Solar Thermal Systems Heat Your Water With The Sun

You can heat your water using just the sun. Not to be confused with solar power arrays, solar thermal systems turn sunshine into relaxing, hot showers.

PHOTO: John D. Ivanko

Why pay for natural gas or electricity to heat your water when, if you have enough, sunshine can do it for free? A solar hot water heating system could meet your needs for heating water in your home, depending on the climate where you live.

Don’t confuse this with solar electric systems that generate electricity from sunlight. Solar thermal systems capture the heat from the sun and use it to heat water. They can even provide space heating in a home.

Using abundant and free sunlight reduces the amount of gas or electricity needed to create your hot water supply. This saves you money while boosting your self-reliance and sustainability efforts.

“Solar water-heating systems work well in any setting,” says Bob Ramlow, co-author of Solar Water Heating (New Society, 2010) and owner of Artha Sustainable Living Center. Ramlow has designed and installed hundreds of solar thermal systems over the past 40 years. “The only requirement is that there must be a sunny place to locate the collectors.”

Reducing your use of natural gas or electricity with a solar thermal system can save an average homeowner 15 to 25 percent on total energy use, according to U.S. Department of Energy and the Lawrence Berkeley National Laboratory.

But before adding a solar hot water heating system, make your use of hot water as efficient as possible. Install low-flow showerheads, aerators on faucets and a water-conserving dishwasher. When replacing a clothes washer, select Energy Star-certified front-loading versions.

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Many people living off the grid wash fewer and fuller loads of laundry and try to take shorter showers.

Read about two Kentucky farmers living their solar-powered sustainability dreams.

Hot Water When Needed

The most common solar thermal systems operate by preheating water in your home, then storing it in an additional insulated tank connected to your existing hot water tank.

This way, on cloudy days, depending on your hot water use, the existing natural-gas- or electricity-heated tank serves as a backup. Most systems have two tanks. But one-tank systems are possible with a backup heater.

In cold climates, and based on hot water use in the home, you can meet as much as 80 percent of your hot water needs with the system. In warm climates, such a system could meet all your hot water needs.

Therefore, most homeowners find that solar water-heating systems can pay for themselves in four to 15 years, depending on the source of fuel used to heat the water.

Solar thermal systems offsetting electric hot water heaters offer the quickest payback, followed by propane (eight to 12 years) and natural gas (10 to 15 years), according to the latest data analysis by Ramlow.

Of course, if the price of natural gas or propane spikes, this would accelerate the economic payback for going solar. Economics aside, solar thermal systems are a direct way to cut the use of fossil fuels, shrink your carbon footprint and help mitigate climate change.

Residential solar hot water systems range in cost from $7,000 to $15,000, depending on the size and configuration of system and type of collectors. So, take advantage of federal or state incentives (see DSIRE in “Resources,” below) to reduce the cost.

The Renewable Energy federal tax credit currently covers as much as 30 percent of the installation cost (credit to be reduced in future years). If you live in rural areas, your project might also qualify for a USDA REAP grant.

solar thermal systems
John D. Ivanko

Systems Options

There are three basic configurations of solar hot water-heating systems, based on your area’s climate:

  1. active, pressurized closed-loop
  2. drainback, non-pressurized closed-loop
  3. passive, integral collector-storage/batch

Closed-loop systems depend on pumps and controls to move a heat transfer fluid through the collectors.

A passive system, in contrast, uses the physical properties of heat transfer instead of pumps. And it has no antifreeze transfer fluid because such systems are located only in places where there’s no danger of freezing.

Active, Closed-Loop

For most of the continental United States where freezing pipes could be a concern, an active, closed-loop solar thermal system is used with a nontoxic, high temperature antifreeze serving as the heat transfer fluid in the pressurized system.

The design includes a number of collectors of various dimensions sized for your hot water needs.

The closed-loop system also includes the following

  • a heat exchanger that transfers the heat from the transfer fluid to your water
  • a storage tank for the solar-heated water
  • a circulating pump powered by electricity (sometimes a small photovoltaic module)
  • a temperature controller
  • other safety and utility hardware.

The heat transfer fluid passes through copper or stainless steel pipe, or PEX tubing.

You can use two types of collectors for active solar thermal systems: flat plate and evacuated tube.

Flat plate collectors—widely and successfully used throughout the country since the 1970s—are glass-covered (glazed), weatherproof and insulated boxes filled with a grid of black, heat-absorbing plates.

The evacuated tube collectors house rows of glass tubes that can heat fluid to higher temperatures than the flat-plate type. But they don’t heat the hot water any faster, explains Ramlow. He suggests reviewing the performance ratings for collectors you’re considering on the SRCC website (see

Evacuated tubes don’t shed snow or frost in the same way as flat plate collectors. This is another consideration for systems in cold climates.

Drainback System

The drainback system operates in a nonpressurized closed loop with an additional small tank that holds the heat transfer fluid.

“When off, the pipes and collectors are empty, which prevents degradation of the antifreeze when the tank upper limit is reached and allows the panels to heat up faster on cold days,” says Brandon Leavitt, who has found great success with such systems that his company, Solar Service Inc., has installed in the Chicago area.

“With fewer components, drainback is more efficient and simpler and can be sized for large and small applications.”

Occasionally, you’ll need a larger lift pump based on the height of the collectors above the drainback tank.

Passive, Integral Collector-Storage (ICS) or Batch System

In warm climates such as Southern California or Hawaii, solar thermal systems can operate passively. They employ only water, no pump and no second tank.

The heat from the water is thermosiphoned through the collectors. The storage tank is placed near the collectors, often on the roof, so additional structural considerations are important because of system weight.

In an alternative design, water can first pass through one or more black tanks or tubes. These preheat the water before it proceeds to a conventional tank.

“The biggest issue is the amount of cloudy weather a certain climate has,” Ramlow says.

“Often, colder climates experience more cloudy days than hot climates do, so the diminished amount of sunlight requires a slightly larger solar collector. Most areas of the U.S. experience freezing conditions at least occasionally, so virtually all systems require some sort of freeze protection.”

Hawaii is one state that can use passive batch systems without worry.

Check out our guide to leasing land to solar developers.

Size, Location & Lifespan

For sizing a solar hot-water system that works well in cold climates, Ramlow recommends allocating about 20 square feet of collector and 20 gallons of storage capacity for each person in the home.

In warm climates, size a typical system with 15 square feet of collector and 25 gallons of storage capacity for each person. Do more than four people live in your household? Reduce the calculations by
10 percent for both climate scenarios.

Leavitt’s Solar Service installs only flat-plate collectors. He estimates two 4-by-8-foot collectors for a family of four to have year-round hot water in most parts of Illinois.

While it’s tempting to place collectors on roofs, doing so can take a more complicated an installation. Should there ever be an issue with the system, it’s also more difficult to repair.

In cold climates, ground-mounted collectors are easy to maintain. And, more importantly, after a big snowfall, you can quickly sweep them clean of snow, ready for the next sunny day.

Consider selecting dealers, designers and installers—usually the same company or person — that have undergone training or certification through the North American Board of Certified Energy Practitioners to avoid problems down the line.

With unobstructed southern exposure between the peak solar window of 9 a.m. to 3 p.m., you can tilt the collectors according to seasonal hot water needs. The collectors are often placed on an aluminum rack in a fixed position about 10 degrees greater than the latitude of your site to optimize their solar gain.

However, sometimes systems are optimized for winter solar gain or for increased hot water use during the warmer months. Every system is unique in this respect.

Properly installed, solar thermal systems should operate for 20 to 30 years with few maintenance issues. The antifreeze solution can probably be replaced during the operation period for closed-loop systems.

In many states, renewable energy setups are exempt from additional property tax assessments.

solar thermal systems
John D. Ivanko

Sidebar: Home Solar Heating

Solar thermal systems are most commonly used for domestic hot water. But you can devote some systems to space heating if additional collectors and a heat exchanger are added. For heating the home, the solar thermal system can interface with either a forced air furnace or a radiant floor system.

A radiant coil is added in the ductwork between the filter and cold-air return on your furnace. When the water in the solar tank is at least 110 degrees and your home is calling for heat, a pump turns on, sending hot water through the coil.

At the same time, a relay switch activates the furnace blower . The blower draws cool air over a hot coil to deliver solar-heated air to your home through your existing ductwork.

Solar thermal systems can also supplement in-floor radiant heating systems. A series of tubes placed in the floor radiate the heat slowly into the room. They can also pair with geothermal systems.

Baseboard or radiator systems, however, will not work with solar hot water systems. The temperatures are rarely high enough.


Find out more from these sources.

Database of State Incentives for Renewables & Efficiency: This is a national listing of state and federal grants, special loans and other incentives for renewable energy systems.

Federal Solar Tax Credit (Investment Tax Credit): For businesses and individuals, there is currently a 30 percent federal tax credit for installing solar water-heating systems, but it’s less in subsequent years. The equipment must be certified by the Solar Rating and Certification Corp. It does not cover heating for pools or hot tubs. 

Midwest Renewable Energy Association: This association hosts the nation’s
longest-running renewable energy fair. 

Solar Rating & Certification Corp.: SRCC evaluates the certified performance ratings of solar collectors.

Solar Water Heating: a comprehensive guide to solar water and space heating systems: by Bob Ramlow and Benjamin Nusz

USDA Rural Development — Rural Energy for America Program: This competitive grant and loan program is for agricultural producers and rural small businesses. Grants can cover as much as 25 precent of the project cost. 

This article appeared in Living Off the Grid, a 2019 specialty publication produced by the editors and writers of Hobby Farms magazine. Living Off the Grid includes stories on permaculture, growing plants without seeds and long-term produce storage. You can purchase this volume, Hobby Farms back issues as well as special editions such as Best of Hobby Farms and Urban Farm by following this link.

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