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Solar Water Heating Overview

New to solar water heating? Start here.

An Introduction to Solar Water Heating – the Other Solar Technology!

By now, we hope you’ve already heard this under-reported fact: solar water heating is a renewable energy technology with a BIG bang for your buck! Why do people overlook this affordable and proven option? Solar water-heating technology may have gotten off to a bad start in the public eye when inexperienced installers in the past poorly installed systems that later failed or never performed up to expectations. Another reason: there is still a lack of public awareness and understanding of the technology.

How Solar Water Heating Works

An example of solar water heating at its most basic level is a garden hose left out in the sun. When you turn on the spigot, the first water to come out of the hose is warm since it’s been baking in the sunlight. A solar water collector works in a similar fashion. Water flows through tubes housed in a solar collector. Throughout the day, a sensor monitors the difference in water temperature between the water in the storage tank (inside the home) and the water in the collector (typically mounted on the roof). At a preset temperature difference, the sensor triggers a pump to circulate the water through the collectors where it absorbs solar heat. In summer months, a solar hot water system can meet all your household hot water needs! Over the course of the year, the system can cover up to 85% of your hot water needs. The typical investment is about $3,000 to $6,000, not including installation. Be sure to investigate tax credits and rebates in your area which can help reduce that initial investment.

a home with a small solar thermal system on its roof

A home with a small solar thermal system on its roof.

Direct vs. Indirect Solar Water Heating

A primary factor in identifying the type of system you need is your location. If you are in an area where it could ever freeze –EVER – then you’ll need a “closed-loop” system. Closed-loop systems are also referred to as “indirect” systems. In a closed-loop system, the fluid that is being heated in the solar collector isn’t the actual water you’ll use; it’s a mixture of water and food-grade propylene glycol that doesn’t come into contact with your domestic water. This glycol-water mixture won’t freeze in the collector – a situation that could burst the piping. This sun-heated fluid then gives its heat to your domestic water supply through a heat exchanger.

If you live in a climate that never freezes – NEVER – you can use an “open-loop”, or “direct” system. An open-loop system is slightly less complex than a closed-loop system, and the potable domestic water is itself sent through the solar collectors to be heated directly. There are also “thermosiphon” systems available. These systems rely on the principle that hot water rises. In thermosiphon systems, the collector and an integrated water tank are located on your roof. As the water heats up in the collector, it will rise to the tank above it. The thermosiphon system is a “passive” system, meaning it does not require a pump.

drawings showing the different methods of solar water heating

Basic schematics for 3 different solar water heating systems.

What Does “Drainback” Mean in Solar Water Heating?

A drainback system allows water to circulate through the collectors without the risk of freezing. When the sun is not shining, the water in the collectors will drain back into an indoor reservoir tank. The drainback system can also offer collector protection in extremely hot environments. At very high temperatures, glycol breaks down into acidic by-products that can corrode the collectors’ copper tubing. Drainback systems can be set to stop circulating the fluid through the collectors when a high temperature set-point is reached, allowing it to drain down into the reservoir tank, preventing overheating. If your climate is truly freeze-safe, you can use distilled water instead of glycol. Water is more efficient at collecting heat, and distilled water won’t leave mineral deposits in the collector tubing, as tap water can do. In hot climates that experience no freezing, drainback systems may be your best choice.

a 15-gallon drainback tank installed on top of an 80-gallon water storage tank

A 15-gallon drainback tank installed on top of an 80-gallon water storage tank.

Solar Water Heating Pumps: AC or DC (PV) Powered?

All solar water heating systems, except thermosiphon systems, require electric pumps to circulate the water or heat-exchange fluid. Pressurized closed-loop systems can often be circulated by a small pump, which could be powered by AC or DC sources. The main advantage of a DC pump is that it can be powered by a small 12 VDC solar electric (PV) module. Complete systems for AC or DC/PV operation are available. Another advantage to a DC/PV system is that even if the utility grid goes down, leaving you without AC power, the pump can still run and you will still have hot water as long as it’s sunny! Open-loop and drainback systems usually need larger pumps to push the unpressurized fluid upwards to the collectors – that will usually require a stronger AC pump.

Solar Collectors & Storage Tanks

Flat Plate Collectors are the most commonly used collectors in domestic hot water applications. The collectors should last well over 20 years and can handle an operating temperature up to 180 degrees F. Surprisingly, a typical flat plate collector holds less than 2 gallons of water! You might think the more fluid-filled tubing the better; but the trick is getting the heat into the water, so the ratio of surface area to water volume is limited by the collector technology. There are many brands and sizes of collectors; most range in size from about 3-4 feet wide to 6-10 feet long and weigh from 90 to 160 pounds dry. It does require some planning (and some helpers) to get these collectors safely to the roof.

Evacuated Tube Collectors are often used in commercial applications or in applications where hotter water is needed, since they are capable of generating temperatures above the boiling point of water. While evacuated tubes have a long life similar to flat plate collectors, they are composed of fragile tubes and are subject to breaking. In snowy climates, the evacuated tubes may not shed snow as well as their flat plate counterparts. Regardless of your collector choice, siting them in the appropriate location is important. Ideally, solar collectors should be south facing, or at least within 30 degrees of south, and have unshaded exposure between the hours of 10 am and 3 pm. As with solar electric modules, the tilt angle is also important: the more perpendicular the collectors are to the sun’s rays, the better. Since system size is usually based on summer temperature maximums and there is far less available solar energy in winter, mounting the collectors at a tilt angle equivalent to your latitude plus 15 degrees will help to balance annual hot water production.

Solar collectors are evaluated and rated by an independent organization called the Solar Rating and Certification Corporation (ICC-SRCC). Many states require that you install an SRCC-rated collector in order to qualify for state rebates and tax credits.

Solar storage tanks are larger and much better-insulated than your average water tanks. The additional insulation allows the water to stay hotter longer. The storage tank in a hot water system is like the batteries in a solar electric system, storing energy (in this case, as hot water) for later use. These are specially designed tanks with extra ports to allow connections to the solar collector piping loop or to external heat exchangers. A household of 1-3 people will typically need an 80 gallon tank. Larger households of up to six people would use a 120 gallon tank. These are big tanks! Make sure to measure the area where you intend to locate the tank and the doorway through which it will have to pass. The tanks are about 5 feet high and 2 to 2 ½ feet in diameter. Some storage tanks have an internal heat exchanger; others must be connected to an external heat exchanger. Choosing a tank with an internal heat exchanger reduces the plumbing work compared to assembly of an external heat exchanger system; but external heat exchangers are more efficient than internal units.

You may be able to use your existing water tank as part of the system; however, you may need an additional tank to achieve the minimum storage capacity. All solar storage tanks have an electric heating element for backup heating when necessary — gas-fired heating of the solar storage tank is not possible. But it is possible to avoid using the electric element by plumbing the hot water output of the solar tank to the cold input of your existing gas-fired tank. Such a system would provide more efficient “finish” heating, as needed, during winter or on cloudy days.

What Size Solar Hot Water System Do I Need?

Sizing a solar water heating system for your home is fairly straightforward. Since it is a supplemental heating system for which you will have some sort of a backup heat source, there is no danger of running out of hot water. As mentioned previously, an 80 gallon tank is recommended for households of three people or fewer. For 4-6 people, a 120 gallon tank is more likely necessary. Allow 20 square feet of collector area for each of the first two family members. Expansion from that point is based on system location. Add eight square feet for each additional family member if you live in the Sun Belt. If you live in the northern US, allow 12 to 14 additional square feet per additional person. For example, for a family of five in Massachusetts, you could use a 120 gallon tank with about 80 square feet of collector (possibly two 4’ x 10’ collectors).

Heating water with the sun is a very cost-effective option. As a DIY project, it is on the more difficult end of the scale for all but the simplest of systems and requires very strong DIY plumbing skills and basic math skills (to calculate friction loss in pipe runs). Be sure to consult a plumber who is familiar with solar water heating systems before you purchase anything.

Solar Water Heating: Comprehensive Guide by Bob Ramlow
Solar Water Heating Systems: Home Edition by Tom Lane
U.S. Department of Energy’s Consumer’s Guide to Solar Water Heating