Expansion Tank

[Stewart]

When using an expansion tank with a pressure cap,should a non pressure cap be fitted to the radiator???

[Guest Alan_builder]

When using an expansion tank with a pressure cap,should a non pressure cap be fitted to the radiator???

 

Yes

 

Also block off the overflow pipe from the radiator of course.

[richardm6994]

Or you can use a pressure cap on both but your rad cap has to have a higher psi rating than your expansion tank.

[Stewart]

If the radiator has a pressure cap how can it suck back the water from the expansion tank when the expansion tank is connected to the overflow

[richardm6994]

Yeah, good point. I got a little muddled up with what I was picturing!

 

Refer to Alan's answer!

[knights_templar]

If the radiator has a pressure cap how can it suck back the water from the expansion tank when the expansion tank is connected to the overflow

 

pressurised tanks are normaly "header tanks" a high point in the cooling system to allow filling, and expantion of cooling fluid, with a cap to control the pressure within the cooling system.

 

what you are describing is an "expantion tank" usualy a low point in the cooling system, not pressurisd to take the fluid that expands past the pressure cap and allow it to be drawn back in when it cools.

 

for the first option you would need to block off the rad cap and expantion outlet, as Alan says, and plumb the tank in as the higest point of the cooling system.

 

for the second option a pressurised tank would be of little use as you already have a cap on the rad, so discard the pressure cap on the tank and fit a lid, a pop bottle would do the same and cost much less.

[Longboarder]

A rad cap has two valves, one operated by increased pressure in the rad, one by vacuum in the rad so under pressure it will allow escape and under vacuum will allow entry.

Try to keep it simple and self bleeding. Expansion tank open to the air.

 

Nigel

[speedtripledan]

longborders description was how my pinto worked with no probs

[stevedohc2b]

In a suck and blow system the tank can have a push on cap

With a fully pressureised system the expansion tank is the high point with pressure cap fitted normally blue on a tin top.

 

Stephen

[Stewart]

A rad cap has two valves, one operated by increased pressure in the rad, one by vacuum in the rad so under pressure it will allow escape and under vacuum will allow entry.

Try to keep it simple and self bleeding. Expansion tank open to the air.

 

Nigel

If the pressure in the rad opens the radiator cap and allows fluid to escape to the expansion tank,how does that fluid then flow back into the radiator as surely when the pressure drops in the rad the cap will seal it off.

[richardm6994]

The water get hot and expands / increases in volume. Once the expanded water is hot enough to create enough pressure on the cap, it lifts the cap and leaves the rad.

 

When the water cools back down, it contracts back to its original volume. This creates a vacuum due to the water which has left the rad and in turn this opens the vacuum valve on the cap (refer to Nigel's diagram) and sucks the water from the overflow tank back into the rad.

[Al Milton]

Simples!

Provided the main seal on the pressure cap hasn't failed of course

[Stewart]

Thanks for the info lads I did not realize that a rad cap was other than a pressure relief valve was not aware it had a vacuum valve.

[Guest mower man]

Not all of them do!! .The suck and blow system works very well on a normal set up i.e.vertical or near vertical rad ,a 13lb cap and at least 1.5 ltr capacity container that breathes to atmos , a header tank etc I would imagine is comp on steeply angled rads , this is with out mentioning good ducting tmake air go through the rad and also to ensure not trapping air under the bonnet ,mower man