Stay Safe and Warm With the Right Space Heater (Updated 10/07)


Oct 17, 2001 (Updated Oct 24, 2007)


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The Bottom Line Space heaters can be a safe and economical way to stay warm if you figure out how much heat you need, buy the right heater, and use it with care.

You probably have an area of your house, garage, shop, or work space which is always too cool during the winter. Not only that, there are probably also mornings during the spring and fall when you don't want to turn on your central heating system but could use a little extra warmth here or there to take the chill off. For these and other applications there are a variety of types of space heaters.

Unfortunately, most heaters are bought as impulse items, which is why stores move them to prominent locations when cold weather moves in. But a hasty choice can mean you won't really get the comfort from the heater that you were wanting, and in extreme cases it could even jeopardize the safety of your family or home.

If, instead of this, you follow a methodical approach and decide how much heat you need, what fuel is best to use to operate your heater, and how to install and use it properly, you'll greatly increase your comfort and level of safety.

HOW MUCH HEAT?

If you call a heating contractor to install a furnace in your house, he or she won't just come out and install the biggest one available. Instead, some calculations will be performed to determine your home's "design heat loss," which is the amount of heat it takes to keep your house at 70 degrees or so on the coldest day of the year. This involves taking into account the number, size, and type of windows, the square footage of the home, the ceiling height, and the home's construction type.

Most people purchasing a space heater do not need a measurement this precise, since the space heater is usually intended to provide a supplemental and not a primary heat source. In addition, heaters used in garages and workshops may be intended only to take off the worst of the chill, since a person working in one of these areas may not need the temperature held at a living-room-like 70 degrees. A temperature of 50 or 60 may be enough to keep a person busily working -- and probably wearing a jacket -- warm enough to be comfortable.

Since many space heaters are electric, a helpful way to think of the capacity of space heaters involves wattage. As a general rule, a decently-insulated area with an 8 foot ceiling and an average number of windows will need about 10 watts per square foot of floor space. For example, an average 200 square foot room will need a total heating capacity of 2000 watts to keep it warm on the coldest day of the year. If your space heater is for supplemental heat only, you can get by with a little less wattage than this -- often 5 to 7 watts per square foot.

For heaters that use fuels such as kerosene and natural gas or propane, heat ratings will be stated not in watts but in BTUs. It's easy to convert BTUs to watts for the sake of comparison. Simply divide the number of BTUs by 3.4 to get the approximate wattage equivalent. (Or multiply the wattage by 3.4 to get BTUs.) Thus, a 10,000 BTU kerosene heater will produce an amount of heat equivalent to an electric heater of just over 2,900 watts.

(The only catch here is that this comparison is possible only between electric heaters and unvented fuel-burning heaters, such as portable kerosene heaters and unvented natural gas or LP gas heaters. This is because unvented heaters, like their electric counterparts, are essentially 100 percent efficient. If the heater is vented, you must factor in the heater's efficiency rating -- often called its AFUE, for Annual Fuel Utilization Efficiency. A 15,000 BTU vented natural gas heater with an AFUE of 80 is 80 percent efficent, so it will actually deliver 12,000 BTU of heat to the room. So, to compare this heater to an electric heater, you would divide 12,000 BTU by 3.4 to calculate its heating power in wattage -- about 3,500 watts.

CHOOSING A FUEL

Once you have a rough estimate of the amount of heat you need, consider what type of fuel the heater will use.

120 Volt Electric Heaters

When I originally wrote this article in 2001, electric heaters were more costly to operate than gas or kerosene heaters, but this is no longer always true. While the price of electricity has remained stable or has increased by a moderate percentage, the price of kerosene is now two to three times what it was six years ago. The price of natural gas has varied a lot from year to year. Also, evolving consumer attitudes and changing legislation have made unvented fuel-burning heaters less attractive to some than they used to be.

Portable electric heaters are widely available and inexpensive. The biggest problem with portable, plug-in electric heaters is limited heating power: Because of electrical code requirements and the limitations of house wiring, portable electric heaters can produce no more than 1,500 watts of heat -- about enough for a 150 - 200 square foot room. If you need more than 1,500 watts, you may need two or more electric heaters, or a big one which operates on its own 240 volt line. Using two 1,500 watt electric heaters on the same household circuit is impossible -- it would cause an immediate, severe overload -- so unless the room where you need is served by two or more separate circuits, you're probably limited to one portable electric heater per room.

Electric resistance heaters deliver heat through the use of heating element(s) that become hot when electricity passes through them. This occurs because the elements are made of types of metal which do conduct electricity, but not very well. When electric current encounters resistance, it generates heat as it forces its way through.

Electric resistance heating is easy to control and regulate, is relatively safe if used with care, and generates no fumes or odors. Electric heating can be economical, too, especially if it's used as occasional, supplemental heat: If you were to use a 1,500 watt electric heater in your home for 4 hours per day, every day for a month, and if it ran constantly with no off-cycle time, it would add $14.40 to your electric bill if you pay the average U.S. electric rate of about 8 cents per kilowatt hour.

Electric heaters range in price from $10 or so to well over $100, depending upon features and design. If you visit any other discount, home, or hardware store, you'll find heaters in a variety of sizes, shapes, and colors.

But just remember that no matter what a heater looks like, a watt is a watt. One 1,500 watt heater produces exactly the same amount of heat as any other 1,500 watt heater. A $99 heater may have useful features and a more stylish look than a $9.99 heater, but it won't get your room one bit warmer, assuming both heaters have the same wattage -- which they usually do: Almost all heaters range from 1,200 to 1,500 watts.

240 Volt Electric Heaters

These are the big brothers of their smaller and more familiar siblings, the 120 volt electric models. Everything true of the smaller electric heaters is true of these heavy duty models, except these big electric heaters require the installation of a 240 volt circuit. You will generally find that plug-in electric heaters up to 4,000 watts (13,652 BTU) will operate on 20 amp 240 volt circuits, while portable heaters up to 5,600 watts (19,112 BTU) will need a 30 amp 240 volt circuit.

There is no shortage of heating power with these big electric units, but their high wattage increases their operating costs: A 5,000-watt electric heater will cost 40 cents an hour to run if you live where electricity costs the national average rate of 8 cents per kilowatt hour.

Risk Factors of Electric Heaters

The riskiest aspect of an electric heater pertains not to the heater itself but to the wiring to which the heater is connected. A 1,500 watt heater draws over 12 amps, and a typical household circuit is rated for 15 - 20 amps. When is it safe to connect an electric heater to regular household wiring: IF the circuit does not have a lot of other loads, IF the house's wiring is in good shape, IF the receptacle is in good shape so the plug fits into it tightly, and IF the heater is plugged directly into the wall and not into a light duty extension cord, an electric heater is pretty safe. But there are a lot of "IF's" to think about there, and many homeowners may not know the answer to all of them.

Electric heaters give a house's electrical wiring a real workout. If the wiring is not up to the task, a fire can result. Unless you have a special one-outlet circuit run for the heater, which is something most people will not do, you are not going to be aware of the condition of every inch of electrical wiring leading to the receptacle where your heater is plugged in. And every inch of that wiring matters: I have seen an electric heater plugged into one receptacle burn up a different receptacle on the same circuit. This can happen because receptacles often are connection points in home wiring. (Currently the National Electrical Code does not in most cases allow this, but most houses were built before that recent change to the code.)

Kerosene Heaters

Many people fail to consider kerosene heaters as an option because of news reports of fires started by heaters of this type. In reality they can be used safely, but because they do use a flame to produce heat, they must be used with care. Moreover, even if a kerosene heater doesn't seem like the best option for your home, it may be exactly what you most need in a big garage or workshop, where the need for quick heat may well exceed what a 1,500-watt electric unit can muster.

Indeed, unlike 120 volt, 1,500-watt (5120 BTU) electric heaters, kerosene models can have much greater heating power, ranging in many cases from 10,000 BTU (2,900 watts) to 23,000 BTU (6,800 watts). While a little 1,500 watt electric heater may do little to warm a big garage or shop, a kerosene heater with two to five times as much heating power may be able to keep such an area warm.

Because they do not need any electric power to operate, kerosene heaters are also a very popular choice for backup or emergency heating. A 23,000 BTU kerosene heater could keep an entire moderately-sized house reasonably warm in any but the very coldest weather.

Kerosene heaters do pose safety questions in two areas: fire and indoor air quality. Because they use an actual flame to produce heat, kerosene heaters must not be located where they could be tipped over by children or pets, and they must never in any case be left operating in the presence of children without adult supervision. They must also be fueled only by clean, clear K-1 kerosene. Using any other fuel -- especially gasoline -- is extremely dangerous. Kerosene heaters must be located well away from flammable materials such as clothing, household decorations such as draperies. Finally, and perhaps most importantly, kerosene heaters must never be fueled inside the house. Spilled kerosene is a major cause of kerosene heater fires. Shut the heater off and take the entire heater outside to fill it up, or in the case of models with a removable fuel tank, shut the heater off, and then take out the fuel tank and take it outside to fill it up.

If you look at reports of kerosene heater fires and injuries, you find that a high percentage involve three types of accidents: Improper fueling of the heater when it is operating, carrying the heater around when it is operating, and improperly attempting to extinguish a kerosene heater that has begun burning unpredictably or out of control. The importance of not carrying a burning heater around cannot be overstressed. When different wind currents hit the heater, as would often occur when moving from indoors to outdoors, the flame can flare up quickly. Fires fueled by petrochemicals are VERY hot, and people just do not realize how quickly they can be severely burned.

Likewise, if a heater is tipped over, NEVER try to set it back up. If kerosene spilled out of the heater when it tipped, it may now be soaking a rug or carpet, creating a perfect "wick" that is very easy to ignite. If you were to approach the heater to set it up as the kerosene ignited, you would be very badly burned. If a kerosene heater tips over, get everyone out of the area and WAIT at a safe distance for the heater's automatic shutoff mechanism to extinguish the flame. Only after everything is completely cooled off should you attempt to set it up again. If in any doubt, call the fire department after leaving the area.

Air quality concerns are posed by the fact that kerosene heaters burn a fuel and consume oxygen, rejecting the products of combustion right into the room. This sounds very bad on the surface of it, but keep in mind that gas stoves, unvented gas fireplaces, and other common gas appliances do this as well.

To avoid air quality problems, avoid using kerosene heaters in very tight (well-insulated, tightly closed) homes, and if you must do this, open a window a little bit in the room where the heater is operating. A necessity for anyone who uses a kerosene heater in a home is a carbon monoxide detector. The detector should be placed near the heater but not directly above or next to it since this could cause false readings.

Operational costs with kerosene heaters are easy to calculate and to compare with electric models. A 10,000 BTU kerosene heater will use a gallon of kerosene every 14 hours, as kerosene yields about 140,000 BTU of heat per gallon. If kerosene sells for $3 a gallon, this 10,000 BTU kerosene heater would cost a little more than 21 cents an hour to operate. An electric heater producing the same 10,000 BTU (2,900 watts) of heat would cost just over 23 cents an hour to run if electricity costs 8 cents per kilowatt-hour. If electricity is a little cheaper than that, as it is in many places, electric heat could actually be less costly than kerosene heat, and it offers the additional advantages of being hassle free, since there's no refueling needed with electric heat, and with electric there are also no concerns over fumes or odor.

While electric heaters and kerosene heaters do not have significantly different operating costs, kerosene heaters do not place demands on a house's wiring, require no special circuits or wiring provisions, and they are simple, inexpensive devices. As such, they are favored by many people who live in older homes, especially those with limited economic resources. In a perfect world, people would not use unvented kerosene heaters indoors, but people on a daily basis take greater risks than that posed by these heaters. If the choice is between freezing and using an unvented kerosene heater indoors, people will choose the heater.

On the other hand, I personally know of people who use kerosene heaters indoors to keep from running electric heat pumps. Since heat pumps are two to three times as efficient as electric resistance heat, you can see the problem here: Kerosene heat is far MORE costly than heat from a heat pump, so there's not much reason to let a kerosene heater pollute the air inside a house to avoid running a heat pump.

Natural or LP Gas Heaters

These heaters are similar to kerosene models in that they can be purchased in larger sizes than electric models, but they require that gas piping be installed and connected by someone experienced in doing such work. The risks for the do-it-yourselfer here are obvious, since the consequences of a gas leak from an improperly installed heater could be particularly severe.

One advantage of gas heaters over kerosene models is that they are available in vented models which reject products of combustion outdoors rather than into the room. This minimizes the risk of carbon monoxide in the home, but reduces a heater's efficiency.

Of course, the disadvantage here is that these heaters are by no means portable. They have to be installed where they are intended to be used, and once this is done they cannot be moved. In addition, natural gas prices have been unstable over the last few years, and utilities are not necessarily dropping their rates in unison with declining wholesale gas prices. At $0.90 a therm, and an 80 percent efficiency rating, gas and electric heat are equal in cost at an electric rate of just under 4 cents per kilowatt-hour. Since there are only a few areas of the country where electric power goes for less than 4 cents a kilowatt hour, gas is still anywhere from a little to a lot cheaper in most places. If gas goes still higher, that break point goes up, and electric could well become cheaper in many areas of the country. On the other hand, if you pay a dime or more per kilowatt-hour, there isn't much chance that electric heat is going to cost less than any other form of heat except, perhaps, burning dollar bills in a fireplace.

Heat Delivery Method

Three major methods of heat delivery are available, regardless of the fuel the heater uses. These methods are natural convection, forced convection, and radiant. Convection heaters -- both natural and forced -- heat the air directly, while radiant heaters warm objects and people in the room before actually heating the air.

Natural convection heaters do not use a fan to circulate air around a room. Heat flows upward and away from the heater and mixes around the room to warm it gradually through convection. These heaters are quieter than forced convection models -- which use a fan -- but they are generally slower to warm an area.

Examples of natural convection heaters are electric radiators, baseboard units, and many kerosene heaters.

Forced convection heaters use a fan to circulate air around a room and push it past heating coils, heating elements, or a heat exchanger. They do generate a small amount of sound from the fan itself, but they offer the advantage of providing rapid heat.

Examples of forced convection heaters are small fan-forced electric heaters and gas wall heaters with circulating fans.

Radiant heaters are a totally different breed entirely from the previous two. Radiant heaters use an orange or red glowing heat surface of one type or another. This could be an electric element or tube if the heater is electric, or it could be a flame-heated chamber if the heater operates on kerosene or natural gas. Radiant heat is transferred directly to people and other objects, and only indirectly to the air. The advantage is that people near the heater begin to feel warm and comfortable even before the heater has a chance to raise the temperature of the entire room.

In a garage, a 1,500-watt electric heater would do little to raise the temperature of the whole garage, but a 1,500-watt radiant heater placed close to someone working in the garage could make the person near the heater considerably more comfortable.

Examples of radiant heaters are electric heat dish type heaters, electric quartz heaters, radiant kerosene models, or gas models with a red glowing plate or chamber.

Installation and Use

Regardless of what type of heater you use, make sure you install it at least three feet (minimum -- check heater instructions for more information) from anything that could catch fire.

Exercise great care using a heater around children, where people are asleep, or while unattended. If you absolutely must use a heater under one of these circumstances, consider an electric model that operates on natural convection, such as an electric radiator or baseboard unit. These models tend to be the safest in these difficult circumstances. Radiant heaters require the most care in locating them properly because their surface temperatures are often higher than other heaters.

As stated above, the riskiest factor of any electric heater involves the household wiring, which is often not up to the additional load posed by an electric heater. As you operate a heater, monitor its cord and plug for excessive warmth: A cord or plug that's slightly warm to the touch is okay, but if either is too hot to touch, or if the cord or plug feels as if it's warm enough that it's becoming soft, turn the heater off, unplug it, and seek the advice of an electrician.

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