How much air gap for a header tank?

[unstable load]

Your header tank can basically be anywhere on the car, it need not be the highest point. It has a pipe that runs to the lowest point in the tank and that pipe connects to the little pipe in the neck of the radiator.
As the system warms up and the expansion occurs, the excess water is pushed past the cap into the header tank and any overflow is vented overboard until it reaches stability when no more water is vented.

Once you switch off and contraction starts the water level in the tank is drawn back into the radiator and as ambient temperature is reached, the level in the header stops dropping, and the resulting gap is the level your system operates at.

Rather than shooting for a particular level, leave the system to find it's own level, all you need worry about is that you have sufficient volume in your tank to hold the volume of expansion plus at least the same amount again to accommodate hot days and high system temperatures when the drain-out will be marginally higher.

Anyhow, that's my take on how it should work, if anyone has any contrary knowledge please correct me.

[DEVONMAN]

Hi John,
As Tom's car is a Dax Rush I don't think it has a connection at the radiator neck and I assume the small pipe is connected directly to the radiator top tank. Other than that I agree with you.

Probably not relevant in this case, if the header tank is way oversized, (ie air space too big), it is possible that the system pressure will not get up to the design value. In this case the coolant will boil sooner than it would otherwise.

Regards Denis

[SuperV8]

My tank only has two connections, vent from rad at the top and the outlet at the bottom which feeds into the bottom hose and then to the pump.

I may have called it the wrong word, header tank or expansion tank?

Did some calcs using the combined gas laws:
cold temp 20degs
hot temp 100degs
cold vol 7L
hot vol 7.2L
cold pressure 14.7psi
hot pressure 18.7psi

That's only a pressure increase of 4psi.

I will try and leave as much gap in the header tank as my levels allow, hopefully between 1/2 and 2/3 and see how it goes?

Thanks for the pointers

Tom.

[DEVONMAN]

My tank only has two connections, vent from rad at the top and the outlet at the bottom which feeds into the bottom hose and then to the pump.

I may have called it the wrong word, header tank or expansion tank?

Did some calcs using the combined gas laws:
cold temp 20degs
hot temp 100degs
cold vol 7L
hot vol 7.2L
cold pressure 14.7psi
hot pressure 18.7psi

That's only a pressure increase of 4psi.

I will try and leave as much gap in the header tank as my levels allow, hopefully between 1/2 and 2/3 and see how it goes?

Thanks for the pointers

Tom.

Tom,
Now that you have pointed out that you have a pipe from the expansion tank to the bottom hose, some of the comments made so far are incorrect.

If the expansion tank coolant level is below the top of the rad then you will not be able to fill the rad. (unless there is a separate filler point on top of the rad.
If you park on a hill and effectively put the expansion tank below the rad then coolant will flow into the expansion tank via the bottom hose connection and air will be displaced via the small hose to the top of the rad. There's not much you can do about this except put the expansion tank as high as possible. However, with the engine running the water pump will suck on the bottom hose connection and also blow the air from the rad back into the expansion tank via the small hose, so no worries.

I'm rusty on the Boyles Law calcs but your pressure calc of only 4 psi seems low as most normal sized expansion tanks on a 4.6 would achieve 10-15 psi.

Thinking further, If the 30mm of air space when cold is compressed by the coolant to say 15mm when hot then the pressure will roughly double. ie 29.4 psi and the cap in the epansion tank will vent at what ever pressure it is set at and control the pressure..
Regards Denis