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When you plug in your block heater, it warms the engine coolant, which warms the engine block and oil.Fred Lum/The Globe and Mail

Is there any benefit to plugging your car’s block heater in when it’s not super cold outside, like only around minus 5 or 10 degrees Celsius? I’ve always just plugged in my car all winter long, and I’m wondering if that’s a bad idea. Could it hurt my car? – Kyle, Winnipeg

When it’s not that cold outside, plugging in your car all the time might help your car get warmer inside a little faster, but it could hurt your power bill.

“It is below minus 15 degrees Celsius, you don’t need to plug your vehicle in,” says Calvin Feist, automotive instructor at the Northern Alberta Institute of Technology (NAIT) in Edmonton.

When you plug in your block heater, it warms the engine coolant, which warms the engine block and oil.

So why plug it in when it hits minus 15 degrees Celsius? That’s the temperature that oil starts to get thick and doesn’t flow normally.

If there’s less oil flow, there’s more friction. That’s when engine parts start wearing faster than they should.

When you plug in your block heater, it warms the engine coolant, which warms the engine block and oil.

So, are there any benefits to plugging it in when it’s above minus 15?

“Yes, the warmer things are the easier they turn over so the less time you have with little oil flow,” Feist says. “But the little gain you get is probably offset by the cost of electricity.”

Plugging in the block heater will also help heat the inside of your car faster.

“It heats the coolant which is used to heat the interior of the vehicle,” Feist says.

If you plug it in when it’s warmer out, it probably won't cause any problems “other than premature wear on the engine heater,” Feist says.

On some newer cars, the block heaters might not come on when it’s warmer than minus 15 degrees Celsius outside, even if you keep them plugged in all the time.

“The heater cords have temperature sensors in them,” Feist says. “They don’t send power to the heater unless it is below a certain temperature.”

Consider a timer

Even when it’s colder than minus 15 degrees Celsius out, you don’t need to keep your engine heater plugged in all the time – there’s no benefit to keeping the coolant warm if you’re not going to be driving your car.

Natural Resources Canada (NRC) suggests plugging in your car two hours before you start it.

The Canadian Automobile Association (CAA) suggests plugging in a little sooner – 3 to 4 hours before you drive.

If you don’t want to go outside and plug your car in at 4 a.m., you can buy a block heater timer. They’re typically under $30.

When it’s cold out, you only need to let your car idle for two or three minutes, whether you plugged it in or not.

In the winter, a cold engine burns more gas and produces more tailpipe emissions. If you idle for 10 or 15 minutes to warm up your car, you're polluting and burning more gas than you would be if you were driving.

It's better to just get in and drive until the car reaches its proper operating temperature, NRC says.

“Idling your vehicle uses lots of fuel and you are driving nowhere,” Feist says. “If it’s below zero, let the engine run for two to three minutes at idle so the coolant is warm and the windows hopefully won't frost up.”

For the first kilometre or so, drive slowly so you can warm up the fluids in your car’s drivetrain, Feist says. The drivetrain only warms up as you drive. When it’s cold, the fluids don’t flow properly and so they don’t work to prevent parts from grinding against each other.

“Consumers have to remember that the drivetrain has oil and fluid in it as well and, in most cases, is a thicker viscosity than engine oil,” Feist says. “You can do a lot of damage by warming the engine for ten minutes then just putting it into drive and heading out on the highway.”

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