Radiator caps and pressures

[John Fitzpatrick]

When you have two caps in the cooling circuit, one on the radiator and one on the header tank, is it better to use a non venting cap on the radiator (or just seal the vent tube) and a pressure relief cap on the header, or a pressure relief cap on both, with the header cap having the lower vent pressure?

[Rick]

Radiator cap must vent and have the lower presure, surge tank to have the higher cap rating.

[Ian Anderson]

Rick

I'd say the exact opposite

Fill the radiator and have a non pressure cap on it (sealed)

Then in the header at the highest point in the system put the pressure cap on so when the water expands it expels any air first so the hot water will fill the system and header

The other way around and your radiator could get air in and not necessarily bleed it back to the header.

Ian

[John Fitzpatrick]

Thanks Ian. My intention is to put a spigot at the high point on the radiator, as suggested on another thread for bleeding, but I didn't know if it was customary to add a secondary pressure relief to back up the one on the header tank. Sounds like a belt and braces approach that isn't needed.

[Paul]

I agree with Ian. For a bleed you could take a small pipe off of a high point on the radiator and run it to the header tank. I also have on mine a small pipe coming off of the top of the retun cooling pipe just before the engine, to bleed any bubbles, again routed to the header tank. both bleed pipes come in low on the header tank.

Regards