Furnace Airflow — Help

How it works

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.

The sensor learns your system's own clean-filter reading automatically — first when it's installed, and again every time you put in a fresh filter. It recognizes the drop in resistance on its own, so there's nothing to set or tap. From then on it compares every reading to that clean baseline; the further above baseline it climbs, the more loaded the filter is.

It answers "when do I change it?" three ways at once, and tells you whichever comes first: the pressure trend (a filter loading up), blower runtime (how long this filter has actually run compared to your last one), and plain age (some filters wear out before their pressure ever moves).

Readings post to this dashboard automatically over Wi-Fi. Nothing to install or check — it just runs.

What's on the dashboard

One number: filter life remaining. 100% is a clean filter on your own system, 0% is the point where it should be changed. Beside it, the filter picture fills with dust to match — how clogged it is right now, measured against that same change point. The chart underneath is the same number over time, with a dashed line showing where it's heading.

Everything else is optional. Tap Settings (top right) to switch on the technical readings, the duct schematic, the ΔP and air-temperature charts, or a list of every sensor — and to open the reading log. Nothing you turn on there changes the sensor; it only changes what this browser shows you.

Changing the numbers yourself

Settings also holds the values the dashboard judges by: the change point (how far above the clean reading counts as "replace it" — a rise of 0.15 in.w.c. by default, which costs any filter type about the same airflow), the caution point (a rise of 0.10 by default), the change-by-age backstop (90 days by default — some filters wear out before their pressure ever moves), and the blower-on threshold below which a reading counts as "the system is off". Move them and the percentage, the colors, and the chart all follow. Reset to defaults puts them back.

What the triangle (Δ) means

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.

Reading the status

The dashboard answers two separate questions, and keeps them apart on purpose.

Is the sensor working?

Does the filter need changing?

The Filter Index is just that pressure drop shown as an easy number — switch on "Technical readings" in Settings to see it. The higher it climbs above where it started clean, the more loaded the filter.

Why does it say it's still learning?

Every home's ductwork is different, so the sensor first has to learn what your system reads with a clean filter. It measures across several separate heating or cooling cycles — usually about a day — before it says anything about the filter.

Nothing is wrong while this shows. The sensor is reporting normally; it's just refusing to guess. Once the clean reading is confirmed, the filter life number and status colours appear on their own.

What does "filter looks undersized" mean?

Sometimes even a brand-new, clean filter shows real resistance — and in most homes that's not a fault in the filter, it's a filter that's small for the amount of air the system moves through it. Field studies find this in roughly half of homes: builders routinely undersize return grilles, so air crosses the filter at twice the speed it was rated for.

If your clean-filter reading comes in high, you'll see a small amber banner on the dashboard. It doesn't mean the filter is dirty, and the sensor still tracks loading normally. It's an insight about the system itself: a larger filter area (a bigger grille, or a deep 4–5″ media cabinet) would let the system breathe easier, quiet the airflow, and stretch filter life. Worth asking about the next time the system is serviced.

Common problems

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.

The filter says "Change now".

That's the sensor doing its job — the filter is restricting airflow. Swap in a clean filter and it returns to green on its own.

A sensor says "Not reading".

It has stopped sending data — usually Wi-Fi, or power to the unit. K&H is notified and takes care of it. Your filter reading beside it is simply the last one it managed to send.