Hot Water: Because It’s Warmer Than Cold Water

Hard core outdoors people are tough.  They’re hard.  They sleep on the ground with no sleeping bag.  They keep warm by coating their skin with grass clippings and animal blood.  They have weather beaten leathery faces.  They drink their own urine and eat the still-beating hearts of the animals they’ve killed with their own bare hands.  They’ll kick your ass.  They don’t take showers, but if they did, they’d be the coldest showers that nature would allow.  Yes, their showers would be cold like the snowy landscapes that they wander with minimalist uninsulated footwear.

We on the other hand, are soft.  Our palms are doughy and smooth like babies’ freshly powdered behinds.  We don’t eat food unless it came from an FDA-approved sterile manufacturing line.  Our sparkling mineral water must be carbonated only with the carbon dioxide from a very specific volcanic vent in the Tuscan countryside.  And our tap water must be hot.  It must be so hot that the skin on our doughy palms stings at the very thought of it.

Okay, none of that is true, but you have to admit that warm water is a pretty nice luxury, right?

RVs often have hot water systems, but they’re powered by propane or natural gas.  We have a small propane tank, but it’s used for cooking, and we’d be filling it up constantly if we were also heating water with it.  Also, the availability of propane around the world is a concern.

As it turns out we’ll be driving around in a little home on top of a machine that generates immense heat and is propelled by thousands of explosions every minute.  Those explosions cause the engine to get hot, so the van has a built in coolant system to cool it down.  Eureka! A heat exchanger that draws heat from the engine was the obvious answer to make hot water.

I decided that the heat exchanger would sit in-line with the coolant coming from the engine.  There are two sets of coolant pipes running under the van; the first goes to the front heater, the second goes to the radiator.  The radiator line would be nice because it carries a larger volume of coolant so it would heat up the water faster, but coolant only flows to the radiator after the engine has reached operating temperature and the thermostat has opened.  Not good for on-demand showers.  I decided to splice into the heater line so we could start heating water right when the engine is started.

To build the heat exchanger I coiled 10 feet of copper tubing and suspended it inside of a 3” copper pipe.  The large pipe was fitted with reducing fittings so that it could be spliced into the heater line, while the smaller copper tube was given fittings to fit the water line.

I slid the whole contraption into a double-walled stove pipe section, insulated, and capped.  The result looks a little bit like a fancy racing muffler.  Or a futuristic flame thrower.


The whole assembly was spliced in, and a shutoff valve was put in place in case we ever wanted to divert flow around the heat exchanger.  I don’t know why we’d ever want to do that, but it’s better to over-engineer.  Water comes from the bottom of the hot water tank, through the van floor, through the heat exchanger, and back into the top of the tank.

Okay, we’ve established that there is a heat exchanger, but how does one operate the thing?  Here’s where the other half of the system comes in.  Before I installed the hot water tank, I installed a thermocouple probe through the tank’s side near the bottom.  The thermocouple measures the temperature of the water.

To display the water temperature I used a PID temperature controller I had laying around.  I mounted the controller in the headliner above the rearview mirror.  Now, with a flip of a switch, we can see the temperature of the water in the hot tank while we’re driving.


If we decide that the water needs to be warmed, there is a second switch next to the PID that controls another solenoid valve.  This valve, when energized, creates a closed loop between the hot water tank and the heat exchanger.  When switched, water is continuously pumped from the hot water tank, through the heat exchanger, and back into the tank.  As we drive along we can watch the water temperature climb on the PID.  Once it reaches the proper temperature, we switch it off and we’re back to normal.  Except now we have 15 gallons of hot water for taking showers and washing dishes.  And rinsing traces of rich port demi glace off of our powdery smooth, doughy non-work-hardened hands.

I conducted a test to see how hot the water tank would get by cycling through the heat exchanger while Nacho idled in our driveway.  After 50 minutes the tank had reached 176 degrees Fahrenheit.  It was still rapidly getting hotter, but as we were quickly approaching the maximum temperature rating of our tubing (200°F) I opted to abort the test and call it a success.  Honestly, I wasn’t expecting it to work this well.


The added benefit of this system is that we’ve basically added a second radiator to our van.  Except that this radiator is superior because it’s water-cooled instead of air-cooled.  If you try to imagine what this does to the space-time continuum, your mind will get stuck in an infinite loop, so don’t try it.  Just know that when we’re climbing a steep hill and it’s 110 degrees outside, we can flip this switch and everything will be okay.  Unless we’re also being chased by Mujahidin assassins, in which case everything will probably not be okay.

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