Tank-in-Tank Technology

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. The inner “tank" is always made of stainless steel. The wall of the inner “tank" 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 hot water storage, low heat losses and reduced footprint

Thanks to the exceptional heat transfer and high storage temperature of the tank-in-tank system, the volume of hot water stored can be reduced. This gives a more compact water heater design and reducing static heat losses via the exterior walls. This cuts initial investment as you can choose a smaller cylinder and reduce operating expenses.

To take full advantage of this capability, a TMV (thermostatic mixing valve) is used on the cylinder outlet.

Reduced scale build-up

The inner tank is made of stainless steel, which means no anode protection is needed and no contamination, leaking or sludging occurs. The corrugated design of the tank also reduces scale build up.


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 limescale on the exchanger surface.

Legionella protection

Legionella bacteria proliferate in warm water (from 20°C up to 45°C). However, they can be eliminated within a few hours (>50°C) or even minutes (>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). When our water heater is operating at 60°C and above it ensures that the hot water remains free of legionella bacteria.

Fast heat up and rapid recovery

The larger surface area enables quick heat up and fast recovery, as well as providing a continuous flow of hot water under stable conditions.

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 one unit.

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.

Take a look at the HeatMaster combined boiler and water heater for domestic and commercial applications.