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bstowers 23-04-2013 20:28

Replaced Water Heater, Only Seems To Work From Shore Power Not Engine
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tl;dr - water heater doesn't heat from engine but does from 120v power, don't know why. Any ideas?

Long version:

I'm pretty new to all this having only started sailing about 1 1/2 years ago and only have had my boat for just at a year now. The wife and I have started doing fairly regular weekend trips once or twice a month and so it was decided that I should replace the water heater that had been removed from our boat by its previous owner due to it being a corroded hunk of junk (his words, more or less). I dug around and found that the heater used in my boat was a Seaward 6 gallon (S-700 is the exact model I got from I was able to locate the 120v shore power wiring that was still hanging out near the unit's original location and reattached those. The plumbing lines to the pressurized water and such were also still nearby, they had just been connected together to close the loop so to speak. I was able to reattach those to the water heater as well and the pump filled the tank in short order. The original lines to the engine were still in place as well, however they had been cut off at the back side of the engine. Since I was going to have to buy line to get them to reach the front of the engine where the connections were supposed to be made, I figured I would go ahead and just replace those lines entirely. I used the same size and grade hose. I connected these to the back of the water heater and to the two bypass connectors that were previously plugged on my Yanmar 2GM20f fresh water pump (see image, red arrows). The outflow bypass had the temp sensor on it, so I used a T connector here so the temp sensor remains in the same place.

Now, all is well and good if I use shore power, I get hot water in fairly short order. However, that won't really do me much good because when we have shore power, we usually don't use the boat water for much anyway because we're in a marina and are using their shower, etc. What we really need it for is when we are anchored, so heating from the engine is what is really useful for us. I had no luck in this area, however. At first, I ran it at fairly low RPM (around 1000 maybe) for 20 or 30 minutes, but never even got the faintest hint of heat out of it. After some time spent checking and rechecking things like coolant levels and force-filling the lines to the heater with water to make sure there was no air in them, I then ran it a bit over 2000 RPM for another 30 minutes or so, but still never even got a hint of luke warm water out of it. After that, I tried it with shore power, which worked fine and rules out a problem with me hooking the plumbing up wrong. So, it seems the issue lies in the lines from the fresh water pump bypasses to the heater unit.

One theory goes that the lines are too long and are losing too much of the heat. They are approximately 12' runs (12' back to the unit, and then 12' back to the engine). This doesn't seem to make much sense to me as I'm using the exact location that the heater was installed in when it was brand new. It worked before, should work now. Besides that, I think if this were the problem that it would at least get warm, just not maybe hot

Another theory goes that the fresh water pump just doesn't have the force to push the water out the bypass and down the 12' run and back. The engine was replaced by the previous owner 4 years ago and the fresh water pump has had no service that I know of since then. I never have overheating issues, however I don't have a real temp gauge only an idiot light, so I can't say how hot it is really running. I did put a bleeder valve on one of the lines where it attaches to the heater unit and when I open that, the coolant that comes out is most definitely not hot. So maybe there's something to this theory? The water in these lines is just sitting there and not being circulated for some reason? I've just re-checked the Seaward manual and it makes no distinction between the inlet and outlet lines coming from the engine, but I wonder if maybe they need to be switched around? I would think they would call out something like that in the installation instructions, but the diagram clearly makes no reference to it at all, so I don't think that matters. Maybe if the lines were hooked up such that the bypasses weren't being used, that would force the issue? Seems like a very bad idea on the face of it, those bypass connectors were put there specifically for this purpose according to the Yanmar diagram.

One last theory was that perhaps idling, even at higher RPMs, just isn't enough to get the heat flowing. I will test this one tomorrow by motoring around for an hour or so. I certainly hope this isn't the problem as running the engine at anchor for half an hour in the morning to get some hot water is something I can put up with. But having to pull up anchor and motor around for a while is most definitely not what I had in mind.

Diagram of the 2GM20f system with bypasses marked for the heater inlet and outlets marked up in red attached.

Anyone offer a noob plumber some advice here? Thanks in advance.

Redcoat 23-04-2013 20:35

Re: Replaced water heater, only seems to work from shore power not engine
Did you remove the plugs from the manufacturer before connecting the hoses?

bstowers 23-04-2013 20:46

Re: Replaced water heater, only seems to work from shore power not engine

Originally Posted by Redcoat (Post 1218674)
Did you remove the plugs from the manufacturer before connecting the hoses?

A friend actually connected up that end, but I didn't notice any plugs when I looked at it before we shoved it back in its hole. Plus, we used a water hose at the end of one line and got quite a fountain out of the other line in return when we checked to make sure there wasn't air in them, so I don't think that's it.

sailorchic34 23-04-2013 20:50

Re: Replaced water heater, only seems to work from shore power not engine
Probably a little air in the system. Make sure to burp the coolant tank, IE remove and replace the radiator cap. Also check the coolant level in the tank. 24' of hose is a fair bit, but it should work a little.

Redcoat 23-04-2013 21:37

Re: Replaced water heater, only seems to work from shore power not engine
I have a ball valve on mine . Right on top of the block next to the thermostat. Might be air with the ball valve you can burp it.

Redcoat 23-04-2013 21:43

Re: Replaced water heater, only seems to work from shore power not engine
I'm running a perkins so I won't pretend to know your engine but. On mine pressure on top return on the low side. If I run them both from the top the coolent goes nowhere. It takes a good hour to heat the water for a full shower in my boat at 900 rpms.from coldwater.


Irish rambler 23-04-2013 22:19

Re: Replaced water heater, only seems to work from shore power not engine
I concur with Redcoat as I'm a Perkins man too. IMO you have a connection wrong, things to bear in mind though are that with 12' of hose & return they would need to be well insulated and nearly level to be efficient. Make sure the tank top is the same height as the top of your engine or you will get thermal convection and that the water pump can cope with the distance,if not fit an electrical circulating pump that only runs when the engine is switched on. An engine just running under no load for long periods is inefficient and bad for an engine as it will not reach proper operating temperature. So check connections and rectify, insulate feed and return, when running the engine in dock tie up securely and run the engine in gear @900/1000 revs, if at anchor run in reverse BUT make sure you have a trip line (with float, to alert other craft nearby) on your anchor as it will 'dig in'.
I hope this helps to guide you.

bstowers 25-04-2013 12:21

Re: Replaced Water Heater, Only Seems To Work From Shore Power Not Engine
Turns out to be pretty stupid. We hooked up one of the lines to the "bypass flow" instead of to the water heater inlet. They are the same size and within inches of each other, but one is oriented horizontally and the other vertically so it's really, really obvious if you know what you are looking at. Attached the line to the correct inlet and it works great now.

Thanks for all the suggestions.

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