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Two coolant expansion bottles (engine and hot water tank) - weird interconnection

Dec 31, 2020
30
Catalina 30 Gig Harbor
Hi all, recent new owner of a 1987 Catalina 30 Mk II. Wondering if someone can help me understand the interaction between the two coolant expansion reservoir bottles on the boat.

The engine and its overflow bottle are down low, of course. The hot water tank is higher, in the port side cockpit locker; and its bottle is positioned above that.

Two things I notice, and I don’t understand the setup. Firstly: the coolant reservoir bottle at the engine never changes its level at all. Secondly: the upper bottle (above the hot water heater) has a tube running from the *bottom* of the bottle, that terminates near the engine and carries a “pressure release” label. It consistently empties coolant into the bilge a little at a time when the engine is running - enough that I’ve stuffed its end in a little plastic bottle so I can keep it out of the bilge and reuse it.

Result of this process is that the upper bottle slowly empties itself. I keep having to transfer coolant from my collection bottle back into this upper reservoir to meet the minimum level identified on the bottle.

On one occasion the upper reservoir emptied out completely without my noticing it. Next thing I knew, I had an air block in the coolant system and the engine began overheating. Did emptying the upper bottle cause that? Coincidence? Dunno.

So, what’s going on here? Should I just allow the upper bottle to empty? Is the “pressure release” tube on the upper bottle attached in the wrong place, needs moving from bottom to top of bottle? The reservoir bottle at the engine has its overflow tube mounted at the top, seems much more logical. “Inquiring minds want to know”…
 
Jan 7, 2011
2,644
Oday 322 East Chicago, IN
I think some pictures would really help. And you don’t mention what engine you haGE…not sure if it makes a difference, but my comments are for a Yanmar (2GM20F).

The only thing I can say is that the engine overflow tank is supposed to mounted higher than the neck of the heat exchanger….The bottle should fill some as the engine temp rises, and then the coolant flows back to the HX when things cool down. I don’t think it is gravity, but siphon or suction pulling the liquid back to the HX as the coolant cools down.

The water heater should have a Temperature and Pressure (T&P) valve that opens when temp or pressure is too high in the water heater…mine doesn’t have a hose on it, but that could be what you describe s the ”Pressure Release” that is draining into the bilge. The T&P valve may be weak and opens prematurely…

The only connection between the engine and the water heater is the engine coolant loop that flows hot antifreeze through a heating loop in the water heater to provide hot water while motoring. The engine can REALLY heat the water, so that could trip the T&P valve.

I cannot picture the reservoir above the water heater… need a photo.

Good luck…

Greg
 
Feb 26, 2004
21,714
Catalina 34 224 Maple Bay, BC, Canada
and I don’t understand the setup.
So, what’s going on here?
Luke, they are two separate systems. You shouldn't be putting coolant in the water heater reservoir, and that doesn't sound right to have one for the water heater, at all. I think you should read your engine and water heater manuals.
The water heater should have a Temperature and Pressure (T&P) valve that opens when temp or pressure is too high in the water heater…mine doesn’t have a hose on it, but that could be what you describe s the ”Pressure Release” that is draining into the bilge. The T&P valve may be weak and opens prematurely…
Sounds correct. The T&P valve may be stuck open. It's draining your drinking water, not coolant.
BTW, what engine do you have?

You should also get to know the io group for C30 skippers. Go to reply #5 here for a link to it: 1992 Catalina 30

And International Catalina 30 Association
 
Last edited:
Oct 22, 2014
15,324
CAL 35 Cruiser moored EVERETT WA
Hi Luke. I read your description and can begin to see the system and working to comprehend the set up.
A couple of pictures would be of immense help.
 
Dec 31, 2020
30
Catalina 30 Gig Harbor
The engine is a Universal M25.
So, let the remaining bit of coolant in the water heater reservoir (that I put there…) empty, and just leave it that way?
If that bottle is intended for hot water not coolant, then it would appear that the T&P valve is OK. It’s only the coolant in there that has been draining to the bilge through that “pressure release” hose.
I guess, then, that I had incorrectly assumed that the air that showed up in my coolant system and required burping, was somehow connected to that upper bottle having become empty…
Thanks for the guidance!