How Does Woosh Calculate Filter Life?

The Woosh Smart Air Filter measures the difference in air pressure between the front and back of the filter as air is moved by the HVAC’s blower. We call this measurement differential pressure. The filter removes dust from the air by trapping it inside the filter media. As dust accumulates, it becomes harder to move air through the filter. This results in an increase in differential pressure, between the front and back of the filter.

Air pressure increases as the filter media loads with dust.

After a period of time, which varies considerably due to environmental factors in the home, pressure will have increased enough to warrant a replacement of the Fold™ filter.

Measuring Differential Pressure

The idea of sampling air pressure to determine filter life is neither novel nor a technically difficult concept. However, building a device that can reliably sample differential air pressure, transmit this data to the internet, run off battery power for more than a year and operate within the confines of a 1” filter frame presents a significant challenge.

We spent 2 years researching and developing a patent pending electro-mechanical solution that enables reliable and accurate sampling of differential pressure. Our electronics are power optimized and sampling rates are tuned to make an optimized tradeoff between battery life and accurate filter life calculations.

Tracking Differential Pressure Over Time

The real magic behind the Woosh solution are the algorithms we have developed that turn hundreds of thousands of differential pressure readings, taken over a long period of time, into a remaining filter life measured precisely from 0–100%. When the Smart Air Filter reports readings to our cloud, they are stored along with the previous measurements taken in the past for that device. Our algorithms then apply a series of digital signal processing steps to eliminate noise and account for variations such as sensor calibration and drift. The processed data is used to compute a remaining filter life based on the increase in differential pressure between when the Fold™ filter was installed and what is currently being sensed.

We made a deliberate design decision to run our algorithms on the cloud rather than on-device. This allows us to continually refine and enhance our solution as we encounter new operating environments, equipment or events. For example, we recently discovered that we are able to detect an evaporator coil freezing solely based on detected changes to differential pressure. We can do this without making any changes to the Smart Air Filter hardware or firmware.

A frozen coil is often the result of larger issues within the HVAC system and can result in catastrophic failures, often requiring whole system replacement.

It’s Way Harder Than it Sounds

Though the operating principle above may sound straightforward, there are many challenges to solve before a 1” filter slot becomes “smart”. Some of the complications our solution accounts for:

    • The effects of temperature on air density and thus pressure.

    • Sensor error (between measured and true values) that drifts over time.

    • Seasonal changeovers between heating and cooling in the system.

    • Variable speed blowers.

    • Multi-zonal systems with dampers that dynamically change air flow within the system.

When we started Woosh, we had no idea just how scientific air filtration was. We continue to learn, research and experiment. At our core we are really a technology company and remain inspired to continually improve the solution we offer to our customers. Keep watching for what’s next!