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  Topic Review (Newest First)
11-03-2008 04:17 AM
Omatako Just to answer your question about the pressure cap on the heat exchanger - the pressure cap is intended to raise the pressure in the system and in so doing raise the boiling point of the coolant. That way, if you do get a slight overheat the coolant won't boil at 100 degrees C and it won't blow out of the system.

In the system on your boat, the pressure cap will allow normal expansion to go back to the expansion tank when the pressure in the system exceeds the value of the cap and the expansion tank will replace the coolant thru a return valve in the pressure cap when the engine cools. It is thus better for the tank to be at least at or above the level of the pressure cap.

Note also that if the pressure cap is removed while the engine is hot (for topping up purposes) the pressure in the system is relieved and if there is a tendency to overheat, it will be exacerbated by the cap having been removed because the hot engine will not be able to build the pressure back (no expansion). That is really the reason why it is bad practise to remove caps from hot engines and not only because you'll get scalded (which could also happen )
11-02-2008 11:38 AM
preservedkillick Thanks Rik & everyone else for the answers.

Rik, I'd not seen that Alberg engine photo before. That looks like a great setup, but my Vitus takes up too much room. I did notice that the middle of the tank is about the same level as where the hose comes out of the heat exchanger.

I'm heading down to the boat now to take another look. I may end up relocating it in a cockpit locker
11-02-2008 08:03 AM
rikhall I am neither a certified mechanic nor the son of one. I did install my Yanmar 2GM20F. I also enjoy searching the web. Here are three pictures, for what they are worth:

From the web -

Here are two from my engine - I think mine were a bit high:
11-01-2008 10:57 PM
Delirious I've had two that were in the cockpit lazaratte about 18" above the heat exchanger - completeley above the engine itself when level - maybe 6" or so. A third was mounted high & inside the engine compartent - maybe 12" above the heat exchanger.
11-01-2008 10:32 PM
SteveInMD Here's a third answer. I was told by a Yanmar certified mechanic that the expansion tank normal level (between the hot and cold markings) should be level with the hose port on the heat exchanger (the hose that runs to the expansion tank). It can be higher if need be, but you have to add a 1/4 turn shut-off valve so you can check the coolant level under the cap (no you can't just look at the tank). He did not say it could be lower than the hose port.

I don't know what engine manufacturer you are talking about, but for a Yanmar it should be at or above the top of the heat exchanger.
11-01-2008 10:10 PM
bobmcgov If space is really tight, could you use a separate microbubble resorber like the Taco or Spirovent? Most models of the Spirovent can take an expansion tank screwed right onto the bottom -- or you can plug that port and stuff the expansion tank nipple-side up on a distant spur, as long as the spur slopes up from tank to circuit so it doesn't become an air trap.

Pressure tanks don't much care where they are located, but you do want to keep the air out of your system. A resorber needs to be located very close to the hot side of your system -- just after the coolant leaves the engine but before any water heater exchanger. In my solar hot water system, the resorbers are not at the highest point but do scrub the bubbles very effectively anyhow.
11-01-2008 09:35 PM
camaraderie can mount it lower. Now you have the choice of two answers!
Of course one answer is probably wrong.
11-01-2008 09:25 PM
Originally Posted by preservedkillick View Post
My question is this, does the expansion tank need to be higher than the engine's heat exchanger?
You question was this, was it not???

My answer was Yes it does...what else or how was not this to your sufficience??

You then don't like my answer???

I know BY EXPEREINCE it does need to be higher..

When we installed my engine a new Yanmar in my boat, the water heater was installed by Vetus, and for weight balance we needed to install it in the stern which is higher than the engine..

Because the expansion tank wasa lower, the engine was allways hot, and sounding the alarm, as the water heater was behaving as the expansion tank..

We moved the tank one foot higher and fixed the issue...

The Yanmar installation manual (the ones Yanmar installers have, not the owner manual),clearly indicates the location, distance and case was a lapse because the heater was installed higher..
11-01-2008 08:45 PM
brak I wonder how important this is on a sailboat where the tank may be higher, lower or the same level as an exchanger depending on level of heel, waves bouncing around etc.
11-01-2008 08:40 PM
merc2dogs The expansion tank also serves the purpose of seperating air bubbles, it has to be high enough that the air in the cooling system will migrate to it. On a car the top of the radiator will be above the highest water passage in the engine, so it does the same thing.

Too low, and the bubbles will form in the exchanger or the engine, and reduce heat transfer which can cause the engine to overheat.

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