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Old 02-04-2013, 12:16 AM   #1
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Why won't my engine get hot enough?

Sorry for the provocative title butI didn't know how else to word the question. A little background first. Our prior boat had a single Lehman 120 that ran at 180 degrees, checked with my inferred at cruise. Water heater water and bus heater were scalding hot. Exact same 120 on this boat with WH and bus heater on the loop and only moderately hot. Replaced thermostat and added circulating pump. Bleed system many times. I've checked the manifold and expansion tanks temps and all seem fine yet I can not get the hot water loop scalding as in the past. Why!!!!
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Old 02-04-2013, 01:07 AM   #2
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Maybe the meter is reading the wrong temp? Is it FW cooled? If it's raw water cooled, it'll run cooler, but not that much.
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Old 02-04-2013, 06:50 AM   #3
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Do you have valves to isolate the water heater and bus heater?

If you don't...you probably should as I need to add some to my loop.

If you can isolate the bus heater...even with no fan blowing...it may be enough to keep the temps down...

Just as they say on a car to turn on your heater in the summer if your car is overheating stuck in traffic...pretty amazing how even tiny ones help cool.
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Old 02-04-2013, 07:28 AM   #4
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The hot water heater probably has some sort of "anti Scald" device , which simply shuts off most of the coolant water going thru it.

A separate coolant supply circuit for each device is required.

Don't ask how I know!
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Old 02-04-2013, 09:46 AM   #5
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Quote:
Originally Posted by Daddyo View Post
Sorry for the provocative title butI didn't know how else to word the question. A little background first. Our prior boat had a single Lehman 120 that ran at 180 degrees, checked with my inferred at cruise. Water heater water and bus heater were scalding hot. Exact same 120 on this boat with WH and bus heater on the loop and only moderately hot. Replaced thermostat and added circulating pump. Bleed system many times. I've checked the manifold and expansion tanks temps and all seem fine yet I can not get the hot water loop scalding as in the past. Why!!!!
does the water heater have a mixing valve at the heater that mixes the hot water at the output with cold to keep the temp to a reasonable level?. Shoot the point that the water loop where the engine loop goes into the heater and the point where it exits the engine on the way to the heater... same temp? and is it close to the thermostat temp after the engine has been running under load for 15 minutes?
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Old 02-04-2013, 02:55 PM   #6
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No valves just an unobstructed loop. Same problem before I added the heater the WH didn't get very hot. No anti-scald valve. In the last boat the system was identical and it was boiling hot.
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Old 02-04-2013, 03:08 PM   #7
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No valves just an unobstructed loop. Same problem before I added the heater the WH didn't get very hot. No anti-scald valve. In the last boat the system was identical and it was boiling hot.
Are the takeoff and return the same as last engine?

Maybe on this engine they are reversed and you are getting the cooler side (post heat exchanger) headed to the loop.
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Old 02-04-2013, 04:13 PM   #8
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It might help to know the actual temperature of the engine coolant. If the engine isn't running as hot as it's supposed to, the water heater won't heat potable water as hot as it should.

An infared non-contact thermometer would be a good tool for diagnosing your issue.
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Old 02-04-2013, 06:26 PM   #9
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Is 100% of the hot water coolant water heating system below the return line where the coolant comes back to the engine? If not air?
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Old 02-04-2013, 07:08 PM   #10
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Same connection points. Water flow is in the correct direction. No air in system (one of the advantages of having the recirculating pump). Checked with my inferred, if I remember right the engine was 180 at the expansion tank and the loop was well less then 130.
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Old 02-04-2013, 08:32 PM   #11
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Maybe the recirc pump is causing too fast a water flow? Many of the heaters have a degree temp rise based on gal/minute waterflow.
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Old 02-05-2013, 12:22 AM   #12
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I considerred that but found the problem the same even if the pump was off. I added the pump after adding the heater. Prior to adding the heater the WH was never hot enough. Someone had suggested to me that the injectors were not up to spec and that was the problem but that concept doesn't make sense to me. Is it possible that the cooling system is so efficent that high heat just isn't going to happen and that on my previous boat the system was compromised and was getting hotter then it should have?
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Old 02-05-2013, 06:33 AM   #13
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if I remember right the engine was 180 at the expansion tank and the loop was well less then 130. __________________

Sounds like the heat exchanger is overcooling the engine , the usual drop thru a cooler is about 20deg.

Is the water sent to the HW heater and box heater plumbed AFTER going thru the cooler?

A valve in the cooling circuit to slow the water thru the cooler will work, but might need a seasonal adjustment .

A Bypass thermostat as used in keel coolers is the "better" solution.
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