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Webasto header tank


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I was shocked at the price of header tanks, infact that's what decided me to go pressurised instead. Prolly doesn't work out much cheaper because you need a few valves etc but then at least you are getting something for your money and a pressurised system is better. Don't be put off - you didn't say if it's a new install or not, but either way it's easily do able.

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1 hour ago, David Mack said:

I saw this one and looks very smart. It is shame the pipe is not coming out of the bottom.

 

But I can only see a 2.5 litre one. The Webasto tanks are 5. On a 60' boat with pipes on both sides I was wondering what size is required? Is 2.5 litre enough?

 

Scrap yards and car header tanks (thanks Sharpness/Ditchcrawler) all good but they seem to come in strange shapes and sizes. I suppose my original question was more to do with whether there is anything special about the Webasto one that I need to consider when finding an alternative. I guess not. 

Edited by rgriffiths
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43 minutes ago, rgriffiths said:

But I can only see a 2.5 litre one. The Webasto tanks are 5. On a 60' boat with pipes on both sides I was wondering what size is required? Is 2.5 litre enough?

 

Scrap yards and car header tanks (thanks Sharpness/Ditchcrawler) all good but they seem to come in strange shapes and sizes. I suppose my original question was more to do with whether there is anything special about the Webasto one that I need to consider when finding an alternative. I guess not. 

Difficult to say, but it is possible to do some sums to see. If you can measure, or find the likely volume of water, you can estimate the size of header tank needed. For example, a run of 15mm OD, 13mm ID pipe running down each side of a 20m long boat would contain 5.3 litres (0.65cm*0.65*pi*4000cm/1000). Add on the volume of water in all the radiators, inside the Webasto boiler and half the volume of the expansion tank, or however high up the normal level is in the tank. You then need to multiply it by the temperature rise when the water is heated up and the volume thermal expansion coefficient of water. Water is weird stuff and its volumetric thermal expansion changes with temperature, getting higher as temperatures go up. For safety, take it at the high end, 80C, where it is 0.00064 per degree. Again, for safety and making the sums easier we will put in a temperature rise of 100C, more than you would get in reality.

 

The amount of expansion for just the water in 40m of 15mm pipe is 5.309*0.00064*100 = 0.34 litres of expansion. Add in the extra expansion for all the rest of the water in the system and see if the volume of air space in the header tank above the normal water level is greater than this.

 

This is all erring on caution, using high end estimates and takes no account of the rise in pressure in the system, which will reduce the volume required. This is just the expansion you would get with the tank cap off and ensuring that it doesn't spill out the top.

 

Hope this helps,  Jen

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