How Do Solar Water Heaters Work in the Winter?

Renewable energy for house

Photo by alphaspirit

Solar water heaters are an effective way to save on energy costs and consumption in many climates. The sale of solar water heaters has increased significantly over the years, but the biggest limitation to more widespread use brings concerns regarding how well the unit will perform during winter.

How do solar water heaters work in the winter? Solar water heaters work in the winter by including features that prevent the circulating water from freezing. The negative effects of cloudy or unsettled weather can be minimized if you use a solar pathfinder to determine the best location for solar collectors. Most solar water heaters also include backup gas or electric heating elements, with some built directly into the heater itself.

More advanced technologies have made solar water heaters a more viable option for northern climates. Here you will find out which features can make a solar water heater an effective energy saver for your home. You will also be introduced to some protective measures to ensure that the system runs smoothly all year round.

Do Solar Water Heaters Work in the Winter?

Cold temperatures do not make solar panels less effective at generating power. Cooler temperatures are technically better for solar panels, provided that the position of the sun is such that solar availability is not greatly reduced.

Cooler temperatures are better for solar panels because high heat actually lowers the efficiency at which panels convert light into electricity. As long as the solar collector is located in an area that receives adequate sunlight, then the system should be able to perform adequately.

The one hang-up is the seasonal positioning of the sun. The farther you are from the equator, the greater the variation in sun positioning. This puts northern zones at a disadvantage when it comes to solar availability. There is also the fact that winter is characterized by more unsettled weather and cloud cover for many regions.

Fortunately, most solar water heaters are constructed with backup conventional gas or electric systems that can heat the water, even when solar availability is low.

How Common are Solar Water Heaters?

Solar-powered technology always has a tendency to be met with some degree of skepticism. One common cause of skepticism is the practicality of solar water heaters, given the climate they live in. Many people live in locations that fall within a temperate climate zone, marked by seasonal variations in available sunlight.

Consider the diversity of climates in the continental United States alone. The city of Yuma, the sunniest in the nation, receives 90% of its possible sunshine each year. Meanwhile, Seattle only receives 43% of its possible sunshine each year. The vast majority of places in the USA fall somewhere in between these two extremes. There is a lot of doubt cast on whether solar water heaters are practical in the cloudy Pacific Northwest cities, including Seattle.

Solar-powered water heaters are more common than you may think they are. It has been estimated that there are as many as 1 million residential and 200,000 commercial solar water heating systems in the United States today. This number will continue to grow as the technology becomes more efficient.

Some Solar Panels Work Better than Others During Winter

Not all solar panels are constructed the same. Solar thermal panels are either flat plate collectors (FPC) or evacuated tube collectors (ETC), with the former being much more prevalent than the latter in solar water heating systems.  The evacuated tube collectors are much more effective in colder climates than the flat plate collectors. They also cost more than the flat plate collectors.

 For this reason, cost has always been a limiting factor when it comes to the practicality of installing these systems in temperate climates. Fortunately, the cost of evacuated tube collectors has been trending downward for several years. This has made it more realistic for homes in temperate climates to be outfitted with solar water heaters.

You can get an idea of the cost of evacuated tube collector water heater system by viewing the price tag for this Duda Solar 30 Evacuated Tube Water Heater Collector. Compare this to the Sunbank Solar Flat Plate Solar Collector.  You can see that the prices are not drastically different by any means.

Active Systems Are Best for Cold Climates

Solar water heaters utilizing an active system are more well-suited for cold climates than passive water heaters. Active systems use an electric pump for circulation versus passive systems which rely upon either gravity or convection to power the circulation system. Those in warm climates may seek out passive systems, but the most common type of system is the active system using an electric pump.

Indirect Heating Systems Are Suitable in Cold Climates

If you are looking for a solar water heater that is efficient in cold climates, you should also be looking to install a system that uses an indirect system to heat the water. Indirect systems make use of a heat-transfer fluid to heat the water. If you do use a system that utilizes a heat transfer fluid, you will need to be cognizant of the specific heat transfer fluid that you end up going with.

In cold climates, you are encouraged to choose a heat transfer fluid that has a low freezing point. Otherwise you will have a real problem on your hands when the heat transfer fluid becomes frozen. An example of a heat transfer fluid with a low freezing point is a glycol/water mixture such as Truegard propylene glycol.

How to Determine If Your Home Can Have A Solar Water Heater

Even if you have been able to find a good solar water heating system for your climate, you will still need to determine whether your home is suitable for a solar water heater. Your house needs to be set-up in a fashion that makes the most out of the available sunlight in your area. The ability of your solar heating system to pick up sunlight largely depends upon how many obstructions there may be in your yard or nearby.

There is a simple way to determine whether your home is ready for a solar water heating system to be installed:

  • Determine True South: true south is not the same as magnetic south. True south is actually 9 degrees east of magnetic south.
  • Determine the altitude (incline) of your solar collector: This is often oriented to the latitude angle
  • Analyze the shading patterns: A solar pathfinder can be used to help you determine the degree to which sources of shade will intrude upon your access to available sunlight

You need to keep in mind the fact that the position of the earth relative to that of the sun will change seasonally. You certainly do not want to install a solar water heater system and then find out 6 months later that your system does not work for a large portion of the year. This why we recommend taking the more analytical approach with the solar pathfinder.

How to Make Your Solar Water Heater More Effective During Winter

There are ways that you can make your solar water heater more effective during the times when the solar availability has been lowered.

Freeze Protection

Solar water heaters in northern climates should be installed with some degree of freeze protection. This is particularly true for passive systems which do not have the built-in freeze protection system that active systems have.

Freeze Protection Valve for Direct Pumped Systems

One of the most affordable freeze protection features for direct pump systems is the freeze protection valve. It is so affordable that you could make the argument that there is no reason to not invest in this feature, even if you feel that it may be overkill. Available freeze protection valves include this one.

In a direct pump solar water heater, one or more solar collectors are installed somewhere such as a roof, and a storage tank is installed somewhere below. Water is circulated from the storage tank to the collector(s) and back down to the house.

This type of system is more susceptible to freezing because there is no loop of heat transfer fluid being circulated to keep the water from freezing as is the case with indirect pumping systems. As was mentioned earlier, this is the reason why solar water heaters in cold climates generally feature indirect pumping systems.

However, there are solar water heating systems in temperate climates that do have direct pumps. Such systems should have a flush protection valve installed near the collector. The valve is set to open automatically when the temperature approaches freezing point, in order to let warm water flow through the collector.

Water Recirculation

Another way to keep solar water heaters from freezing during winter is to install a water recirculation system. This system is jump-started when the exterior temperature dips under 40 degrees Fahrenheit.

The water recirculation system works like this:

  • A collector freeze sensor activates a pump
  • The pump then circulates warm water through the collectors
  • A security blanket for power failures is a drain valve that will manually drain the collector

Add Insulation to Your Solar Water Heater

You can improve the performance of your solar water heater during winter by adding insulation to the system, as is recommended by SF Gate. You will start out by measuring the dimensions of your system. First measure the height and circumference of the water heater to determine how big of a cover you will need.

Then look for a fiberglass hot water blanket such as this one. The fiberglass hot water blanket should be equivalent to the height of the water heater while being slightly larger in terms of circumference.

You can also apply fiberglass insulation to all piping outside the wall for added heat security. This insulation should only be kept on during the coldest months of the year. The water in a solar water heater can heat up to 140 degrees or even more without the insulation. Remove the insulation during warmer months to avoid overheating.

Do I Still Need a Conventional Water Heater as a Backup?

This is a good question that reflects one of the most common concerns among homeowners with regards to solar water heaters. There is always concern that the solar system may not be effective enough to completely replace a conventional water heater.

Do you need a backup for your conventional water heater? Yes, nearly all homeowners will have some kind of backup system in place for your solar water heater, as is reflected in this report by the U.S Department Of Energy.

Actual need for this system will depend upon a variety of factors. How much warm water do you actually use? In the average-sized home servicing a full family, you will probably need a backup system to keep your water from going cold. If you are the owner of a tiny house running off the grid, you have probably already figured out how you will make the most out of the resources that you do have.

Your need for a backup system also depends upon the location you live in. If you live in a location that is frequently cloudy, such as the Pacific Northwest, then you will need a backup system. Take these factors into consideration when you are looking into buying a solar water heater.

What Does a Solar Water Heater Backup System Look Like?

Solar water heater backup systems vary in design. You should be able to find a backup option for your solar water heater that doesn’t break the bank. Backup systems usually consist of gas or electric water heaters. In the alternative, you may consider installing a rooftop tank with a thermosiphon system.

Thermosiphon System

Thermosiphon systems are passive by design, not needing any pumps to circulate water throughout the system. They consist of a water tank paired with a flat-plate collector. The unique design feature of this system is the fact that the water tank is placed on the roof, immediately above the collector.

The thermosiphon system doesn’t need an electric pump because cold water has a higher density than hot water, meaning that it will sink. Cold water then flows down through the collector and is heated, causing it to rise into the storage tank. The hot water from the top of the tank is the water that is distributed into the house.

Hot water is available in a limited quantity in such a system. For this reason, thermosyphon systems are generally only recommended for small-scale hot water needs. The water doesn’t circulate at a rate fast enough to service the needs of full family homes during winter, although they may be practical for a remote vacation home or off-the-grid cabin.

Solar Water Heater with Electric Backup

You can often find solar waters that have an electric backup system built-into the water heater for times when solar availability is so low that the water heater is unable to generate enough hot water for your home’s needs.

The solar water heater found here is one of those systems. In models with electric backup, there is a thermostat that controls the backup heating element. This is designed to ensure that the electric heater is only used when absolutely necessary. You can find a passive solar water heater with electric backup here.

Solar Water Heater with Gas Backup

This Ultra Force Solar Water Heater comes with a gas backup system. If you look at the specification sheet, you will notice that this solar water heater has a connection port for the solar loop. This is a feature characteristic of indirect solar water heating systems.

In an indirect solar water heater, a propylene glycol (antifreeze) cool is run through the solar collector and then circulated through the water heater tank itself. When solar availability is low, the water heater’s control system will switch on the gas power system.

This particular unit is more suitable for large residential and commercial settings, as is stated in the spec sheet. You can see how these systems work in this video of a residential installation of a similar system.

Health and Safety Benefits of Solar Water Heating Systems

One of the greatest benefits of the solar water heater is the end of one of the more overlooked dangers in the home. The overlooked danger I am alluding to is carbon monoxide poisoning. There is always a potential, even if it is a low potential, for this to occur. Carbon monoxide is a deadly by-product of a malfunctioning gas combustion process in an appliance fueled by natural gas.

According to UCLA, there were an average of 430 carbon monoxide poisoning deaths per year in the United States from 1999-2010. Carbon monoxide detectors significantly reduce the occurrence of carbon monoxide poisoning. Solar water heaters remove the threat entirely.

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