Hayden,
Here is an article about adding a bus heater to the coolant loop on a C34.
http://c34.org/wikiwp/?rdp_we_resource=http://c34.org/wiki/index.php?title=Cabin_Heater
Please note that Bob’s diagram shows ½” hose from the 3/8” lines at the top of the coolant pump and back to the thermostat housing. The text says 5/8”. My water heater takes 5/8” hose from that reducer bushing. Most Kuuma and Seaward heaters take 5/8”, too. Comments on hose size of ¾” should make no difference.
But note that Bob’s system has the bus heater in series with the water heater and it works just fine.
I replaced my Seaward water heater with another Seaward in 2013, 15 years after we bought our boat when the OP replaced his 12 year old heater. The heaters have a reported life of between 10-16 years (I know, a wide variation).
My M25 has always heated our water in less than 45 minutes to scalding hot. I moved from San Francisco (57 water temp) to British Columbia (47 water temp). In a year of living here I found no appreciable change in how long it takes to heat the domestic water under power. I have found that the water in the tank cools off more quickly once at anchor, ‘cuz it’s colder “down there” where our heater is under the galley sink.
I can’t conceive of how you were running your M25XP at 140 degrees. I don’t know of any thermostats that would fit an M25XP other than 160F and 180F thermostats. But you seem to have that behind you.
The hose size you see is 3/8" and is only for the first couple of feet and then the rest of the circuit is 5/8". On the M25XP, the bypass ports are 3/8" so it needs to start and end with that if I use the bypass. I prefer to keep it in the bypass as a safety measure. much shorter run of the critical hose and if I get a leak in the bypass loop I can cut out 6" of good hose and return to straight bypass hose eliminating any problems in the heaters.
So you got the hose sizes right. I couldn’t view the videos because my plug in wasn’t working. Just to confirm: your engine has the “regular” connection to the heater as shown in Bob’s diagram: from the top of the coolant pump to the heater and then back to the thermostat housing. Is this short 3/8” hose what you mean by “bypass?”
For the first 30 minutes I ran in gear at 1900 rpm which is full cruise speed on my M25XP.
1900 sounds awfully low for cruising speed on an M25XP. I run at 2450-2800 on our M25 engine. WOT is 3000. My thermostat opens and gets up to 180F engine operating temperature when I run in gear in about 5 minutes. I get scalding hot water in 30-45 minutes.
It has never heated water well from the engine but gets fully hot on 120v in about 30 minutes.
Unless it’s because of your 140 operating temperature. But now that you got your 180F thermostat, you should get scalding hot water in less than an hour. You may well have a dud of a heater.
Seaward still makes heaters, they are step above the Kuuma.
http://c34.org/bbs/index.php/topic,7893.0.html
My M25 was plumbed in series with the engine coolant and you just have a newer version of that engine. Why not take the flow from the top of the thermostat that goes back to the coolant reservoir and run that hose to the water heater and then from the water heater back to the coolant reservoir? You'll think you are in the hot springs at yellowstone park with all the hot water you'll be basking in. Then just use the bypass loop for the cabin heater instead of trying to run both off that minimal flow?
I don’t understand this at all. What bypass loop? If you follow Bob’s sketch and Dave’s first sentence here, it’s the same thing. Coolant pump>>3/8” hose>>3/8” to 5/8” bushing>>water heater and like that back to the thermostat housing.
Good luck.