solar hot water
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Solar hot water

Heating water using the power of the sun is what we do to provide an almost constant flow of hot water.

Types of Solar Hot Water

  • Thermosiphon
  • Indirect Solar Hot Water System
  • Direct Solar Hot Water System
  • Indirect Air 
  • Drain-down 

Water Feed 

All the solutions below require a mains or pumped cold water connection to the bottom of the hot water storage tank (Cylinder). As hot water is drained from the top of the tank, it is replaced by the cold water feed at the bottom.

Thermosiphon Hot Water System

Indirect Thermosiphon

  1. The sealed system fluid inside the solar collector panels is heated by the sun. 
  2. The natural rising of the heated fluid travels to an indirect solar loop coil inside a hot water cylinder which is positioned higher than the solar collector.
  3. Siphoning circulates the water throughout the indirect loop system
  4. Transferring solar heat to the mains fed stored water
  5. The loop circuit cools and returns to the panel
  6. Hot water rises and Cold water falls pushing fluid through the system without power.
Thermosiphone Indirect solar hot water schematic
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Direct Thermosiphon

A direct solar hot water system is a very simple concept. The direct thermosiphon is the same principle as indirect thermosiphon. There is no indirect coil in the tank so the actual water is heated and circulated. The circulated water flows through the panel thus heated by the sun then rises to the tank. The hot water rises to the top of the tank where it is stored and used. 

thermosiphon direct solar
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Positives of the Thermosiphon

  • No electrical current is required to operate a circulation pump 
  • Less risk of overheating because of a power cut
  • A simple system with less risk of failure.

Negatives of the Thermosiphon

  • Lack of control
  • Works best in warm climates
  • Solar panel position must be below hot water storage cylinder

Indirect Solar Hot Water System

indirect solar water heater plan
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  1. The sun heats the fluid in the panel or tubes 
  2. This fluid is pumped or siphoned through a hot solar loop inside the hot water cylinder
  3. The water inside the cylinder is heated by the solar loop coiled inside.
  4. Hot water transferred to the cylinder from solar loop cooling it
  5. Cooler water is returned to the lower pipework feeding the solar panel to be heated again.

Direct Solar Hot Water Systems

Indirect thermosiphon solar hot water schematic
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Unlike the indirect solar hot water heater, the water that is heated in the panel is the same water supplied to the home. Direct solar hot water systems are common in the Philippines. These are very simple setups which are often supplied as a complete unit. 

Positives of direct solar hot  water

  • Simple installation
  • Efficient heats water directly from the water supply

Negatives of direct solar hot water

  • Low used direct hot water system risks of Legionella
  • Unsightly on roof
  • Not great for ground mount

Solar hot water requires a control unit.

The ideal set-up is to have a differential thermistor measured pump switching unit. This works by measuring the temperature in the tank and the panel. IIt will control the pump when:

  • Panel temperature higher than the Tank – Pump is ON
  • Tank temperature lower than the Panel – Pump is off

This is good because the tank is not cooled when the panel temperature is cooler than the tank.

There are other ways to control the circulation pump?

Solar Photo Voltaic Powered Pump

Non-Thermosiphon pumped solar hot water circuits requires power. this pump needs to be powered. The pump can be:

  • Mains 110v
  • Mains 230v
  • 12v DC
  • Solar generated 12v 

Simple systems can benefit from the use of a solar-powered pump for circulation. By using a solar PV panel you can eliminate the need for a control circuit. How this works is that the panel only produces power when the sun shines. This is a double whammy as the heat is generated during sunny conditions as to the pump will start to circulate the nearly heated fluid.

Calculate the solar panel size for your pump

  • Small 12v circulation pump can run using a 50watt PV Solar panel. 
  • RS15-6 hot water circulation pump will need at least 150watt plus a 200watt inverter to convert DC to AC 110v 
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Timed Solar circulation pump

A simple plugin type timer can be all you need. The biggest problem with not having a way of turning off the pump when the sun goes down is that the panel will cool the water that you have just heated. Nighttime is the worst time for your pump to be running. A timer set to come on in the morning and off by the evening works ok. I have used this method as a temporary solution when my programmer needed replacing.

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Recommended Solar Installation Components

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Duda Solar 30 Tube Water Heater Collector 45° Frame Evacuated Vacuum Tubes SRCC Certified Hot

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RS15-6 Hot Water Recirculating Pump 110V Circulation Pump 3/4-Inch NPT 3-speed Recirculation Pump 9.5 Gpm for Water Heater System

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Low Noise pump 780L/H 206GPH 5M/16ft for off-grid Home or RV solar System. This pump can be powered by a 12V solar PV electric panel.

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Steca 0301U Solar Hot Water Control: LCD Display, Power Cord, and 2 sensors

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100 Liter 26 Gallon Stainless Steel Solar Hot Water Heater Tank Single Copper Coil Heat Exchanger. If you intend to install in conjunction with a wood or fuel boiler then choose a double coil model.

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Complete 4 Panel Hybrid Solar Water Heater Kit is an ideal solution for smaller installations such as off-grid remote locations or Cabins. The pump is powered by a solar panel.

Disclosure: Some of the links in this description go to Amazon which we receive a small commission.

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