Blocked Filter Detection


The majority of heating, ventilation and air-conditioning (HVAC) systems, both residential and commercial, have built-in filters that refine air by catching dust and other contaminants. The filter’s job is to provide end users with healthy air as well as making it easier to keep the HVAC system clean.

Unfortunately, filters can become blocked over time and this can be problematic for two key reasons. Firstly, the product performance decreases, leading to a noticeable reduction in both airflow and the unit’s capacity for heating and cooling. Secondly, a blocked filter is unable to effectively remove contaminants from the air, which can be risky for end users with respiratory problems.

HVAC filters are typically cleaned on a time basis. Most commonly, the system, such as a residential air-conditioning unit, will notify the user to clean the filter after a certain period of operation. This approach often results in inaccurate cleaning notifications for the user – they are likely to receive a cleaning notification too early or too late.


Blocked filters are a consistent pain point in the HVAC industry. Receiving a cleaning notification based on time is somewhat outdated and unnecessary in an age where more advanced technology can provide a more accurate solution. Fisher & Paykel Technologies’ HVAC Motors feature an intelligent built in micro-processor. Allowing us to replace conventional detection technology and costly sensors with new software.

Graph showing the airflow vs. filter blockage

The micro-processor’s software enables our motor to monitor the reduction in airflow through load sensing using an algorithm we’ve called Blocked Filter Detection (BFD). As the filter collects contaminants, the airflow is reduced and this decreases the fan load. The motor can detect that load change over time and uses that to judge the status of the filter. When that load reaches a predetermined level, it will send an accurate notification to the end user.

In an environment where there is a large volume of dust and/or high contaminant levels, the end user will receive more frequent cleaning notifications. Conversely, in a clean environment, the end user will receive fewer notifications. BFD helps the end user to get the best performance out of their unit at all times, avoiding unnecessary cleaning as well as the health risks and discomfort that can be caused by an underperforming system.

For an individual using a single residential air-conditioning unit, BFD offers accurate notification – resulting in greater comfort and easier maintenance for the user. On a larger scale, for example, an office building with a centralized HVAC system, BFD means maintenance only when and where it is needed. This is more cost-effective since preventive maintenance can be planned more accurately, and unnecessary maintenance can be eliminated.


There are a large number of environmental factors that influence the airflow going through a product. These include ambient air temperature, atmospheric pressure and humidity. These factors alter the air density, which in turn affect the load on the fan or blower. To ensure our algorithm was accurate, we extensively investigated and tested these factors and fine-tuned the BFD algorithm accordingly.

Beyond the environmental factors, product characteristics such as fan or blower type, fan or blower size, heat exchanger, and filter type can also impact the airflow. We measure the airflow and loads at different conditions and blockage levels, resulting in a matrix that enables us to refine our algorithm to a specific application.

When we develop a brand new product, we run our motor in a specialised test mode to determine the parameters of the detection algorithm. From there we tweak the software to suit the new product and ensure a high level of accuracy.


To create this load detecting algorithm, we needed to dive deep into our motors’ capabilities and explore a huge range of different parameters. This same algorithm, with the right adjustments, is suitable for many applications to detect changes in filter blockage, airflow or motor load. In these situations the motor can be programmed to respond intelligently to the conditions or give a warning.  This adaptability and intelligence makes it a unique and dynamic feature.

As a software based feature, we’re able to continually update and improve our algorithms for new models, and stay up-to-date with product adjustments made by our customers. For example, if a customer wants to change to a more efficient fan, we re-evaluate the product’s thresholds and loads before applying a software update to make sure BFD is optimised for their application.

Our motor also has the ability to automatically modify the algorithm by identifying the product it’s installed in and adapting accordingly. This means the same motor can be used throughout an entire product range without having to make any changes to it – this is very cost-effective for our customers.

Our built-in micro-processor offers numerous options and opportunities to introduce new features to the HVAC market that weren’t possible before. We are committed to innovation of new solutions and the evolution of our existing technologies, not just for added value to our customers, but to their customers as well.