The Passive Solar Water Heater

The passive solar water heater is the simplest and cheapest solar water heating system. Find out whether you can install a passive solar water system where you live. Find useful installation tips and data on the expected cost, available tax credits, available loans and the expected Return on Investment period
passive solar water heater

A passive solar water heater is a direct system.(By direct I mean water is used exclusively, with no resort to anti freeze fluid, heat exchange and other components). It is the simplest solar hot water heater. It is called passive because no external energy is required to circulate the water between the water storage tank and the solar collector. If one adds to it a solar hot water pump and a differential temperature sensor, one will end up with a direct active system. This will allows (adding a little to the cost) to install the collector higher than the tank (typical situation - the collector is on a slant roof and the tank is horizontally installed in the attic)

Can you consider passive solar water heating system for where you live?

Yes, under the following conditions:

  • You live in the Sun Belt States or other sunny regions

  • You don’t expect freezing conditions where you live.

  • The collector can be mounted bellow the storage tank (like what you see in the picture

If all three conditions apply to your specific location, passive solar water heater is your best choice
View next paragraph about installation considerations

Installation Considerations

The Passive Solar Water Heater system has two components, the solar hot water collector and the solar hot water tank.. Plumbing and running pipes is a straight forward proposition.

Top consideration (if you did not add a pump) – the storage tank must be installed above the collector.

Make sure to place the collector in an area that is not shaded. Installing on the roof (whether flat roof or slanted roof) is the most popular choice. However you can consider a non shaded area in the garden.

The optimum orientation of the collector face is toward South, it can be compromised though. South East orientation or even East orientation will usually provide satisfactory results.

The solar collector is slanted relative to the horizon to capture more sun radiation. On a slanted roof, the collector is installed parallel and close to the roof surface. It might deviate from the optimum theoretical slant angle, in practice you won't loose much in performance.

The Solar Hot Water Tank

The solar hot water tank is usually bigger than the conventional heater storage tank. Usually, it is installed outdoor, higher than the collector. The tank usually has inside it a backup electrical heater for cloudy days. (I mean real cloudy, even 2 to 3 hours of partial sun shine are enough to heat up the water.) The tank usually has over pressure protection and a thermostat that cuts the circulation to the collector (over heat protection) for collector and tank sizing click here

Water Pipes and plumbing

Overall, 4 pipes are connected to the system.
Two are going into the tank and out from the tank. The first is connected to the main cold water supply to the home. The second is hot water outlet that goes for the household hot water usage. It is recommended to insulate the hot water pipe.
The other 2 pipes are between the tank and the collector. The lower one carries colder water from the tank to the collector, the other one returns warmer water to the tank. This is a close loop that continues until the water reaches the desired temperature. Upon reaching the desired temperature, the thermostat senses the event and cuts off the circulation. Only when hot water is used by the family members, fresh (new) cold water enters the tank. no pump is needed, the circulation is carried on by the thermo siphon effect.
It is recommended to insulate the loop pipes to avoid losing heat to the environment

In case you expect rare freezing conditions, e.g. over night, a work around is to arrange a valve in the collector that drains the loop water slowly just before the water temperature reaches the freezing point. Water bellow the freezing temperature might damage the pipes inside the collector; I prefer to use an indirect solar water heater system, a system that uses antifreeze liquid (such as glycol) in a close loop between the collector and the tank and a heat exchange to transfer the heat to the water.

Cost Considerations

Typical cost of a Passive Solar Water Heater kit can run between $2000 to 3000$ for a family of 2-3 people and up to $3500 for family of 4-6 people. On that you need to add installation cost. If a pump is needed, consider added cost of $200 to $400. You can be allowed tax credits from the IRS as well as some of the states

Assuming tax credits the heating water system can pay itself in 3-4 years,

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