How Does a DIY Solar Water Heater Work?

The Department of Energy estimates that heating water accounts for 18 percent of a typical home’s energy usage, so if you could use the sun’s energy for this task, you could enjoy significant savings on your energy bill. That’s what a solar water heater does, and while it may look like a PV panel from the ground, it’s much easier to build and install.

Judging from the number of panels and storage tanks I saw on rooftops when I lived in Japan in the ’90s, homeowners in some other countries are more tuned into this energy-saving strategy than Americans. Al Ruggie of Phoenix-based ASAP Restoration laments: “For sure, we have never installed a solar hot water heater on a home for a client because we wouldn’t be able to warranty the unit since (as far as I know) they aren’t made by any single company that offers one. And (sadly, I’ll admit) despite being in Phoenix where it’s sunny year-round, I rarely see solar hot water heaters on local properties.”

The California property on which I live was off-grid when we moved in (it still is, but we’re working on that), and besides PV panels, it had a large solar water heating panel. It was homemade, too bulky and unwieldy to put on the roof, so we never used it. And that begs a caution for anyone considering a DIY solar water heater: Don’t forget to include hoisting it onto the roof, securing it against high winds and connecting it to your hot water supply as part of your overall plan.

What Is a Solar Water Heater?

A solar water heater heats water using the sun’s energy and circulates it into the household’s hot water supply. There are several ways to build one, but one of the most common is to construct a collector panel with an in-built network of tubing through which water can circulate. Cold water enters at one end, and water heated by radiation absorbed by the collector comes out the other.

Solar hot water systems of yesteryear were often leaky, failure-prone and ineffective (like our bulky panel), but the industry has matured. Today, you can choose between super-efficient, professionally installed collectors and less-expensive DIY-friendly solar hot water heater systems.

How Does a Solar Water Heater Work?

All solar water heaters have one feature in common: the ability to absorb heat from sunlight and transfer it into water. Systems differ, however, in the way they store and circulate water.

Passive systems rely on the thermosyphon effect — the fact that hot water is less dense than cold water and tends to rise above it. These systems usually include a holding tank positioned above the solar collector (this is the type of system I often saw on Japanese rooftops). On a sunny day, water heated in the collector rises into the tank, and an outlet feeds the heated water into the building by the force of gravity.

Active systems circulate water using a mechanical pump, and there are two types:

• A direct system circulates water into a holding tank in the building, which is usually just a standard tank-style water heater.
• An indirect system circulates a heat-transfer fluid like ethylene glycol (anti-freeze) through a heat exchanger, which is simply a system of coils installed inside a water heater tank. The fluid exchanges heat with the water in the tank rather than heating the water directly. This type of system prevents water contamination and is better for colder climates because it resists freezing.

Will a Solar Water Heater Work for Your Home?

As with all things solar, the number-one prerequisite for a solar water heater is enough direct sunlight to make it work. If you have a sunny spot on your roof or property large enough to set one up (it doesn’t have to be on the roof), you can save energy by feeding heated water into your existing water heater or circulating it through your plumbing directly from a rooftop tank.

You can benefit even if you live in a cold climate because you can either install a freeze-proof heat-exchange system or a system you can drain in the winter when temperatures are below freezing.

How Much Does a Solar Water Heater Cost?

This depends on how much of it you DIY. If you build your own collector, install it yourself and hook it up to your existing water heater, the cost could be as low as $1,000, depending on the materials you use. The cost to purchase the components as a kit is between $1,500 and $6,000, and if you have the system professionally installed, add about $3,000.

Solar water heaters qualify for the Renewable Energy Tax Credit, a federal program that reimburses 30 percent of the cost of purchase and installation. In addition, some states and communities may provide additional incentives. To qualify, the system must be rated by the Solar Rating and Certification Corporation (SRCC) or a comparable entity, which may include some commercial DIY kits.

How To Make a DIY Solar Water Heater

As mentioned, the most common DIY solar water heater consists of a collector, a coil network, a cold water inlet and an outlet for hot water. Here’s a very basic outline of the procedure for building one:

1. Build the collector box using lightweight plywood (1/4 to 3/8 inches thick) for the base and 1-by-4-inch cedar for the frame. Seal all joints with silicone caulk.
2. Insulate the interior of the box with 1-inch foil-backed foam insulation with the foil facing outward.
3. Construct a coil that fits snugly in the box using 3/4-inch copper tubing and elbow fittings. Maximize the number of turns in the coil to lengthen the path of water flowing through the collector.
4. Extend one end of the coil through a bottom corner of the box for the cold water inlet and the other end through the diagonally opposite corner for the hot water outlet. Solder an adapter onto each end so you can attach a garden hose or plumbing pipe.
5. Cover the collector box with glass and use silicone caulk to seal the edges and keep out rain.
6. Mount the collector on the roof or the ground in a southerly direction to maximize solar exposure.
7. Connect a cold water hose or supply pipe to the inlet and a hot water hose or pipe to the outlet. Run the hot water supply to a holding tank, which may be in the house or on the roof. The plumbing connections vary depending on the configuration.

Pros and Cons of Solar Water Heaters

Pros

• Energy savings: You’re heating water for free. Whether you use it directly from a rooftop tank or use it to fill your water heater, you’ll save money.
• Low maintenance: Once the system is up and running, you can basically forget about it. If you’re circulating a heat-transfer fluid, however, you may have to replace it periodically.
• Eco-friendly: A solar water heater is an effective way to reduce dependence on fossil fuels for home heating.

Cons

• Depends on the weather: A solar water heater won’t work on cloudy or rainy days or at night. Insulating the storage tanks helps mitigate this drawback.
• Winterization: You need to prepare for freezing temperatures, either by using a heat-transfer fluid or draining the pipes and shutting down the system in the winter.
• Cost: In general, a solar water heater costs significantly more than a conventional one, even if you build it yourself.

FAQ

How long does a solar water heater keep water warm?

In a properly insulated storage tank, water heated in a solar water heater during the day should stay hot overnight and possibly longer, depending on how much you use.

How much solar power do I need to run a solar water heater?

The amount of solar energy a solar water heater uses depends on a number of factors, including the amount of available sunlight, your water usage and the size of the collector panel. As a rule of thumb, plan on 10 square feet of panel space for each person in the household.

What’s the lifespan of solar water heaters?

A solar water heater can last from 10 to 25 years, although this depends on climate, the configuration of the system and the materials used to build it.

Do solar water heaters need a pump?

If a solar water heater’s storage tank isn’t mounted above the collector to take advantage of the thermosyphon effect, you need a pump to circulate water through the coil and into the tank. A solar-powered pump doesn’t use any extra energy, but if you plug a circulation pump into your electrical system, it will consume from 25 to 150 watts of power when it’s running.