The following article was provided by Markus Ritter, a United States Power Squadron (USPS) member of District 10 and the Door County Power Squadron.
Markus had a constant overheating problem on his Mariner 36 stemming from the connection of his hot water tank in series with the diesel engine heat exchanger. He did a lot of research on-line and ended up connecting his hot water heater in parallel and changing the thermostat. He reports that it solved his problem and the engine achieves normal operating temperature quickly and then stabilizes and he has plenty of hot water. Since the retrofit he has not had a single incident of overheating.
Types of Water Heater Connections On Sailboat Diesels
Abbreviations used in the following discussion:
• WH – water heater
• HE – heat exchanger
1. WH in Thermostat bypass mode (water heater starts heating before thermostat opens) same as on an automobile and the recommended method per Westerbeke, who refers to this type of installation as the "positive flow, hot water heater connection in the thermostat bypass circuit"
2. WH in series with the HE (WH restricts flow through HE)
3. WH in parallel with the HE (WH robs some water from HE)
4. WH is heated by sea water that has passed through the heat exchanger
Type 1 Modes (Bypass)
1-A. Thermostat closed (All flow goes through WH and nothing goes through HE) WH hoses are warm and HE hoses are cold
1-B. Thermostat starting to open (lots of flow still goes to WH and some flow goes to HE)
1-C. Thermostat fully open (Lots of flow goes to HE and some still flows in WH)
Type 2 Modes (Series)
2-A. Thermostat closed (No flow in either WH or HE) great flow in 5" long bypass hose.
2-B. Thermostat starts to open (some flow in both WH and HE)
2-C. Thermostat fully open (max and equal flow in both limited by the WH hoses and orifice sizes)
NOTE: Some boats have a water heater bypass valve with the type 2 series connection. When open the coolant can flow directly from the engine to the heat exchanger without having to go through the water heater. A small amount of coolant still flows in the water heater. Therefore, when the valve is open the WH can still get warm just not as fast. These bypass valves were popular on the M25 powered Catalina’s in the 80's and early 90's.
A friend with a Catalina 36 experimented and found that with the valve open the max temp was 165, but closed the max temperature increased substantially indicating that the thermostat was no longer in control.
This bypass valve with the heater in series is still recommended by Raritan, but they also caution to use the engine manufacturers recommendation. Both Westerbeke and Yanmar now prefer Type 1 installations.
Type 3 Modes (Simple Parallel)
3-A. Thermostat closed (no flow in either WH or HE) high flow in 5" long by-pass hose
3-B. Thermostat starts to open (more flow in HE than in WH)
3-C. Thermostat fully open (max flow, HE has much more flow than WH)
Type 4 Modes (Sea Water)
4-A. Thermostat closed (No flow to HE and sea water does not warm up) high flow in 5" long by- pass hose)
4-B. Thermostat starts to open (increasing flow of anti freeze in HE )
4-C. Thermostat fully open (max flow in HE and sea water starts to get warm as it passes through the HE)
I have not figured out ANY merit to this system. I am certain that sea water will corrode the Water Heater much faster. The guy who had this set up on his boat also had an angry wife as the water never warmed up beyond luke warm.
Method for Bleeding the System: