In serious hot water

Devising a multiple-input system for producing hot water

Tierra del Fuego, Land of Fire, might have a warm name, but the images of glaciers and penguins give me the chills. We are sailing our way toward the southern tip of South America, and while my husband, Patrick Childress, sees this as a great adventure, I am more practical and first require heat and hot water on our Valiant 40, Brick House.

To warm the air, we have installed a radiator “car heater,” which transfers heat from the diesel engine cooling system and blows warm air in the main saloon when the engine is running. A wood-burning stove from Cubic Mini Wood Stoves was also installed in the main saloon for heat at anchor.
 
Cold-water showers in the equatorial tropics are quite a different sensation than those near Antarctica. Our original water heater got very hot when the diesel engine ran for a long time, or when plugged into 120-volt AC shore power. But going forward, it will be very rare to have any shore power, and it is unreasonable to run the engine just to heat up water. Nearly any original component still on our 44-year-old sailboat has long outlived its expiration date; with every passing day, we felt our original water heater was one day closer to a bursting disaster.

It was time to update our water heating capabilities. Traditionally, marine water heaters work by either heating an element with electricity, or by circulating hot engine coolant through heat exchanger coils inside the water heater. But why not have the ability to heat water from the cooling system of the diesel engine, shore power, and also multiple ways from our 12-volt system? Why not utilize free excess energy from our wind generator and solar panels to heat water? We had just installed a new Freedom Won lithium battery bank, which gave us more usable energy due to more efficient charging.

When I contacted various water heater companies, the only one that was receptive to a 12-volt element being installed in their heater was Torrid Marine. They were keen to assist with an off-grid water heating solution, and were already making hybrid water heaters — the Explorer Series. The Explorer can have dual electrical elements, and can even be used in combination with a hydronic diesel heater or stove equipped with heating coils, passively capturing heat without any additional power being consumed.
 
Torrid Marine set to work with svHotWire.com, which supplied a 12-volt DC/120-volt AC element, and built us a custom water heater with it. The Torrid Marine heaters are glass lined and foam insulated with a stainless-steel outer casing, and that’s the durability we required. With more research, and by talking to other owners of Torrid Marine heaters, we knew working with this highly responsive company would be worthwhile.

A schematic showing the elements of Brick House’s new hot water system and how they interconnect.

Going down in capacity
Our old water heater was 11 gallons, but we purposely bought Torrid Marine’s smallest unit, simply because we practice water conservation anyway; six gallons gives us at least three showers. In addition, six gallons would be much more efficient to heat using the off-grid methods we had planned. Torrid Marine also explained that hot water rises — just like hot air — and since the outlet is at the top, the first bit of hot water effectively floats to the top and flows out of the outlet first while the rest of the water heater is still heating up.

After going through many details with Torrid Marine, the water heater was shipped to us in Africa as quickly as if we were in a marina in the U.S. We received it just three days after it was built, which is unusual in Africa.

The first step was removing the old water heater and installing the new one in its place. Not wanting to buy a propane torch and soldering supplies for a small one-time job, we hired a plumber for an hour to make fast work of connecting one pipe and supplying the one needed hose adapter. Everything else, we did ourselves; improvisation like this is a mandatory skill for cruising the world.

Then, the electrical connections began. We first connected the heater direct to 12 volts with a 25-amp marine switch to enable hot water on demand (running for two hours, our batteries went down a total of about 40 amp-hours). Since we do not have access to 120-volt power, (most countries in the world use 220/240 volts) nor a built-in marine inverter or transformer, we left the 120-volt input disconnected. Rather than make a fatal mistake of zapping the system with 240 volts one day, it seemed best to leave it out of the system for now.

Running the engine produced hot water, as did the direct 12-volt connection. All was good. The easy part was complete, and we could have stopped there and sailed away happy! But I’ve always wanted our extra wind and solar energy to be put to good use to heat water, rather than just dumping power when the batteries were fully charged.

A digital thermometer with a remote probe was installed to monitor water temperature.

Using load diversion
I decided to add another, slightly more complicated, but more gratifying way to heat hot water via load diversion. Most, if not all, load diverters are simply charge controllers set into a different mode than traditional charge controlling. Most controllers do just one of three jobs: charge control, load diversion or load control — i.e., low voltage disconnect — but not all three. A more technical description of this really is that there are basically two good ways to control battery voltage with wind and solar systems using the TriStar pulse width modulation (PWM) charge controller. With solar panels, the circuit can be turned off when the battery voltage reaches the maximum. What happens with most solar power systems is the controller turns on and off rapidly, changing the amount of time the circuit is off during each cycle (or pulse). With wind generators, we need to keep an electrical load on the generator, and so wind generator controllers usually divert the excess energy into an electrical load like that of a water heater. Charge controllers that divert the excess are called diversion-type controllers. Solar systems can also use diversion-type controllers, but wind generators cannot use PWM controllers, except when in load diversion mode.
 
We installed Morningstar’s TriStar 60-amp PWM charge controller in load diversion mode with a simple change to the DIP switches on the unit itself. We installed it under the bunk, away from the water heater and batteries. We ran appropriately sized cables from the diverter to the battery terminals. The controller was installed as per the Morningstar instructions with the addition of one component.

An additional component
One wire run was modified, and that was the negative wire from the wind generator. That wire still connected to the negative battery terminal, but it was run through an 80-amp isolating diode on a heat sink first. This would mean that in order for the controller to be energized to do any diverting, the wind had to be blowing; otherwise, the controller was off. Because the isolating diode only allowed power to run in one direction, it caused the controller to only be “on” and diverting when the wind generator was spinning. If we didn’t install this diode, all excess power for all charging sources, even including the alternator, would always be powering the hot water heater element — and the water could get very hot, very fast, with no way at all to shut it down, short of powering off all chargers. Now if we turn off the wind generator, or if the wind stops blowing, no more power is diverted because the diverter is not powered up.

We didn’t place a thermostat on the 12-volt side of the hot water heater because when the battery voltage is high enough, the generated power needs somewhere to go or else the batteries could be overcharged. The battery management system (BMS) on our lithium batteries would, of course, shut the batteries off if the voltage got too high. But, if the thermostat had also shut down the water heater, there would be nowhere for the wind generator power to go to, no battery and no load. This could possibly harm the generator, or more likely the TriStar controller itself.
 
The BMS units are designed as a final attempt to protect the batteries in an emergency, an absolute last line of defense only. Without a thermostat, the hot water heater would keep heating and heating, and then finally pressure release the hot water, soaking everything in that compartment, including the electrics of the water heater itself. Short of an alarm and closely monitoring the temperature, or possibly using a thermostat to switch the diversion to a separate resistive diversion load like a small heater, there seemed no perfect solution.

Experimenting with voltage
To finish the installation, we routed wires to power the hot water heater load from the controller (see diagram). When the batteries become fully charged to the preset voltage on the TriStar diverter, those wires are energized to heat the water. We experimented extensively with the best voltage to set the TriStar to divert. We studied what voltage our lithium batteries would sit at with full solar, as well as 10 to 20 knots of wind, and determined that a setting of 13.8 volts would work best, while all our other charger controllers/regulators on board were set at 14 volts. This would cause each charger to continue trying to charge the batteries to 14 volts — which is the battery manufacturer’s recommended charging voltage — but nothing would ever get to 14 volts because the controller, when on, would continuously divert power at 13.8 volts. In practice, 13.8 volts is not achieved until the batteries are about 97 to 99 percent charged; so this, for us, is a perfect time to start making hot water. We will only be switching diversion on when sunny, windy weather is expected and when we need hot water, so the batteries still get to 14 volts to complete their cycle often. Sound complicated? It took a bit for us to figure this out.
 
The off-grid charging solution on our boat is small compared to those on many sailboats, and I was always skeptical that, with this little power, we would be able to make hot water with our 12-volt system. Our wind generator produces 300 watts maximum, and our one solar panel 265 watts maximum. We realized two things: 1) That the wind alone would rarely blow strongly enough and consistently enough to produce hot water, and 2) Sometimes, the wind and solar would be enough to get some hot water! So, using the same negative wire from the diverter to the battery, we bypassed the diode by installing a second negative cable on the back of the diode onward to the negative battery terminal. We then installed a selector switch so that we could essentially “divert all” charging sources using that negative wire that bypasses the diode.
 
Come to find out, we were a little astray on what the diode was really accomplishing. If we switched to go through the diode, the only thing that can power the diverter is the wind generator. Once it’s powered, however, we believe it then diverts all extra power over 13.8 volts to the load, not just power from the wind generator. In other words, the diverter keeps the batteries at 13.8, and anything extra that it has coming from any charging source goes to the hot water heater with no discrimination as to where that extra voltage is coming from.
 
We thought originally it would only divert power from the wind energy when more than 13.8 volts. We only learned of our erred thinking when we experimented by setting the controller to divert at 13.2 volts — our batteries’ resting voltage for most of their state of discharge — while the batteries were already at nearly 100 percent state of charge (SOC), which is 14 volts. It was then switched to wind, while blowing a very minimal 10 knots, and it diverted power from all charging sources, including all the power it wanted from the batteries themselves, resulting in more than 20 amp-hours draining from the batteries every hour, even when only five or seven amps were recharging from the wind.

Brick House’s previous 11-gallon water heater.

Diverting any power
So, it was clear at this point that it was not just diverting wind power — it was actually diverting any power it could find, with even the battery itself as a power source. This is due to lithium batteries having such a flat discharging curve; to keep the batteries at 13.2 volts, you could easily take 50 amps per hour for four or five hours from the 400 amp-hour total, and you’d still be at 13.2 volts (though SOC may fall from 99 to 40 percent). The same would not be true of lead-acid batteries at all, as their voltage steadily declines with use. If my lithium batteries are at 40 percent SOC (i.e., 13.2 volts), I don’t want to be making hot water. But if my batteries are at 99 percent SOC (which is also 13.2 volts) I definitely do want to be making hot water.

Morningstar, which makes the TriStar controller, explained that the diverter is simply a voltage reader/battery voltage maintainer. It cannot discriminate where power is coming from; it just diverts all power from the sources it has that isn’t needed for keeping the batteries at the specified voltage. The only thing that the diode does is depower the controller when there is no wind input and the selector switch is set to “Wind.” We can depower the wind generator and set the switch to “Wind,” essentially shutting off all diversion. We could not do that without the diode and selector switch. Regardless, we do need to be vigilant to not run the engine or plug in shore power with the battery selector switch on to “Divert All,” which would heat the water too much if unmonitored.

Right now, we manually turn the water heater on and off in anticipation of needing it, and we have installed a digital thermometer with a remote probe so we can easily monitor the temperature. We also have multiple timers with alarms so we don’t forget to look at the thermometer and switch things off. Positioning the selector switch to “Wind” by default prevents the system from working except when we are purposely and protectively monitoring it. This is equivalent to being diligent and frugal when using any system on a boat, such as propane.

We received great technical support from Torrid Marine, svHotWire.com, altEstore.com and Morningstar, and this project never would have been possible without these companies answering endless questions. I thank them all!
 
It warms my heart (and my body) to have at least six ways to get hot water now for our trip to the fjords of Tierra del Fuego!

Rebecca Childress is a regular contributor to Ocean Navigator and lives aboard the Valiant 40 Brick House with her husband, Patrick. They are currently in South Africa. Patrick produces a cruising video series available on YouTube.