Heat Your Home With Solar Hot Water

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This Wisconsin home has 320 square feet of solar collectors that provide home heat and domestic hot water.

When Terry McIlveen built his home in Maine in 1997, he made the unusual choice to install radiant floor heating. “People thought I was nuts,” he says. However, since that time, radiant floor heat has become increasingly popular, and it’s easy to understand why. This type of heating system works by pumping hot water — or water and propylene glycol (antifreeze) — through a system of tubing in the floor. That means houses using this heating system get warmer from the floor up. In the winter, there are no cold floors underfoot — instead the floor is the warmest part of the home.

McIlveen soon discovered an additional benefit — radiant floor heat is a great match with solar hot water. If you already heat your home with hot water, it’s just one more step to heat that water with solar energy. In the spring of 2010, McIlveen hired ReVision Energy, a southern Maine solar company, to install rooftop solar collectors to help heat his home and produce his domestic hot water, thereby cutting his fuel oil consumption by up to 25 percent each year.

The system cost $20,000, so McIlveen chose to finance it through a loan. His exact savings on fuel each year will depend on how much he has to run the heater in the winter, as well as the ever-changing price of fuel oil — but he knows that if he saves just two fuel tanks a year, he can cover his loan payments.

Using solar hot water for space heating won’t supply 100 percent of your heating needs. You’ll still need a supplemental heat source, so you won’t be able to scrap your furnace or boiler. But in new construction, that supplemental heat source can be much less expensive (a smaller system, for example). However, under the right conditions, a solar thermal system can replace a significant portion of conventional energy sources for both space heating and domestic hot water.

Tim Merrigan, senior program manager for solar heating and cooling at the National Renewable Energy Laboratory, says a solar thermal system can carry up to 40 percent of the annual heating load for some homeowners. Bob Ramlow, who is a solar consultant with extensive experience in solar heating systems, says that most people in well-insulated, tightly constructed homes can save up to 50 percent.

Using more solar energy and fewer fossil fuels has many environmental benefits. By investing in such a system, you’re reducing greenhouse gas emissions and other forms of pollution that come from mining and burning fossil fuels. But there are often cost savings, too, because you’re heating that water with free sunshine, and any savings on your fuel bills will add up. That’s why Ramlow emphasizes that if you’re pricing a system, to look not just at the initial price tag, but at the return on investment over time. The return will vary depending on a number of factors, including the price of heating fuel, your home’s solar exposure, and its average heating loads. Ramlow estimates that for the solar heating systems he has installed in Wisconsin, a typical return on investment is between 10 and 15 percent a year.

Domestic Hot Water, Plus, Home Heating!

You can install a solar heating system in an existing home as a retrofit, but it works even better if it’s designed into new construction (more on that in a bit). Either way, it’s almost always part of a system that also provides domestic hot water. If you’re going to invest in a solar hot-water system, you don’t necessarily have to include space heating, but if you’re looking at solar space heating, you almost certainly will want to get domestic hot water as well.

The reason is simple economics: Why install a solar water heating system that’s used only for heating (a seasonal need), when you could also use it for domestic hot water (needed year-round)? A combined system provides both. In winter, when the solar potential is lowest, collectors provide some space heating and some of the domestic hot-water supply. In summer, when the heating load is zero, collectors should be able to provide most, if not all, of the hot water for showers, laundry and your sinks.

Combined domestic hot-water and space-heating systems can be put together in many ways. Specifics hinge on the demand for hot water, overall heating loads and the type of supplemental heat. But one constant is the need for a way to store solar energy. When the call for heat is at its highest — for instance, a bone-chilling January night — solar potential is at its lowest. So systems typically include one or more water storage tanks where heat from solar collectors can be banked during the day and used at night or on cloudy days.

Water in the storage tank (or tanks) is warmed by a fluid, such as propylene glycol, which circulates through solar collectors and a heat exchanger. When the thermostat calls for space heat, the system taps into the solar tank first. Tanks can be large, with a capacity of 1,000 gallons or more, but are often much smaller.

Solar collectors are central to any system, and they’re usually mounted on a south-facing roof. Along with collectors, a storage tank, pumps and controls, solar space-heating systems also include a source of supplemental heat. That could be a small boiler, furnace, heat pump or even a woodstove — anything that can take over when the demand for heat outstrips your solar resources.

Radiant Floor Heat and Other Alternatives

Heating and storing water lies at the heart of any solar heating system, but homeowners also have to decide how best to distribute that heat throughout the house. There are several options.

Radiant floor heat distribution is tailor-made for solar collectors because water temperatures can be relatively low — 120 degrees Fahrenheit or below. That’s well within the range of heat that solar collectors can comfortably produce. This type of heating system also typically includes high-mass materials such as concrete and tile. This thermal mass helps store heat and evens out fluctuations in indoor temperatures by releasing the heat gradually.

Conventional radiators seem as though they would be another good option, because they too rely on heated water. However, radiators can’t be married as successfully to solar collectors. Whether they are baseboard fin-tube or the freestanding cast-iron variety, radiators are typically designed for water temperatures of between 160 to 180 degrees. Reaching that temperature is a real stretch for solar collectors.

However, according to DIY solar expert Gary Reysa, it is possible to design a system that uses baseboard heating to produce useful heat with lower water temperatures, by using special baseboard units that work with lower-temperature water.

You can also use solar collectors in a forced-air heating system by adding a hot-water coil heat exchanger to the air return duct. Ed Murray, president of Aztec Solar in Rancho Cordova, Calif., has used this strategy for retrofits. He adds a dual-stage thermostat that draws on solar hot water first, and resorts to the furnace only if the heat potential in the solar storage tank is too low. If the system is designed correctly, a homeowner should never experience cold air at the registers, Murray says.

When I contacted Viessmann Manufacturing Co. in Waterloo, Ontario, I was told that these alternatives to radiant floor heating are possible, but they shouldn’t be your first choice. They’re just not as efficient as hydronic heat distribution through tubing under the floor.

That said, Ramlow offers another point of view. He explains that if you already have a forced-air heating system, it makes sense to use it with the solar hot-water system because it’s so much less expensive than installing radiant floor heating (and air conditioning is easier with forced-air delivery systems). Ramlow has installed a number of solar heating systems that use forced-air heat, and he’s been satisfied with how well they perform.

Start With Home Energy Efficiency

The amount of fuel oil or natural gas saved by a solar space-heating system depends on a variety of factors, but none is more important than how the house was built. If little heat leaks through the exterior walls and the roof of the building — what an energy designer would call a low heating load — solar collectors can make a big difference. If heating loads are high, which is frequently the case in older homes, the fraction of heating bills that can be reduced by solar collectors is much smaller.

If you have a conventionally built house with skimpy amounts of insulation, uncontrolled air leaks and drafty windows, you should look at insulation and air sealing as the first step toward a solar space-heating retrofit. Adding insulation is a cost-effective way to immediately reduce your heating bills, and if you are ready to invest in a solar heating system, the heat it produces will go much further toward heating your home.

Fortunat Mueller, an engineer with ReVision Energy in Maine, explains that the company sends out a fact sheet to potential customers that recommends three prerequisites for solar heating: insulation of R-30 in the walls and R-50 in the ceiling, a low-temperature distribution system and high-efficiency backup heat.

“We ask people to first consider insulation upgrades to get the load down,” Mueller says. “Then we can design a solar system that actually tackles 30 to 40 percent of the load, whereas in a leaky old 18th-century farmhouse, I could cover the roof in solar and all I can do is 2 percent of the heat load.”

This is one reason solar space heating is so well-suited to new construction. In a new home, you have to buy a new heating system and solar water heater anyway, so right at the outset you can choose options that work well with a solar heating system. You can also start out with high levels of insulation and build a tightly sealed home with low heating loads.

Still another option for new homes is to incorporate passive-solar building principles that will reduce the heating loads. Passive-solar design strategies include facing the long axis of the house south, sizing roof overhangs appropriately, and using high-mass building materials and high-performance windows. In a well-designed passive solar home, heating loads will already be minimal, so solar collectors should be able to provide even more of a home’s heat.

Costs and Savings With Solar Hot Water

Key factors determining the cost of a solar heating system include the size of the house, how much of the heating load the system was designed to carry, local costs for collectors, pumps and controls, and how competitive the solar market is locally.

Terry McIlveen’s solar system for his 3,300-square-foot home in Maine cost $20,000 installed. That’s what Ed Murray estimated a similar system in a 2,000-square-foot home would cost in Sacramento, Calif. Another installer in Connecticut charges about $17,000 for a combined domestic water and space-heating system in a 2,000-square-foot home.

As it happens, these three systems all came in at about the same price, but coming up with an “average cost” for a solar heating system is a stretch. At least no installer or renewable-energy expert I spoke to would make a guess. Each system is essentially custom-designed for a specific house with specific energy needs. Local labor and equipment costs vary, as do tax incentives. The current federal tax credit on solar water heating systems used for domestic hot water is equal to 30 percent of the cost, but in some locations, state and local incentives will reduce the cost even more.

While installation costs can be fairly high, maintenance costs should be low. The solar hot-water storage tank may need periodic inspection and topping off, and various valves, pumps and sensors can sometimes need replacement. Overall, though, solar heating systems should require little attention for long periods of time.

As previously mentioned, when it comes to cost, the crucial factor to consider is long-term return on investment. Those numbers will vary according to the specifics of your individual house and climate. In general, the more fossil fuel that a solar heating system can replace, the more attractive it becomes financially.

While no one can precisely predict the future cost of fuel oil, propane, natural gas or electricity, it’s clear that in the long-term the prices for those ever dwindling resources will continue to go up. In contrast, solar space-heating systems, like other renewable-energy systems, have predictable costs that you can amortize over the life of a mortgage. The cost of providing domestic hot water and a portion of space heating will be the same for the life of your mortgage as it is the day the system is installed. That’s an advantage a conventional heating system simply can’t match. 

DIY Solar Space Heating

If you’re interested in a solar space-heating system and are pretty handy, consider building your own! Contributing editor Gary Reysa heats his home with a solar water-heating system combined with radiant floor heating. Reysa’s plan, which was featured in the December 2007/January 2008 issue, includes mounting solar collectors to a free-standing building with good solar exposure, such as a shed or other storage building. He was able to install this entire system, including the radiant floor heating, after the home was built. Because this design allows you to place your solar collectors wherever they get the best solar exposure, it’s flexible enough to work in many different situations. Find detailed instructions for building this system in Solar Heating Plan for Any Home.

How to Buy a Solar Heating System 

If you decide to invest in a solar-water and space-heating system, your first step will be to locate a solar installer or system designer who can put together a system that matches your needs. But as you may know, solar is hot right now! Many solar installers are just entering the business and don’t have a lot of experience. So how do you find a good one?

The Solar Energy Industries Association provides an online list of some of its members who are installers. The North American Board of Certified Energy Practitioners keeps a state-by-state list of installers who have passed the organization’s certifying exam. Both of these industry groups are good places to start, but it’s likely that not all of your local installers will be listed, so an Internet search is also a good idea. After you have a few names to start with, asking the right questions can help you find the best prospects: How long have they been in business? How many systems have they installed? Does the company have an in-house solar engineer?

While independent installers can pick and choose whatever components they like, manufacturers that make most (if not all) of the components themselves sometimes offer packaged systems. For example, when I contacted Viessmann to request a quote, they provided a sample hot-water and space-heating proposal tailored for a hypothetical house in Portland, Maine, in only a few hours. The document listed all of the Viessmann components and included both a schematic drawing and estimates on energy savings and solar production, along with an estimated cost for the equipment. The service is convenient, and it may lower labor costs on site because some of the components come preassembled. Whether a packaged system is attractive or not may depend on what local talent is available. One solar engineer told me the packaged systems he has seen are “somewhat disappointing in performance and — in my opinion — unnecessarily expensive.” He suggested that these packages would be better for new construction than retrofits.

Manufacturers of renewable energy components also may be able to help find local dealers or installers, and they may even offer design services directly. Here are a few you can check out:

Butler Sun Solutions
Stiebel Eltron
Thermo Technologies
Trendsetter Solar Products

More Resources

Learn more about federal financial incentives for renewable energy (search for “tax credits”). You can also find incentives specific to your state or utility.

Read more about solar water and space heating options in Solar Water Heating by Bob Ramlow and Benjamin Nusz.