Door open: the cheapest, most effective alarm you can install

Of every sensor we install, the door sensor is the cheapest. A magnetic reed switch costs a few coins, has no complex electronics, draws almost no power and doesn’t break. And by a comfortable margin, it delivers the highest return per unit spent.

The problem is banal and expensive

Cold room doors stay open for trivial reasons: unloading goods, restocking a shelf, someone who stepped in for a box and would be back “in a minute”. Nobody leaves the door open out of malice. They leave it because their hands are full and they’ll be right back.

The equipment, meanwhile, goes into continuous operation. It doesn’t know the door is open — it only knows temperature isn’t falling, and it responds the only way it knows how: by working harder.

Three consequences stack up:

Consumption. The compressor leaves its normal cycle and runs without stopping. In large rooms we measured 25% to 40% increases in consumption during extended open-door periods.

Humidity and ice. Warm, humid air entering a cold space condenses and freezes on the evaporator. Accumulated ice reduces air circulation, which makes the compressor work harder still. It’s a self-reinforcing loop.

Wear. Continuous operation outside design duty directly shortens compressor life.

And the cruel detail: for most of that time, temperature stays within range. The system is paying dearly to deliver the right number. A temperature form would record everything as normal.

What we measured in the field

On one of the first installations with door sensors, the customer estimated doors stayed open “about five little minutes a day”.

The measured average in the first week was 19 minutes per day per cold room, with peaks over 40 minutes on delivery days. None of those openings were irresponsible — it was normal operation, with a rather optimistic sense of time.

The effect is behavioural, not technical

Here’s the interesting part. We installed a local siren — a beep starting after three minutes of open door — and a WhatsApp alert to the manager after six.

Within two months, the average dropped from 19 to 4 minutes a day. A 79% reduction.

No sophisticated technology did that. What changed is that the information started existing at the right moment, for the right person. Before, nobody knew the door had been open for eight minutes — least of all the person who left it open, who genuinely thought two had passed.

That’s the difference between a system that records and a system that interrupts.

How to do it properly

Three details separate a useful alarm from one the team disables in week two:

Stepped escalation. A soft local beep at three minutes. An insistent one at five. A manager notification at six. Don’t start at the top.

Muting with a reason and a deadline. Receiving a delivery legitimately requires the door open for twenty minutes. The system has to allow muting, but with a recorded reason and a defined deadline that expires on its own.

Counting, not just alarming. The long-term value isn’t the beep, it’s the weekly report of accumulated open-door time per room and per shift. That’s the number that changes behaviour durably.

The best first step

If your operation is instrumenting cold for the first time and needs to choose where to start, our recommendation is almost always the same: temperature and door, together.

Temperature because it’s the regulatory obligation and the primary indicator. Door because it’s the cheapest sensor, it addresses the largest source of waste and it produces a visible result within weeks — which tends to be decisive for the project surviving internally.

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Want to estimate how much open-door time your operation accumulates? Talk to us — we can usually give an order of magnitude in the first conversation.