Blog · 26 September 2026 · 5 min read
Esquentador and Carbon Monoxide: Warning Signs and What to Do
Short answer: carbon monoxide (CO) is produced when gas burns without enough oxygen. You cannot see it or smell it. With an esquentador, the signs are indirect — a yellow or orange flame, soot or black marks, condensation on windows, and headaches, dizziness, nausea or drowsiness that improve when you leave the room. If you notice any of them: switch the heater off, close the gas, open the windows, get out into fresh air and do not use it again until it has been checked.
If anyone feels unwell, call 112. This is not a moment for troubleshooting.
Why a water heater can produce carbon monoxide
A gas burner needs a steady supply of fresh air and a clear way out for its fumes. When either fails — blocked room vents, a blocked or disconnected flue, a dirty burner, a badly adjusted heater — combustion becomes incomplete and produces carbon monoxide instead of harmless carbon dioxide.
Open-flued heaters (the atmosférico type) take their air from the room, so they depend directly on the room being ventilated. Room-sealed (estanque) heaters take air from outside and are much safer. See the types of esquentador.
Warning signs on the heater
Flame colour: a healthy gas flame is blue. Yellow, orange or lazy, floppy flames indicate poor combustion.
Soot or black marks on the casing, around the flue or on the wall above the heater.
The heater switching itself off repeatedly — many models have a safety device that cuts out when fumes are not escaping properly.
Heavy condensation on windows in the room while the heater runs, or a stale, heavy feel to the air.
Warning signs in people
Headache, dizziness, nausea, tiredness, confusion or shortness of breath while the heater is running — especially if several people in the home feel the same, or if symptoms improve when you go outside.
Pets being unusually lethargic can also be an early sign.
These symptoms are easy to mistake for a cold or tiredness. That is exactly what makes carbon monoxide dangerous.
What to do immediately
Switch the heater off and close the gas valve (or the bottle).
Open windows and doors, and leave the room or the flat.
If anyone has symptoms, call 112 and tell them you suspect carbon monoxide.
If you smell gas (rotten eggs), do not switch lights or appliances on or off, do not use a lighter, and leave — then call the gas emergency line from outside.
Do not use the heater again until a qualified professional has inspected it.
The bathroom trap: heaters inside bathrooms
Some older Lisbon flats still have open-flued water heaters inside or next to the bathroom. A closed bathroom door, a steamy long shower and a heater drawing air from a small room are a dangerous combination.
If your heater is in or open to the bathroom, keep the room ventilated, never block the vents, and ask your landlord or a professional whether it should be replaced with a room-sealed model or relocated.
How to prevent it
Never block permanent ventilation openings in the room, even in winter.
Have the heater serviced regularly: burner cleaned, combustion checked, flue inspected.
Do not store things on top of the heater or around the flue.
Install a carbon monoxide detector near the heater and in sleeping areas. They are inexpensive and give a warning long before symptoms become dangerous.
When replacing an old heater, prefer a room-sealed estanque model where possible.
What a professional check involves
We look at the flame and the burner, clean what needs cleaning, check the flue connection and draw, check the room ventilation and the safety cut-out, and test for gas leaks at the connections.
If the heater cannot be made safe, we tell you so clearly and give you replacement options. See water heater repair and replacement.
Choosing and placing a carbon monoxide detector
Buy a detector that complies with the European standard for CO alarms (EN 50291) — it is printed on the box. Battery models are easy to install; sealed-battery models last several years without a battery change.
Place it in the room with the water heater, about 1 to 3 metres from it and at head height or on the ceiling, following the manufacturer's instructions. Add one in or near bedrooms. Test it monthly with its button.
A detector is not a substitute for maintenance. It is a warning that something has already gone wrong.
Why winter is the risky season
In winter people close windows, block vents to keep draughts out and take longer, hotter showers. That is exactly the combination that starves an open-flued heater of air. Many incidents happen in bathrooms and kitchens with the windows shut on cold days.
Keep permanent ventilation openings clear all year round.
If you rent: what to ask your landlord
When the heater was last serviced; whether it is an open-flued or room-sealed model; whether the flue and ventilation have been checked. If the heater shows warning signs, report it in writing and stop using it until it is inspected — your safety comes before any disagreement about who pays.
Seen any of these signs? Stop using the heater and have it checked. We inspect, repair or replace esquentadores in Lisbon.
Water heater check and repairFrequently asked questions
Does an esquentador need a carbon monoxide detector?
It is strongly recommended for any gas appliance with a flame inside the home. A detector costs little and gives an early warning.
Can an electric water heater produce carbon monoxide?
No. Carbon monoxide comes from burning fuel. Electric tanks have other risks, but not this one.
My flame is a bit yellow at the tips. Is that dangerous?
Small yellow tips can happen, but a mostly yellow or orange flame is a sign the heater needs checking. When in doubt, have it inspected.
Who is responsible for the heater in a rented flat?
The appliance belongs to the property. Report any warning signs to the landlord in writing and stop using it until it is checked.
How often should a gas water heater be serviced?
Every one to two years is a sensible interval, more often for older open-flued heaters.
My CO alarm went off. What do I do?
Switch off gas appliances, open windows, get everyone outside, and call 112 if anyone feels unwell. Do not use the heater until it is checked.
Services Lisbon plumbing team
Guides written by the plumbers who do this work in Lisbon homes every week — drains, leaks, water heaters and pipework.
