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Old 07-18-2022, 03:50 PM   #1
Bus Nut
Join Date: Jun 2015
Location: Toledo OH
Posts: 781
Year: 2006
Coachwork: Thomas
Chassis: MVP-EF
Engine: Cat C7 + Allison 3000PTS
Coolant heater for domestic hot water - what kind of heat exchanger?

Hi all, I'm contemplating picking up a Chinese knockoff of a Webasto Thermo Top to heat a (separate) coolant loop. This loop will keep my fresh water tank thawed via an in-floor coolant loop, and also heat my domestic hot water using some sort of heat exchanger. However, I'm not really sure what kind of heat exchanger I need.

I'm thinking something like this on Amazon would work:

However, it's surprisingly expensive, and I'm not sure how many plates I really need?

The alternative would be something like this:

I don't really need the 120V option but I guess it's not bad to have.

Stupid me, I had a Webasto Scholastic but I sold it and took the coolant loop apart, so at this point I don't plan to loop in the engine coolant as well like somewhereinusa did. I'm not THAT great with plumbing and would rather minimize complexity.

Any pointers would be greatly appreciated, I'm just starting to figure this out as of today and have plenty of time to plan.

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Old 07-18-2022, 05:37 PM   #2
Bus Geek
Jolly Roger bus 223's Avatar
Join Date: Sep 2017
Location: Swansboro,NC
Posts: 2,965
Year: 86
Coachwork: Thomas
Chassis: Ford B700
Engine: 8.2
Rated Cap: 60 bodies
a solar water heat exchanger is basically nothing more than a coil wrapped around the water heater tank inside the shell so a glycol system warming up the outside of the tank inside the shell and insulation.
must are stainless coils on stainless tanks but open up a water heater and wrap copper around it with something like thin rubber as a spacer to separate the two metals put the insulation back on and put it back together.
its not that easy because of the new stub outs in the casing but is doable and remember that heat rises so the entering water/glycol should come in towards the bottom and the outlet should(opinion) actually be ran all the way to the top and and back down the side of the heater inside the casing before exit to create a heat trap to retain as much heat as you can.
dont get rid of the elements because that is your backup and make sure your pressure relief valve is up to the task the diesel heater might carry more BTU than the elements it was designed.
make sure its piped to a safe location but any dribbles can be noticed.
they can only handle so many discharges before needing replaced.
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Old 07-18-2022, 05:44 PM   #3
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Join Date: Oct 2021
Location: Jacksonville, FL
Posts: 672
Year: 2003
Coachwork: BlueBird
Chassis: TC2000 28ft
Engine: Cummins ISB 5.9 24v, MD3060
Rated Cap: 14
Im doing the Kuma. Unsure to go with 6gal or 11 gallon. Its the cheapest option i have found and i plan to primarily use the 120v element when on shore power and diesel for offgrid.

I thought about buying your Webasto for this, but i think the brand names ones are overly complex and ill treat my chinese one as disposable.

Anyway, below is my thread on my plan..
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Old 07-21-2022, 11:42 AM   #4
Bus Nut
Join Date: Apr 2018
Location: League City, TX
Posts: 321
Year: 1994
Coachwork: Thomas
Chassis: International 3800
Engine: DT408 6.7L L6
Rated Cap: 14
I'm doing something similar. But I'm a slow build path, so don't hold your breath.
My Build Thread!
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