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Block heater question

My dad always said 2 hour max on plugging in a block heater otherwise it damages the heater or seals. I think overnight being plugged in does no damage. Who is right.

It doesn’t matter. I’ve had block heaters plugged in for 48 hours and never caused damage to them.


You are correct. In just 2 hours, the block heater might not even raise the temperature of the coolant significantly.

These devices are designed to be plugged in overnight, and if you do this, the coolant should be at about 100 degrees in the AM. That will make the car a whole lot easier to start, and will give you heat very quickly. Your father’s approach is unlikely to achieve much.

Block heaters are pretty fail-safe…The time needed depends on the wattage of the unit…A 500 watt heater will warm up an engine easily in 2 hours. A 100 watt unit should be just be left on overnight. The larger heaters can have an impact on your electric bill so putting them on a timer makes sense…

In North Dakota there is absolutely no way any one of us would have done anything other than leave the heater on all night. It’ll have just sufficient energy to keep the coolant a mediocre if warm, not enough to do damage to itself or to anything else.

They’re designed to be run overnight.

Agree; where I live even the airport has plugs for block heaters, and I have left the car plugged in for up to 5 days. The airport does cycle the power on and off at one hour intervals. Two hours is enough to warm up the jacket water for an easy start and good oil circulation, anything more is a waste of electricity.

A block heater plugged in over four hours is wasting energy. After four hours the block heater isn’t going to raise the coolant temperature any more for the ambient temperature. If you plug the block heater into a timer so it comes on four hours before driving the vehicle, you can save on your energy bill.


I agree you don’t need it plugged in more than 4 hours before you start. All it is doing is heating the coolant up so if it were going to damage the engine, getting an engine warm during operation would do the same thing.

Time required to reach equilibrium temp depends on the heater, the car, the ambient temperature and somewhat on ambient wind speed. I know a guy that determined times for a car in a garage. At higher temps, a couple of hours might be sufficient. I don’t recall what his max was.

If the heat of a block heater damaged the car, imagine what all the heat would do when you run the engine.