1.1 The plan

Cat W/C Air Filtration System · 12 Oct 2026

This is everything I think the project needs, in roughly the order I'll do it. Some of it will change once I've measured the room and read more.

Research

The research comes first, because it decides what I buy and what I design around. Ammonia is the main part of the smell, but I want to know how much the sulphur compounds matter, and which carbon holds ammonia best and for how long: plain activated carbon, carbon impregnated for ammonia, zeolite, or a mix. The HEPA layer has to be fine enough for litter dust and dander without slowing the air too much. The baffles need testing too, to see whether a longer, folded path through the carbon catches more than it costs in airflow. I'll use AI for this, check what it finds against the sources, and write up the results here.

Measuring

Before I design anything I need numbers for the room and the air in it. The room's size sets the airflow, the wall decides how the unit mounts and where its power comes from, and the door's thickness and material decide which cat flap fits. The most important number is a baseline: the sensors on their own, logging for a week or two before the filter exists. Without it I can't say whether the filter works.

The fan has to move the room's volume $V$ some number of times an hour, $n$:

$$Q = V \times n$$

As a guess until I measure: a WC of 1.5 × 1.0 × 2.4 m is 3.6 m³. At 10 air changes an hour that's 36 m³/h, about 21 CFM. That's through the filters, not in free air, which matters for the fan.

Sourcing filter material

The air passes through three layers. A cheap, washable pre-filter comes first and catches cat hair, so it doesn't clog the HEPA layer behind it. The HEPA can be a ready-made panel or sheet cut to size, and the carbon can be a honeycomb panel like FÖRNUFTIG's, loose granules in a printed cassette, or both. One option is to design around FÖRNUFTIG's replacement filters, which IKEA sells separately. They'd be easy to replace, but they'd fix the size of the unit and the choice of carbon.

Printing

I'd like it in white PLA, so it looks like the concept on a white wall. PLA is fine at room temperature, and the fan won't warm it much. The A1's bed is 256 × 256 mm, so a case any bigger has to print in parts and join, and the joins need to be hidden or made into a feature. The perforated front is the hard part: lots of small holes, printed so they come out clean.

Electronics

Power comes from a 12 V DC supply on a plug, with a converter down to 5 V for the ESP32 and the sensors, so there's no mains inside the unit. The fan runs on 12 V with PWM speed control and a tach output, so the ESP32 can set its speed and read it back. It has to cope with the resistance of the filters and baffles, so its static pressure matters more than its free-air rating. The sensors cover particles, a VOC or gas reading, and temperature and humidity, with ammonia too if I can find a sensor that's good enough. A pressure reading across the filters would show when they're clogging.

Software

The firmware on the ESP32 reads the sensors and sets the fan speed. Most of the time it should run quietly, then speed up when the readings rise after a tray gets used and settle again once the air clears. It reports to a server, with a dashboard that shows the readings over time, and it takes updates over Wi-Fi, so the unit doesn't have to come off the wall to change the code.

Fitting it

The cat flap goes in first, because the cats have to be happy using it before anything else matters. Then the unit goes on the wall, with its intake low and near the trays and its outlet away from them, so it doesn't blow litter about. The power cable runs tidily down to the socket.

What success looks like

These are targets to start with, not promises.

If it works, the room just won't smell, and the numbers will show why.