Does the hot water system have a pressure relief valve with a manual lever that allows you to bleed air from the tank like a domestic system has?

Looks like you already knew the problem (and the answer)

On a trip last year aboard a friends cruiser on the canals in europe the hot water system failed similar to yours , being resident fixit i set about the solution and found it to be caused by the reset / breaker at the tank (easy to test if you can get a voltmeter on the element terminals) which explains why no contribution while on AC power. But not if you are using engine to heat the water.
The reset button was not at all obvious and i had to remove more than i would have liked to get to it and extremely fiddly to get back together. good luck

I found the manual and it states that the pressure lever is to be opened to fill the tank, do I need to to this every time the tank is empty? My last hot water service isotherm was exactly the same system but I did not need to fill the tank manually each time ? Scratching my head again!

southace said..
I found the manual and it states that the pressure lever is to be opened to fill the tank, do I need to to this every time the tank is empty? My last hot water service isotherm was exactly the same system but I did not need to fill the tank manually each time ? Scratching my head again!

Hi southace, if the HWS is similar to the one on my boat, it has two coils inside the tank. One is the heating coil using engine coolant, the other is used to heat the water that comes out the taps.
The pressure relief valve dumps water into the tank proper which is the storage medium for the heat.
So the engine heats the water in the tank via a coil, which stores the heat which in turn releases the heat to the water flowing through the other coil on demand.
This gives you much more hot water than if you used the water out of the tank, BUT you do have to regularly exercise the valve to ensure that the tank is full or the performance is greatly diminished.
If yours is like this they are great systems.
Cheers, Bob.

I lovely update! Not!

after filling the tank asumming this was the problem with the relief valve , this morning at 5 am I was looking forward to a nice long hot shower before work!
Ummmm nope boil the kettle have a shave and pommy shower! Water stone cold!

Now I assume the heater element has burnt out for some reason! I guess I can run the mains but after putting up with A V8 fiat main for 6 hours today I really don't want to hear my little perkins running for another hour just so I can have a hot shower! I just love boats! Not!

southace said..
I lovely update! Not!

after filling the tank asumming this was the problem with the relief valve , this morning at 5 am I was looking forward to a nice long hot shower before work!
Ummmm nope boil the kettle have a shave and pommy shower! Water stone cold!

Now I assume the heater element has burnt out for some reason! I guess I can run the mains but after putting up with A V8 fiat main for 6 hours today I really don't want to hear my little perkins running for another hour just so I can have a hot shower! I just love boats! Not!

Well woodys marine hot water have been out of business for 4.5 years. There is only one company in Australia that has limited amount of replacement parts! $400 later and cold showers till Friday I should have hot water again!

During my absence I have had some big learning curves and major disappointments while trying to repair my failed hot water service.

After spending $400 on new parats for the discontinued Woodys marine hot water system I found i still had a bleeding problem as the water was not replaced with cold while I used the hot eventually leaving my tank dry and only allowing one litre of hot water while showering. I replaced my Main water pressure pump $280 and assumed I had fixed the problem but the following day I was back with the same problem!

The following day I decided to try something different. I filled the tank with the pressure relif valve , turned the water pump off and went to work for my 12 hour day. Upon arriving home I immediately hit the pressure relif valve and as my suspicions told me the tank was dry!

i then while filling the tank put my finger over the overflow tube and pressured the tank while filling, I could then see at the bottom of the tank water leaking from a choroded joint that I was a little worried about! Right O ruptured pressure vessel! Dam!

its a copper tank and I rang a few boilermaker mates and they suggested replace the tank!

This morning I ordered this Italian made 40 litre hot water service @ $1400 and another 6 days wait!
http://www.bcaaust.com.au/Marine-Water-Heaters

gumtree woodys hot water replacement parts!

I LOVE BOATS!

southace said..
Finally all my problems have been rectified thanks to this new ubute system! Now I'm broke! :(

I could post you a bar of soap south ace quiet happy to do so how you off for soxs?

southace said..
I will swap u a bar of soap for my old woodys heater! All new parts but the copper pressure vessel tank which is ruptured! You could use some boat coat! Or fixteck maybe. Just pay the shipping.
regards
southace

Ill sneak up on to your boat and run a hose over to mine when I want a hot shower .. So now your finished your boat you can come here and help me S.A.
Masts are coming maybe this weekend also the anchor winch will be here tomorrow