Choosing a dehumidifier for a store

A dehumidifier is the most commonly bought and most commonly misapplied piece of equipment in small collections. The reason is that the headline specification is quoted under conditions no collection store ever sees.

The temperature problem

A compressor dehumidifier works by cooling air below its dew point on a refrigerated coil so water condenses out. That works well in a warm humid room. As the room gets colder, the gap between the air temperature and the achievable coil temperature narrows, extraction falls, and eventually the coil frosts and the unit spends much of its cycle defrosting rather than drying.

Most collection stores are cool by design. A unit rated at, say, twenty litres per day at 30 °C and 80 percent may manage a small fraction of that at 12 °C and 60 percent, which is a realistic winter store condition. The machine is not faulty; it is being used outside the conditions its number describes.

Desiccant units work differently. Air passes through a rotor coated with a moisture-absorbing material, and a heated airstream regenerates the rotor. Performance holds up at low temperature. The costs are higher electrical consumption and waste heat, the latter being either a bonus or a nuisance depending on the store.

The practical rule: warm space, compressor. Cold or unheated space, desiccant. Anything near the boundary, ask for a performance table rather than a single figure.

Sizing to the load, not the volume

Room volume tells you almost nothing, because a sealed modern store and a stone cellar of identical volume have wildly different moisture loads. What matters is how much water enters the space per day.

The sources are ground moisture through floors and walls, infiltration of humid outside air, and occupancy. For most stores the first two dominate. A rough handle comes from your own data: if the space is known to run at, say, 68 percent when the target is 55, and you know roughly how much air the space exchanges, a supplier can size to that. What you should resist is a recommendation based on floor area alone.

Undersizing means the unit runs continuously and never reaches setpoint. Oversizing means short cycling, which is hard on the compressor and produces a sawtooth in the humidity record rather than a stable line. Both are visible in logged data afterwards, which is a good argument for logging before and after installation.

Control is where most installations fail

The humidistat built into a dehumidifier sits inside the machine’s own warm, dry airflow. It is not measuring the room, it is measuring the machine. A unit set to 55 percent on its own dial may hold the room at 45 or 65.

The fix is an external control sensor placed at object height, in the position you would put a logger. Where the unit does not support one, the alternative is to set the dial empirically against a separate logged record, adjusting until the logged value is right, and to accept that the relationship will drift.

Either way, a dehumidifier holding a collection space should be independently logged. The machine’s own display is not evidence, and a wireless sensor with an alarm turns a silent failure into a same-day problem.

When not to buy one at all

If the space is damp because of a specific defect, fix the defect. A dehumidifier running permanently against an active leak is an expensive way to avoid calling a builder.

If the record shows the space is within a defensible band and merely feels damp, do nothing. A store at 60 percent, stable, with no mould and no corrosion, does not need intervention, and drying it will cost money and introduce a new risk. Target humidity ranges covers where that judgement gets made.