Pipe - expansion tank to heater circuit return

[widdowson2008]

The small pipe - expansion tank to heater circuit return. Exactly what is its function? I have my thoughts but ideas welcomed.

[haydn callow]

Isn't that the heater feed ???? Hot coolant from top of rad to tank then out via the pipe in question to the heaters...to the rear first so the passengers get tostie warm as quickly as possible

[widdowson2008]

Isn't that the heater feed ???? Hot coolant from top of rad to tank then out via the pipe in question to the heaters...to the rear first so the passengers get tostie warm as quickly as possible

No Haydn - definitely return line which goes to the stat.
Question is, why is it necessary? what does it do? I think the answer is simple when you (clue coming up) consider what would happen if it were NOT there.
In fact, I believe it has 2 purposes - just need someone to come up with the same reasoning.

[haydn callow]

Sorry...wondered what you were getting.....you seemed to be saying the small pipe from the tank was a return....it is a "out" from the tank

[widdowson2008]

Sorry...wondered what you were getting.....you seemed to be saying the small pipe from the tank was a return....it is a "out" from the tank

In effect it is a return, but the orifice in the header tank that this pipe is connected to is only 5.5mm diameter which compared to the surrounding pipework is VERY small, so not a return as such.
My thoughts on this pipe are:
If it were NOT there, then
1 - this leg of the system could not be bled using the see saw method
2 - because the pressure on the flow side of the pump is higher than the return side due to the pumping action, then the expansion tank cap would probably excede 1.1bar on a regular basis. This small pipe equalises (to some extent) the pressure on the 2 sides of the pump.

[haydn callow]

But....untill the stat opens the only way any coolant circulates to anywhere is via this small outlet pipe from the tank.....this is why the tank gets so hot so quickly.....because HOT coolant from the cylinder head goes to the rad top tank and then up to the ex tank....if the tank did not have a outlet this could not happen and it would be a "old fashioned" simple ex tank.

The small outlet is 5.5mm

[widdowson2008]

But....untill the stat opens the only way any coolant circulates to anywhere is via this small outlet pipe from the tank.....this is why the tank gets so hot so quickly.....because HOT coolant from the cylinder head goes to the rad top tank and then up to the ex tank....if the tank did not have a outlet this could not happen and it would be a "old fashioned" simple ex tank.

The small outlet is 5.5mm

Not true Haydn.
Coolant circulates through the heater circuit permanently - whilst the engine is running. (cct 1)
Also, until the stat is FULLY open, coolant also circulates from the head, via the stat housing, and back to the pump. (cct 2)
Water will always take the easiest route, which is through the circuits mentioned.
It will also circulate via the header tank/ heater return/stat (cct 3), but at a very low rate.
Just done some calcs on pipe areas and the volume passing through cct 3 (from header tank) represents, at very best, 8% of circulating coolant. In the grand scheme of things, naff all.

Still reckon this small pipe is just to prevent excess pressure building up in the expansion tank. All I'm trying to do here is get folks opinions on the small pipe.

[jaylee]

But....untill the stat opens the only way any coolant circulates to anywhere is via this small outlet pipe from the tank.....this is why the tank gets so hot so quickly.....because HOT coolant from the cylinder head goes to the rad top tank and then up to the ex tank....if the tank did not have a outlet this could not happen and it would be a "old fashioned" simple ex tank.

The small outlet is 5.5mm

Not true Haydn.
Coolant circulates through the heater circuit permanently - whilst the engine is running. (cct 1)
Also, until the stat is FULLY open, coolant also circulates from the head, via the stat housing, and back to the pump. (cct 2)
Water will always take the easiest route, which is through the circuits mentioned.
It will also circulate via the header tank/ heater return/stat (cct 3), but at a very low rate.
Just done some calcs on pipe areas and the volume passing through cct 3 (from header tank) represents, at very best, 8% of circulating coolant. In the grand scheme of things, naff all.

Still reckon this small pipe is just to prevent excess pressure building up in the expansion tank. All I'm trying to do here is get folks opinions on the small pipe.

Don't ask me Steve, the last time i was asked my opinion on a "small pipe" i pretty much thought i had the universe sorted out!

.... Apart from where i put the biscuits...?!!

[widdowson2008]

.......
Don't ask me Steve, the last time i was asked my opinion on a "small pipe" i pretty much thought i had the universe sorted out!

.... Apart from where i put the biscuits...?!!

[Northern Bongolow]

just wondering about the 8% steve. if the pipe from the rad is larger (slower and of lower pressure) than the small pipe out to the heaters (faster and higher pressure) wouldnt this pipe (inject) the contents into the return flow pipe to the stat.
i think this pipe is crucial, as it returns the hottest coolant back to the stat thus controling the whole system.
the difference in pipe sizes is actually there to create back pressure in the exp tank to eradicate any air and degases the coolant.



maybe

[The Great Pretender]

Just done some calcs on pipe areas and the volume passing through cct 3 (from header tank) represents, at very best, 8% of circulating coolant. In the grand scheme of things, naff all.

Interesting, can you post your findings (as simply as possible for non tec peeps) of how you arrive at a maximum of 8% flow through the small pipe please.

[haydn callow]

I do know that if the pipe was not there the coolant alarm sensor would not work in the position we now fit it.

[jaylee]

Does it allow trapped air in the heater matrix's exit the system during bleeding...? & when running some sort of pressurized equilibrium in the header tank, so when cold on a relatively healthy system coolant levels can be checked more accurately??

The only air that should be in the system occupying the space between the full line & the header cap..?

In the case of an extreme overheating episode the level in the header would rise & boil like bu66ery!?? like this.. http://www.igmaynard.co.uk/bongo/forum/ ... =3&t=49511

Maybe i worded that wrong..! Just avin a crack at a guess..

[widdowson2008]

just wondering about the 8% steve. if the pipe from the rad is larger (slower and of lower pressure) than the small pipe out to the heaters (faster and higher pressure) wouldnt this pipe (inject) the contents into the return flow pipe to the stat.
i think this pipe is crucial, as it returns the hottest coolant back to the stat thus controling the whole system.
the difference in pipe sizes is actually there to create back pressure in the exp tank to eradicate any air and degases the coolant. maybe

Agree that the smaller hole will create a faster flow but the volume is less than the larger hole and is quite clearly shown in this little experiment I did on the expansion tank? Do you remember it?

[widdowson2008]

Just done some calcs on pipe areas and the volume passing through cct 3 (from header tank) represents, at very best, 8% of circulating coolant. In the grand scheme of things, naff all.

Interesting, can you post your findings (as simply as possible for non tec peeps) of how you arrive at a maximum of 8% flow through the small pipe please.

Up 'till the stat opening, the pump shoves coolant basically through 3 different routes at the same time, and these are 1.header tank, 2.heater circuit, 3.bypass from head, and water will always favour the easiest route, ie biggest pipe as demonstrated in this pic.


The pipe diameters for the above 3 circuits are
1.header tank, 5.50 dia (area 24 sq mm)
2.heater, 13 dia (area 132.5 sq mm)
3.bypass 13 dia (area 132.5 sq mm)
Total area of all 3 circuits = 289 sq mm
Therefore, as a proportion, header tank flow to total area flow is 24 divided by 289 = 8% (ish)

Of course, once the stat opens, it presents a 4th path for the coolant to go down. this will reduce the 8% even further.

Does this make sense to you?

Didn't mean this to get too heavy a discussion - just intrigued by the purpose of the small pipe.