Configuration of coolant hoses

[Jon Weywadt]

When filling coolant, after installing stainless steel coolant pipes some years ago, I wondered about the configuration of the small hoses entering the expansion tank.
The small hose from the thermostat housing connected to the top of the expansion tank, while the small hose from the top of the radiator connected to the bottom of the expansion tank.
Now, after replacing the water pump for the second time (see other posts about that) I again wonder why the small hoses are connected as they are.
To me it would make sense if the hose from the top of the radiator would connect to the top of the expansion tank, thus allowing air in the radiator to be vented out of the system. Connecting it to the bottom of the tank makes coolant flow back into the pipe, and block air from escaping the radiator.
Therefore, are there any of you who have the hoses connected as I suspect is the correct way? Namely Thermostat housing to the bottom of the expansion tank and top Radiator hose to the top of the expansion tank?

[roy4matra]

When filling coolant, after installing stainless steel coolant pipes some years ago, I wondered about the configuration of the small hoses entering the expansion tank.
The small hose from the thermostat housing connected to the top of the expansion tank, while the small hose from the top of the radiator connected to the bottom of the expansion tank.
Now, after replacing the water pump for the second time (see other posts about that) I again wonder why the small hoses are connected as they are.
To me it would make sense if the hose from the top of the radiator would connect to the top of the expansion tank, thus allowing air in the radiator to be vented out of the system. Connecting it to the bottom of the tank makes coolant flow back into the pipe, and block air from escaping the radiator.
Therefore, are there any of you who have the hoses connected as I suspect is the correct way? Namely Thermostat housing to the bottom of the expansion tank and top Radiator hose to the top of the expansion tank?

Hello Jon,

Both these hoses are air vent or degassing hoses, the one from the thermostat housing is venting air from the engine just as the other is the radiator venting hose.  The real problem I suspect is that the header tank was taken from something else, and doesn't have the connections just where they are really required in this application.  The hose from the thermostat housing is tiny compared to the connection on the header tank, showing it wasn't designed for it, so they had to use two different size hoses and join them with a reducer (which was plastic and eventually broke as it became brittle with age and heat) and you ended up losing coolant if you didn't spot it, or replace it before it cracked.

Once the system is originally bled of air, the system should remain air free unless you get a leak.  However, the most likely place to introduce some air into the system would be the engine or water pump and if this is bled out straight away (from the thermostat housing) it won't even reach the radiator to need bleeding from there.  So I suspect the engine degassing has priority and hence the reason that hose goes to the higher connection.  Another reason may be that the radiator hose connection on the header tank is higher than on the radiator even when it is connected at the bottom, whilst the engine degassing hose if it was connected at the lower connection would be almost the same level and wouldn't provide as much upward flow.  With the hoses connected as they did at the factory, there are reasonable differences in height for upward flow on both degassing hoses.

Roy

[GP]

For further clarity yes these are the correct positions: Thermostat housing to the bottom small forward facing connection of the expansion tank and top small Radiator hose to the top larger forward facing connection (which uses the brass reducer originally) of the expansion tank. I spoke to Roy yesterday and he recommends fitting a hand vacuum pump or similar to the top small hose going forward to the top of the radiator and sucking the air out. He uses a small hand held pump made by "Silverline" with a collector for the for the coolant ahead of the vacuum inlet. Even gave me a demonstration too!

 
 

[roy4matra]

For further clarity yes these are the correct positions: Thermostat housing to the bottom small forward facing connection of the expansion tank and top small Radiator hose to the top larger forward facing connection (which uses the brass reducer originally) of the expansion tank.

No these are the wrong way around Graham.  The top larger hose on the header tank with the reducer goes to the thermostat housing, and the lower small connection on the header tank goes to the radiator. (and the reducer was always plastic from the factory - people replaced them with something more solid as the plastic ones used to break)

I spoke to Roy yesterday and he recommends fitting a hand vacuum pump or similar to the bottom small hose going forward to the top of the radiator and sucking the air out. He uses a small hand held pump made by "Silverline" with a collector for the for the coolant ahead of the vacuum inlet. Even gave me a demonstration too!

Roy

[Jon Weywadt]

Hi Roy.

I am surprised both should be venting hoses. Right now I have the thermostat housing connected to the top of the expansion tank. The tank is one fabricated here on the forum some years ago and is aluminium with a small stud on both top and bottom. Thus I have no reducer pipe in the connection. As you will see on the photo below, there is a steady stream of coolant coming from the thermostat housing into the expansion tank, even at at idle.
I don't see how air can possibly escape from the top of the radiator when the hose goes to the bottom of the expansion tank. There is coolant in the tank and a hight difference producing a pressure that would prevent any air from escaping the radiator.
In the past I have used my brake bleeder glass and a vacuum to suck air out through that hose, but I can't help but believe it never worked as intended that way.
The stream of coolant coming out the top hose now, shows that the pump puts pressure on that connection, thus removing any air from the engine. That does not happen from the radiator.
Perhaps it was a mistake during assembly. Wouldn't be the only one (i.e. 2nd. gear synchro)

[Oetker]

What they told me.(don't know if it is true but it sounds logical).
The reducer is to trim the force of water that come from thermostath to the top of the expantiontank.
It has so much force that airbubbles are created in the water in the tank.
These bubbles are sucked in the circulation creating a airproblem.
Some  expantion tanks has a seperat compartment to prevent that, for example the Clio bottle I use.

regards Herman

[Jon Weywadt]

What they told me.(don't know if it is true but it sounds logical).
The reducer is to trim the force of water that come from thermostath to the top of the expantiontank.
It has so much force that airbubbles are created in the water in the tank.
These bubbles are sucked in the circulation creating a airproblem.
Some  expantion tanks has a seperat compartment to prevent that, for example the Clio bottle I use.

regards Herman

My expansion tank is an aluminium tank with two chambers. The top inlet is smaller in diameter than the original, thus I have no reducer in the hose leading from the thermostat housing.
But your comment makes me think that the reducer (actually expander since the water flow is from the smaller hose to the larger) is in the flow to allow air bubbles to collect together before entering the expansion tank. This must then also be why it is not connected to the bottom of the tank.

I still think the air hose from the radiator also should have been connected to the top of the tank for the air in top of the radiator to escape.

But what if I installed a fuel pump in the air line from the radiator. Maybe with a timer to let it run a few minutes after start to remove any air. Since it is not critical for the coolant flow, it would not matter that the pump is not running all the time. But it would ensure that no air is hindering the proper flow through the radiator.

[Oetker]

I think modification is not needed.
The circulation system is fine as it is.
I mounted the Clio bottle only because I couldn't find a original one.
I know found one and will remove the iron outputs because rust on the outputs can kill the bottle.
I will replace them for stainles or copper.

regards Herman

[roy4matra]

I still think the air hose from the radiator also should have been connected to the top of the tank for the air in top of the radiator to escape.

But what if I installed a fuel pump in the air line from the radiator. Maybe with a timer to let it run a few minutes after start to remove any air. Since it is not critical for the coolant flow, it would not matter that the pump is not running all the time. But it would ensure that no air is hindering the proper flow through the radiator.

No, you don't need this Jon.  And the radiator bleed hose does not need to be connected to the higher header tank connection.  When you initially fill the system, you use the radiator hose to bleed the air from the top of the radiator.  After that the radiator should have no air in it since this is a sealed system and unless the radiator has a leak no more air should get in there.  Air does not flow downwards and the only way air could travel from the engine to the radiator would be for it to go down under the car.  However, it would take the easier route and flow back to the header tank via that bleed hose.  I have had a Murena from new and never had a problem with the cooling system getting air into the radiator during normal use.

So don't start inserting extra things where they are not needed, and giving potentially more problems.

Roy

[krede]

I am replacing my header tank with an aluminium one, that only has a connection at the bottom, and one at the top , above coolant level. (And one for overflow at the cap).
From what I hane read here it seems that it would be ok to simply connect both the radiator venting line and the one from the thermostat ,to the upper one by means of a t-piece ?

[roy4matra]

I am replacing my header tank with an aluminium one, that only has a connection at the bottom, and one at the top , above coolant level. (And one for overflow at the cap).
From what I hane read here it seems that it would be ok to simply connect both the radiator venting line and the one from the thermostat ,to the upper one by means of a t-piece ?

No, you should leave the radiator bleed hose at the bottom of the header tank where it is under the coolant level.  If the fluid level dropped owing to a leak you want the radiator to remain free of air, so having it under the fluid level at the header tank would do this, until the tank was completely empty.  If it was connected at the top where there is always air, it could get air back into the top of the radiator as the level dropped.

The extra height of the connection at the bottom of the coolant tank compared to the top of the radiator will always allow a small flow of any air bubbles that appear at the top of the radiator to vent back to the header tank.  Don't forget all this system is under pressure at normal running.

If you were going to use a tee-piece for the two to the upper connection, what were you going to do with the bottom connection?  Just cap it off?  Why?  It is better to leave the hoses connected the way they have always been, as Herman says, which we know works fine.

Roy