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Old 12-04-2016, 08:48 AM   #1
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heat exchanger hot water using rear heat

I recently bought a passenger van with rusted rear ac lines so the seller (a dealer ) just capped the lines right under the doghouse and cut them out at the rear heat/ac. I plan to eventually replace the rear ac lines but being winter I'm thinking of doing the heat lines much sooner, and they are rubber hoses crimped to metal lines. I'm thinking of just running the missing length with rubber heater hose rather than replacing the metal lines under the van, for simplicity. I'll post a pic soon but the way they just cut the rear heat lines out and rejoined them under the doghouse with an elbow it should be easy to undo. In the rear I still have some rubber hose exposed so I should be able to reconnect there with some fittings.

Here's my question, can I not tap into this run under the van somewhere and run it through a heat exchanger for hot water? Isn't the coolant always flowing though these lines regardless of whether the rear blower is running? I found a cheap heat exchanger and plan to read a few long posts on plumbing mixing and cut off solutions for the heat exchanger but I'm thinking with rear heat I'm already halfway there.......or am I.

I would buy a hot water heater for this van since I plan to add propane eventually but then I found this, which seems like quite a deal for a heat exchanger:

https://www.amazon.com/B3-12A-Stainl...75A9C31VHB8Z90
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Old 12-04-2016, 09:20 AM   #2
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I seem to be eating a lot of popcorn these days...
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Old 12-04-2016, 09:43 AM   #3
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Sounds like a fun project however if your going to use a flat plate heat exchanger I would encourage you to look into using a double wall type. Double wall exchangers have an air gap between the different fluids so if there is a leak, the leaking fluid will drain out. They are less efficient because the heat must travel across the gap in air and more money than a thin wall single type. IMO single wall isn't worth risk of mixing water and coolant if there was an internal leak. In the first year I had my van (used van) we got sick from a small amount of coolant in the domestic water. The first van trips were just skin rashes then on one trip flu/food poisoning like symptoms, we got pretty sick. We used the van water for washing and cooking but not straight drinking at the time. The coolant was completely unnoticeable until one day the leak got bad enough that we could smell coolant in the sink water and BINGO figured it out. It's kind of embarrassing to admit because it seems from a logic standpoint that we should have figured it out sooner or even thought about it as possible before it happened. I assume there are a lot of folks using flat plates for water heating with success but I just wanted to mention it's not impossible to have a cross contamination. Just wanted to throw this into the conversation if it could prevent someone else from having our experience.

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Old 12-04-2016, 09:47 AM   #4
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I say absolutely and oddly enough I was thinking about this the other night. Musing with some thoughts about passive solar hot water heating too, but that engine heat is waste on all but the coldest days for the most part and could heat up 20 gallons in not all that long of a time.

Wonderboy did some pretty convincing calculations on 12V vs Propane water heating and I think he modified a small 120V water heater to work on 12V.

Maybe a heat exchanger either internally in the hot water reservoir or externally and has a small pump to feed into an electric or propane or diesel unit.
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Old 12-04-2016, 02:32 PM   #5
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What ShuttlePilot said is correct. That is why the plumbing code requires double wall heat exchangers for domestic plumbing systems. It can be done, just depends how much fear you want to live with.

That being said I think using the engine heat to generate hot water is a great idea. Have seen other overland rigs with similar systems tapped into the heater hoses. They used the temperature slider on the dash to control the water temperature.
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Old 12-04-2016, 03:03 PM   #6
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Maybe I should be doing a yearly springtime water test. I use the on board water for the shower/washing but I was led to believe you'd be able to detect it easy. Never really thought of a small leak. They make water test kits
Antifreeze Detector (Water Only) (1 Test)

I wonder if the air gap type flat-plate type exchanges would do better if they were enclosed in an insulated box? Are they much larger?
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Old 12-04-2016, 03:27 PM   #7
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From Double Wall Brazed Plate Heat Exchanger Manufacturers | Fabrication, Design, Consulting | AIC - L-Line

L-Line DW Double Wall Heat Exchangers

Double Wall Brazed Plate Heat Exchangers - With visual leak detection

The design of the double wall brazed plate heat exchanger prevents cross contamination of fluid streams if there is an internal leak within the unit.

Two stainless steel plate walls separate the fluid streams, with an air gap in between. In the event of a leak, the affected fluid flows into the air gap and escapes out of the heat exchanger unit, enabling visual detection of the leak.

Special corrugation plate patterns within the heat exchanger promote highly turbulent flow. Turbulance dramatically improves heat transfer rates and reduces the amount of deposits inside the unit.
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Old 12-04-2016, 04:54 PM   #8
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Worth looking into. Thanks.
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Old 12-04-2016, 05:32 PM   #9
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Thanks Eric and everyone else. That's great info. I guess I should beware but this is a budget project. My initial research showed that quality heat exchangers are several hundred dollars. If it goes over 2-300 for this entire idea I'd rather just put a simple, reliable propane HWH in my van. I thought about the one I linked since it's so cheap, but cheap construction always has it's disadvantages as pointed out above.

Anyway, if someone has done this or similar I hope they chime in. Meanwhile I've still got a few threads at Expo to read up on.

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Old 12-04-2016, 06:31 PM   #10
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I am using the same heat exchanger with 1/2" ports.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I was aware of the risks of a single wall exchanger when I bought it, but was cheap and wanted to make sure it would do the job before spending a bunch of money. I did put a check valve in front of it and was only using it for shower and not drinking water. Although with some current changes it can supply hot water to my sink now.
With that said this heat exchanger has no problem heating water, in fact it works so well I installed a tempering valve after it to keep from scoulding people with 145 degree water. It has been in use for a year now.
I plumbed the coolant lines it to the back using silicone heater hose, I used 1/2" copper to go through the floor of the van and to connect to the exchanger. My entire water system is inside the van for winter use.
Idon'thave rear heat so to 'T' in to the front heater hoses and to keep the coolant from flowing through it when not needed I installed this bypass valve:
http://www.amazon.com/dp/B000C9DDIA/...I3QWQN21X9T5BV
It is vacuum operated so I used this actuator to operate it from a standard 12v switch:
http://www.amazon.com/Dorman-600-104...JBT1DRTX21X6N1
So far everything is working great.
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