The “Tank-in-Tank" concept

Tank-in-Tank: what is it?

Tank-in-Tank: how does it work?

The boiler is connected to the primary circuit of the water heater: the hotwater storage tank thermostat switches on the pump between the boiler and the water heater when the temperature falls below the selected set-point. The heat generated by the boiler is transferred to the storage tank via the heat exchanger (the wall of the inner "tank") and heats the DHW to the desired temperature.(Fig.2)

The benefits of the “Tank-in-Tank" system

• All the hot water you need: the secret to highly stabile hot water supply
  
  The large surface area of the heat exchanger (often twice that of a coil) enables the use of all of the boiler’s output to heat DHW very rapidly. That avoids the necessity of stopping the boiler whilst the tank is being refilled, which blocks the heating system and causes the boiler to start and stop repeatedly, thereby impairing fuel economy and increasing the rate of harmful emissions. (Fig.3)
  
  
• Minimum storage and reduced losses
  
  Thanks to the exceptional heat transfer characteristics of the Tank-in-Tank system, the volume of hot water stored can be reduced, resulting in a more compact water heater design and reducing static heat losses via the exterior walls: that cuts initial investment and operating expenses.
  
  
• Sanitary and maintenance-free
  
  At ACV, the inner tank is always made of stainless steel: stainless steel is sanitary (it is generally used in hospitals and in the pharmaceutical and food and beverage industries), requires no maintenance whatsoever (it does not require anode protection), and enables the use of high storage temperatures.
  
  
• Self-descaling: high efficiency and long service life
  
  Each DHW draw-off creates a brief underpressure in the tank (at the moment that the draw-off tap is opened), followed by a slight overpressure (upon closure of the draw-off tap). As the inner tank is free to move, its walls expand and contract very slightly under the influence of these pressure changes and prevent the formation of lime scale on the exchanger surface: thorough testing and the testimony of numerous users in regions with calcium-laden water confirm that ACV Tank-in-Tank water heaters are truly effective in preventing the formation of lime scale deposits as opposed to traditional coil systems that necessitate frequent descaling.
  
  Thanks to this self-descaling feature, the heat transfer efficiency of our water heaters and their hot water output remain high throughout the service life of the installation. (Fig.4)
  
  
• Anti-legionellae
  
  Legionellae bacteria are micro-organisms that can cause serious diseases of the respiratory tract. Legionellae remain in a latent state in cold water (below 20°C) and can proliferate in warm water (from 20°C up to 45°C). They can be eliminated (in a few hours), however, at temperatures higher than 50°C – or in a few minutes above 60°C. It is recommended never to store the hot water thereafter at a temperature below 60°C.
  
  The “Tank-in-Tank" system prevents the formation of warm pockets because even the base of the inner tank is a heat exchanger that remains at the temperature of the boiler (typically from 60 to 80°C): thus, any “Tank-in-Tank" water heater operating at 60°C ensures that the hot water remains free of legionellae and safe for the consumer. (Fig.5)
  
  
• Installation in any position
  
  Tank-in-Tank systems can be installed in any orientation because the temperature is uniform throughout the tank. Note that when electrical heating element booster systems are fitted to Tank-in-Tank water heaters, they are always located in the primary circuit: this keeps the hot water perfectly clean and prevents the formation of lime scale deposits on the heating element. Yet another example of our focus on sanitation and long, maintenance-free service life!