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Engine Heater Effectiveness?

I have a 2005 Subaru Outback with a winter package that includes an engine heater. It’s parked outside (no garage). During the winter I usually remember to plug it in the night before. But some days I forget. Is it worthwhile plugging in for only 5 minutes in the morning before I start it up? Or does it take more like 30 minutes to make a difference? On average, morning temperatures are around 20 degrees. Thanks! Carol

If the temperature is 20* you don’t need the block heater at all.

What if it is zero degrees (F) or lower? On those mornings will five minutes of plugging it in make any difference or does it need longer to have any meaningful impact on the fluids?

Minimal. I have a heater for my tractor. The effectiveness of mine is measured in hours, not minutes. IMO, 20degrees may not be worth the inconvenience unless the early warmth to you personally is important. If it is, I would get a remote starter. Asemaster is right as far as the car is concerned. To gage the effectiveness on your car, experiment with different times over an hour, which would be my minimum time.

It depends on the ambient temperature on how effective the engine heater is. The colder it is the longer it takes to heat up the mass of the coolant and the engine. I live in Minnesota, and when I use to park my car outside I had my engine heater on a timer. So when it was 20 degrees outside I’d set the timer for one hour before using the vehicle. When it was zero out I’d set the timer for two hours. And when it got down to 10 below or lower the timer was set for four hours before using the vehicle.


In my opinion if the temp gets down to zero F you’re going to need hours for the heater to be effective. Even at 20* 5 minutes will gain you nothing.

Now it depends what the goal here is–engine wear and operation or passenger comfort. If your concern is keeping the engine and oil warm enough to operate without significant friction and wear on the cold starts, I’d use the block heater for a couple of hours when the temp hits zero. If your primary concern is keeping the fog off the windshield and the passenger compartment warm soon after start-up, use it when the temp hits 20.

If you regularly experience temps at zero, I would also use the lowest viscosity oil recommended for your car.

I use a block heater at 15 degrees and below, I would figure a minimum of 2 hours to gain any benefit.

Thanks all, for the feedback. I have both goals of a warm interior (my comfort!) and I would like to put less friction on the engine. I’ll look into a timer as it sounds like a great solution. Carol

Based on my limited experience, you might want to turn the heater on shortly after you turn off the engine. Keeping the engine warm is easier than trying to warm it. Your owner’s manual might have some useful ideas about the ‘winter package’.

5 minutes isn’t going to do anything. As Tester said more like two hours if it is below zero. On non-fuel injected cars I used to use the heater even in a garage when it got down to 10 and below but not with fuel injected. It’ll just help take the strain off of things but won’t make it start or not unless it is really cold like -20. I don’t want to think about it.

I have both goals of a warm interior (my comfort!) and I would like to put less friction on the engine.

Of your two goals, the comfort goal is the only one that makes sense. While it’s true that you’re putting less friction and wear on the engine, you’ll be very hard pressed to measure it. As someone once wrote, it might be the difference between getting 310K vs 315K miles from your engine.

I agree with the others who stated that ~2 hours is really the minimum for the block heater to actually do anything significant.

Also–I have a very practical bit of advice, so that you don’t wind up in a situation of driving away with the extension cord still attached to the block heater. This is what I used to do, back when I had an engine block heater:

After plugging the bright orange extension cord into the plug for the block heater (located underneath the front bumper), I draped the extension cord over the side-view mirror on the driver’s door. This gave me a VERY obvious reminder that I needed to unplug the wire before backing the car out of the garage! Otherwise, in my usual rush to leave the house, I could have overlooked the cord running from underneath the bumper to the wall outlet in front of the car.

@vdcdriver that is what I do also, mostly because my wife might not remember if I plugged her car in or not. luckily the cord unplugged from the car ended up in the drive when it happened.

Another helpful hint, blue cords (not the ones with the light on the end) stay flexible in cold temps.

Five minutes won’t do much good; you need 1 1/2 hours or so. Most people think a block heater is only needed to ensure the car starts. The other benefits are many:

  1. Less load and wear on battery and starter motor
  2. Less oil sludging and improved (more rapid) oil circulation to critical parts, resulting in much less engine wear. Those first frew minutes after a cold start cause very high rates of engine wear.
  3. You get heat from the heater and defroster quicker

Having used a block heater for over 47 years, we have yet to dispose of a car because the engine wore out.