Glossary

ACV applies technologies best-suited to meet the demands of modern life with regard to stabile hot water supply. Its wide range of domestic hot water (DHW) generators and combined boilers and water heaters offers hot water generating solutions to fulfil every conceivable need. Thanks to their exceptionally large heating surface area, Tank-in-Tank systems guarantee a maximum stability of hot water supply.

The annual operating efficiency of a boiler indicates its energy performance over the course of a year and takes into account the heating function (about 70% of the annual energy consumption) and the domestic hot water (DHW) generating function (about 30% of the annual energy consumption). This annual operating efficiency takes into account the number of starts and stops over the course of the year, the boiler’s auxiliary equipment (the sanitary circuit pump in particular), and the heating control system.

In an atmospheric boiler, the flow of gas under pressure is what draws in some or all of the oxidant air required for combustion. The part of the air that is introduced in this manner is called the primary air. The balance, the secondary air, is added in the burner chamber. This is the combustion system that is most commonly used in traditional boilers. It is relatively inexpensive, but presents several drawbacks: the combustion efficiency tends to be rather low, the combustion rate of the burner is constant most of the time, there is a greater quantity of harmful emissions and combustion gas discharge must accommodate the constraints associated with draught. ACV offers a complete range of atmospheric boilers but recommends, when the user’s budget allows, a boiler with a pre-mix burner: the latter is more expensive but suffers none of the drawback listed above.

Also known as a “balanced flue boiler", the sealed boiler enables you to operate without a chimney, either in a residence with no chimney (new construction) or in one with a decommissioned chimney (renovation). Piping for flue gas discharge and fresh air intake comprises either a double concentric pipe of about 12 cm in diameter (for the most common residential boilers) or two separate pipes (one for fresh air, the other for combustion gases). The external connection is either on an exterior wall or through the roof. ACV offers a number of sealed boiler models, both gas-fired (fitted with pre-mix burners) and fuel oil-fired (fitted with transparent flame fuel oil burners). ACV sealed boilers are approved from 6 to 240 kW (depending on the country).

A basic boiler provides only heating to facilities and includes no hot water generating function. In order to produce hot water, a water heater can be connected to it.

This refers to a mechanical burner that diffuses the gas and air in the combustion chamber of the boiler. It is an inexpensive system but it pollutes more and is louder than a pre-mix burner. For this reason, ACV has eliminated this burner technology from its range of residential boilers and uses it strictly on medium and high output boilers.

Also known as “hot water tank" or “water heater", a boiler heats and stores domestic hot water at elevated temperatures. It is connected to the heating circuit of a boiler for heating and/or to a heat source (solar panels, heat pump, air conditioner condensing unit, ...). Most ACV boilers are of the stainless steel Tank-in-Tank type. Their capacity ranges from 60 to 1000 litres. Their heat exchange surface area enables them to absorb from 23 to 112 kW and to heat domestic hot water very rapidly. By heating very rapidly and maintaining the hot water at a high temperature in the hot water tank (which is only possible with stainless steel), the volume of water being stored can be reduced: this reduces investment and space requirements along with operating costs (very limited heat loss through the tank walls).

The boiler aquastat, commonly known as a “boiler thermostat", controls the water temperature in the boiler and provides the operating instructions to the burner of the boiler as a function of energy demand. The boiler thermostat is, at a minimum, supplemented by a room thermostat, which may be programmable. The room thermostat is located in a room where it controls the boiler operation from a distance.

The boiler thermostat, also known as an “Aquastat", controls the water temperature in the boiler and provides the operating instructions to the burner of the boiler as a function of energy demand. The boiler thermostat is, at a minimum, supplemented by a room thermostat, which may be programmable. The room thermostat is located in a room where it controls the boiler operation from a distance.

The combined boiler and water heater supplies energy to heat facilities and generates domestic hot water simultaneously. It is an economical and practical solution because only one unit need be installed and maintained. In contrast to traditional “water bath" boilers, which have of hot water systems with strictly limited capabilities, ACV offers innovative Tank-in-Tank boilers that provide, along with all of the standard features of modern heating systems, unequalled stability of domestic hot water supply (up to 175 litres per minute at 40°C) and a level of economy in operation that is second to none!

Combustion gases that are generated by the burning of fuel oil or gas contain water vapour. The energy contained in this water vapour represents approximately 10% of the Net Calorific Value (NCV) of the fuel. In a condensing boiler, the combustion gases are cooled upon contact with heating water return circuit: in cooling, the gases give up their latent energy to the heating water and the thermal efficiency of the boiler increases accordingly. The thermal efficiency of a boiler can, when expressed relative to NCV, exceed 100% and attain a theoretical maximum of 112% with natural gas – higher than the efficiency of a traditional boiler.ACV offers several types of condensing boilers, including a gas-fired model (HeatMaster TC) that operates in condensing mode both for heating and for domestic hot water generation, thereby attaining an efficiency near the theoretical maximum. This heating and hot water production unit is the only one of its kind and delivers the highest efficiency of any gas-fired boiler available today.

The combustion of fuel oil and gas produces two gases that contribute to the greenhouse effect: carbon dioxide (CO2) and nitrogen oxide (NOx). By consuming less energy (thanks to higher efficiency) and a complete combustion, a modern boiler pollutes the air three times less than an older boiler. This is true of ACV boilers with pre-mix (gas) or transparent flame (fuel oil) burners. And the burners used in ACV boilers have particularly low sound emission levels as well. Last but not least, the incorporation of modern, programmable climate control systems with room thermostat and exterior temperature sensor enable the user to optimise energy consumption. Thus ACV boilers are particularly eco-friendly and participate in the sustained development of heating installations.

Water expands when heated. An expansion vessel absorbs this expansion and prevents overpressure conditions in the heating circuits.

ACV floor-standing boilers are sturdy and compact: by integrating heating and hot water generation into a single, completely insulated block, they make it possible to save considerable space.

The GCV is equal to the NCV plus the latent heat of the water vapour produced by combustion. The energy contained in this water vapour is equivalent to 10% of the NCV of the fuel: the GCV is thus equal to about 110% of the NCV. By condensing this water vapour (i.e. by cooling it to a temperature of 58°C for natural gas), it is possible to recover this heat, thereby increasing the thermal efficiency of the boiler accordingly (see condensing boiler).

One or more heating circuits distribute the energy that has been transmitted to the “primary" water in the boiler to the rooms being heated. Several heating circuits can be connected to a single boiler and work at different temperatures (at low temperatures for floor heating system in living rooms, for example, and at higher temperatures in rooms equipped with radiators or convectors). These heating circuits are controlled as a function of outside temperature (outside temperature sensor) or room temperature (room sensor or room thermostat). ACV offers a complete range of control systems and hydraulic kits to meet all your sophisticated heating requirements.

The heating control system governs the operation of the boiler and the heating circuits as a function of heating and hot water generation demand. ACV offers 5 types of control systems on all of its boilers:

• simple control by means of a room thermostat that controls the heating pump: this is the simplest and least expensive system to install, but it does not optimise the operating cycles of the boiler (unnecessarily frequent starting and stopping of the burner), nor does it tailor the heating temperature to the instantaneous demand;
• temperature control by means of a room thermostat: when a heating demand occurs, the boiler modulates its output (see modulating output) to maintain the heating temperature at the specified level. This control method is the one most commonly used in heating system renovations;
• climatic control with outside temperature sensor: the heating temperature varies as a function of the outside temperature and the period (day/night). This is the usual climatic control method with an outside temperature sensor, and the one most commonly selected for new installations;
• climatic control with outside temperature sensor and room thermostat (“booster" mode): the heating temperature modulates automatically as a function of the outside temperature but also takes into account the temperature inside the home. This control method provides a higher level of comfort for no more than the cost of an additional room thermostat;
  
•  differential control of multiple heating circuits at different temperatures: this system enables control of two or more independent circuits with multiple temperature sensors: radiators, floor heating, swimming pool, supplementary water heater. ACV offers standard control modules and hydraulic kits for this, the most sophisticated solution. Alternatively, ACV will be glad to advise your installer or engineer on the design of specific projects.

The instantaneous heating efficiency of a boiler is the ratio of the useful output transmitted to the heating circuit versus the power consumed by the boiler (heat input). The instantaneous efficiency of a boiler depends on several parameters: the quality of the burner, the performance of the boiler’s heat exchanger, the quality of the boiler’s combustion and its cleanliness, the control of the boiler, the boiler’s insulation. Modern fuel oil and gas-fired boilers deliver efficiencies ranging from 92 to 95 %. Condensing boilers provide even higher efficiency (from 98 to 105%). Instantaneous efficiency is nonetheless only one data point relative to the appreciation of a boiler’s performance. The annual operating efficiency, which considers both the heating and the domestic hot water systems, gives a better indication of the energy performance of a boiler.

Heating system mixing valve — A heating system mixing valve mixes, in a fixed or variable proportion, the water at the boiler outlet with from the return line: whether for continuous monitoring of the temperature of the water sent to the heating circuit or to raise the temperature of the water being returned in order to prevent condensation inside the boiler as it is potentially sensitive to corrosion or to operate heating circuits at different temperatures

The safety group combines three functions: a manometer to indicate the water pressure in the installation, a safety valve to prevent overpressure conditions and a vent to evacuate air trapped in the lines.

The hot water efficiency of an installation is the ratio of useful energy content of the domestic hot water (DHW) to the power consumed (heat input) by the boiler or hot water generator. In the case of DHW supplied by means of a hot water generator, the hot water efficiency depends on several parameters: the heating efficiency of the boiler, the performance of the water heater’s heat exchanger and the water heater’s insulation. ACV water heaters and combined boilers and water heaters operate at levels of efficiency higher than any traditional DHW generating systems for the three following reasons:

1. The large surface area of the heat exchanger 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.
2. 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.
3. Self-descaling: each time hot water is drawn off, the inner tank walls oscillate slightly, preventing the formation of lime scale deposits. The efficiency of the Tank-in-Tank system remains high throughout the service life of the installation. Testimony from numerous users confirms 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.

All ACV combined boilers and water heaters, along with a large number of ACV boilers for heating only, include a domestic hot water priority (DHW) function straight from the factory: whenever a substantial DHW demand occurs, the priority system shuts off the heating pump and dedicates all of the boiler’s output to the generation of DHW. Any temporary interruption of the heating function goes unnoticed because the building provides adequate thermal inertia.

At ACV, the storage tank of a Tank-in-Tank system 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.

In addition, the storage tank of a “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.

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.

The use of stainless steel assures ACV heating and hot water generating systems of a long service life. Stainless steel combines outstanding resistance to attack by water and to corrosion: it is synonymous with high performance throughout the service life of the installation. Due to its extensive experience with this technology, ACV is uniquely qualified in the application of stainless steel in Tank-in-Tank systems.

Modern, so-called "low temperature" boilers deliver modulating output as function of heating demand and a temperature between 30 and 60°C. They provide a higher level of comfort and reduce energy consumption. All ACV boiler models can be operated at low temperature with suitable hydraulic controls.

The NCV of a fuel is the energy created by burning one litre of fuel oil or one m³ of gas. Generally speaking, the NCV of one litre of fuel oil or one m³ of gas is about 10 kWh (the amount of energy corresponding to an output of 10 kW during one hour).

The outside sensor is used in a “climatic" heating control system. The sensor is installed on an exterior wall in a neutral part of the building that is relatively protected from exposure to wind and sun. The sensor measures the outside temperature, taking into account, in particular, the amount of sunlight, and transmits this information to the controller. The latter automatically adjusts the heating temperature by anticipating the heating demand: the result is a better regulated temperature and more comfortable heating.

The output of a boiler generally designates the useful output actually transmitted by the boiler to the heating circuit. Output is expressed in kW  or in kcal/hour (1 kcal / hour = 1.16 kW).

Depending on the country and the quality of insulation in the building, the output required to heat an average home varies from 10 to 35 kW. The output required to generate domestic hot water should also be taken into account, however, as this often represents a critical factor in the selection of a boiler. And although the output requirements for heating decrease with improved quality of insulation in the home, the sanitation demands of modern life are constantly increasing and more and more often represent the determining factor in the choice of heating and hot water generation installations. ACV’s detailed advice can be found under “Select your installation". ACV offers boilers from 6 to 3000 kW that cover practically all heating and hot water generation needs.

In a pre-mix burner, the gas and all of the primary air are mixed before being injected by a fan into the burner manifold inside the combustion chamber: the air/gas mixture is perfectly homogeneous and allows for excellent combustion with extremely low levels of pollution emissions. Apart from that, this type of burner enables high-output operation with very high efficiency in a relatively small combustion chamber (and therefore in a compact boiler). And finally, the output of a pre-mix burner can be modulated to continuously tailor its operation to the actual heating demand whilst maintaining a higher level of output in reserve to address hot water demand. ACV has been a pioneer in pre-mix technology: the first ACV pre-mix burners were commercialised in 1993. Today ACV offers this technology on its boilers from 20 to 240 kW.

Also called a circulation pump, the pump makes the heating water circulate faster between the boiler and the rooms being heated. All modern heating installations are equipped with a heating pump. With ACV Tank-in-Tank boilers, there is no pump between the boiler and the water heater because the latter is integrated into the boiler: this lowers investment and installation costs and reduces energy consumption.

PVCC tube placed at the domestic cold water inlet to the boiler or the hot water generator, which serves to inject cold water into the stainless steel storage tank. The shape and position of the end of the tube is designed to maximise the heat transfer between the hot water of the tank and the cold water entering and to maintain the stratification inside the tank.

The heating control system modulates the temperature of the boiler as a function of the outside temperature sensor (in general) but does not adjust according to room temperature. The room sensor that is installed in a living room measures the room temperature and transmits this information to the controller. The controller thus constantly corrects the temperature set-point, taking into account both the predicted heat demand (as indicated by the outside temperature sensor) and the “instantaneous" room temperature.

The room thermostat measures the temperature of the room in which it is installed and transmits commands to the heating pump or the boiler control system to maintain the desired temperature in the room. This is the simplest temperature control method and the one most commonly used in heating system renovations. The room thermostats offered by ACV enable the user to program specific requirements over the course of the day, by day of the week and for periods of absence.

Also known as a “sealed boiler", the balanced flue boiler enables you to operate without a chimney, either in a residence with no chimney (new construction) or in one with a decommissioned chimney (renovation). Piping for flue gas discharge and fresh air intake comprises either a double concentric pipe of about 12 cm in diameter (for the most common residential boilers) or two separate pipes (one for fresh air, the other for combustion gases). The external connection to the balanced flue is either on an exterior wall or through the roof. ACV offers a number of balanced flue boiler models, both gas-fired (fitted with pre-mix burners) and fuel oil-fired (fitted with transparent flame fuel oil burners). ACV balanced flue boilers are approved from 6 to 240 kW (depending on the country).

Depending on the model, the heating bodies of ACV boilers are either of heavy gauge, high strength steel or stainless steel. AVC is a world-renowned specialist in the design and manufacture of heating and hot water generation systems made of steel and stainless steel: our factories feature the most advanced technologies coupled with an uncompromising approach to quality, which serve to ensure the unparalleled service life of our installations.

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 is a heat exchanger between the primary circuit (connected to the boiler) and the DHW storage tank. Details about the operation and advantages of the Tank-in-Tank system can be found under “ACV technology".

The thermostatic valve of a radiator or a convector measures the room temperature. It takes into account temperature variations due to exposure to sunlight or to the presence of a cooking unit or other installations that give off heat. The thermostatic valve automatically changes the flow of hot water in the radiator or convector as a function of the local temperature.

The function of a burner is to convert fuel oil energy into heat. A traditional fuel oil burner is inexpensive and produces a yellow flame: the level of harmful emisssions (NOx) from such a burner is unnecessarily high. ACV offers “transparent flame" burners that deliver better efficiency and produce a substantially smaller quantity of polluting emissions.

A "transparent flame" fuel oil burner features a recirculation system that returns combustion gas to the flame to cool the latter. All sealed boilers in the ACV product range are equipped with transparent flame burners.

The air vent is generally placed at the highest point in the installation and permits evacuation of air trapped in the heating circuit. The vent can be manual or automatic.

The wall-hung boiler is mainly intended for flats or individual homes of medium size. It makes it possible to save floorspace and can be hidden in a cupboard or an attic.

Also known as a "water heater tank" or "boiler", a hot water generator heats and stores domestic hot water at elevated temperatures. It is connected to the heating circuit of a boiler for heating and/or to a heat source (solar panels, heat pump, air conditioner condensing unit, ...). Most ACV hot water generators are of the stainless steel Tank-in-Tank type. Their capacity ranges from 60 to 1000 litres. Their heat exchange surface area enables them to absorb from 23 to 112 kW and to heat domestic hot water very rapidly. By heating very rapidly and maintaining the hot water at a high temperature in the hot water tank (which is only possible with stainless steel), the volume of water being stored can be reduced. this reduces investment and space requirements along with operating costs (very limited heat loss through the tank walls).

See also: Steel and stainless steel

ACV recommends the installation of a safety valve or a safety group at the cold water inlet of its water heaters. The integrated safety valve limits the increase in pressure and evacuates excess water due to expansion during heating.

ACV specifies the installation of a safety valve or a safety group at the fresh water inlet of its hot water generators. The safety valve limits the increase in pressure and evacuates excess water due to expansion during heating.