My cooling system runs without a header tank. I have a filler neck in the top hose with a pressure cap with a valve and a vent presumably to deal with pressure relief and expansion. Do I run a pipe from this outlet to an expansion tank? If so is this effectively just an overflow tank? Any links to something suitable? Do I need the tank plumbing in anywhere else in the system too?

Tank is normally plumbed into the bottom hose and the vent you've got goes to the top breather on the expansion tank.

What I don't understand though is if I run a tank to the bottom hose the system will no longer be pressurized as the header tank can't have a pressure cap on aswell as the other cap? I thought there was a difference in running a header tank and an expansion tank. I also believed you could run without either?

Yes an expansion tank is effectively and overflow tank. No it doesn't need to be plumbed in any where else apart from the connection on the filler neck. The bottle should be open to atmosphere and should have enough coolant in it to cover the hose from filler neck to ensure that air is not drawn in to the cololing system as it cools down.

Coozer is talking about a header tank which is part of the cooling system and has a constant flow of coolant through it at all times.

You need one or the othe or you risk drawing air in to the system.

Davie

Good man, just what i thought.

Next question does it matter at what height this goes relative to everything else in the system?

No. It will pull coolant back in under a vacuum as required and there is no issue with air locks because it's not part of the actual cooling system. Use an anodised drinks bottle like me. Wilkinsons do some nice ones and you can even use a bike bracket to hold it if you want.

cheers

worth putting a touch of science around this one as it seems to come up again and again and few seem to understand what's going on!

First the fact that causes the issue - water expands as it gets hotter!!

next, it needs some room to expand. No room and it will make some though the weakest joint!

The pressure cap is that weakest joint.

With an overflow system, the main pipework up to the pressure cap is "full" of water. As it heats up, it expands, the pressure rises very rapidly and water is forced out of the pressure cap into the explansion tank via a pipe (that needs to be fully to the bottom of the tank). As the temperature falls the expelled water is drawn back in via the immersed pipe.

With a header tank system (MUCH better in my opinion) there is a vessel (doesn't have to have flow through it at all) within the system that contains a fixed volume of air at ambient temperature. As the water in the system heats up, it expands, and because unlike water, the air is (easily) compressible, it gets compressed and makes room for the water. The pressure cap then only opens once the air is compressed up to the opening pressure of the cap.

As long as the air volume is sufficient to accomodate the water expansion, the system will be fine.