Car heaters

[Neatheyc]

When using the heaters in a car, does it consume any extra petrol?

I always thought the heat came from the already hot engine?

[Mortartube]

It shouldn't use any more petrol. When you turn the heater on, you open a valve which allows the cooling water from the engine to run through pipes covered in fins (called a heater matrix). Cool air blows over these fins and is heated, it is then directed into the car by pipes Air conditioning on the other hand can certainly increase fuel consumption.

[Bonzo 2000]

As mortartube says, the heat comes for free.  Use of the fans to blow the heat through the cabin will increase the load on the electric system and therefore the alternator, and therefore increase fuel consumption.  If you have a rev counter, you will see it dip slightly if the engine is idling and you suddenly turn the fans on to full blast.  The revs quickly recover to normal idle speed, but only by pumping a bit more fuel into the engine.

[Neatheyc]

Bottoms. Looks like I lose my bet with my girlfriend then!

Cheers guys.

[New Forester]

I think you do win your bet, really.  When driving along the heater will work by the ram effect without the fan.  The fan will blow cold air without the heater.  I think it's the fan not the heater which uses fuel.

 

In any case, at tickover the engine is using very little fuel to start with, so quite a small extra load will show up -- anything much more than the radio or the interior light (you don't need a rev counter -- you can hear the change in pitch).  The power the fan uses is tiny compared with the power produced by the heater as heat.

 

No heat is truly "free" of course.  However, the engine produces waste heat anyway (being only, I think, 60% efficient or less).  Usually that heat is dumped to the air -- through the radiator, by convection around the engine and in the hot exhaust.  The flow to the rad is thermostatically controlled, so when you turn on the heater the thermostat will close slightly and dump the equivalent amount less heat to the radiator.

 

All this assumes, of course, that you have a water-cooled engine.

 

Air-conditioning uses part of the power produced by the engine, so as Mortartube says, this does increase consumption.