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Old 01-20-2018, 11:30 AM   #1
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City: Baltimore
Join Date: Sep 2017
Posts: 33
Onan 8kW heat exchanger relocation

Any of us who have the 1980s Onan diesel generators know just how buried the heat exchanger is buried behind the engine. Because of this, the previous owner didn't bother with it much. At sea trial, seawater was mixing with the coolant, so I knew a replacement was needed. Upon disassembly, I decided there must be a better way to put this thing back together. The zinc is about impossible to reach without removing the starter, exhaust header, and sound enclosure. After visualizing changes to the piping arrangement, I believe the simplest way is to have the heat exchanger upstream from the raw water pump. So, water first passes through the sea strainer, then the heat exchanger, and then on to the pump. My question is: can I do this? Or does the pump have to come first? Any ideas or experiences?


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Old 01-20-2018, 12:25 PM   #2
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City: Inside Passage Summer/Columbia River Winter
Vessel Name: Alaskan Sea-Duction
Vessel Model: 1988 M/Y Camargue YachtFisher
Join Date: Jul 2012
Posts: 7,082
Room would be an issue and it is designed to have the heat exchanger downstream from the pump and thermostat assembly. Last year I removed and replaced the exhaust tank, heat exchanger, water pump, thermostat and all hoses. Dropped 2 boat dollars on parts. Did the work my self.

Conduct periodic preventive maintenance and will last for years.

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Old 01-20-2018, 02:48 PM   #3
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City: Freeport, TX.
Vessel Name: Rum Away
Vessel Model: '89 Bayliner 3888
Join Date: Mar 2017
Posts: 14
I would avoid putting the exchanger (i.e. pressure drop) on the suction side of the seawater pump. I think this would reduce total seawater flow. And may also cause pump priming issues. My $0.02
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Old 01-20-2018, 04:05 PM   #4
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City: Between Oregon and Alaska
Vessel Name: Charlie Harper
Vessel Model: Wheeler Shipyard 83'
Join Date: Jun 2016
Posts: 2,328
ditto on pump suction problems. Pulling water thru tubes is more restrictive than thru a hose or pipe. In time as the usual corrosion builds before cleaning, suction will become more difficult. With the exchanger after the pump, water is pushed thru the tubes with a little pressure.
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