Space heaters, convection versus radiant: how each heats, and when one is not a whole-home plan

Convection vs radiant space heater: how each one heats

Convection is heat carried by moving air. North Dakota State University Extension says convection heat relies on circulating heated air currents. In plain words, a convection heater warms the air near it, the warm air spreads out, and over time the air in the whole room gets warmer. You feel the heat because the air around you is warmer.

Radiant heat is heat that travels in a straight line from a hot surface to whatever it reaches, without needing to warm the air first. Infrared is the name for this kind of invisible heat energy. The same extension page says radiant heat provides warmth by heating the objects in front of the heater, just as a sunbeam heats whatever it touches. You feel the heat because your skin and clothes, and the chair or floor near you, are being warmed directly.

That difference explains how each type tends to feel. A convection heater works on the room's air as a whole. A radiant heater works best on whatever sits in front of it, which is called its line of sight, meaning the things the heater can directly face with no wall or furniture in the way.

Is a radiant heater more efficient?

Ads for space heaters often make strong claims about efficiency, which here means how much useful warmth you get for the energy the heater uses. The extension sources we read are careful on this point.

North Dakota State University Extension says radiant heat may be slightly more efficient because it can be used to heat only the objects in the line of sight of the heater, reducing the need to heat the entire room. Notice the words may be slightly. The possible advantage comes from warming only what is in front of the heater, not from any magic in the heater itself. If you need the whole room warm, that advantage shrinks.

The same release is titled Be Wary of Portable Heater Claims, and the University of Missouri Extension published its own piece titled Study Space Heater Claims Carefully. Both titles point the same way: read the marketing on the box with care. This guide does not repeat any savings figures, because the sources do not back them.

In practice, the choice comes down to how you use the room. If you sit in one spot, such as a desk or a reading chair, warming you and the things near you may fit the job. If people move around the room, warming the air may fit better. The sources do not rank one type as the clear winner.

Safety: clearance, tip-over shut-off and cords

The safety advice from extension services and fire agencies comes down to space around the heater, a shut-off feature and how the heater is plugged in.

Clearance is the open space kept around the heater. Mississippi State University Extension says to keep combustible material such as beds, sofas, curtains, papers, and clothes at least 3 feet from the front, sides, and rear of the heater. Combustible simply means able to catch fire and burn. The Missouri Division of Fire Safety says the three-foot safety zone includes furniture, drapes, and electronics, anything that can burn. The Hartsdale Fire District in New York gives the same distance and names papers, clothing and rugs.

A tip-over shut-off is a switch inside the heater that turns it off if the heater falls over. The Missouri Division of Fire Safety says to buy a unit with a tip-over safety switch, which automatically shuts off the heater if the unit is tipped over. Mississippi State University Extension also lists a heater equipped with a safety tip-over shut-off feature among the things to look for. The sources present this as a feature to shop for, not as a legal requirement.

Cords matter too. The Hartsdale Fire District says to plug space heaters directly into a wall outlet. It also says not to use an extension cord or power strip, which could overheat and result in a fire. An extension cord is a separate cord that lengthens the heater's own cord, and a power strip is a block of several outlets on one cord. Mississippi State University Extension puts it bluntly: extension cords or power strips must not be used under any circumstance.

Last, turn the heater off when no one is watching it. The Missouri Division of Fire Safety says to turn off portable heaters whenever leaving the room or going to bed.

Why a space heater is not a whole-home heating plan

A whole-home heating plan is the main system that keeps every room of a house warm on its own, such as a central furnace, so that no room depends on a portable unit. A space heater is not built for that job.

Mississippi State University Extension says space heaters and other types of portable heaters are a great way to heat a single room, or supplement a building's main heating system. To supplement means to add to, not to replace. The Missouri Division of Fire Safety says portable space heaters are commonly used when the main heating system cannot adequately heat a home or when relying on central heating alone is considered too costly. In both descriptions, the space heater sits next to the main system.

The numbers show why. A BTU, short for British thermal unit, is a standard unit for measuring heat, and BTU per hour is how fast a heater puts out heat. The University of Missouri Extension says a well-insulated, 1,500-square-foot home will require at least 70,000 BTUs per hour to heat the entire house. The same article says infrared heaters typically operate on 110 volts and use 1,585 watts of power for maximum output, which works out to 5,410 BTUs per hour. A watt is a unit of electric power. That is a small share of what the example house needs.

The heat also stays put. The University of Missouri Extension notes that when you move to a different room, you need to bring the space heater with you.

The catch with turning the rest of the house down

Some people plan to run a space heater in one room and turn the main heat down for the rest of the house. The University of Missouri Extension warns that lowering the heat to the rest of the house too much can cause frozen pipes and excessive condensation on walls and ceilings. Condensation is water that forms when moist indoor air touches a cold surface, the same way water beads on a cold glass.

So if you use a space heater as a supplement, keep the thermostat for the main system at a reasonable setting rather than turning it far down. The thermostat is the wall control that tells your main heating system when to run.

If you find yourself relying on a space heater every cold day because one room or the whole house will not warm up, that is a sign to have the main heating system looked at. The Missouri Division of Fire Safety's description above, a main system that cannot adequately heat a home, is exactly the situation where a licensed contractor can find the cause.