Overview of Solar Water Heating Systems
In the United States, heating water takes up a major chunk of a building’s operating expenses. Nationwide, about 18% of energy use in residential housing goes toward heating water, but only 4% of energy use in commercial buildings.
Currently, solar-heated water only meets about 2% of the potential solar water heating systems for all residences in the United States.
Generally, you can use solar heating facilities at residences with a southward-facing (or near to it) roof or unshaded grounds to install a collector (to face the sun’s rays and absorb the solar energy).
In 2021, those states were the most popular for the solar heating market: California, Texas, North Carolina, Florida, Arizona, Nevada, New Jersey, Massachusetts, Georgia, and New York.
The New Jersey, Massachusetts, and New York mentions show that the solar water systems don’t always have to be in a sunspot!
Interestingly. On a worldwide basis, as of 2017, Barbados, Austria, Cyprus, Israel, and Greece are the leading countries of solar-heated water by capacity per person.
Are DIY Solar Water Heaters Worth It?
DIY means do-it-yourself, as opposed to hiring a construction company to do your work.
A solar water heater doesn’t connect to the electrical grid to collect power; it absorbs energy from the sun and feeds it back to the water heating system in your house.
To be clear, a solar water heating system only provides heated water and can’t power your electrical appliances, for example. So one option is to install a solar photovoltaic system instead, which would supply solar energy into your electric water heater.
Using a photovoltaic system means you can keep your regular hot water system, but you can do solar panels and solar water heating systems!
A solar heater uses a solar collector on your roof at an angle that best captures the sun’s rays and directly heats the water in your house.
A typical solar water heater can reduce conventional electric energy costs by about 40-50% and save money on your energy bills.
Understand that the solar water heater doesn’t eliminate grid energy—it acts as a backup, one that can potentially save a lot of money since you’re not paying for the grid energy that you no longer need when it’s sunny out.
The DIY solar water heater and the electrical grid run side by side, acting as partners, especially in warm climates where the sun is powerful for long hours each day.
DIY solar hot water systems indeed cost money to install; however, they can pay back their investment when you need less grid energy and hence have lower utility bills by saving on energy costs.
Once the investment has paid back itself with this free energy, you can enjoy nearly free hot water for the remaining lifetime of the solar water heater!
Plus, in the United States, Congress has offered an investment tax credit, known as the federal residential solar energy credit (keep checking as to availability—the latest indication is that it will expire December 31, 2024).
This tax credit is a dollar-for-dollar reduction in your income taxes. So, for example, if you get a federal tax credit of $1,000, and your federal taxes, otherwise owing is $1,000 or more, you get a $1,000 reduction in your federal taxes.
You can also apply for a state tax credit, depending on your state.
For more details on investment tax credits for solar energy, visit this website.
Solar panels are a different matter. Solar panels convert sunlight into electricity, which powers your house. Here, we’re just covering solar water heating systems.
The Economics of DIY Solar Water Heaters
DIY solar water heating systems are usually more expensive to purchase and install than gas water heaters or conventional electric water heaters.
However, using a DIY solar water heating system can save you money in the long run.
The economics of savings for solar water heating systems rely upon many different factors:
· Level of consumption of hot water
· Performance of the solar water heating system
· Geographical location and sun exposure and hence solar energy available
· Available government tax credits and financing
· Comparative costs of gas water heaters and conventional electrical heaters
Factors used to estimate the energy efficiency of a solar hot water system are solar energy factor and solar fraction.
The solar energy factor is calculated by dividing the energy delivered by the overall system by the amount of conventional energy input into the system.
If the factor is 5, that means the solar water heating system is pumping out five times more energy than the energy consumed by the gas or electric water heating systems that back up the solar hot water system.
Usually, the solar energy factor is 2 or 3.
The solar fraction is the proportion of the total hot water heating load provided by solar heating. So, naturally, it would range between 0 to 1.0.
Solar water heating systems usually have a solar fraction of 0.5 to 0.75.
What are the Different Types of DIY Solar Water Heaters?
There are many different types of DIY solar hot water systems. First, there’s the active solar water heater, which usually requires a pump. In contrast, we also have the passive solar water heater, which doesn’t use a pump; instead, it uses convection.
The passive solar water model allows hot water to rise and cold water to sink, so a pump isn’t needed.
There are two main types of passive models:
1. Integrated collector storage. These are also called batch heaters and use tanks that act as storage and solar water heating collectors (collector meaning that it exposes the water to the sun’s rays, thus collecting the sun’s heat). (They are affectionately referred to as “breadboxes” because of their bulky appearance).
2. Convection heat storage. Those are mainly flat-plate collectors, and the tank is located in the basement. Thus, the collectors and the storage tanks are separate. The storage refers to where the hot water is kept.
We will be focusing more on passive solar water systems because they’re cheaper and easier to install.
Further, we’ll focus on the flat-plate collectors’ next sub-category because they’re easier to build and install than batch heaters. It’s just good to be aware of the different possibilities of DIY solar water heaters.
The flat-plate collectors are also called thermosyphon systems because they rely on the premise that “heat rises.”
The warmer water rises to exit the system to heat the house, and colder water rushes in at the bottom through a lower pipe to replace the warmer water.
Gravity powers convective flow in a thermosyphon system. For best results, place the top of the collectors at least one foot below the bottom of the tank.
Greater height differential will result in greater flow. However, larger pipes, shorter runs, and gentle bends will make for an adequate flow rate.
If you require freeze protection, it’s not hard to do. The collectors can be filled with an antifreeze solution (propylene glycol is the most common and non-toxic).
You can transfer the heat to the domestic water via a heat exchanger.
Do DIY Solar Water Heaters Work?
Yes, but be mindful of the climate zone where you live. For example, thermosyphon systems do not work in cold climates where the winter is freezing because they do not have freeze protection.
In that case, you are best to drain the water from the solar water heater to not freeze during the winter. In this way, they are ideal for cottages or second homes that are not used in the winter.
Solar water heaters that meet the American National Standards Institute or the International Standards Organization will have protection at both extremes of temperature.
In the summer, automatic controls prevent the water from overheating.
In the winter, antifreeze systems use special, non-freezing fluid to run through the collectors and drain away from the heat from the collector into the tank so that no water is actually piped outside.
It is best to mount the solar water heater on the side of the roof that faces south and set them at an angle of 18º to 50º down from the horizontal plane for the best direct sunlight. That way, you maximize solar power using free energy.
To determine the size you need, consider 30 gallons of hot water per person in your household. If your family is very conservation-minded, you might get by with ten gallons of hot water per person. And you save energy costs too!
A typical 80-gallon electric hot water tank serving a family of four will consume approximately 150 million BTUs in its seven-year lifetime.
The hot water tank is usually sized to handle one day’s worth of consumption. For a family of four, an 80-gallon tank would supply 20 gallons of hot water per person per day.
Also, for solar thermal collector sizing based on your climatic region:
· In the Sunbelt, use 1 square foot of collector per 2 gallons of tank capacity (daily household usage).
· In the Southeast and mountain states, use 1 square foot of collector per 1.5 gallons of tank capacity
· In the Midwest and Atlantic states, use 1 square foot collector per 1.0 gallon of tank capacity.
· In New England and the Northwest, use 1 square foot collector per 0.75 gallons of tank capacity.
Based on these rules of thumb, a household of four with an 80-gallon tank will need approximately 40 square feet of the collector in Arizona, 55 square feet of the collector in South Carolina, 80 square feet of the collector in Iowa, and 106 square feet of the collector in Vermont.
For those active systems (and hence require a pump), a controller device attached to your solar water heater will sense direct sunlight and check if the water has reached the desired temperature.
When the controller is ready to stream hot water into the storage tank inside the house, it will activate the pump.
In the same way, in cold winter climates, if the controller senses the temperature will freeze the pumps, it will drain the water from the solar water heater through drainage pipes to outside.
The Important Components of a Solar Water Heater System
The major parts of a closed-loop system include solar collectors, circulating pumps, a differential control with sensors, heat exchangers, and a storage tank.
Lesser but essential parts include an expansion tank, pressure relief valve, check valve, drain and fill assembly, and pressure and temperature gauges.
Controller and Sensors
When the controller senses that one sensor is warmer than the other, it will energize a switch that can turn on a pump or other electrical device.
When there is no longer a difference in temperature or very small, the differential thermostat will turn off the pump.
In a solar hot water system, one sensor (placed to measure hot water) is on the piping very close to the outlet of the solar collector, and the other sensor (placed to measure cold water) is near the bottom of the storage tank.
When the sun comes up, the collector will become hot very fast. The collector’s sensor will activate the switch in the controller to turn on a pump.
When the sun goes down, dark clouds appear, or the storage tank becomes hot enough, there will no longer be a large “differential” between the two sensors. Then the thermostat will shut the pump off.
The exchanger transfers solar heat from the solar-heated closed loop to the domestic water. Factors that increase heat transfer are:
· Greater surface area
· High thermal conductivity
· Maximum temperature differential between the two fluids
Exchangers may be categorized as a single wall or double-wall, which refers to the number of barriers between the two fluids exchanging heat. For example, systems that use glycol should not use a single-walled exchanger because of the potential for contamination of the potable water.
An exchanger may be as simple as a copper coil within the storage tank, where single wall heat exchangers are permitted. Solar heated water is circulated through the coil with the help of a circulating pump.
Exchangers are installed in a counter-flow configuration whereby the two fluids flow in opposite directions through the heat exchanger. This maximizes thermal heat exchange by maintaining the greatest temperature differential between fluids.
The selection of an exchanger is based on its capacity to transfer heat (in BTUs per hour) produced by the solar collectors.
The manufacturer or distributor of the exchanger will specify which model is adequate, depending on the total square footage of the collector to be installed.
Manufacturers will also specify a recommended minimum flow rate on the waterside of the heat exchanger to achieve an adequate rate of heat transfer.
A check valve permits fluid to flow in one direction only. It prevents heat loss at night by convective flow from the warm storage tank to the cool collectors. Check valves may be of the “swing” type or the “spring” type.
An expansion tank allows the fluid in the closed-loop to expand and contract in the heating and cooling cycle. Without the expansion tank, the plumbing could burst when the fluid is heated.
How Do You Make a Solar Water Heater?
For this article, since there are so many variations of do-it-yourself solar water heaters, we’ll focus on the flat-plate model, especially the type that heats a pipe that runs in the shape of a spiral.
Design the Solar Water Heating Collector
If you’re inclined to pursue the do-it-yourself route, you can make your own collector. Soft copper pipes are ideal for heating the water inside the collector because copper carries water well and can bend to your liking.
Cross-linked polyethene (“PEX”) replaces traditional copper and galvanized steel pipes in home renovation projects and the mainstream construction sector. Here’s a video for the PEX option.
You can mount these pipes (made of soft copper or PEX or other materials) inside a handcrafted large insulated box and use a tempered glass pane as the cover to allow the sun’s rays to enter easily.
It would be wise to spray-paint the inside of the wooden box and the pipes to increase the absorption of the sun’s rays.
Here’s a sample video on how to construct your own flat-plate solar water heater. There are many other instructional videos similar in application.
Interestingly, the pattern used for the copper tubing in the video reminds us of a garden hose lying on the lawn. You’ll recall when you picked up the garden hose lying down for watering the lawn on a nice hot day, you felt the warm water as it exited the hose.
Imagine what the flat panel can do for the water inside the copper tubing when it’s on your roof! And keep thinking of saving those energy costs.
Another option, although more expensive, are the evacuated tube collectors. This is just another term for vacuum tubes, which work along with the similar operation of coffee thermoses. They consist of two layers of glass with a vacuum in between the layers.
In the evacuated tube collectors, the potable water is heated quickly as the tubes are structured to absorb the sun’s rays very efficiently.
Mount the Solar Water Heater Collector on the Roof
In general, any standard roof can support a flat-plate collector mounted on top. Check that the shingles and the sheet and truss system are structurally sound before attaching the collector box. If it’s a DIY solar water heater system, that could be a wooden box.
Attach mounting clips to the surface of the roof. Then you will hook up the collector box to those mounting clips. Finally, you should insert the mounting slip under shingles to protect the roof from sun and rain.
You should insert the lag bolt into a truss support beam for maximum strength. In addition, you should spread sealing material around any point of contact to prevent rainwater from leaking underneath the roof.
Flat-plate collectors can weigh between 100 to 150 pounds, so two people using ladders are often needed to lift it onto the roof.
The collector is then hooked up to the mounting clips and is secured into place, ready for action.
Insert Pipes Into the Roof for Carrying the Water Flow
Then you will drill two holes into the roof, one for the collector feed line (cold incoming water running into the collector) and another for the collector return line (hot water returning to the house to feed into the hot water tank).
It’s important to buy particular flashing—sheet metal or plastic designed to direct water away from the affected area—that fits around a pipe where it intersects the roof.
Usually, flashing for a pipe looks like a cone-shaped structure, with the tip cut off in a big way. Or you could use a cap instead.
As well, this cone-like structure has a wide collar at its bottom—giving you the impression it’s really trying to direct water away.
Once the flashing is secured around the point where the copper pipe enters the roof, you can finish off all touchpoints using a sealant.
For those who are careful as to unwelcome outside pressures on the solar water heater, it would be a good idea to add the following:
· Air vent (to prevent air from being trapped into the solar water heater)
· Freeze prevention valve (to open only when water temperatures hover near the freezing point, to drain the pipes, so they don’t burst when freezing)
· Pressure relief valve (to let out excess water should the water pressure be too high inside the pipes, and hence prevent them from bursting)
Insert Pipes from the Collector on the Roof to the Basement Tank
Now that we’re here, we can address the division of labour if you’re using a team. One or two people could install the collector onto the roof, while the other one or two work in the basement, installing piping and hooking it up to the hot water tank.
Quite often, soft copper pipes are used since they can run through the attic down to the basement without the need for soldering inside the attic.
Copper tubes inside the attic are sheathed with insulation to prevent heat escape.
Air ducts inside the house are often good companions to pipes since they already have a network through the house, including furnace ducting.
The water heater in the basement serves two purposes. First, it stores the heated water that has been piped down from the roof. Secondly, it will have a backup energy source (electrical heating) if the solar water collector is not producing any heated water.
The conventional hot water tank in the basement does not have the collector feed or return pipes required to exchange with the collector on the roof, so the plumber has to retrofit the hot water tank.
The installer will install the new hot water tank and solar collector piping. Once all the piping has been installed, you should cover it with fibreglass insulation material to prevent unnecessary heat loss.
It would help if you placed the new solar water heater as close as possible to the regular hot water system so that they can synergize in proximity.
The last step for the water heater is to set the thermostat to prepare for the automatic switchover from the solar water heating to electrical heating, as necessary.
Check out this video to see a good example of installing and preparing a water heater in the basement.
Set the Controller and Sensors Near the Hot Water Tank
We’ve already talked about the differential controller and what it does. Here, though, this controller must be installed near the water heater, and it comes with LED displays (light-emitting diode), digital readouts, and other options.
The controller has several settings. These include the high-temperature setting, the temperature differential, and the “off” setting.
As to another electrical item that must be installed, sensors come with their own wiring, and if they are routed outside, they must be protected from the weather with plastic or rubber sheathing.
The sensor wires can run along the pipes on the outside as they’re essentially sharing the same path anyway, which is more efficient.
You can use the thermal paste to provide a good bond between the sensor wires and the surface of the pipes.
The collector feed sensor is usually placed at the hottest point of the flat-plate collector to record the maximum possible temperature.
Set the Pumps and Valves Near the Hot Water Tank
There are many other critical components of the solar water heater, such as pumps and valves. One is the system pump (circulator) to circulate the system fluid (water or propylene glycol) from the basement tanks to the collector that’s mounted on the roof.
Valves that are important to the solar water heater are;
· Check valve (prevents the thermosyphon process from recirculating the hot water through the collector at night).
· Drain valve (if the collector needs to be drained, for example, if winter sets in and it’s not adequately protected against freezing).
· Isolation valve (since its only possible options are fully open or fully closed, its role is to stop the circulation of the hot water as a safety stop, without draining the system).
· Anti-scald valve (optional, meant to prevent the hot water from entering the house circuits at too high a temperature).
Finally, should store the operation manual for the solar water heater in an accessible place.
Also, should affix the label explaining the freeze protection method for the solar water heating system in a visible spot onto the heater itself.
Extra Material: Batch Solar Water Heater Systems
So far, we’ve mainly covered flat-plate heating systems because they are far more compact. However, batch heaters are also popular. The key thing to remember is that they are far bulkier, which is why often they’re not mounted onto a roof.
Instead, as mentioned, a batch water heater could use a large tank inside and keep far more hot water than a flat-plate system.
The batch water heater is best for southern climates and has been around for hundreds of years, as they are relatively simple to design. Reportedly, the first batch water heater in the United States was constructed in Baltimore in 1896!
The batch heaters are popular in seasonal homes and recreational facilities where the property is the only summer. However, during the winters, especially in cold climates, you should drain the batch heater.