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 runthrough its centre. The tank is made of stainless steel and holds alimited volume of water to meet short term demand. The tank wallbecomes an indirect heat exchanger for generating DHW.
As with all ACV stainless steel Tank-in-Tank systems, the boiler operates at high domestic hot water temperatures(above 90°C if necessary), and is available in versions that arespecially designed for very corrosive water such as sea water. Ofcourse, the boiler has a primary circuit and provides heat during theheating season. (Fig.1)
The “Tank-in-Tank" type combined boiler and water heater: how does it work?
Theboiler operates in heating mode just like any traditional boiler: theboiler thermostat controls the burner and lights it when the boilertemperature drops below the set-point.
In hot water draw-offmode, a domestic water thermostat senses the introduction of cold waterto the tank, cutting off the heating pump in order to save all theboiler’s output for heating. Its large heat exchange surface areaallows the tank to absorb all of the burner’s output to gradually heatthe water as it enters the tank: hot water supply is exceptionallystabile and the periods of interruption in heating mode are limited toa strict minimum. (Fig.2)
The benefits of the “Tank-in-Tank" system
- All the hot water you need: the secret to ensuring highly stabile hot water supply
The tank’s large surface area and the burner’s high output guarantee an unparalleled stability of hot water supply, very rapid refilling of the tank and minimal interruption of heating mode.
- 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 compact boiler 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 inhibit the formation of lime scale on the exchanger surface. Thanks to this self-descaling feature, the boiler’s efficiency and hot water output remain high throughout the service life of the installation.
- Anti-legionellae
The tank 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 operating at 60°C ensures DHW that is free of legionellae and safe for the consumer.
- Fuel oil, gas and electricity
ACV Tank-in-Tank boilers can be operated with fuel oil or gas and there is even a range of electric Tank-in-Tank boilers! That enables users to adapt to changes in the market (which can change significantly over the course of an ACV boiler’s long service life) and to convert their boilers to a different fuel than the one selected originally.
It comes as no surprise that ACV combined boilers and water heaters are selected by our customers in more than 40 countries situated at all latitudes, from Siberia (outside temperatures as low as – 65°C) to Mexico and the Persian Gulf (outside temperatures over 50°C). Do you know of any other integrated system that provides heating in the winter and hot water 365 days per year, all the while delivering exceptional performance and stability throughout the service life of the installation?
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Combined boiler and water heater
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