Tank-in-Tank: what is it?

The Tank-in-Tank is a Domestic Hot Water storage exchanger (DHW) completely immersed in a steel outer tank that contains primary water. At ACV, the inner “tank" is always made of stainless steel. The wall of the inner “tank" itself acts as the heat exchanger between the primary circuit (connected to the boiler) and the DHW storage tank.

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

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: both attributes combine to cut initial investment as you can choose a smaller cylinder and reduce operating expenses.

Sanitary and maintenance-free

At ACV, the inner tank is always made of stainless steel, which means no anode protection, no contamination, leaking or sludging caused by cracked glass linings. Also, because the water is held at consistently higher temperatures this discourages the growth of legionellae bacteria.

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.


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 and above ensures that the hot water remains free of legionellae.

The “Tank-in-Tank" type combined boiler and water heater: what is it?

A combined boiler and water heater is a boiler that integrates the heating function together with domestic hot water generation (DHW).

In a “Tank-in-Tank" boiler, the tank is ring-shaped and the flue pipes run through its centre. The tank is made of stainless steel and holds a limited volume of water to meet short term demand. The tank wall becomes an indirect heat exchanger for generating DHW.

As with all ACV stainless steel Tank-in-Tank systems, the boiler is capable of operating at high domestic hot water temperatures (above 90°C if necessary), and is available in versions that are specially designed for very corrosive water such as sea water. Of course, the boiler has a primary circuit which provides heat during the heating season.

The “Tank-in-Tank" type combined boiler and water heater: how does it work?

The boiler operates in heating mode just like any traditional boiler: The boiler thermostat controls the burner and lights it when the boiler temperature drops below the set-point.

In hot water draw-off mode, a domestic water thermostat senses the introduction of cold water to the tank, cutting off the heating pump in order to save all the boiler’s output for heating. Its large heat exchange surface area allows the tank to absorb all of the burner’s output to gradually heat the water as it enters the tank: hot water supply is exceptionally stable and the periods of interruption in heating mode are limited to a strict minimum, and as the reheat time of the unit is so quick users generally would not notice the interruption at all.