel issues, from restricted flow (which can be an issue with the infrastructure) to the component within the furnace malfunctioning (which is also a common reason for the yellow pilot light on gas boilers.

Heat exchangers

These can deteriorate due to bad air flow, age* or a combination of factors – leading to poor unit performance both in terms of heating and energy consumption and furnace shutdown.

*Note that the typical life of a furnace is 15-20 years.

Faulty Component

There are many smaller components in a furnace, from ignitors to capacitors – all of which are prone to wear and tear.

Repair and Maintenance

At Acosta Heating and Air Conditioning, we have seen nearly every kind of furnace and every kind of breakdown. We recommend incorporating the housekeeping tips above to ensure a longer and more effective life for your furnace. The cost of maintenance is nearly always cheaper than that of a repair and, even better, you decide when it is performed.

Of course, should the worst happen, you can call us at Acosta for a professional, speedy repair and advice about the ongoing maintenance of your furnace.