Alternate Water heater plumbing lines

Hi all, I'm looking to hook up the coolant lines on my hot water heater in order to heat the fresh water from the engine coolant (Currently it's shore power only). The typical points for fittings on my Mercruiser 7.4 FWC are the plug at the water pump & the port in the thermostat housing. My motor has a custom type thermostat housing fabricated out of pipe, therefor there is no such fitting. In addition, the plug in the water pump is non removable. I really don't want to replace the water pump just to plumb the heater. Does anyone see a problem with splicing a "T" fitting into one of the hoses coming out of the thermostat and an additional "T" in the return line from a manifold to the heat exchanger? The net result on the system would be similar regarding the direction of flow. Would this approach introduce any significant imbalance on the coolant flow through the manifolds?

 

yes, your heater needs to be under the thermostat not in the line to heat ex after it

 

Thank you Poweabout. Just out of curiosity, what do you think the outcome would be if I attempted this?

 

Why a T , simply cut what you think is the hottest source and lead it to and back the HW heater.

Beware the FW will be as hot as the engine coolant after a short time.

180F to 200F can scald very easily.

An anti-scald valve should be put in the domestic HW line .

An thermostat mixing valve IS NOT an anti scald valve.

The A-S valve will shut down the hot FW if there is not enough cold to mix , the thermostat mixers will not.

 

Thanks Fred, that's more or less what I was considering by using T fittings in the in/out lines on a manifold. I run a 160 T Stat & the motor, she never gets over 180. That is quite hot none the less, an anti scald valve is a great idea.

 

like to see a photo of the thermo housing
how about the small hose from the water pump that is the bypass on a big block?
Good place to plumb the hotwater service in

 

Here's a photo.

 

that hose that comes out the front, does it go to the water pump?
it should also be under the thermo, is that how it works now?

 

You can get a proper thermostat housing for $150 or less. It doesn't make sense to risk overheating the engine or burning your riser hoses.

 

@powerabout, yes that is the bypass hose.
@Gonzo, funny you should say that, I've been looking at them online. The previous owner of this boat cut so many corners and "rigged" things. I've had her for 7 years, and I'm still correcting all of the mistakes. Based on the serial number, the motor was a raw water cooled stern drive motor. It was rebuilt and converted into a FWC inboard. I know the t-stat housings are different for FWC vs. RWC. I uess it was easier for him to mock up this "housing" then it was to put the correct housing on the engine.

 

It it is a FWC you can use the regular automotive housing.

 

That won't have the fittings for the 2 hoses though no?

Essentially that is how this system works now though. The T-stat is below this fitting right on the manifold. This "housing" is bolted down on top of the t-stat, to the intake manifold. The 2 hoses go to each exhaust manifold, same as the OEM one. This works, just doesn't have the threaded port for my water heater connection.

 

So the exhaust manifolds are FWC, and the risers get raw water?

 

That means the bypass is above the thermo
So what happens when the thermo is shut?
Also means when thermo open a certain amount of water goes straight back into the block as that is the path of least resistance

 

How its plumbed depends on whether it has centre rise manifolds or not.
Then on which version heat ex it has