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post #1 of 9 Old 06-11-2016 Thread Starter
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Install water heater

We have an Oday'35 with a Universal M25.

We had to replace the water heater.

The concern is that the engine uses 7/8" hose but all the new water heaters use 5/8" hose.

We are concerned about restricting the flow by forcing it through the water heater.

https://www.dropbox.com/scl/fi/16r99...u8kum1R0A&sm=1

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post #2 of 9 Old 06-12-2016
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Re: Install water heater

A couple of thoughts. How is the heater connected? Series or parallel? If parallel, then all the water does not go through the heater. Look for "tees" to determine your hookup. Even if series and all the water does go through the heater, you should be fine. My Yanmar 3GM30F which has similar power to your M25 has all 5/8 inch hoses. Don't forget hose size is based on ID not OD.

Last edited by JimsCAL; 06-12-2016 at 09:24 AM.
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post #3 of 9 Old 06-12-2016
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Dave, I am VERY familiar with how the water heater is plumbed on an O'day 35 with an M25. The factory did this poorly, by plumbing it between the Heat Exchanger and the Exhaust Manifold. IIRC, the HX uses 5/8 hose, and the WH uses 1/2 inch. While you can get a fitting which allows you to connect the two, a better way for the engine, and the WH is to connect the HX output directly to the exhaust manifold. Connect the WH to the 3/8 nipple for the thermostat bypass (at the front of the engine). The engine temperature will be more consistent, you'll get hot water faster, and of there is a leak, it will be in a smaller diameter hose.
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Rereading your post. Yes, the HX and EM use 7/8" hose. Do not plumb the WH in this hose. Plumb it to the 3/8" hose at the bypass.
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Re: Install water heater

Dave; read and study THIS SITE

What the factory did, was to create a TYPE 2 water heater / heat exchanger connection.


- credit Markus Ritter owner of a Mariner 36 s/v Sankaty check out his website at Sankaty

What you want is a TYPE 1 water heater / heat exchanger connection.

- credit Markus Ritter owner of a Mariner 36 s/v Sankaty check out his website at Sankaty


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post #6 of 9 Old 06-12-2016
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Re: Install water heater

Quote:
Originally Posted by eherlihy View Post
Rereading your post. Yes, the HX and EM use 7/8" hose. Do not plumb the WH in this hose. Plumb it to the 3/8" hose at the bypass.
Our heater goes from these 3/8" lines to the 5/8" lines via a simple pipe connector (expander that goes from 3/8 to 5/8) right in the engine compartment.

Stu Jackson, Catalina 34, 1986, M25 engine, Rocna 10 (22#), Maple Bay, BC, Canada
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Re: Install water heater

And Dave, no one but you can follow the link to your DropBox.

This is what it looks like when you have plumbed the water heater with a Type 1 connection;


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Re: Install water heater

Thank you all.

It looks like a pretty definitive solution.

The lesson from the Icarus story is not about human failing.
It is a lesson about the limitations of wax as an adhesive.
If you have an engineering problem solve it.
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post #9 of 9 Old 06-12-2016
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Re: Install water heater

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Originally Posted by JimsCAL View Post
A couple of thoughts. How is the heater connected? Series or parallel? If parallel, then all the water does not go through the heater. Look for "tees" to determine your hookup. Even if series and all the water does go through the heater, you should be fine. My Yanmar 3GM30F which has similar power to your M25 has all 5/8 inch hoses. Don't forget hose size is based on ID not OD.
I also have a 3GM30F and an Isotemp water heater with 5/8" hoses. I've had it for maybe 15 years and haven't had any issues.
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