How does a room heater work?

Every electric heater does the same simple thing: it pushes current through a wire with resistance, and the wire gets hot. A 2,000 W heater on 230 V mains has about 26 Ω of wire, from P = V² ÷ R (see OhmsLawClear). All of the electricity becomes heat, so every electric heater is 100% efficient, exactly like the element in a geyser (WaterHeaterClear). What differs is how the heat reaches you.

Radiant heaters (the rod, quartz, halogen and carbon types) make something glow. Their infrared shines straight onto you, like sunshine, and a shiny reflector aims it forward. They feel warm in seconds, but only where they point.

Convection heaters warm the air. A fan heater blows air past a hot coil, just like a big HairDryerClear; a PTC ceramic heater uses clever stones that can't overheat; a convector lets warm air rise out of its top. The oil-filled radiator (OFR) heats oil sealed inside steel fins: slow to start, gentle, and still warm for a while after you switch it off.

The angithi, a pot of burning charcoal, is the old way. It gives real heat, but it also fills a closed room with invisible carbon monoxide. More on that in chapter 5.

Heat moves in three ways (all three are in HeatClear). A heater uses the first two.

Radiation is heat carried by light, mostly infrared that you can't see but can feel. Everything warm glows; hot things glow far more, because the power follows the Stefan–Boltzmann law: P = εσAT⁴, with T in kelvin. Double the temperature and the glow is 16 times stronger. Infrared is the same kind of electromagnetic wave as the radio signals in RadioClear, FMClear and TVStationClear, only with wavelengths a million times shorter. Above about 525 °C some of the glow becomes visible: dull red, then orange, then yellow-white. A halogen filament at 2,200 °C is bright; an oil radiator at 75 °C glows only in infrared.

Infrared crosses the air without warming it and lands on you, the floor and the walls. So a radiant heater warms whatever it points at, fast. Step out of its beam and you feel cold again.

Convection is heat carried by moving air. Air touching something hot warms, gets lighter and rises; cooler air flows in below. A fan simply pushes harder. Convection heaters warm the room air, so the whole room slowly gets comfortable, not just one side of you.

Conduction is heat passed on by touch. In a room it works against you: heat seeps out through walls, windows and the roof to the cold outside.

An oil-filled radiator has no oil to burn. The oil is sealed in for good. An element at the bottom heats it; hot oil rises inside each fin, gives its heat to the steel and sinks again: convection inside. The fins then warm the room air: convection outside.

About 3.5 kg of oil and 9 kg of steel make a big thermal mass: over 10,000 joules for each degree. So an OFR takes many minutes to warm up where a fan heater takes seconds, runs at a gentle 70 to 80 °C, and keeps giving heat for 15 to 20 minutes after you switch it off.

Its thermostat is a bimetal strip: brass and steel bonded together. Brass expands more when warm, so the strip bends and flicks a contact open. Turning the knob moves the contact. A tip-over switch in the base cuts the power if the heater falls, and a thermal fuse is the last line of defence.

A PTC ceramic heater uses stones of doped barium titanate. Below about 220 °C they conduct well; just above it, their resistance shoots up ten times every 12 °C. So they draw full power when cool and throttle themselves when hot. Block the airflow and the power simply falls. A plain nichrome coil would keep getting hotter until a cut-out saved it.

A room is a leaky box. Heat escapes through the walls, the window, the roof and every draught. The rate is roughly U × A × ΔT: how leaky each surface is, how big, and how much warmer inside is than outside. Add it all up and a typical Indian bedroom loses about 100 watts for every degree you hold it above the outside air.

So a heater can only warm a room until the leaks match its power. That is the steady temperature. A 2 kW heater in a leaky room might never reach 20 °C; the same heater in a small, sealed room can make it too hot.

The walls matter too. Brick and concrete soak up heat for hours before they warm, which is why a heater warms the air quickly but the room still feels chilly near the walls. Heaters with a thermostat switch off when the air is warm enough, which saves money; plain radiant heaters just run flat out.

An AC in heat mode is a heat pump: it moves heat in from outdoors and gives about 3 units of heat for 1 unit of electricity (see ACClear). Every other heater here gives exactly 1. The energy itself follows EnergyClear: every joule you pay for ends up leaking outside.

Dry air. Warm air can hold much more water vapour than cold air: about twice as much for every 11 °C. Heating a room adds warmth but no water, so the relative humidity (RH) falls. Foggy Delhi night air at 7 °C and 85% RH, warmed to 22 °C, drops to about 32%. Below about 30 to 40%, many people get dry eyes, dry skin and a scratchy throat. Radiant heaters feel worst because they also dry the skin they shine on. A bowl of water helps a little; opening a window now and then helps more.

Carbon monoxide (CO) is the real danger. Charcoal, coal and wood in an angithi or sigri never burn completely (why, see CombustionClear); a glowing charcoal bed makes a lot of CO. It has no colour and no smell. It grabs haemoglobin in your blood more than 200 times as tightly as oxygen, forming COHb, so your blood carries less and less oxygen.

Asleep, you feel nothing. At 10 to 20% COHb comes a headache; at 30% you are too confused and weak to get up; above 50 to 60% it is often fatal. In a closed room this can happen in two or three hours. Every winter, families across North India are found dead this way. Delhi Fire Service counted 15 angithi deaths in Delhi in January 2024 alone. A study of autopsies at AIIMS New Delhi found about 80% of CO deaths happen in winter, nearly all in closed rooms. Official NCRB figures list only about ten CO deaths a year for all of India, so most are probably recorded under other causes. The true toll is not known, but it is far higher.

Most heater accidents are fires, burns and poisoning, and each has a simple rule.

Keep 1 metre clear. Curtains, quilts, sofas, clothes drying on a chair: infrared heats them just as it heats you, and they can't move away. Never dry clothes on a heater or cover it.

Plug it straight into a wall power socket. A 2 kW heater draws 8.7 amps for hours. A 6 A socket or a cheap extension board with a thin cord gets hot at the pins and in the wire, and that heat is where many house fires start. Heaters belong on a 16 A power socket on a 16 A MCB (see MCBClear). Don't run a heater and a geyser on one circuit.

Switch it off when you sleep, or use one with a thermostat and a timer. Never sleep in a closed room with a fire: an angithi, coal or wood needs a window open, and even then it is safer to put it out and take it outside before bed. A CO alarm costs less than a heater.

Children and pets touch things. Choose heaters with a guard, a tip-over switch and a cool-touch body, and keep them out of reach. Buy one with the ISI mark: in India, room heaters must meet IS 302 (Part 2/Sec 30) and be certified by BIS.