Installing a 30L Dewhot Fan Forced Gas Geyser (OD30DH) in series with and after an existing electric geyser is an excellent, highly efficient hybrid setup for large homes, ensuring high-volume, continuous hot water while saving on energy costs. The 30L model is smart, automatically controlling both gas and water flow for accurate temperatures. 
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Here is a breakdown of how this configuration works, its benefits, and key installation requirements.
How the Series System Works
In this setup, the electric geyser acts as a pre-heating storage tank, and the Dewhot gas geyser acts as a "booster". 
 
Cold Water enters the Electric Geyser, heating to a set temperature (e.g., ).
Warm/Hot Water exits the electric geyser and flows into the Inlet of the 30L Dewhot Gas Geyser.
The Gas Geyser measures the incoming water temperature. If it is already hot enough, the burner stays off. If it is too cold (e.g., in winter or after high usage), the burner turns on and boosts the water to the final desired temperature. 
 
Advantages
Reduced Gas Consumption: Because the gas unit receives warm water, it uses significantly less gas to reach the final temperature.
Endless Hot Water: Even if the 30L electric tank is depleted, the gas geyser provides continuous hot water.
Optimal for High Demand: The 30L fan-forced unit can serve multiple taps, showers, and appliances simultaneously.
Load Shedding Protection: During power outages, the 30L fan-forced unit only requires a small amount of electrical power (approx. ) for the fan and display, which can be powered by a small inverter or battery.
Consistent Temperature: As a "smart" unit, the 30L model adjusts gas usage to ensure a constant temperature. 
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Key Installation Requirements
Registered Installer: In South Africa, the installation MUST be done by a registered SAQCC Gas installer to receive a Certificate of Compliance (COC).
Water Pressure: A minimum of  (2 bar) of consistent water pressure is required for the gas geyser to operate properly.
Temperature Limits: If the electric geyser heats water above , a tempering valve might be needed to avoid damaging the gas geyser, although usually, a 30L fan-forced unit handles pre-heated water well.
Outdoor Installation: The 30L fan-forced model is designed for outdoor use and must be installed with a weather box (cover) to protect it from harsh weather.
Pipe Insulation: Hot water pipes should be insulated to improve efficiency.
Safety Bypass: It is advisable to install bypass valves to isolate the gas unit for maintenance without losing all hot water. 
https://dewhot.co.za/wp-content/uploads/2023/05/Dewhot-30L-Outdoor-Fan-Forced-Gas-Geyser-Manual.pdf?srsltid=AfmBOop0CAVa6vY40sduXG11n7DDtQCE1jIKawunVNxRo9J-q03oJb3V
Technical Specifications for 30L Dewhot
Model: OD30DH
Water Output:  / min
Efficiency: ~88%
Electric Power:  ()
Includes: WiFi-enabled App Controller and remote temperature controller. 
https://dewhot.co.za/wp-content/uploads/2023/05/Dewhot-30L-Outdoor-Fan-Forced-Gas-Geyser-Manual.pdf?srsltid=AfmBOop0CAVa6vY40sduXG11n7DDtQCE1jIKawunVNxRo9J-q03oJb3V