Engine cools hot water

[harbin2]

I have a raw water cooled Volvo MD7A engine in my Islander 30 Bahama. I have a domestic hot water heater that runs on 110 volt shore power and is plumbed into the engine cooling water. The cooling water exits the thermostat housing, through a heat exchanger in the water heater and then out to the exhaust for exhaust cooling.
I usually heat the water in the heater before I leave the dock. However, when I run the engine, the engine cooling water cools the water in the heater. This happens because the engine runs cool and because there is a bypass line in the thermostat housing that is always keeping “cold” water running through the line.
I am considering inserting a bypass line with a 3 way valve that will allow me to bypass the heater. When the heater contains cool water and I’m running the engine hard for a while, it does warm the water a little – so I could redirect the flow through the heater for that case.
I’d be interested in hearing how others have addressed this same problem.
Thanks in advance.
Ron
Islander 30 - What I Do

[CatalinaRob]

hi Ron - raw-water cooling systems are not designed for hot-water heating coil system as you describe. Your PO must have set this configuration up. Raw water cooled engines are designed by default to run much cooler than closed system - heat exchange type systems (which run much hotter). The reason is that salt sea water (raw water) will form scale / salt deposits at higher temperatures within the engine block; therefore the thermo is set to keep the system much cooler. I don't have the exact numbers / degrees but believe its about 40 degrees cooler.

You would need to either bypass/disconnect the run to the HW tank and redirect to the exhaust system OR convert your cooling system to a closed-loop fresh water system (which involves much more).

Good luck!

- Rob

[Faster]

A RWC engines need the thermostat bypass to continue to cool the exhaust while the engine 'warms up' - and as a result as mentioned the water temp is always cooler, certainly cooler than the electric element will heat your hot water tank up to.

As Rob indicated I doubt there's a good 'fix' other than converting to FWC - possible but a bit pricey and a bit of an engineering project as well.

[harbin2]

Thanks guys.
As usual, this forum so valuable. I think Rob is correct - RWC systems are not DESIGNED to heat water. My thermostat is designed to be fully closed at 140 and fully open at 167. Untill this last winter, the engine had a minor overheating issue when I ran it long and above 2200rpm (max is 2,600). I removed the exhaust manifold and found lots of blockage so I'm fairly confident that problem will go away - and make the one we're discussing here worse.
I'm certainly not going to convert to fresh water ($$) although I wish I had a fresh water system. I think I may just rug up a bypass and keep it bypassed most of the time.
Thanks again.
Ron

[tomperanteau]

Also consider this. I have read that salt water is not good for circulating in the coil of a hot water heater. I did not read why, but it must of course be because of the metals generally used for the coils and the corrosiveness of the salt water.

[harbin2]

I surely concur with that theory. When I first got the boat 3 years ago, I was removing one of the heater hoses from the heater and when I twisted the hose, the fitting broke off right in my hand. I'm surprised it hadn't been leaking because it was corroded almost completely through around the entire circumference of the fitting. The heater worked fine electrically but I made the mistake of going right out and spending $300+ for a new heater - and all the work to replace. I should have just bypassed the heater then. Live (and spend) and learn.

[JimsCAL]

If you ever consider a fresh water conversion, here's an inexpensive way to go.

Atomic 4 Fresh Water Cooling System - Electric Antifreeze Pump