Originally Posted by edstill
I had to replace the hose returning fluid from my hot water heater to the heat exchanger on my Universal M25.
If you are sure about the hose, then yes, you must replace this liquid, otherwise You won't get hot water any more. Your boat has (I assume from your comments) a raw water intake, this flushes over your heat exchanger to help cool the fluids. Internally your engine is cooled by a separate, constantly circulating fluid, often called "fresh water" but it normally contains some coolant liquid/anti-freeze - hence the pink colour you are seeing.
So Separately from your raw water /sea water system, your heat exchanger is full of little pipes that are connected to pumping that fluid you just drained. The primary job of that fluid is to circulate around the engine and draw off the heat (separately the raw water then cools the fluid from these pipes in your heat exchanger).
On your boat you are using this "internal" cooling fluid for a 2nd job - You get free "hot water" since the circuit has been extended to your hot water heater from what you say... Of course you'll only get hot water if your engine has been running recently. Plain water is rarely used in these internal pipes due to the risk of freezing, but neither is 100% coolant/antifreeze either as this is actually a poor conductor of heat, and you wouldn't get your hot water... Normally it is a 50/50 mix of water and antifreeze. A little compromise.
I hope i've described that so it makes sense.
So - now you have to replace that liquid AND get any air out of the system (it will settle in the pipes at the highest point, and if you are lucky there will be a vent there). There should be an expansion tank/reservoir of this fluid somewhere near your engine (at least there is on mine) - you can simply top up into that little tank. It normally has a min fill level, and a max level - just fill to the min fill level and maybe 1/3 upto the max fill line - when you run your engine the fluid expands and then it might reach the max level. Run the engine for a little bit and make sure you've cleared any air pockets - check on the level and top up if necessary.
Separately when you winterized your boat (if applicable for your environment) you would ensure the raw water system is flushed through with antifreeze, so (I bet you can guess) it won't freeze and damage anything during the winter cold. But from what you say this isn't the hose you disconnected.