Why Your Dehumidifier Stops Working in a Cold Room

Every winter we get the same call. The dehumidifier has been running for four days in an empty new build, a garage or an unheated site room, the tank is barely wet, the walls are still damp, and the customer is sure the machine is broken. It almost never is. It is doing exactly what the physics allows at that temperature.

Why cold stops a dehumidifier

A refrigerant dehumidifier works by making a surface colder than the air, so moisture condenses on it and runs away as water. Two things go wrong when the room is cold.

1. Cold air holds far less moisture to begin with. Air at 20°C can hold roughly twice as much water vapour as air at 10°C. Even at the same relative humidity, there is simply less water in the air for the machine to catch — so the tank fills slowly and it looks like nothing is happening.
2. The coil ices up. To condense moisture out of already-cold air, the coil has to run below freezing. Frost forms, blocks the airflow, and the machine has to stop drying and run a defrost cycle. On a cold morning a basic unit can spend more of the hour defrosting than extracting.

This is not a fault and it is not a brand problem. UK government guidance on drying buildings gives the working ranges plainly: refrigerant dehumidifiers 15 to 28°C, and desiccant dehumidifiers — a different technology that uses a moisture-absorbing wheel instead of a cold coil — 0 to 25°C. Below about 15°C you are outside what a refrigerant machine was built for.

Two features push that limit down. Hot-gas bypass defrost clears the frost using the machine’s own heat instead of shutting down, which is why the Dri-Eaz DrizAir 1200 keeps working to around 1°C. And an LGR (low grain refrigerant) unit like our 63L LGR keeps pulling water out of air that is already dry, which is the other place standard machines give up.

What happens at each temperature

Is it broken, or is it just cold?

Six symptoms, and what each one usually means. If you recognise the first four, the room is cold, not the machine.

The fix, in order

1. Measure the room temperature. Not the outside temperature, not how it feels. A cheap thermometer settles the argument in five minutes.
2. Get the space to 18 to 22°C and hold it. UK drying guidance suggests running central heating at around 22°C during a drying job. In an empty building that means hiring heat.
3. Close the space up. Heating a room with the door open to an unheated hallway is heating the hallway. Drying only works as a closed system.
4. Use a machine built for the conditions. Hot-gas defrost for cold rooms, LGR for the late stage when the air is already dry.
5. Add air movement. Warm air sitting still does nothing. Air movers put it against the wet surfaces.
6. Give it a steady week before you judge it. Turning the heat off overnight resets the building every morning.

Find your situation

Open the one that matches the building. Each gives the kit and the real daily hire price, delivered across London from our Greenford UB6 depot.

Empty new build over winter

Plaster, screed and blockwork all giving up water into a building with the heating not yet commissioned. This is the classic case where a dehumidifier looks broken.

Kit: 3kW oil-filled radiator at £19.99 a day to hold the temperature, plus a a 55 litre DrizAir 1200 at £49.99 a day. Add an air mover per room.

More on this in new build damp in the first winter and drying new plaster with a dehumidifier.

Building site with no mains heating yet

110V only, or three-phase available but nothing plumbed in.

Kit: 3kW 110V site fan heater at £19.99 a day if you are on transformer power, or a 9kW three-phase heater at £24.99 a day where you have 3-phase. Pair with a the 63 litre LGR unit at £59.99 a day.

Our winter construction hire checklist covers the rest of the cold-weather kit.

Unheated garage, lock-up or workshop

Concrete floor, uninsulated walls, sits at outdoor temperature all winter.

Kit: 3kW fan heater at £19.99 a day and a 43L Dri-Eaz Cube at £39.99 a day. Seal the up-and-over door gap with anything you have — that gap is most of your heat loss.

Cold basement or cellar

Below ground, stable but cold, often 10 to 13°C year round.

Kit: heat first, then the 63L LGR unit with pump-out so nobody carries a tank up stairs. Full detail in how to dry out a basement.

Empty rental between tenancies

Heating off to save money, windows shut, and by February the walls are black. It is condensation, not a leak.

Kit: 38L dehumidifier at £29.99 a day with background heat. Landlords should also read Awaab’s Law and the damp deadlines — the response times are now legal ones.

Cold warehouse or unheated store

Large volume, low temperature, stock at risk. Ferrous metal starts corroding above 60% RH and mould begins around 70%.

Kit: 95L building dryer at £77.99 a day, with a 15kW three-phase heater at £29.99 a day or a 40kW unit at £64.49 a day for the volume. See warehouse and office humidity problems.

A flooded property in winter

The worst combination: soaked fabric and a building with no working heating.

Kit: heat plus LGR plus air movers, run continuously. Do not judge progress in the first 48 hours. The full sequence is in what to do after a house floods.

Which heater to pair with it

The heater is not there to make the room comfortable. It is there to lift the air into the band where the dehumidifier works. Prices are per day from our Greenford depot; a refundable deposit applies and comes back in full when the equipment returns undamaged.

Not sure which output you need for the room size, our heater sizing guide works it out from volume rather than guesswork. And if you are wondering whether you need both machines at all, heater versus dehumidifier for drying explains why the answer is almost always both.

What the heater actually costs you

The objection is always the same: another machine, another hire line, more electricity. Look at it as time instead.

A dehumidifier running in a 10°C room is not doing nothing — it is doing a fraction of its rated work, and you are paying full hire for it. Adding £19.99 a day of heat to get that room into its design band is the difference between a job that finishes and a job that drifts. On a two week hire, a heater that saves a single week has more than paid for itself.

The electricity side is a fair question and we have worked it through properly in what it costs to run a hired dehumidifier.

Five mistakes on cold jobs

1. Judging the machine in the first 24 hours. In a cold building the first day is spent warming the fabric, not drying it. Give it a settled week.
2. Heating in bursts. Six hours on, eighteen off, and the walls never come up to temperature. Steady and continuous beats hot and intermittent every time.
3. Leaving the door to a cold hallway open. You are heating and drying the whole building instead of the room you booked the kit for.
4. Blaming the dehumidifier for a ventilation problem. A trickle vent or a broken window undoes both machines at once.
5. Using a gas or paraffin heater indoors to dry a room. Burning fuel produces water vapour — roughly a litre for every litre of fuel. You are adding moisture while trying to remove it, as well as taking a carbon monoxide risk. Electric heat only for drying work.

Cold-weather drying questions

At what temperature does a dehumidifier stop working?

A refrigerant dehumidifier is rated for roughly 15 to 28°C. Between 10 and 15°C output drops noticeably. Between 5 and 10°C it spends much of its time defrosting. Below about 5°C a standard unit removes very little. Machines with hot-gas bypass defrost, such as the Dri-Eaz DrizAir 1200, keep working down to around 1°C.

My dehumidifier is not collecting any water. Is it broken?

Check the room temperature first. Cold air holds far less moisture, so a slow tank in a 10°C room is normal rather than faulty. Also check that the room is closed, that the filter is not blocked and that the drain hose is not kinked or running uphill. If the room is 20°C, closed, and the humidity is above 60% and the tank is still dry after a day, then call us.

Why is my dehumidifier icing up?

Because it is condensing moisture out of cold air, which means the coil has to run below freezing. Frost forms and blocks the airflow. The unit then runs a defrost cycle to clear it, which is time spent not drying. Warming the room is the fix. Units with hot-gas bypass defrost handle it far better than basic ones.

Should I use a heater or a dehumidifier?

Both, on a cold job. Heat lets the air carry more moisture; the dehumidifier is what actually removes it from the building. Heat alone just moves the damp to the coldest surface in the property, which is usually a window reveal or an external corner. The full comparison is here.

What is a desiccant dehumidifier and do you hire them?

A desiccant unit uses a moisture-absorbing wheel rather than a cold coil, so it works from around 0°C upward and does not ice up. It is the right technology for genuinely cold spaces such as cold stores. Our fleet is refrigerant and LGR, so on a cold job we pair a dehumidifier with a heater to bring the space into its working band, which for most London buildings is the cheaper and simpler answer.

What temperature should I set the heating to when drying a room?

Around 18 to 22°C, held steady. UK guidance on drying buildings suggests running central heating at about 22°C during a drying job. What matters more than the exact figure is that it stays there — a room that swings from 22°C to 6°C overnight loses most of the day’s progress.

Can I use a gas or paraffin heater to dry out a room?

No. Burning gas or paraffin produces water vapour as a combustion product — roughly a litre of water for every litre of fuel burned — so you are adding moisture to a room you are trying to dry. There is also a carbon monoxide risk in an enclosed space. Use electric heat for drying work.

Does an LGR dehumidifier help in a cold room?

It helps with the other half of the problem. LGR stands for low grain refrigerant, and it means the unit keeps extracting when the air is already dry — the late stage where standard machines stall. It is not a substitute for heat below 10°C, but on a cool job that has to finish properly, an LGR unit at £59.99 a day is the one that gets you to the end.

How long should I run it before deciding it is not working?

Give it a settled week at a steady temperature. The first day or two in a cold building goes into warming the fabric rather than removing water. If the humidity has not moved after seven days in a closed room held at 18 to 22°C, something else is wrong — usually ventilation, a live leak, or a machine too small for the volume. Sizing guide here.

Can you deliver a heater and dehumidifier together, today?

Yes. We deliver across Greater London from our Greenford UB6 depot with same day and next day delivery available, and heat and drying kit go out on the same drop. Tell us the room size, the floor type and what power you have on site — 13A plug, 110V transformer or three-phase — and we will put the right combination together. Call 020 3375 4048 or message us on WhatsApp.

Related guides

• Heater or dehumidifier for drying — what each one actually does to the moisture.
• What size heater do I need — output by room volume, not by feel.
• Winter construction hire checklist — the rest of the cold-weather kit list.
• Condensation on the windows every morning — the domestic version of the same physics.
• How long it takes to dry out a room — realistic timescales once the temperature is right.
• How many dehumidifiers do I need — for jobs bigger than one room.

A pool hall closed down for the winter combines both problems: a cold room and a constant moisture source. Holding a closed pool room under 60% RH covers what to run and why.