Mold is a water problem. Take the water out of the air.
For food and tobacco warehouses, food plants, shops and the shared spaces of condominiums. A Karnot reheat fan coil dries the air first, then warms it back to a comfortable temperature. Held below 60% humidity, day and night, mold does not get the damp it needs.
Why the mold keeps coming back
The air here is damp most of the year. Mold spores are always present. The one thing you can take away is the water.
An air-conditioner cools. It does not dry.
It works to a temperature. When the room is cool enough, the compressor stops, and the drying stops with it. The fan often keeps blowing over a wet coil and puts the water back into the room. Adding more units makes each one stop sooner.
The damp settles at night
When work stops, the building cools. Cool air cannot hold as much water, so the humidity climbs and the water lands on walls, ceilings, packaging and stock. Do that every night and the building never dries out.
Dry the air first. Then warm it back.
A reheat fan coil has two coils, one behind the other. The first is cold and takes the water out. The second is warm and brings the air back to room temperature. Both are fed with water from heat pumps outside.
Swipe sideways to follow the air.
Chill
Damp air passes over the cold coil, fed with chilled water at 7 °C. The air drops below its dew point, the temperature at which it can no longer hold its water.
Wring
The water runs off the coil into a tray and away to a drain. It leaves the building for good.
Warm
The air is now dry but too cold for the room. The second coil, fed with hot water at 45 °C, warms it back up. The room gets dry air at a comfortable temperature.
A humidity sensor runs the unit, not a thermostat. It keeps drying for as long as the room needs it. Temperatures shown are a typical duty; every design is sized to the room and what is stored in it.
Where it fits
Anywhere damp air meets something that spoils, or people who have to breathe it.
Food and tobacco warehouses
Dried food, grain, cured tobacco, paper and packaging all take up water from the air. Dry air keeps the stock in the condition you bought it.
Warehouse climate controlFood plants and bakeries
Steam from cooking and water from the nightly washdown go straight into the air. Preparation and packing rooms stay dry between shifts.
BakeriesShops and commercial spaces
Doors open all day and let the damp in. Stock rooms, restaurants and shop floors stay dry without being made cold.
Building climateCondominiums and dormitories
Lobbies, corridors, basements and ground-floor shops are where the damp collects. They run from the same central water plant as the rooms.
Condominiums & dormitoriesThree ways to fight damp
Each one has its place. The last row says where.
| Question | More air-conditioners | Portable dehumidifiers | Karnot reheat fan coil |
|---|---|---|---|
| What it does | Cools the room. Dries only while the compressor runs. | Dries the air, and gives its heat back into the room. | Dries the air, then warms it back to room temperature. |
| What controls it | A thermostat. It stops when the room is cool. | A humidity switch on each machine. | A humidity sensor in the room, logged day and night. |
| Where the water goes | Some to the drain. Some back into the room when the compressor stops. | Into a tank someone has to empty, or a hose to a drain. | To the drain, piped. |
| The power bill | Rises with every unit added, and the room is over-cooled. | High. The air-conditioning then works to remove the heat they add. | One plant does the cooling and the drying. The warm coil runs on a heat pump: about four units of heat for every one of electricity. |
| Where it wins | Cheapest to buy. Fine where the air is already dry. | Quick to buy and plug in. The right move in an emergency. | A room or building you will run for years. It is a piped system, not a plug-in machine. |
Questions we get asked
What humidity stops mold?
Mold needs damp that lasts. Hold the air below 60% humidity, day and night, and it does not get the moisture it needs to grow. The nights matter most: that is when a building cools and the damp settles.
Why does my air-conditioner not stop the mold?
An air-conditioner works to a temperature. When the room is cool enough the compressor stops, and the drying stops with it. The fan often keeps blowing over a wet coil and puts the water back into the room.
What is reheat, and why is it needed?
To take water out of air you have to chill it well below room temperature. That air is too cold to blow into a room. The second coil warms it back up, so the room gets dry air at a comfortable temperature and the drying can run for as long as it is needed.
Does the reheat waste electricity?
Not the way an electric heater would. The warm coil is fed with hot water from a heat pump, which gives about four units of heat for every one unit of electricity.
How do I know it is working?
A humidity sensor in the room runs the unit, and the readings are logged day and night. You can see the record, and so can your auditor.
Is it only for big buildings?
It suits a room or a building you plan to run for years: a warehouse, a food room, a shop floor, a lobby. It is a piped system fed by heat pumps, not a machine you plug in for a week.
Fighting mold in a room or a warehouse?
Send us the size of the room, what you keep in it, and a photo of a humidity reading if you have one. We tell you what it takes to hold it dry.