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SV Demeter 04-06-2010 13:50

Installing Water Heater with Perkins 4.154
Our new to us boat came with a CNG fired instant hot water heater but it does not work well and needs to come out to make the insurance folks happy. I want to install a conventional Isotherm unit that uses either 110v or engine coolant. My question is how to set up the bypass circuit for the engine coolant? Where and how do I tap into the engine coolant circuit on this engine? I am thinking I need a bypass circuit as I dont want to be routing all of the engine's coolant through the hot water heater right? Engine in question is a 1984 Perkins 4154.


Stillraining 04-06-2010 15:30

I have same engine and set-up...Ill see if I can find a picture

osirissail 04-06-2010 16:30

Basically one hose comes off the fresh water/coolant pump on the engine. There is usually a pipe plug already installed in my marine fresh/coolant water pumps for this purpose. The the return line goes back into the cylinder head.
- - I am ever paranoid about blown cooling systems, so I installed ball valves on the engine for both hoses to the hot water heater. This allows me "isolate" the water heater hoses away from the engine should one or the other hose leak or rupture.
- - Also it allows me to "regulate" how long I let the engine "heat" the hot water. Remember diesel run at about 160-180 degrees or more when under load. There is the potential for "over-heating" the water in the hot water tank and causing injury to somebody taking a shower - or - even triggering the "overheat/relief" valve on the water heater and dumping near boiling water into your bilge/galley/cabin/whatever. Usually a half hour of so of engine running (at full engine temperature) is enough to heat the water tank. Then I turn off the ball valves.

Stillraining 06-06-2010 18:20

1 Attachment(s)
Sorry forgot...Here ya go.

SV Demeter 07-06-2010 09:12

Thanks Stillraining. My engine right now just has a single hose connecting the two points you reference. If I set up the heater as you suggest will I be running the entire FW cooling system through the water heater? Is this okay? Osirisail I hear you and like the idea, will have to take a close look at the FW coolant pump and see what I have to work with.

Stillraining 07-06-2010 11:02

1 Attachment(s)

Originally Posted by SV Escape Plan (Post 464869)
Thanks Stillraining. My engine right now just has a single hose connecting the two points you reference. If I set up the heater as you suggest will I be running the entire FW cooling system through the water heater? Is this okay? Osirisail I hear you and like the idea, will have to take a close look at the FW coolant pump and see what I have to work with.


Here is another you can see, it is part of the thermostat housing ( Water pump not yet installed in this picture)....So it circulates only a good portion of the water pumps flow rate before the thermostat is open and then a lessor amount once it does open and the flow takes the path of least resistance.

The ball valves are a good idea for obvious reasons of service and isolation issues to the W/H that is all...not to control flow

SV Demeter 07-06-2010 11:21

Thanks that's what I was thinking. This is port that the water heater is t'd into is simply the bypass loop for the engine coolant right? When the engine is cold and the t-stat is closed the coolant bypasses the t-stat and just goes back to the header tank/heat exhanger. Once the t-stat opens the majority of flow goes through the t-stat to the rest of the motor and cooling system. I will probably have to mount the hot water heater above the header tank/heat exchanger so I will probably either install a remote header tank in the hot water heater circuit or at least a valve to allow me to bleed air from the system. Thanks for the help, a picture is worth 1000 words.

Stillraining 07-06-2010 11:28

1 Attachment(s)
Here is a better expination on how your engine cools itself.

osirissail 07-06-2010 13:49

Basically coolant in the engine is "sucked" out of the cylinder head by the fresh water coolant pump and then pumped to the heat exchanger where it is cooled by raw water. Then the coolant is sent back to the cylinder head and engine cooling "galleries" to again be sucked out by the fresh water coolant pump. A closed loop.
- - Raw sea water is sucked from the ocean through the strainer by the raw water pump and is then sent to the raw water side of the heat exchanger where it cools the fresh water coolant. After that it is either sent through the exhaust manifold cooling jacket or directly to the injection elbow in the engine exhaust pipe and used to cool the exhaust gases so that rubber hosing can be used.
- - A water heater needs to have the fresh water coolant "pumped" to it so a "take-off" somewhere very close to the output of the fresh water coolant pump is necessary to get the hot coolant to the water heater. Since the hot water heater is in essence a "heat exchanger" the output from the hot water heater can be directed back to a port in the engine cylinder head or block water jacket. While in operation the hot water from the fresh water coolant pump travels two paths - one to the engine heat exchanger and the second path to the hot water heater and back. Yo don't want to have the hot water heater return line come back to the hose from the fresh water coolant pump to goes to the heat exchanger. This would set up a "closed loop" circulating hot water before it gets to the engine heat exchanger which would reduce the temperature of the water in the water heater.
- - With the extra ball valves installed you can isolate the water heater hoses from the normal engine system to either regulate the temperature of the water in the hot water heater or to prevent loss of coolant in the engine due to hot water heater or hose failures. Additionally, most hot water heaters are mounted above the level of the engine fresh water coolant reservoir tank. If the ball valves were not there it is possible for the extra coolant contained in the hot water heater hoses to drain back into the engine coolant tank and overfill the tank resulting in loss of fluid overboard.

SV Demeter 07-06-2010 13:58

I was with you all the way to the very last part about heater being above the header tank and the valves somehow preventing coolant from draining back and overfilling the header tank. Are you telling me that you open and close these valves everytime you run your engine? I like the idea of the valves being there in case of a problem but I had simply planned on installing a remote header tank at a location above the hot water heater. This is the way my last boat was plumbed. The remote header tank t'd into the bypass loop that supplies the hot water heater with hot coolant and allows air to be bled from the system. I replace the existing vented cap on the heat exchanger(header tank on a perkins) with a non vented cap and place a 14lb cap on the remote header tank. (guessing at the 14lb rating need to see what perkins calls for).

osirissail 07-06-2010 14:32

I started my original post with a comment about my being paranoid about loosing my engine cooling system. Over the past 19 years of having this boat I have found that if I leave the both valves open then I forget and a few hours later of motoring I have 180 degree water in the hot water tank. One time the overtemp relief valve let loose and filled the galley with hot water. There was a thread some time back about fitting a temperature regulator valve to the coolant lines from the engine to keep the hot water tank temperature at an acceptable 110 degree plus/minus. But it got awful complicated.
- - With a small engine and short run times it is unlikely that the hot water tank will get too hot as compared to my big 6 cylinder Perkins. So leaving the valves open might not be a problem - except for - the drain back problem if the water heater is higher than the engine coolant tank. Ideally with an "tight" hose system there should not be any backflow to the coolant tank. But the world is rarely ideal, so I keep the valves closed all the time and only open them when I am about an hour or two's motoring away from an anchorage/harbor. Then during the shutdown, put everything away phase I close the valves.
- - But as I said I am paranoid with my engine's coolant system. Most production boats have hot water heaters with hoses from the engine and do not have any valves what-so-ever in the system. And for those boats everything works just fine. So it is strictly a judgment call on your part which way to go as far as the valves are concerned. And additionally a couple of gallons of coolant stored away would be your guarantee that should you get a coolant draining back to the engine coolant tank you can easily replace any lost fluid.
- - If your engine coolant reservoir is above the hot water heater then there is not a problem with back flow from the water heater. Then the valves would only be there for emergency use should a water heater or hose break. With experience actually operating the engine you need to check the temperature of the hot water heater water after engine shutdown to make sure it is not too hot for safety. If it is then there are two options - 1. use the valves to turn on and off the hot water heater loop; and 2. partially close the pressure side valve to reduce the flow of engine coolant to the hot water heater until your average temperature is acceptable.

- - I am not a big fan of "pressure caps" as I got severely burned once when one let loose on my car. That kind of accident would be catastrophic onboard a boat in the middle of nowhere.

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