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Old 01-24-2021, 02:00 PM   #1
Mini-Skoolie
 
Join Date: Jul 2018
Posts: 10
Hydronic system questions

I'm trying to get a layout going for my bus conversion. I've got two roof top A/C's. I may replace with a heat pump in the future when they wear out. For the time being, I have to figure out the heat and hot water situation. The bus has a 45k btu proheat engine heater. After reading threads and fb groups til I'm dizzy, I think I may have a a decent layout plan. To keep it as simple as possible, I'm thinking about installing a 20 gallon marine water heater in the rear engine bay with the proheat. I have valves going to the engine now. I can tie into that and have an engine coolant loop going through the marine heater. The normal domestic water side would have boiler antifreeze. I would have one pump that went to a manifold and have valves that go to 2-3 hydronic sections through the bus. On that loop I would also have a plate heat exchanger to heat the DHW. I'm thinking when running down the road the engine will keep the 20 gallon water heater up to temp. When parked the 120v heater element will be used. If it can't keep up, the proheat will be turned on. When we need hot water, turn the circulation pump on, and we're good to go. Anybody see any problem with this layout? I'm thinking the 20 gallons should hold enough heat to be able to get a shower without having to use the diesel heater. Also need to find a way to control the proheat to turn on when the tank gets cold for when we're using the heat.
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Old 01-24-2021, 02:56 PM   #2
Skoolie
 
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Join Date: Aug 2018
Location: Rhode Island
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Year: 1980
Coachwork: Crown Coach
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Your heading in the right direction. Issues I see with the plan are the flat plate exchangers ability to heat DHW at a reasonable flow rate. Not saying its impossible but will take some work to size properly based on set temps, flow rate, and rate of rise. Also the 20 gallons of capacity will get exhausted pretty quickly without additional heat input either from the Proheat, engine, or 120v element.

Use of the system for hydronic heating should work well as you don't need the rapid temp rise like you do for domestic hot water. Couple of things to remember, you need to build in expansion capacity and excess pressure/temperature relief just like in a traditional heating system.

I have been toying with a similar plan these last few weeks, radiant floor heat powered by an espar/webasto/proheat or similar with a flat plate heat exchanger to the engine loop to provide cold-start pre heat or even secondary radiation to dump excess engine heat in an emergency overheating situation just like putting on the heaters but with more btu capacity. Keep us posted with your ideas.
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Old 01-24-2021, 05:42 PM   #3
Mini-Skoolie
 
Join Date: Jul 2018
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I’ll get the biggest plate exchanger that’s reasonably priced. Unless we needed the heat, hopefully the electric element would keep up. Another option is just to add another either 11 or 20 gallon marine water heater just for domestic water. There seems to be a lot of fear of having engine coolant only 1 step removed from domestic water, but that’s what the marine water heaters are built for. I plan to add some form of expansion tank. I assume the tank pressure relief valve could be vented to that tank, but it shouldn’t even be needed if it’s basically vented to atmosphere. Two heaters would eliminate all the guesswork. There’s also the option to use the DHW as the heating medium for the hydronic heating. Not a huge fan of that idea though I suppose it would work.
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Old 01-24-2021, 08:03 PM   #4
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Location: Swansboro,NC
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Year: 86
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for any plate heat exchanger or solar water heat exchanger
plate exchanger is a cast plate with the heated fluid running through the exterior of the plate while the water to be heated runs/leaks through the interior tubing of the plate being heated.
my experience.
no one knows there is a problem until its a problem and melts supply lines to sinks or whatever.
solar water systems use glycol for many reasons and the water piping is wrapped around the outside of of a normal water heater tank but of course inside the insulation and wrapped around the tank. DIY heat rises so bottom is inlet.
if you dont have it right summer will let you know.
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Old 01-25-2021, 08:43 AM   #5
Mini-Skoolie
 
Join Date: Jul 2018
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Quote:
Originally Posted by Jolly Roger bus 223 View Post
for any plate heat exchanger or solar water heat exchanger
plate exchanger is a cast plate with the heated fluid running through the exterior of the plate while the water to be heated runs/leaks through the interior tubing of the plate being heated.
my experience.
no one knows there is a problem until its a problem and melts supply lines to sinks or whatever.
solar water systems use glycol for many reasons and the water piping is wrapped around the outside of of a normal water heater tank but of course inside the insulation and wrapped around the tank. DIY heat rises so bottom is inlet.
if you dont have it right summer will let you know.
I’ve read this at least 4 times. Still not 100% sure what you’re trying to convey, but here’s what I’m getting. You don’t like the chance of a leak in a plate heat exchanger. You like the safety of having things further isolated like the solar water heating system. Not sure what summer has to do with any of it.
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