Hot Water Heating Systems

March 11, 2013 · 4 comments

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Domestic hot water will account for about 30% of your home’s energy footprint. Only a few decades ago there were very limited options for the off grid home’s water heating system. Back then the only options were a gas hot water heater, some form of wood boiler or the often high maintenance solar hot water systems.

Recent advances in technology, the availability of more efficient appliances and drastic price reductions give the solar homeowner more options than ever before. However with more options comes more confusion and more chance of making the wrong decision.

First, it needs to be mentioned that you will not be able to use electricity for water heating. Home produced electricity is expensive and limited. Unless you have a massive solar system or wind turbine, you will need some other form of water heating system.

The second thing to consider is that it might make sense to use what you have and add to it if possible. If you already are operating a wood boiler, why not add a heat exchanger for your water heating. This is called a combination heating and hot water system. Most homes have some type of boiler for home heating and another system for water heating. It makes no sense to operate two complete systems and keep two large tanks of water hot when you could combine the two and only have one.

Our favorite combination is the open-direct system using a Polaris gas water heater.

Let’s look at the options:

Solar Water Heaters

As we are all doing our best to reduce our energy consumption you would think solar hot water systems would be the most popular by far. It’s not true, at least right now. The majority of “off the grid” solar homes use other options for heating their water. Solar hot water has traditionally been very complicated and high maintenance making it not the popular choice you would think it would be. Hot water collectors leak, pumps fail and issues like freezing seem to plague these kinds of hot water heaters.

Another issue with solar hot water is the fact that they perform poorly in the spring, fall and winter. Almost everyone who has installed a solar hot water heater has had to rely on some form of backup making the entire system expensive. If you already have a functional hot water system it might make sense to add solar to reduce your dependence on fossil fuels. It is not impossible to design a reliable system to produce 100% of your hot water needs but it is expensive. Learn more about solar hot water systems and how to install them click here.

Wood Stove Hot Water Heaters

You might be surprised to find out your standard “off the shelf” wood stove can be converted to produce unlimited hot water in addition to home heating. If you are using a wood stove for space heating in the winter months it makes a lot of sense to add a hot water front or coil to your wood stove. Using your wood stove for water heating can be a great compliment to your solar hot water heating system as it will heat water during the colder months, when the solar doesn’t perform so well. Learn how to heat your water with your wood stove.

Wood Boiler Water Heaters

It is very common for the off grid home to use some type of wood boiler for home heating. Some use outdoor wood boilers and some use the standard inside wood boiler but both are awesome for water heating. When purchasing your wood boiler look for one that includes an extra coil just for domestic water heating. Usually the extra coil mounts somewhere on the side of your boiler and is located inside the water jacket. They are a very common option for most types of wood boilers.

If you cannot locate a proper water heating coil for your wood boiler, you can add a heat exchanger. A heat exchanger is a unit that transfers heat from your dirty heating system water to your clean domestic water supply. The two fluids never mix, they just transfer heat from one to the other.

If you heat your home with a wood boiler, solar hot water might be the best choice for you. The wood boiler can take up the slack in the poor solar months. Tips for heating your water with a wood boiler coming soon.

Propane and natural gas have been the most common methods off grid home water heating since the beginning of solar homes. It is not that we like fossil fuels, but they are convenient and have the ability to store large amounts of energy in a small space.

However it is not wise to just buy the cheapest gas water heater you can find. There is a huge difference in efficiency and reliability between various gas water heaters. That $200 you saved buying a hot water heater on eBay will be irrelevant when your wife is trying to clean poop off your child and there is no hot water. You will pay in the end.

Gas Boiler (Propane and Natural Gas)

When most people imagine a gas boiler they picture some huge industrial machine that fills your entire living room or a small box that fits on the wall. Today’s gas boiler can be either. There is a huge variety of propane and natural gas boilers in North America. Their efficiencies can very from lows of 40% to highs of 95-96%. Imagine buying a boiler only to dump 60% of the heat out the chimney. Yuck! It happens every day.

There are two ways heat is lost in a gas boiler:

  1. Heat going up the chimney, vent or flue
  2. Heat lost though the walls of the boiler

You can do a quick do-it-yourself efficiency assessment by looking at the venting system. Double wall steel vent or chimney is bad. Two (2) or three 3 inch plastic pipe is probably good.

Gas boilers can also use any amount of electricity from none (pilot operated) to many kWhs per month (glow plug, fancy controls and high capacity fan).

The typical home gas boiler looks more like a hot water heater than a boiler.

You must do your best to find a high efficiency boiler with a very low electricity requirement.

We recommend the Polaris condensing hot water heater/boiler by American Water Heaters as it is very reliable, only loses 4% out the vent (96% efficient) and operates on very little electricity. The Polaris is large enough to heat most homes and the domestic hot water making the system very simple, space saving and efficient.

If you already are using a gas boiler for home heating, you could add a heat exchanger as your primary water heater or setup something like an open-direct heating system.

Gas Water Heater (Propane or Natural Gas)

The gas water heater is not much different than a gas boiler. A tank style gas water heater looks much like an electric tank water heater but has a flue or venting system. The typical gas water heater is about 50-70% efficient and is built as cheaply as possible.

Like the gas boiler, there are two sources of loss in the gas tank water heater:

  1. Heat going out the vent, chimney or flue
  2. Heat leaving the tank through the walls (sides and top) of the tank

You can still buy a propane tank water heater that operates from a pilot and has no electricity requirements. This is the most common hot water heater used in the off the grid home and it is unfortunate as a lot of the gas’s heat goes out the flue or vent.

The better choice is the condensing hot water heater that can be up to 96% efficient. They look the same but can save a substantial amount of gas. As mentioned above, our favorite is the Polaris as it loses almost nothing out the vent and even less through the walls of the tank.

Gas Tankless Water Heater

The propane/natural gas tankless water heat, also known as demand or instantaneous hot water heater, has gained a lot of favor by off grid homesteaders in the past decade or so. If you are not familiar with them, they heat water as the water passes thru rather than storing a large amount of hot water in a tank.

They are much better than the standard tank style water heater as their efficiency is usually in the 70-85% range. This is accomplished as there is only one source of loss which is the heat going out the flue, or vent. As there is no tank of water, there is no heat lost thru the walls of the tank.

But is 70 or 80% acceptable? Maybe.

The tankless gas water heater was our recommendation for over a decade until we discovered the condensing gas water heater which is much more efficient. However the tankless hot water heater might still be a good choice for you. There are many variables when choosing a tankless hot water heater. Learn how to choose the correct gas tankless water heater.

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{ 4 comments… read them below or add one }

Carl March 11, 2015 at 12:35 am

Hi Jody,

Being as brief as possible, I am off grid Sw Oregon and needing a better way to produce hot showers then current wood fired/propane setup (now failing). Current house juice is mid 90’s tech, you know, 12 volt panels, pwm controller etc, Outback inverter. Primitive. Cloudy winters, but drought getting pretty bad. No snow

So, thank you so much for the excellent info you have provided on choosing elements, found nowhere else. Have figured all this as well as done the math on how many watts of panels needed to produce so many btu’s. Need real simplicity (no pumps, etc.) and freeze resistance so my only choices are a passive evacuated tube (so freezing not too dangerous) system or buying a new string of pv panels plus tank. Not talking about heating as a dumpload or as an adjunct to my current pv system.

My memory is (figured this last week) that 1500 watts of panel would put almost the same # of btu’s into tank as a 25 sq foot thermal panel would. Price would be about same figuring $1/watt but pv would take up 5 times the area.

Problem with thermal is have looked at both Dudadiesel and Solarbank and really feel pretty wary about long term reliability here. Tubes lose vaccum at unspecified rate but clear that in 10 or 15 years you’ll notice it and what are chances you’ll be able to get new tubes? And other murkiness. Tanks, hidden Chinese manufacturers. Bah

Plus, both firms are skittish. They’re dancing. They are only choices for passive evacuated tube though. And even if one was willing to settle for seasonal use of a flat panel system such don’t seem available in States tho haven’t looked real hard.

Sorry, failing in being brief. Am writing you because you told a questioner that you could sell him the required dc thermostat and perhaps switches, so if that offer still open please inform. Found a thermostat elsewhere but looked kind of cheap so maybe since you have looked at this subject you have a better one.

And to the main point of this letter. –You told said questioner only that he would be very pleased with the results of hooking a 120 volt string directly to a 120 volt element. like I said, the math seems to bear this out. I’d probably be up around 150-190 volts if can afford more then 4 panels after buy tank so all to the better.

My request to you is could you elaborate on this to me, if possible. I.e, do you have any direct experience of this and if so what happened under what climate conditions? Also, can you make any comparisons to thermal under same conditions?

Math ain’t everything. If Pv direct connect really works and is practical seems like more people would have done it but finding no tales. Of course, if one is grid tie, uses a heat pump and can sell excess back to utility in summer it’s a slam dunk but we’re not on that page, thank God. High-five emoticon

Obviously I’m a little overly frustrated here. Hope you found this rant mildly amusing or at least not a total waste of time. Would appreciate any ruminations but if not, that’s okay too, and good day to you. Smile emoticon

Carl Henson
Cave Junction

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Jody Graham March 11, 2015 at 12:40 am

Hi Carl,

I am so sorry for the long delay. The reason I operate this website instead of a real job is I have a chronic illness that can leave me useless for weeks at a time. You have probably forgot about me by now. There is not alot of info out there about solar photovoltaic (pv) hot water systems. I have a 30 evacuated tube system but it is a total pain in the butt. There are always controllers/sensors not working or the pump not working or the pump staying on or freezing if you do not have enough antifreeze. I will never waste my time with solar thermal again. Too much maintenance. The only experience I have with heating water with pv is in dump load situations. The batteries get full and we dump the extra power into hot water. It is very simple. When solar modules were $16 per watt which was the cost of my first 720 watt array in 2001, using pv was far too expensive.

But with solar below a $1 per watt I think we really need to start designing hot water systems with pv. My solar hot water collector cost around $2000 for approx. 3600 watts of heating capacity. Add the piping, an expansion tank, pump, controls, antifreeze and heat exchanger and you could easily add another $1500 to the cost if you pay no labor. That is pretty close to a dollar per watt. With solar pv we would need to add some wring, heating elements and some way to either disconnect the solar modules when the water gets too hot or dump the hot water. The easiest by far is to dump the hot water. Using a cheap off the shelf AC aquastat (thermostat for water) you could simply open an AC solenoid valve that would dump the water down the drain before it boils. Another option is to bury a water loop outside. Using a cheap AC circulating pump and AC aquastat you could dump your hot water by the pump circulating it thru the ground. These two systems would allow you to connect your solar modules with no fancy dc stuff except a fuse or breaker and use off the shelf AC equipment to protect against boiling.

You could also disconnect the modules when the water gets too hot as in the drawing below:

http://solarhomestead.com/?attachment_id=2286

In the above drawing, the big power goes thru the big wires. The aquastat doesn’t see more than a few milliamps as that is all it takes to operate a solid state relay. I generally NEVER use an AC component for DC but in this application they seem to last forever. I do have a lot of experience with that.

When buying modules the goal would be for the mppv voltage (maximum power point voltage) of your modules to add up to the voltage of the AC/DC heating element. If you have a 2000 watt 240 volt element try to get modules that have an mppv that adds up to 240 or close. That way you will not need any mppt controller at all on the solar side. Just connect the solar modules directly to the water heating element.

I assume you have read these but here they are anyway:

http://solarhomestead.com/dump-loads-for-solar-wind-and-microhydro/

http://solarhomestead.com/dump-load-diversion-load/

http://solarhomestead.com/ac-elements-as-dump-loads/

Hope I answered at least something. Please keep up the fight and make something happen. We went off grid 15 years ago here when everyone thought we were idiots and most still think we are. But it is a way of life I don’t want to change. Sure I wish I could have paid today’s prices for the solar modules but I have gotten 15 years of power out of them which has to count for something. We didn’t even have a real phone until this week when a new cell tower was put up. We don’t rough it really we just don’t waste as much as most.

Cheers,

Jody

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Owenclont April 19, 2013 at 8:15 am

Thank you. It is hard to find a website that can explain things in a way that I can understand it. Most solar websites are either very technical or just have the same info everyone else has. It is obvious you have experience living off the grid.

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Jody Graham March 11, 2015 at 12:33 am

Thank you for the kind words. I really appreciate it. Jody

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