Types of
Water Heater Connections
on sailboat diesels

1. WH in Thermostat by pass
         (water heater starts heating before thermostat opens)
    same as on my automobile and the recommended method per Westerbeke
    Westerbeke, refers to this type 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


1.A. Thermostat closed
       (All flow goes through WH and nothing goes through HE)
            WH hoses are warm & 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)

Click here for implementation details on 1980 Universal 5432


Type 2 Modes (Series)


2.A. Thermostat closed
      ( No flow in either WH or HE)
       (Great flow in 5" long the by pass hose)


2.B. Thermostat starts to open
      (some flow in both WH and HE)


2.B. Thermostat fully open
      (max and equal flow in both limited
         by the WH hoses and orifice sizes)


Some boats have a water heater by-pass 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 by pass valves were popular on the M25
powered Catalinas in the 80's and early 90's.  Click here
for a picture of a by pass valve.

One of the Catalina 36 owners 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. 
Westerbeke and Yanmar now prefer Type 1.


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 (WH in 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 sure sea water will corrode the Water Heater
much faster.  The guy who had this  on his boat had
an angry wife as the water never warmed up.










High Water Heater
Click here to return to higher continuous top speed page.
Created Oct 5, 02 

Added type 4 example  Sept. 6, 06' that was actually found on a real boat.
Added High water heater example May 23. 08'
This page was last updated on: November 4, 2012