How It Works: Block Heaters

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Just as athletes stretch their muscles before they perform, vehicle engines are happiest when they’ve been given a bit of a warmup before they’re started on very cold days. That’s the job of the engine block heater.

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It was invented in 1947 by Andrew Freeman, who’d lived through enough winters in North Dakota when he ran down his Ford’s battery trying to start it in the bitter cold.

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He used an electric heating element to warm the fluids in the engine, and while his rudimentary design has been improved over the years, block heaters still work on the same basic principle.

What exactly are we heating?

The name “block” heater refers to the lower part of the engine, known as the block. Thanks to gravity, oil collects in the pan at the bottom of the block when the engine’s shut off. Not all heaters go into the block, but most people use the term anyway.

Oil gets thick as it gets cold. This makes it harder to start the engine since the moving parts have to overcome this resistance — and any extra cranking time will use more battery power.

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It also takes longer for the oil to get up to the top of the engine, and even though it’s only for seconds, that increases engine wear. Synthetic oil flows better than conventional oil and helps to prevent this, but it can only do so much when the thermometer drops deep into negative temperatures.

Block heaters aren’t necessary in milder climates, but if the temperature falls below about -15°C, they’ll make a difference for your engine’s start-up, whether it’s first thing in the morning or if it’s been sitting outside all day at your workplace.

Most types of block heaters actually warm up the engine coolant rather than the oil, but this also warms up the engine, which in turn takes the chill off the oil. Some are located in the lower rad hose, while others are screwed into the block through an expansion plug hole.

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Do all block heaters fit into freeze plugs?

Also known as “freeze plugs,” the popular story behind expansion plugs is they’re meant to protect your engine if it’s cold enough that your coolant freezes: the plugs pop out so the ice doesn’t expand inside and crack the engine block.

That’s the theory, but even in rare circumstances where an engine freezes, they don’t always pop out as planned. In reality, it’s because engines are sand-cast, with a die pressed into sand to make a mould that’s then filled with molten metal. The holes are part of the mould, and the sand is shaken out of the engine casting through them.

The rad hose or expansion plugs are the most common locations for factory-installed block heaters. Other types include heaters that go into the engine oil dipstick tube; oil pan or engine heaters held on with magnets or glue; heaters that screw into the oil pan drain hole; and electric blankets that go over the top of the engine and are removed before you drive away.

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There are also transmission heaters, which warm up the fluid to help the unit shift gears more easily and efficiently, as well as battery heaters to keep these warm for better starting power, although they’re generally far more popular on work trucks than on everyday passenger vehicles.

How do I get a block heater in my car?

Thanks in part to improvements in engine oil, and in the cost of including something that most people don’t use anyway, automakers are more likely to list block heaters as extra-charge options rather than a standard feature.

If you’re adding an aftermarket one, then unless you’re really handy around a tool chest, get it professionally installed if it’s the type that goes into the engine block or cooling system, to help avoid the potential for leaks.

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The block heater plugs in using its attached cord, and if it’s your first time using it, you’ll have to find the cord under the hood. While it may be convenient to simply plug it in before you turn in for the night, the heater only needs to be on for about three or four hours to do its thing. Installing and setting a timer on the outlet will cut down on the amount of electricity used. Don’t forget to unplug it before you drive away in the morning (don’t ask how we know this).

In addition to component protection, an engine that’s already been warmed a bit by a block heater will come to operating temperature quicker, saving on fuel and emissions.

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Whether you use a block heater or not, once you’ve started your vehicle, don’t let it sit and idle until it’s fully warmed up. It’s running richer than usual, and because it’s just idling, unburned fuel can wash away oil in the cylinders and result in engine wear.

A modern engine actually warms up faster when it’s being driven. Start it, let it idle for a minute or so, and then drive moderately for the next few kilometres, accelerating smoothly instead of stomping on the throttle. That quicker engine warmup also means your heater will blow hot air sooner, and both you and your engine will be better off for it.