How to not burn down your old house with a plug-in space heater

Minimizing other loads sharing the circuit, avoiding extension cords and using proper cords when necessary, replacing receptacles whose contact spring-tension has become loose, and carefully managing the heaters’ controls can all improve your odds of avoiding a tripped breaker or house fire.

1. Balance your loads. Map your home’s electrical system, and plug your high-power appliance into a lightly-loaded circuit (and then avoid adding other loads to that circuit, whether they be one-time (a corded tool) or semi-permanent (a floor lamp or entertainment system).
2. Get connected. Don’t use an extension cord unless you have to. The most legitimate use of an extension cord for a space heater would be to place it beside your desk or reading chair, and extend its cord within the same room to reach a dedicated or lightly-loaded receptacle not otherwise within reach of the manufacturer’s cord. When using an extension cord, use one rated for 15A or better (not the smaller household type cord). Use the shortest cord available, that will reach. Go out and buy one that is the right size (15A or greater rating) and within 3′ of the right length. Use a 9′ cord for a 9′ job and a 6′ cord for a 6′ job. Never daisy-chain cords together, especially for something like an unattended, high-power appliance like a space heater: buy a cord that is long enough. The contacts at each coupling are a weak link in the chain of contacts in the circuit, and a frequent point of ignition in house fires — so much so that much of the National Electric Code (NEC) for residences amounts to rules that prevent you from ever needing extension cords in newly-built homes and compliant renovations.
3. Keep snug. Check the spring-tension of contacts in any receptacle you will be using. If the cord-end doesn’t sit snugly in the receptacle, have the receptacle replaced with a new one (or replace the extension cord or multi-outlet strip with the loose socket). Loose contacts may result in open connections and result in arcing across tiny gaps between the receptacle and the prongs of the cord-end plug. An arcing contact gap may operate the heater, but it scorches the contacts, further degrading the connection, and heats the metal parts more than they would heat if connected firmly. This type of loose, arcing connection causes many fires each year, a problem for which AFCI breakers are a solution.
4. Stay in control. Most plug-in electric space heaters feature a thermostat and a power setting. Many people never take time to consider how best to use either setting. They turn everything up full blast and then turn it all off 10 minutes later when their shins start to get scalded by the excessive heat.
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     Instructions on appliance cords tell you how much power the item consumes and what type of cord to use.

     You may be able to achieve a comfortable temperature without resorting to the highest power setting of the heater — it’ll just take a bit more time. The power setting of a heating appliance is related to time. Power = work per time. How many degrees will the appliance change the room temperature (work) in a given amount of time? More power (work/time) faster temperature change per time — but has no direct connection to the final/equilibrium temperature. Some heaters indicate right on the control, the wattage associated with each setting. A common control configuration is a 500W switch and a 940W switch. Individually, they do what they say: 500W or 940W of heating. Together, they draw the full 1440W for which the heater is rated (the entire capacity of a typical 15A residential circuit that shares several receptacle and lighting locations throughout the house. 500W is ‘low’; 900W is ‘medium’; and both together, 1440W, is ‘high’. Plan your heating. Turn on the heater on low or medium before you need the heat, and then use high only if you still need it when you settle down to use the room. If you know your circuit is overloaded by setting your heater to the ‘high’ setting, remember that ‘overloaded’ means ‘enough of a risk of fire that a respected national fire-prevention association studied more than 100 years of fire statistics and determined you should never do this.’ It’s not that you will start a fire. It’s that the small chance of starting one is unacceptable. Compare to cigarette-smoking and cancer. It’s a risk thing, involving a small chance of a big catastrophe.

   • The thermostat actually works. You can turn on the heat ahead of time, and the thermostat will turn it off automatically when the room temperature reaches its target. Many thermostats on space heaters lack a degree scale, but you should be able to hear the thermostat click in and out as you adjust it with the heater on. Set the thermostat ‘too hot’ and leave it there just until the heater brings the temperature to ‘just right’. Then dial the thermostat slowly back down until you hear it click (off). It is now set for your comfort temperature.

Sometimes, even these precautions won’t mitigate circuit overload sufficiently. It doesn’t matter that a space heater ‘always worked in the past’. It doesn’t matter that ‘nothing else is turned on, on that floor’. Knob-and-tube wiring, especially, tends to serve multiple floors (there is no reason for any type of circuit wiring to be confined to one floor), and knob-and-tube wiring is very old. Overloading it for decades could work out fine, but eventually the years of excessive heat from overload can cause it to fail. Alternatively, a system normally capable of handling a slight overload might have another marginal failure (such as spring tension wearing out on an old receptacle, resulting in a poor contact to the inserted cord plug-end), such that the combination of two or more factors, though unlikely, eventually does occur, after decades of good fortune, and a receptacle or appliance cord catches fire (or a breaker trips that never tripped before).

 Upgrade your electric, if necessary

If you can’t be sure that your receptacle (and the circuit serving it) has capacity for your plug-in electric space heater’s high or medium setting, then don’t use it. Have a new circuit installed, and stop worrying.

See also:

• Learning to live with multi-generational wiring in Philadelphia homes
• Philadelphia Top Injury Lawyer: Space heaters create fire risk in Philadelphia
• Philadelphia Fire Department: Fall (heating season) safety tips

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