Domestic Hot Water

Solar hot water systems are most commonly used to heat water for basic household needs such as laundry, bathing, and dishwashing.

Solar hot water systems are most commonly used to heat water for basic household needs such as laundry, bathing, and dishwashing. These systems are commonly referred to as “domestic hot water systems” or DHW. DHW systems typically use solar energy to pre-heat the water that is incoming to a conventionally fueled hot water heater. The warmer the water from the solar heater, the less conventional fuel will be needed to provide the household’s hot water needs. During the summer months in most of the nation, a properly sized solar hot water system will provide 100% of a household’s needs. In the winter, or during extended cloudy periods, the amount of hot water provided by the sun may be 50% or less. APG SOLAR desires to install systems that meet between 50% and 100% of your annual water needs.

The size of the solar collectors suitable for your site will depend upon the geographic location and your hot water demand requirements, but typically they will require between 40 and 80 square feet of mounting area for a standard residential application. The most common mounting technique used today is installation of the collectors flush with the roof on a south facing exposure. Alternatives, such as ground mounting, or rack mounting on flat roofs are also possible.

We recommend installing a tankless heater in parallel with the storage tank in order to avoid conventional tank water heater standby losses. These losses occur when the entire tank contents are heated to the set temperature, while only a portion of the contents are used. The remainder of the heated water temperature is cooled and lost. This primarily happens overnight. The primary advantage of the tankless water heater is that it does not incur standby losses. The other advantage is that it utilizes a modulated flame, meaning the intensity of the flame, or amount of fuel being consumed (BTU Rate) is directly related to the volume and temperature of hot water called upon.

APG SOLAR also recommends the use of evacuated tube collectors due to several advantages. The first being higher efficiencies over conventional flat plate and another being less roof loading due to their lighter weight.

To give you an idea of how big this industry is, 190,000,000 sq. ft. of hot water panels were installed worldwide in 2007.  Hawaii, for example, leads all states within the US with 22,500 residences enjoying the advantages of SHW systems. As a cumulative result of SHW in Hawaii the benefits are as follows; avoidance of 63 million kwh, 105,000 barrels of oil, 55,000 tons of CO2 emissions & customer savings of $8.8 million.


TYPES – ACTIVE & PASSIVE (show this text and have read more option)

Active Systems: Open Loop Recirculation, Closed Loop Drain back & Glycol, Closed Loop Combination

  • Open Loop Recirculation

Open Loop Recirculation systems are most common in the North Central Valley. Their principal of operation works as follows; In the event a higher temperature exists on the roof in relation to the tank, a differential controller turns on a pump that circulates well or city water through flat plate collectors and back to a conventional water heater or the supply side of a tankless. This differential temperature comparison takes place between 2 temperature sensors, one mounted on the roof & one in the tank. When the differential is minimal the controller signals the pump to cease. In the event another roof mounted sensor (snap switch) determines a freezing condition, the controller signals the pump to operate thereby sending warm water from the tank to the roof in order to prevent panel damage, thereby recirculating.

  • Closed Loop – Drain back System

Drain back systems use water as the heat-transfer fluid within the collector loop. The water is forced through the collectors by a pump and then is drained by gravity to the storage tank and heat exchanger. These systems have no valves to fail and when the pumps are off, the collectors are empty, thereby assuring freeze-protection and auto shut-off if the water in the storage tank becomes too hot.

  • Closed Loop – Glycol System

Closed loop systems use a heat-transfer fluid to collect heat and a heat exchanger to transfer the heat to domestic water. Active closed loop systems use electric pumps, valves, and controllers to circulate the heat-transfer fluid, usually a glycol-water antifreeze mixture, through the flat plate collectors. This glycol-water antifreeze mixture makes closed-loop glycol systems effective in areas subject to freezing weather down to below zero F temperatures. For this reason, closed loop systems are preferred for year round use in areas that experience common freeze.

  • Closed Loop Combination

This system could be a variation of the above designs however must contain multiple heat exchangers.


Passive Systems: Open Loop Thermo Siphon & ICS (Integrated Collector Storage) also called Batch.

  • Open loop, batch or ICS

Open loop systems heat and circulate household (potable) water directly in collectors prior to distribution in the household. One type of open loop system is a batch heater that is simply a tank inside an insulated housing or a series of smaller tanks surrounded by an insulated box with a glass lens filled with water and positioned preferably on a south-facing roof system where it absorbs solar energy. The housings may incorporate a selective surface that absorbs the sun but inhibits radiant loss.

In climates where freezing occurs, batch heaters must either be protected from freezing, drained for the winter, or contain a thermostatically controlled heating element. Batch heaters are considered passive, and perform using the principle of thermo siphon.  They are less expensive as a result of fewer components, and they require less maintenance and experience fewer failures. These systems are good economical choices for seasonal applications such as summer camps. Installed system costs are approximately $5000.00.


COMPONENTS (show this text and have read more option)

Solar hot water systems are made up of collectors, storage tanks, piping, controls, and in some cases pumps. Active systems use pumps to circulate water or other heat transfer fluid through the system. Passive systems have no pumps and rely on water pressure, gravity, natural convection or thermo siphoning to circulate water.

  • Evacuated Tubes

The newest solar hot water systems use evacuated tube technology. A heat pipe transfers heat to a copper header pipe inside of an insulated manifold. A pump circulates water through the manifold, warming the water. To learn more about this technology and how it can benefit your specific need, please view the following brochures: Agriculture, Business, Home.

  • Tanks

Most solar hot water systems require a well-insulated storage tank. Many systems use converted electric water heater tanks for storage, or plumb the solar storage tank in series with a conventional water heater. APG SOLAR recommends the use of tankless (otherwise referred to as “point of use” or “on demand”) systems fed by solar heated water to obtain the highest level of cost savings and efficiency.

  • Controls / Pumps

A differential temperature controller monitors the temperatures at the solar collector outlet compared to the storage tank. When the collectors are warmer than the tank, the control turns on a fractional hp pump which circulates a heat-transfer fluid, usually water or a water-glycol mixture, through the collectors and back to the tank or heat exchanger.

  • Heat Exchangers

A heat exchanger transfers heat from the heat-transfer fluid (usually either water or a water-glycol antifreeze mixture) to the household water supply. The heat exchanger itself is a series of copper or stainless coils submerged in the water storage tank or a compartment outside of the storage tank.


For more information regarding tankless water heaters and evacuated tube collectors, call us today!

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