Eberspacher header tank

[blackrose]

I'm currently installing a Webasto Thermotop C to replace an eberspacher DW10 which stopped working long ago. I'm using the same central heating system and I'd always assumed that since the system has a header tank it was an unpressurised system. But when I unscrewed the cap off the header tank today the coolant level in the tank rose by about 5mm. (I haven't actually run the heater yet as I'm still connecting the electrical system.)

 

I'm not sure how this eberspacher header tank cap works - it doesn't appear to be a spring loaded pressure cap but it's obviously blocking the breather nipple on the side of the header tank neck.

 

Can anyone enlighten me on how it works?

 

 

 

Edited October 2, 2021 by blackrose

[matty40s]

Most eberspacher header tanks only have one attached pipe,  the webast works best with an in coming and an outgoing pipe,  ideally with an isolatable bypass pipe joining them for when you have bled the system.

Your eberr tank may well have a moulded 2nd pipe site which will be sealed, consider using this on your installation.

 

The cap works just like an engine pressure cap, once the pressure builds, it pushes the inner creating a gap for excess pressure to be released.

[blackrose]

Thanks. Yes my eberspacher header tank does have a second blanked off pipe at the bottom.

 

But one of the diagrams in the Webasto installation manual for a non-pressurised gravity plumbing system, shows a single pipe from the header tank so I'd assumed that would be ok?

[blackrose]

I think I'm possibly getting confused with the terminology. In the manual Webasto show pressurised systems without header tanks,, but it looks like they use mains water pressure? 

 

I'd taken the pressure cap on my header tank to mean the system is pressurised but perhaps not? 

 

Anyway the only movement on that cap is the inner part which spins but it doesn't compress. I'm still unsure as to how it works.

[Cas446]

3 hours ago, blackrose said:

I think I'm possibly getting confused with the terminology. In the manual Webasto show pressurised systems without header tanks,, but it looks like they use mains water pressure? 

The 'mains' water pressure can be readily provided by your water pump with an accumulator. However as per the manual you need an expansion vessel as opposed to a header tank, the advantage being it can be positioned low down in the boat.

[blackrose]

Thanks, I probably didn't explain properly, but I don't want a pressured system.

[ditchcrawler]

34 minutes ago, blackrose said:

Thanks, I probably didn't explain properly, but I don't want a pressured system.

What was the pressure source that pressurised your old system?

[blackrose]

5 hours ago, ditchcrawler said:

What was the pressure source that pressurised your old system?

 

The old system wasn't pressurised. It's the same C/H system. Unless we're saying that the pressure cap pictured equates to a pressurised system? I'm still no wiser if that's the case or not?

 

According to the Webasto manual diagrams mine isn't a pressurised system but it doesn't go into any detail on whether the header tank is open or capped. For example, isn't the closed loop cooling system of an engine with a pressure cap classified as a pressurised system?

 

Perhaps this is just a difference in use of terminology?

Edited October 3, 2021 by blackrose

[Tony Brooks]

5 hours ago, blackrose said:

 

The old system wasn't pressurised. It's the same C/H system. Unless we're saying that the pressure cap pictured equates to a pressurised system? I'm still no wiser if that's the case or not?

 

According to the Webasto manual diagrams mine isn't a pressurised system but it doesn't go into any detail on whether the header tank is open or capped. For example, isn't the closed loop cooling system of an engine with a pressure cap classified as a pressurised system?

 

Perhaps this is just a difference in use of terminology?

 

I agree with whoever said it looks like a modern car type pressure cap and if you inspect it you may well find the pressure. More likely something like 250KP than PSI. However, that does not mean it is a pressure cap, it may not have the pressurisation parts inside it, it could just be an automotive expansion tank plus a modified automotive pressure cap that can't hold pressure.

 

The easiest way to check is to put it in your mouth and seal around the rubber ring with your lips. Then give t a good blow. If air passes through it is not a pressure cap, if not, then it is.

 

If it is, I would suggest it will be set to vent at something like 15 PSI.

 

Using it  as a pressure cap MIGHT reduce the evaporation rate when hot but would make leaks a bit more likely, but I would be happy with it. I don't think this is what is meant by a pressurised system in central heating terms, that uses an "accumulator" to hold the top-up water and allow for expansion, just like a domestic combi-boiler. That presents a problem with repressurising with a ready mixed antifreeze mixture. Whereas all you have to do is remove the cap and pour a bit more in.

 

If you really do not want any pressure in your system then a small drill up the hole in the centre, bottom of the cap to destroy the valve would do the job.

 

 

Edited October 3, 2021 by Tony Brooks

[blackrose]

Thanks, there's nothing on the cap to tell me if it's pressure activated or not. I tried blowing into it as you suggested and air doesn't pass through it. 

 

Once I get the system up and running I'll see if it makes any difference with the cap screwed down tight and then just loosely screwed on with a turn or two. 

[ditchcrawler]

19 hours ago, blackrose said:

 

The old system wasn't pressurised. It's the same C/H system. Unless we're saying that the pressure cap pictured equates to a pressurised system? I'm still no wiser if that's the case or not?

 

Sorry I thought you said it was pressurised 

[Tony Brooks]

I think it was pressurised when hot since it was fitted, in the same way the engine cooling system is, but I don't think that is what is normally meant by pressurised in resect of boat heating systems. It's an accident in specifying the header tank and cap originally.