Coolant overflow header tank

[blackrose]

I've just been changing my antifreeze and noticed that the bottom of a coolant header tank that I installed several years ago because antifreeze kept going all over the floor, is slightly below the height of the top of the engine's built in tank. I'm just trying to work out why I did it that way?

 

It's connected with a small hose from the overflow on the side of the pressure cap fitting to the bottom of the header tank. The header is normally empty and just accommodates expansion and I vaguely remember being told that the coolant gets sucked back into engine as it cools, rather than under gravity. But I still don't understand why the bottom of the tank isn't above the engine? Did I just get it wrong?

[Tony Brooks]

Your reasoning is OK as long as the cap on the engine is either a plain cap with a simple rubber seal, both in good condition. Pressure caps have a vacuum relief valve built in that allows air in the system when the coolant cools and contracts. If the engine has a pressure cap on it, even a higher pressure one than the one of a low expansion tank, it may suck air in via the engine cap rather than coolant back from the tank. If you are using a pressure cap on the engine then that must have a rubber seal under its cap but then you do not need a pressure acp on the expansion tank.

 

Your way is a quick way (as long as the hose on the expansion tank expanson tank can suck from the bottom of the tank) and usually works with a plain cap on the engine.

[WotEver]

4 hours ago, blackrose said:

I still don't understand why the bottom of the tank isn't above the engine? Did I just get it wrong?

I can’t see why it would matter either way. Above or below it’s still allowing room for expansion. 

[blackrose]

Ok, I just intuitively thought that a header tank should always be above the rest of the system, but if any expanding fluid in the tank is drawn back into the system by negative pressure then I guess it makes no difference where it it.

[Guest]

2 hours ago, blackrose said:

Ok, I just intuitively thought that a header tank should always be above the rest of the system, but if any expanding fluid in the tank is drawn back into the system by negative pressure then I guess it makes no difference where it it.

The plastic header tank on ours is just slightly above the engine (Vetus.) I would say if it's not broke, don't fix it. I understand your thoughts though, because I too have an inquisitive mind.

[Flyboy]

3 hours ago, blackrose said:

Ok, I just intuitively thought that a header tank should always be above the rest of the system, but if any expanding fluid in the tank is drawn back into the system by negative pressure then I guess it makes no difference where it it.

I had an Austin 1300 in the distant past and this had an expansion tank level with the sump. The radiator had a plain cap and the expansion tank had a pressure cap. The radiator was always full so must have sucked up the coolant ok.

[Johny London]

Convert it to a sealed, pressurised system, it's not hard.

[blackrose]

4 hours ago, Johny London said:

Convert it to a sealed, pressurised system, it's not hard.

 

Isn't it already a pressurised system?

 

I can't see any problem on this thread that you're trying to solve?

[Johny London]

Obviously not - if it has a header tank.

And the problem I am trying to solve is this: The op wondered whether he had failed to put his header tank high up enough, with a pressurised system he wont have a header tank to worry about.

[Guest]

4 minutes ago, Johny London said:

Obviously not - if it has a header tank.

And the problem I am trying to solve is this: The op wondered whether he had failed to put his header tank high up enough, with a pressurised system he wont have a header tank to worry about.

My Vetus has. 

[blackrose]

9 minutes ago, Johny London said:

Obviously not - if it has a header tank.

And the problem I am trying to solve is this: The op wondered whether he had failed to put his header tank high up enough, with a pressurised system he wont have a header tank to worry about.

 

You may think what you're saying is obvious, but actually you're wrong.

 

The fact it has a header tank doesn't mean it isn't a pressurised system. The engine has a pressure cap on its own coolant tank (i.e. it's a pressurised system) and the connection to the external header tank is made with the hose that connects from the pressure cap fitting. I thought I'd made this clear in my original post but perhaps not?

 

So in order for coolant to go into the external header tank the pressure cap has to be activated. 

 

Edit: Most engines as I understand it have a built in header tank and a pressure cap. So most engines do have pressurised systems AND header tanks. I just have an additional external header tank to avoid coolant spillage on the engine room floor.

Edited June 15, 2019 by blackrose

[Tony Brooks]

2 hours ago, Johny London said:

Obviously not - if it has a header tank.

And the problem I am trying to solve is this: The op wondered whether he had failed to put his header tank high up enough, with a pressurised system he wont have a header tank to worry about.

That is far from true, especially on skin tank cooled boats. The coolant in the system is going to expand with heat. Lots of coolant = lots of expansion and often the header tank on the manifold has insufficient volume to contain the expansion - hence coolant flowing onto the base plate.

 

Many skin tank cooled boats need a  header tank otherwise when they cool the suck so much air in (because the hot coolant has been expelled) it builds up in the skin tank and causes overhaeting.

[Guest]

43 minutes ago, Tony Brooks said:

That is far from true, especially on skin tank cooled boats. The coolant in the system is going to expand with heat. Lots of coolant = lots of expansion and often the header tank on the manifold has insufficient volume to contain the expansion - hence coolant flowing onto the base plate.

 

Many skin tank cooled boats need a  header tank otherwise when they cool the suck so much air in (because the hot coolant has been expelled) it builds up in the skin tank and causes overhaeting.

My one-time neighbour's boat needed two plastic header tanks on the Vetus pressurised system as one couldn't offer enough volume for expansion! He teed them off the pipe from the existing one which cured the problem (as would have a bigger single tank). I am told that Vetus use Renault car items.

[pearley]

Our boat has a Beta 43 and as delivered had a plain cap on the engine header and a plastic header tank (a Renault one) with pressure cap. When having some work done 3 years later at a different boat builders they remarked it should have a pressure cap on the engine. I spoke to Beta who said a 10psi cap and no header tank. I now just run the overflow tube into a Tesco milk bottle. I find the level in the engine header needs to be about 30mm below the top of the tank.

 

Edited June 16, 2019 by pearley

[Tony Brooks]

7 minutes ago, pearley said:

Our boat has a Beta 43 and as delivered had a plain cap on the engine header and a plastic header tank (a Renault one) with pressure cap. When having some work done 3 years later at a different boat builders they remarked it should have a pressure cap on the engine. I spoke to Beta who said a 10psi cap and no header tank. I now just run the overflow tube into a Tesco milk bottle. I find the level in the engine header needs to be about 30mm below the top of the tank.

 

 

But that is specific to your boat. That sounds normal for a heat exchanger cooled boat and possibly one with a proper keel cooler but once you get to skin tanks and "home made" keel coolers the volume of coolant comes into the equation. Similarly of the calorifier is a long way from the engine, especially if it is connected with large bore pipes.

 

If you want your engine to suck the coolant out f the Tesco bottle you need a rubber seal under the cap so it forms an airtight seal to the filler neck. They are available but might take some finding.

 

This sort of thing crops up in advice time and time again. Very well meaning members make a post based one their own boat or very limited experience, often with insufficient information from the questioner. Every boat, even production boats, tend to be different so advice based on just one boat stands a good chance of being somewhat misleading.

 

[pearley]

Yes it is specific my boat. I wasn't offering advice, just giving my experience.

[Johny London]

I've just realised the op was referring to the engine - and not the heating system, (talk of the header tank confused me) which I hope explains my somewhat strange thoughts on the matter!

[Guest]

3 hours ago, Johny London said:

I've just realised the op was referring to the engine - and not the heating system, (talk of the header tank confused me) which I hope explains my somewhat strange thoughts on the matter!

I wish I had a pound for every time I have done that! 

[Tony Brooks]

42 minutes ago, catweasel said:

I wish I had a pound for every time I have done that! 

ditto