We use a modern wood cook stove to heat our home, cook & bake our food, and heat our water (Learn how we heat our water: Hot Water for FREE). Can't imagine how we lived without it. But then summer arrives...What then? One could always cook with a propane or solar oven, but what about hot water? This is an issue we just faced this last summer. In this article we will look at the different options that are available. We finally found and tested an inexpensive solution. It requires very little additional work to install and functions well during the times we need it most.
The problem with heating hot water is that many homes are utilizing a standard 220 volt electric hot water heater that consumes massive amounts of electricity. When you live off the grid, this is a problem. It would require a massive renewable energy system that would be cost prohibitive for most people. And there is no reason why one would need to use such a water heater. There are many other options.
A standard propane water heater would work just fine, as long as one has propane (obviously!). Better still would be a propane tankless (a.k.a. on-demand) water heater. This would be a much more efficient option requiring significantly less propane.
But our goal is to become as independent as possible. This means a water heater that is powered by something we don't have to buy.
So let's look at some truly independent options...
There are many options for solar water heaters that range from inexpensive home-made jobs to high-end state of the art commercial models.
The home-made options are typically not suited for use during times of the year when temperatures dip below freezing. You would need to winterized them before any threat of fall or winter weather arrives. There are probably hundreds of variations that may be found on the internet. They range from coils of black plastic pipe on one's roof to coils of copper inside a box with outer glass cover (creating a greenhouse effect). Some of these utilize a DC electric circulating pump. It pushes the heated water into a storage tank and bring fresh cool water into the heater. This pump may be activated by a switch that is temperature controlled. But some are directly connected to a solar panel and only run when the sun is shining (precisely the same times that the water is being heated and needs to be circulated).
Commercial units come in two main varieties--evacuated tube and flat panel. Evacuated tube models are composed of numerous rows of glass tubes with another smaller tube inside each of the larger tubes. A vacuum is pulled between each of the two tubes which greatly lessens the loss of heat to outside weather. Water or an antifreeze solution is then circulated through the inner tubes. The advantages of evacuated tubes lie in greater efficiency and less heat lost. So they work quite well even during surprisingly cool temperatures. However they are somewhat fragile and may not be as suited to areas with a great amount of snow and ice due to the potential for breakage from buildup.
Flat Panel Solar Heaters
These heaters look much like solar electric panels but contain tiny water passages which enable a great amount of surface area to come in contact with the heat from the sun. While not as efficient as evacuated tubes, flat panels still work very well and are certainly more rugged when faced with heavy snow and icy conditions.
Typically an antifreeze solution is circulated through the tubes or panels and the hot fluid is routed inside to a heat exchanger which transfers the heat of the fluid to the home's hot water. There are some varieties of evacuated tube heaters that are called drain down systems, which do not require the use of antifreeze because the water drains away from exposed areas once the water cools off. And just for the record, it is possible to set up a solar water heating system that doesn't require an electric circulating pump. This would be called a thermosiphon system, operating on the principle that heat rises.
This was the type of system I was planning on installing for summertime use in our cabin. The disadvantages would be the amount of work involved in installation, significant cost of commercial systems, and reliance on an electric circulating pump (unless it is set up as a thermosiphon system). For us, being in a temporary location, I was hesitant to purchase and install the equipment and then be faced with tearing it out again in a year or two.
Our Final Solution
That brings us to the option we chose. Our winters are cloudy with short days and it takes a substantial solar system to make any significant impact on power usage during the months of November, December, January, and sometimes even February. Since we have a solar system that is appropriately sized, we have plenty of excess electricity during the long, hot, sunny days of summer. Often our batteries are fully charged each day by noon or even before and that means we were wasting tons of power for the rest of the day. What to do with all that wasted power? Why not use it to heat water? So that is what we did.
DC Heating Element in our Range Boiler
I purchased a DC 12 volt water heating element that can burn either 30 or 60 amps (depending on how it is wired). There is also a 24 volt model available. Currently I have a water coil in the wood cook stove that is plumbed into a heavy duty range boiler tank and circulates without any electric pump as a thermosiphon system (read all about it here: Hot water for free--from the wood cook stove!). It just so happens that there is an additional threaded port in the middle of the tank. Our DC water heating element screwed right into that port.
I then purchased an inexpensive Schneider (used to be Xantrex) C-60 charge controller. This unit may be operated either as a conventional charge controller (disconnecting the charging source from the batteries once the battery voltage reaches a certain threshold) or as a diversion load charge controller (once the batteries reach a certain threshold, any excess power is diverted to a diversion load and "burned off"). For applications using solar panels to charge the batteries, the conventional charge controller mode would be used.
Once the batteries are full and voltage reaches the set point, the charge controller disconnects the solar panels from the batteries. But for our application, we will use the diversion load mode. We have excess power that we want to burn off and that is exactly what a diversion load charge controller accomplishes. The charge controller is connected to the battery bank by wiring it into our system's DC disconnect box, which adds the safety of breakers to the system.
I then purchased 2 strands of 2 gauge copper wire to run from the charge controller to the water heating element. The size of wire you use will depend on the distance to be covered, whether you wire the water heating element to run only one or both elements, and whether you use the 12 volt or 24 volt model. If you are not familiar with properly sizing wire, have the company you buy your heating element from assist you. Here is an online calculator.
Since there are two heating elements on this unit, it may be wired to run only one or both. This will consume roughly 30 amps or 60 amps respectively. To use both elements, making them one 60 amps element, simply make sure that the connecting bar is connecting both elements together. If just one is desired, simply remove the connecting bars and connect the electrical wires to one or the other element. Our unit is set to run both elements for a maximum of 60 amps.
Next we need to set the charge controller. Since I still have my main charge controllers connected and operating with the system, I need my new diversion load controller to start diverting power to the water heater BEFORE my main charge controllers reach their threshold and begin disconnecting the solar panels from the batteries. If this step is not taken, your diversion load controller will never divert power to the heating element. This is something you will need to play with for a while, but eventually you will find a setting that works fairly consistently.
As the solar panels begin charging the batteries in the morning, our voltage begins to rise. As the batteries become more and more charged, we approach the absorption set point. This is the point at which our main charge controllers begin restricting power to our batteries and make sure the voltage does not rise above this set point. For a 12 volt system this set point is often 14.8 volts (29.6 volts with a 24 volt system). But since our diversion load controller is set to kick in at perhaps 0.1 or 0.2 volts lower than the main controllers, we divert all that power to the water heater.
If our solar panels are producing so much electricity that they are powering the water heater and still have more left, the voltage will continue to rise until it reached the set point of the main charge controllers and then they will begin restricting the flow of power into the batteries. When properly set, all the controllers operate together harmoniously.
We have found this setup to work quite well in our climate. There are only about 2 or 3 months during the summer when running the wood cook stove is uncomfortable. During spring and fall, we can build a flash fire early in the morning to heat water and cook, without making the house too uncomfortable. So our difficulty was during that 2 or 3 months of full summer weather. In this area, that is also the time of year that is very sunny.
We have found that by the end of the day, we have half a tank of hot water if we reserve the hot water for showers alone. Of course that means little or no dish washing with the hot water. The reason that only half the tank is heated is because of the location of the port on our range boiler tank. It is located half way up, so everything from that point on up is heated.
Few Extra Notes
I insulated around the tank to help the hot water stay warm during the night. However I must note that we do not have an abundance of hot water with this system. But it is sufficient for the two of us. And I might add much better than the alternatives (taking a cold shower or heating up the house on a hot day). It was also useful to close both valves on the wood cook stove's thermosiphon system. This prevents a potential backwards flow of hot water, thus diluting the hot water in the top with the cool water in the bottom of the tank. IMPORTANT - Be certain to open these valves before firing up the wood stove!!! Failure to do this could result in the water coil exploding in side the firebox.
One improvement that could help this system to operate significantly better would be to custom order a range boiler with an extra port near the bottom. This would enable the heating element to heat the entire tank. Mine only heats the top half. And the reason I say an extra port, is that you could still have to option of installing a second heating element in the location mine is currently installed. If you have a significant solar system (at least 2,000 watts), it is certainly possible to power two elements with surplus power on a sunny day. Customization of the range boiler tank is available when ordered from Stoves & More. That's where I purchased mine. My tank came with the standard configuration since I didn't know I needed a custom setup at the time. I also didn't realize it was possible to do so.
Installing good insulation on the range boiler tank would also help significantly. I added some makeshift insulation for a while and this definitely helped, but permanent heat resistant insulation would be much better.
Both of these improvements could go a long ways toward providing more hot water. Which is nice for a larger family or one that uses more hot water than we do. And while this system probably does not produce as much hot water as a good solar hot water system, it works and was a much easier installation. Also it costed us far less than a solar hot water setup as well. That was because we already had the range boiler tank plumbed in. So if you find yourself thinking of ways to use your extra power and need some hot water, give this a try!