A small sensor sits across your furnace filter and measures the pressure drop — how hard the blower has to pull air through the filter. A clean filter is easy to pull air through; a loaded filter fights the airflow, and the pressure drop climbs.
At setup, the sensor records the pressure of a clean filter as its baseline. From then on it compares every reading to that baseline. The further above baseline it climbs, the more loaded the filter is.
Readings post to this dashboard every 30 seconds over Wi-Fi. Nothing to install or check — it just runs.
Δ
The triangle is the Greek letter delta. In engineering it means "change" or "difference." So ΔP ("delta-P") is the difference in pressure across the filter — the air pressure on one side minus the pressure on the other.
A clean filter has a small ΔP. As dust builds up, air can't pass as easily, so the pressure difference across it grows. That growing ΔP is exactly what tells you the filter is loading up.
The Filter Index is just that pressure drop shown as an easy number. The higher it climbs above where it started clean, the more loaded the filter.
The reading shows 0 (or nearly 0).
The furnace/AC blower is off, so no air is moving and there's no pressure to measure. It jumps back to a real number the moment the system runs.
The page won't load on my phone.
Usually a DNS cache holding an old answer. Turn Wi-Fi off and load it on cellular, or try a private/incognito tab. It clears on its own within ~10 minutes.
It says "waiting for first reading."
The sensor is registered but hasn't posted yet — it's booting or joining Wi-Fi. Live data appears the moment it sends.
It's showing yellow or red.
That's the sensor doing its job — the filter is loading up (yellow) or needs replacing now (red). Swap in a clean filter and it returns to green on its own.