Do non-radiant heaters dry out the air in your home?
Dry lips, itchy skin and static shocks in winter. Heating does not remove moisture from air, but it does lower relative humidity, and some heaters make it far worse than others.
Dry lips, itchy skin, a scratchy throat in the morning, and static shocks off the door handle. Every winter, and every winter people blame the heating.
They are partly right, and the way in which they are wrong is the interesting part.
Heating does not remove water from the air
This is the piece that gets misreported constantly. Warming air does not take moisture out of it. The absolute quantity of water vapour in the room is unchanged.
What changes is relative humidity. Warm air can hold more moisture than cold air, so the same amount of water in warmer air represents a smaller fraction of what the air could hold. Heat 5 °C outdoor air to 21 °C indoors and the relative humidity falls dramatically, without a single molecule of water having gone anywhere.
That falling relative humidity is what your skin, eyes and throat register as dryness. The comfortable band is 40 to 60% RH. In a cold climate, air heated from below freezing can land nearer 20%, and in a well-ventilated space during a sustained cold spell it can drop lower still.
So the honest answer is: the drying effect is mostly a consequence of heating cold air at all, not of which heater you chose.
Where the heater does make a difference
Two things separate one heater from another, and both are real.
How high you have to run the air temperature. Since the effect is driven by how far you warm the air, a system that makes you comfortable at a lower air temperature produces less of it. A radiant system warms you and the surfaces directly, so most people are comfortable two to three degrees lower than a convection system requires. Two or three degrees less air heating is two or three degrees less relative humidity lost.
Whether it moves air. A fan heater or forced-air system is continuously mixing room air with colder, drier air drawn in from elsewhere, and continuously passing air over a hot element. It also lifts dust and allergens off surfaces and circulates them. That is a different and much more noticeable dryness than the thermodynamic effect.
Heater by heater
Electric convection and ceramic heaters. The element is small, so it must run very hot — typically 150 to 250 °C — to deliver useful output. At those temperatures the element scorches airborne dust, which is the burnt smell you get when a heater is switched on after the summer, and it degrades air quality. They heat air and only air.
Fan heaters. The same, with air movement added. The worst of the options for perceived dryness, which is why people who use them so often end up buying a humidifier too.
Gas heaters. Unflued gas heaters actually add moisture, since water vapour is a combustion product. That is not the advantage it sounds like: they also add combustion gases to a room, and in a poorly ventilated space that is a genuine hazard rather than a humidity strategy.
Infrared. The element runs at around 55 °C rather than 250 °C, spread over a large surface rather than concentrated in a coil. Nothing is hot enough to scorch dust, nothing is burned, and no air is moved. The room is warmed by radiation absorbed at its surfaces.
The condensation problem at the other end
It is worth saying that low humidity is not the only failure mode. Cold moist air meeting warm surfaces — or warm moist air meeting cold ones — produces condensation, and condensation produces mould and rot.
Radiant heating helps here too, for the same underlying reason: it warms the surfaces, and a surface above dew point does not condense. The cold corner behind the wardrobe is where the problem lives, and it is the one place forced air never reaches.
Practical advice
Whatever you heat with:
- Fix the fabric first. Draughts through gaps, leaky windows and poorly sealed floors pull cold dry air in continuously. That does more damage to indoor humidity than the heater does.
- Do not over-heat the air. Every degree costs you relative humidity as well as energy.
- A humidifier is a reasonable tool alongside a convection or fan heater, provided it is cleaned. An unmaintained humidifier is its own health problem.
- Plants and open water help a little, and cost nothing.
In short
No heater physically removes water from your air. But a system that has to drive the air temperature high and blow it around your rooms will make a house feel much drier than one that warms the surfaces and leaves the air alone.
If you want to know how the surfaces in your building would need to be heated, send us a plan.
