Last Updated on October 26, 2026 by Umar Farooq
Do boilers create humidity? A normal flued gas boiler makes a lot of water vapour when it burns gas, but it sends that vapour out through the flue, so it does not raise the humidity inside your home. If a boiler is the reason the house feels damp, something has gone wrong with it, or the real culprit is somewhere else.
It is an easy mistake to make, because the symptoms arrive together. The heating comes on, the windows steam up and the boiler gets the blame. Most of the time it is just standing nearby looking guilty.
Do boilers create humidity? Where the water from burning gas goes
Burn natural gas and you get carbon dioxide and a surprising amount of water vapour, because the hydrogen in the gas turns into water. Green Building Advisor, writing about gas furnaces, puts it bluntly: the combustion process produces a lot of water vapour, but it does not get into the house, because it leaves through the flue. The same article notes that unvented gas heaters are different, because they release that moisture indoors. That article is about furnaces rather than UK boilers, but the combustion chemistry and the flue principle are the same.
A modern condensing boiler goes further. It runs the flue gases cooler so that more of the water vapour condenses inside the boiler, and the condensate is sent to a drain. What is left leaves as a white plume at the flue terminal. Worcester Bosch’s installer FAQ says that plume is normal, and that the flue gases of condensing boilers exhaust at around 55 degrees C, so they cool quickly in the open air and look like breath on a cold morning.
So the steam outside the house is the boiler doing its job. It is the moisture you do not see indoors that is worth worrying about.
Open flue or room-sealed: does it change the answer?
Not for moisture, but it matters for safety, so this comes first.
Never block, cover or box in a boiler flue, air vent or the grille of a boiler cupboard, and do not seal up the gap round a flue. If you notice soot staining round a boiler, a yellow or orange flame instead of blue, or a persistent smell of burning, turn the boiler off and call a Gas Safe registered engineer. Fit a carbon monoxide alarm near any gas appliance.
An open-flue boiler takes its combustion air from the room and sends the products up a chimney or flue. A room-sealed boiler, which is what most homes have now, brings air in and sends exhaust out through a sealed duct. Either way the water vapour from combustion is carried away with the flue gases. What changes is how the boiler interacts with the room’s air supply, which is why an open-flue boiler needs a permanent air supply and why you should never block its vent.
When a boiler can add moisture indoors
A flued boiler in good order does not add humidity, but a few faults and set-ups can put water in the house. These are my reading of how the system works rather than numbers from a study, so treat the list as places to look.
- A leaking flue joint. If the flue gas escapes into a cupboard, loft or room instead of the outlet, its vapour goes with it. Stained plaster or damp around a flue run in a roof void is a reason to get it checked.
- A leaking or blocked condensate pipe. The drain has to carry that acidic condensate away. If it leaks, freezes or overflows, the result is a wet floor or wall, not humid air, but it looks like damp.
- Pipework and radiator leaks. A weeping valve or a pinhole in a pipe adds water to a cavity or floor, and that shows up as damp patches and a boiler that keeps losing pressure.
- An unflued gas or paraffin heater. People often mistake these for “the heating”. They put combustion water straight into the room, which a flued boiler does not. Do electric heaters reduce humidity explains the contrast.
Why it feels like the boiler is causing the damp
Three things travel with the boiler that look like cause but are not.
The hot water. A combi boiler heats the water for showers, and the shower is the moisture source. Showers, cooking and drying clothes are the standard household moisture sources, and the boiler is not on that list. The boiler provides the heat; the person in the cubicle provides the vapour.
Warm air holds more moisture. When heating comes on, the air warms and relative humidity falls, even though the water content has not changed. When it goes off, the air cools, the relative humidity rises and the cold windows collect condensation. The boiler did not add that water, but its schedule decides when it shows up. When to turn heating on to stop damp explains why a steady low heat often does better than short bursts.
The cold shoulder. If the boiler is running to keep the house at a low temperature, the walls and window glass stay cold, and the indoor moisture from cooking, breathing and washing settles there. Does heating reduce condensation looks at this trade-off.
Is it the boiler? Three easy checks
- Look at the boiler and its pipework. Dry cupboard, no drips from the condensate pipe, no pressure loss on the gauge, no stains round the flue. If any of that is wrong, call an engineer; it is not a humidity problem to solve with a dehumidifier.
- Compare the times. Does the dampness arrive with showers, cooking or laundry rather than with the heating? Write down when the windows steam up for three days. If it follows the activities, the source is people, not the boiler.
- Rule out unflued heaters. If any portable gas or paraffin heater is in use, that is the likely moisture source. Stop using it indoors, then see what happens.
If all three come back clean, the cause is ordinary household moisture. What causes high humidity in a house lists the usual sources, and how much moisture a household produces puts numbers on them.
A working boiler is the one thing in the house that is not making it damp
A flued boiler burns gas, makes water vapour and sends it all outside. It adds to your damp only if the flue, condensate drain or pipework is leaking, or if the heat in question comes from an unflued heater. Otherwise the humidity is from showers, cooking, breathing and laundry, and the answer is extraction and ventilation, with a dehumidifier for the stubborn cases.
FAQ
Does a combi boiler cause damp?
Not by itself. A combi boiler heats hot water on demand and sends its combustion vapour outside through the flue. Damp comes from the moisture you create with showers, cooking and laundry, or from a leak in the boiler’s pipework or condensate drain.
Why is there steam coming out of my boiler flue?
On a condensing boiler it is normal. The flue gases are cool enough, around 55 degrees C according to Worcester Bosch, that the water vapour condenses into a visible white plume outdoors, especially on cold or still days. It indicates the boiler is working as designed.
Does central heating make a house damp?
The heating itself does not add water to the air. Condensation can still appear on windows and cold walls when warm, moist air meets a cold surface, so the damp is a ventilation and temperature issue rather than a boiler output.
Should I put a dehumidifier near my boiler?
Not as a fix for the boiler. If the boiler cupboard is wet, find the leak. A dehumidifier is for reducing the general moisture in the rooms, and it should not be boxed in or placed where it blocks the boiler’s ventilation.
Can an old boiler leak water vapour into the house?
A badly sealed or damaged flue can release combustion products into the building, which is a safety issue as well as a moisture one. Turn the boiler off and call a Gas Safe engineer if you suspect that.
