# Physics explanation of why air in cooling system causes heater not to work?

I was listening to the first part of last weekend’s show today via podcast, and one of the first callers had a problem with no heat in their car’s passenger compartment. It is a pretty interesting call, as the caller seemed to have replaced pretty much everything replaceable in the cooling system, to no avail. So the brother’s conclusion was it was an air bubble in the heater core circuit.

But I wondered what is the physical explanation for this? What would a physics professor say as an explanation? I can see how an air bubble might prevent the water pump from working correctly. The pump ain’t pumping no water if the impellers are spinning in air.

But flow through the heater core is stopped just because of an air bubble in the heater lines? My pin-head thinking is that if there’s an air bubble there, the pressure from the water pump will compress the air bubble until it has the same pressure as the coolant pressure, at which point the air bubble should press the coolant on the other side of the air bubble with the same force as if the air bubble wasn’t there. So an air bubble might delay the flow at first, but eventually the air bubble will compress enough that full flow will return and move the air bubble through the heater core circuit and out.

But I have a feeling I’m not understanding something. Because I’m aware for example if an air bubble gets into a person’s blood, it can block off the flow of blood in that artery. That seems to indicate that “yes”, an air bubble in the heater circuit can indeed prevent the coolant from flowing through the heater. Anybody able to offer up some pointers on what I’m missing?

The cooling system is mostly a serious job of engineering while the heater is more like a weekend hobby installation. There’s not much flow through the heater core to start with so any problem might keep the water from flowing.

Excellent logic, but if the bubble would need to get pushed downward to clear the path and the coolant has an alternative path to travel with no resistance, the coolant will choose the alternative path and not push the bubble through. With heater cores, that’s sometimes what happens.

Try a few experiments.

Holding a play ball filled with uncompressed air under water takes effort. Pushing a bubble downward into a coolant path is analogous.

Also, you can easily set up parallel paths for water between two vessels (a clear plastic canister with a screw-on cover and the sink) with two clear plastic hoses, one large and the other small, with the smaller one elevated in the middle with an air bubble. See how high you have to turn the faucet on to clear the bubble… if you can.

Principles of fluid dynamics can also be used to your advantage. When my kids were little I had a 14 foot ceiling and used to cut down trees to match. Trouble is, in the first 24 to 48 hours after cutting they draw a huge amount of water. I was unable to find a stand with sufficient capacity to not run dry overnight, so I set a gallon water bottle next to the stand’s bowl, filled both to the same level, and put a plastic tube between the two, sucking the air bubble out. Thereafter, as the level in the bowl dropped it drew water from the milk bottle, keeping the levels the same. It essentially greatly increased the water available for the tree. Problem solved. As a pleasant byproduct, I could fill the system by just pouring water in the milk bottle; no more bending under the tree.

The same principle is used to establish level over distances. If you put a container at each end of a tube with equal level markings on each container, and run a plastic line between the two with the air sucked out, the level between the two points will be that point at which the water is equal between the two vessels.

As I recall, C&C didn’t tell the caller the most probable cause: blown head gasket.

Ah Ha ! You are absolutely right TSM, my theory would only make sense if the pipes were horizontally situated. I failed to consider the buoyant force of the bubble pushing upwards. Good explanation. Thanks.

Thanks.
Have a great holiday.

Couldn’t resist. Happy holidays to everyone!

Any one suggest a stuck flow control for coolant to the heater core? My wag. In my world of physics the air bubble would float to the top, and heater cores are usually lower than the top of the radiator.

That’s true, barky, or else nobody would ever be able to bleed a heater core. However, both the inlet and the outlet of heater cores come out the topside at the same level, and if the inlet or outlet outlet path has to take a slight dip to proceed along the coolant path, a bubble can form there. There does exist for the coolant an alternative path through the engine.

I admit that I’ve never had, or even seen, a blockage. But my practice is to elevate the vehicle’s front end by using ramps in order to ensure that the radiator fill hole is the highest point in the system, and I always turn the heating system on when purging the air out. I suspect that many people don’t turn the heater on when they purge the system, and I suspect that’s how some of them end up with an air bubble. I accept the general consensus that it happens, even though I’ve never had it happen. There are countless configurations of heater core installations out there, probably tens of thousands, . I have no doubt that some could be prone to this problem.

Note that I’m open to a better explanation if you have one.