Glossary Of Terms | Dux Hot Water




Combustion of gas or liquid fuels requires the presence of oxygen in adequate supply. Normally this is supplied from the air. Air used in combustion is of two types, (i) primary air (which air is mixed with the gas or liquid fuel prior to combustion) and (ii) secondary air (air admitted to the flame without prior mixing with the fuel). The term “excess air” refers to the air passing up the flue and not actually used in combustion. A certain amount of excess air is required, but not too much, as it is a source of loss of heat.


The amount of water available from a water heater over a specified time period. The value is derived from the addition of the amount of water available from storage (rated delivery), and the amount of water available from the recovery rate (heater output). This value takes into account the temperature rise of the water and the allowable temperature variation.


The ambient temperature is the average temperature of the atmosphere in the vicinity of an appliance. Performance of water heaters is generally checked with reference to ambient temperature rather than a fixed temperature to allow for a statement of performance which will hold true for summer and winter performance. The generally accepted figure for discussion of performance is 15°C for air and water temperature.


A device designed to reduce corrosion in lined steel water cylinders and pipework due to galvanic action or electrolysis. It is normally made from magnesium or aluminium and has a higher potential (positive) than the material it is protecting (negative) which is called a cathode e.g. Vitreous enamel lined cylinder. An anode will “sacrifice” or “dissolve” in order to prevent corrosion of the material it is protecting. Its life span is limited and will decrease with the rise in temperature, worsening water quality and materials used in the system. An anode may be in the form of an expendable electrode used to reduce or prevent corrosion (see under “Galvanic Action”) or as a source of material electro plating.



A component designed to control or deflect the flow of a fluid or gas (e.g.: flue gases in a gas heater). In the case of Dux gas storage type water heaters the flue baffle has been designed to ensure maximum heat transfer of energy contained in hot flue gasses to the water in contact with the flue surface while maximising efficiency of gas consumption and windy area performance reliability.


The boiling point refers to the temperature at which a liquid changes to vapour by the addition of heat. The boiling point depends on the pressure at which the liquid is held; and it increases as the pressure increases. As the atmospheric pressure reduces as altitude increases so the boiling point of water in an open vessel decreases with rising altitude.



A non-return valve is intended to allow a fluid to flow in only one direction in a pipe, should conditions occur which might lead to flow in the reverse direction the valve automatically closes.


Storage type gas water heaters are rated in terms of their capacity expressed in litres – that is, the volume of water they contain. This is not a measure of how much hot water is available from the water heater.


Combustion takes place when gaseous, liquid or solid fuels react at an elevated temperature with oxygen by burning, thus releasing heat.

For perfect combustion an adequate supply of oxygen must be available, and all the carbon in the fuel will be converted to carbon dioxide and all the hydrogen to water vapour.

The presence of carbon monoxide in the flue gases and/or “sooting” indicate imperfect combustion. Imperfect combustion results from poor burner aeration where there is an incorrect quantity of air being supplied for every volume of gas supplied to the burner. That is, there may be insufficient air or excessive gas flow or pressure.


Convection refers to the transfer of heat by means of a flow of fluid. The fluid (which may be a gas as air, or liquid such as water) is heated in one place then moves to a place where it can give up its heat. Convection can be either natural or forced.

Natural convection is caused by the expansion of a fluid with heat, making it less dense than the surrounding fluid and allowing the heated fluid to rise. This principle is used in the heating of water by applying heat at the bottom of a vessel and allowing convection effects to carry the heated water to the top of the tank from where it is drawn off. This configuration in Dux water heaters allows draw off of the hottest water in the tank without the incoming cold water (refer Displacement) at the bottom of the tank diluting the temperature of water at the outlet.

Natural convection is also the mechanism that is relied on in the flueing of gas water heaters to draw the hot gaseous products of combustion away from the burner and up the flue thereby heating the contained water.

Forced convection refers to the transfer of heater fluid by pump or fan and is used where natural convection is either inadequate or unsuitable. Typical applications include pumping of water in some heater systems and “Power Flues” in some gas water heaters such as the Deliverance range.



The term used to define performance of an electric storage water heater. It is measured in litres and is the amount of water a heater can deliver on standby mode, before a significant (12-14°C) drop in temperature. General rated delivery rates are nominated by regulatory bodies. Acceptable rates as defined in the Australian Standard for Storage Water Heaters, AS1056 are as follows.


25 31.5 40 50 63
80 100 125 160 200
250 315 400 500 630


Hot water is much less dense than cold water and this is used in the displacement principal on which most storage water heaters operate. If cold water is fed into the bottom of a tank full of hot water it will displace an equal quantity of hot water out of the top of the tank, and if the heater is correctly designed the hot water will float on the cold water for a considerable time without mixing. All Dux storage water heaters in current production use the displacement principal.


An angled tube of high temperature polypropylene or copper positioned in Dux storage type systems to ensure that the hot water is drawn from the highest point in the tank where, thanks to convection, the hottest water is located. These tubes enable us to maximise the temperature of hot water available and its quantity.


A device fitted in the flue way of a gas appliance to prevent updraught, down-draught or secondary flue blockage from obstructing the escape of products of combustion, or otherwise affecting normal operation of the appliance.


The deterioration of many brass alloys in contact with hot water is caused by the mechanism of “dezincification”. This results when the zinc content of the brass is leached out leaving a spongy copper structure, which may leak or burst under pressure. For this reason it is recommended that all plumbing fittings used on Dux water heater installations are made of “DR Brass”. These can be identified by “DR” stamped on the fitting.


A section of pipework that does not form part of a constant circulation system. Water in this section is stagnant until a flow is activated by opening a tap.



Efficiency is a measure of performance expressing the results obtained as a percentage of effort or energy put in. On this basis thermal efficiency is the percentage of the potential heat content of a fuel which becomes usefully available as hot water (or hot air in the case of a space heater). Thermal efficiency is a measure of performance under stated conditions, and the efficiency will vary as the conditions are varied. Accordingly, official or test thermal efficiencies should not be quoted or used unless the circumstances are comparable to the method of test. The Energy Rating or “Star Rating” scheme used in the labelling of gas water heaters (no such scheme applies to electric water heaters) is intended to assist the consumer in comparing the relative efficiencies of gas appliances on offer in the market. The Star Rating assigned to a product on the basis of a test, which models the hot water usage of a “typical” household.


The element used in Dux storage type electric units is basically a coil of resistance wire in a copper sheath to make a heating unit which produces heat by the passage of an electric current through it. All Dux electric water heater utilise a sheathed (copper or Incalloy) element where the resistance wire is contained within the sheath and is protected from the corrosive effects of hot water under elevated pressures.


This is a safety device fitted to a water heater and designed to cut off the energy supply to the heater (i.e. gas or electricity) should the thermostat cease to function. The energy cut-off is intended to prevent the occurrence of excessive temperature in the water heater. This device is incorporated in all electric thermostats (the cutout temperature is denoted “ECO..”) and gas control valves used in Dux water heaters.


The rate in which energy is supplied to an appliance. The hourly consumption (HGC) is the basic or nominal input at which a gas appliance was tested and to which published performances figures apply.

The HGC is measured by the one-hour input in megajoules (MJ). Orifice size supplied and published gas pressure at the orifice give HGC on one particular gas only and adjustment may be needed on other gas types. Electrical appliances are rated in kilowatts or watts (1 kilowatt equals 1,000 watts). Again this applies only to the voltage for which the element is designed. That is, a 2.4kW element in a Dux electric water heater will only provide 2.4kW when connected to single-phase electricity. (240 volts of 50 Hz)



This term is often used to express the amount of hot water available from a gas storage type water heater and is derived by adding the capacity (contained volume in litres) of the water heater to the recovery rate (litres/hour). This is a somewhat misleading figure because it is only possible to supply this quantity of water all as hot if the water heater is full of hot water and stabilised at full temperature from top to bottom prior to the first hour of use.


The passage through which combustion or exhaust products are conveyed from an appliance to a discharge point.


Just as the human body can break down if exposed too long to an adverse environment or repeated stress, so also can materials fail due to fatigue. A material can fail by repeated exposure to a stress well below its normal breaking point purely by cycling the stress on and off. Water heaters can be exposed to repeated pressure fluctuations during their life and must be designed to resist the effects of fatigue.


A device which senses the presence of flame and causes the fuel to be shut off in the event of flame (e.g. Pilot) failure.



If two unlike metals are immersed in an electrolyte an electrical current will flow. The metal, which becomes the anode of this cell will corrode and dissolve, while the other metal, the cathode will be protected from corrosion. The two metals do not have to be vastly different for this effect to take place. It can occur with, say, copper and copper alloy. It can even occur with two different parts of the same sheet, tube or rod. The purpose of the sacrificial anode in Dux water heaters is to provide a metal (magnesium or aluminium) more reactive than the steel, brass or copper of the tank, element or valves for the corrosive effects of galvanic action to concentrate. That is, by providing an anode all other metallic components of the product become cathodes and are protected from corrosion.


This definition will be confined to the fuel gases for which our appliances are designed. There are five basic types of fuel gases:
a) Natural Gas
b) Town Gas and Processed Natural Gas
c) L.P.G
d) T.L.P or Tempered L.P
e) S.N.G or Simulated Natural Gas.

Natural Gas : Widely used throughout the world is obtained from bores sunk into the earth either on land or under the sea. Although natural gas varies in different parts of the world, it is basically methane, but it may contain a number of other contaminants that may have to be removed before it can be distributed. A “wet gas” contains hydrocarbons, which are liquid at normal temperatures (e.g. Petrol fractions); a “sour gas” contains hydrogen sulphide. As finally distributed natural gas is substantially methane, it has a high H.V (heating valve) of around 40 MJ per cubic metre and is not toxic.

Town Gas : Is of very variable composition, depending on the plant and the raw materials used in its production. Although originally based on coal gas, coal is becoming less significant as a raw material and reformed gas is becoming more important. Town gas has a H.V around 20 MJ per cubic metre and is generally toxic due to its carbon monoxide content.

L.P.G : (Liquefied Petroleum Gas): Consists of commercial (propane (C)3(H)8() or butane (C)4(H)10(), or a mixture of both. These gases are obtained as a by-product from the processing of raw natural gas or from oil refining. The gases can be distributed as a gas, or more frequently compressed to a liquid under pressure and transported in pressure vessels. L.P.G, when sold under a trade name such as Elgas, Heatane etc., is usually commercial propane, but butane may be supplied in hotter regions. L.P.G has a high H.V (95 MJ per cubic metre for propane, 118 MJ per cubic metre for butane) and a high relative density (1.5 propane, 1.9 butane). Being heavier than air, L.P.G does not disperse as readily as town gas, and this makes L.P.G more hazardous than town gas.

T.L.P :or Tempered Liquefied Petroleum Gas: This is a mixture of propane (or butane) and air for distribution by piped supply from a central source, as with town gas. It is being used to replace town gas, particularly in country areas, due to the ease in which it can be produced by automatic plant operation.

S.N.G or Simulated Natural Gas: A mixture of L.P.G and air for distribution by piped supply from a central source. As the name implies, it approximates the characteristics of natural and is used as a substitute for natural gas either before natural gas is used or as a peak load substitute.



A heating system using circulating hot water.


Also known as indirect storage or “coil-type”, this is a water heater in which water is heated by passing through exchanger (usually in the form of a coil of copper tubing) immersed in static heated water.


A device that transfers heat from one medium (e.g. fluid or air) to another medium thereby cooling the first and warming the second. A heat pump is a device that regulates water or air temperatures for heating purposes. It operates much the same as a reverse cycle air conditioner in that it uses refrigerant as a means of heating and cooling. When heating is required, the refrigerant is compressed and the resulting heat energy created by this action is released into the room by forced air movement or to water by conduction. If cooling is required, the system is reversed and the heat energy in the room is absorbed and expelled outside.


When fuel is burnt in air, heat is generated. The heating value, formally referred to as the calorific value, of a fuel is the amount of heat developed when unit a quantity of the fuel is burnt.


Another name for head or pressure under static conditions, i.e. with no flow of water or gas. The other type of pressure is “dynamic” i.e. measured when water or gas is measured while flowing.


A type of water heater in which heat energy is added only when water is flowing through the water heater. (In contrast to storage type water heaters).



Materials with such poor ability to conduct heat that they are used to prevent heat being lost from an appliance. Most insulating materials depend for their effect on the presence of numerous air cells, so held that convection is restricted or prevented.




A measure of the amount of energy used, and is equal to 1 kilowatt operating for 1 hour, 2 kilowatts for ½ hour, ½ kilowatt for 2 hours, 100 watts for 10 hours etc. The kWh is the unit of energy by which electricity is sold to the consumer. One kWh is equivalent to 3.6 Megajoules.



Legionella are bacteria. They can be found in all types of water supplies and under certain conditions such as warm temperature and slow movement can proliferate to hazardous levels resulting in the possibility of susceptible people contacting Legionnaires Disease. They can also be found in moist soil and air. In water systems where stagnant water and warm temperatures (22°C to 55°C) are experienced, Legionella can multiply rapidly. There are systems available that can help control Legionnaires Disease hazards through temperature control and through dosing systems utilising chemicals to kill the bacteria.



An instrument used to measure low gas pressure.


This is a heater connected directly to the cold water mains so that hot water is delivered at the same pressure as the cold water mains.


Unit of energy equivalent to 1 million joules. (J). One Megajoule is equivalent to 0.2778 kWh.


The maintenance rate is the rate at which heat or energy has to be supplied to an appliance to maintain its temperature at the required level without the unit being used. In the case of a storage heater, it is the heat input required to maintain a specified temperature (generally 45°C above ambient on gas heaters and 55°C above ambient on electric heaters). The maintenance rate may be measures in MJ per hour for gas or kWh per 24 hours for electricity. The temperature above ambient must always be quoted. In reference to electric water heaters the term “heat loss” or “standing heat loss” is usually used.



The flow produced by the tendency of warm/hot air/gases to rise.


Dux twin element electric storage units are wired so that the boost (upper) element (intended for connection to continuous tariff electricity so that the boost ability is available any time of the day or night in the event that all the water heated by “off-peak” power has been consumed) will never operate at the same time as the lower element which is connected to “off-peak” electricity and performs most of the heating of hot water but only during the periods that “off-peak” power is available. The electricity supply is only available to the lower element when the upper element’s thermostat is OFF – this ensure that the current drawn by either element does not exceed the capacity of the supply wiring or the switchboard. The upper element only heats the volume of water above it whereas the lower element will heat the entire volume of water contained in the tank.



A safety device incorporated in a water heater that stops the supply of electricity or gas and / or the flow of water when the outlet water exceeds a preset temperature.



Intended for drinking or cooling purposes.


A permanent pilot burns to ignite the main burner automatically as required. Heat from the pilot is absorbed by the water. As the typical gas consumption of a pilot burner is less that 1MJ/hr it only costs a few dollars per year to operate.


A measure of the acidity of alkalinity of aqueous solution on a scale of 0 to 14. A pH value of 7.0 indicates a completely neutral solution. Below 7.0 the solution is acidic; the lower the pH the more acidic it is. Above 7.0 the solution is alkaline and the higher the pH the more alkaline it is.


A device used to provide a high-voltage spark to ignite the pilot flame on gas water heaters. Pushing the igniter button permits a spring-load plunger to strike and deform a piezo crystal. This generates a very high voltage, which causes a sharp spark to jump across the gap between the electrode and the pilot burner. The spark ignites the pilot flame.


A form of pressure reducing valve, which automatically reduces inlet water pressure within the acceptable limits at the outlet under static cold-water conditions, but only when supply pressure exceeds a predetermined minimum.


A valve, which automatically reduces inlet water pressure to a specified value at its outlet under static cold water conditions.


The portion of the total air for combustion which, is induced into the throat of an aerated burner and mixed with gas prior to ignition on the burner ports.


Also known as a “pressure and temperature relief valve” or P.T.R Valve or T.P.R Valve or T & PR Valve. It is a device which automatically opens a relief vent when the pressure and/or temperature rises to a predetermined value and closes when the pressure and/ or temperature falls below this value.


The primary flue is the flue passage within an appliance, which conducts the flue products from the combustion chamber to the draught diverter or a balanced flue terminal. The primary flue in a conventional storage water heater passes through the centre of the storage cylinder and provides a heat transfer surface area.




The quantity of water (litres) that can be heated through a specified temperature rise (e.g. A 45°C rise = heating water coming in at 15°C to an outlet temperature of 60°C) in 1 hour. This is largely determined by the rate of energy input (gas consumption or element rating) and the ability of the water heater to transfer the energy to the water (thermal efficiency).


The difficulty with an electric current will pass through a material. Even the best conductors have some resistance. The passage of an electric current through a resistance produces heat, and the amount produced is given by the following formula: Watts = volts(2) / Resistance on ohms


A liquid used to cool devices by absorbing heat from surrounding air or liquid as it evaporates. RING MAIN An industry term for a flow and return circuit transporting fluids or gases. SAFETY SHUT-OFF VALVE A device on a gas appliance which shuts off the gas supply to the appliance to prevent a hazardous situation. A flame failure safety shut-off (or safety pilot) operates when the actuating flame becomes extinguished. A “100%” shut-off valve cuts off all gas, including the main pilot burners. Other types may cut off only the gas supply to the main burner(s).



This is a term used to describe the formation of scale (or lime) on the inside of water heaters and associated pipework or fittings. It is a result of minerals such as calcium carbonate being deposited out of the water when the water is heated. The propensity of water to scale is defined by the saturation index.


Progressive rise in temperature of water in the top of the storage water heater caused by frequent draw-off of small quantities of hot water from the outlet. This situation can occur in gas water heaters where heat is absorbed into the water above the level of the thermostat. Precautions are built into the design of the heater to prevent the water at the top of the cylinder from becoming excessively hot.


A water heater incorporating a storage tank such that water can be heated whether water is flowing thorough the heater or not flowing (as contrast to instantaneous or continuous flow water heaters).


This term is used to describe the arrangement of a body of water into two or more layers of different densities. Water expands, as it is heated (see Thermal Expansion); therefore hot water has a lower density than cooler water. In a storage water heater the less dense hot water “floats” above the cold water. This is a very stable arrangement and very little mixing can occur without mechanical agitation.


A multiple element electric storage type system where all elements are able to operate simultaneously e.g. the “Titan” range


A device that is fitted to a circuit to alter flow characteristics of the medium. It has an electrically operated motor which is externally controlled, usually by a thermostat, which opens or closes a gate or ball in the body of the valve that the medium flows through.


Certification of a product as “StandardsMark” approved indicates that the product is fully compliant with all requirements of the appropriate Australian Standard pertaining to details of performance, design, manufacturing, quality and safety of operation. Some products may claim they are manufactured in accordance with the Australian Standard but this does not mean that they necessarily comply with the Standard or would pass if tested to the requirements of the Standard. At present all Dux gas storage products and 250 and 315 litre electric storage units have StandardsMark approval to AS1056 “Storage Water Heaters”.



A device that ensures delivery of hot water at a specified temperature to an outlet by mixing two different streams of hot and cold water to provide a cooling effect in the situation that the hot water from the water heater exceeds the desired outlet temperature. These are a common method of ensuring that an installation meets the 50(0)C maximum outlet temperature regulation applying to all new buildings as per AS3500.4:2003 Heated Water Services (Part1.9.3)


On being heated, all solids and liquids expand; the exact amount of expansion varying with different materials. The expansion of metals with heat is used to operate solid stem thermostats where two materials of different expansion characteristics are involved. The different expansion of the two materials with heat is made to operate a valve or switching mechanism at an appropriate temperature. The expansion of water on heating has three effects. First, there is an increase in volume for the same weight of water. This increase is allowed for in Dux mains pressure storage type water heaters by provision of the Pressure and Temperature Relief Valve, which allows the extra volume of water to leave the storage cylinder. The amount produced is proportional to the temperature rise and indicated in the following table:

Temperature Rise Relative Density of water Expansion of 1 Litre
when heated from 20°C to temp.
20 °C 0.998 0.006 litres
40 °C 0.992 0.015 litres
60 °C 0.983 0.027 litres
80 °C 0.972 0.0421 litres

The second effect is that expansion with heat makes the liquid less dense, hence heated water is lighter than cold water and it rises by means of “convection currents”. This effect is used in circulating water heaters, which are storage cylinders fitted with flow and return connections. In these installations the hot water rises from the heating unit up the “flow” pipe to the storage tank, and cold water flows from the tank down the “return” pipe to the heating unit. The third effect is that hot water, being less dense than cold water, will float on top of cold water without mixing for some time. This principle is used in all displacement water heaters and allows cold water to enter the cylinder (while hot is taken from the top) without mixing with the hot water.


When two unlike metals are joined to form a circuit and there is a temperature differential between the junctions an electric current is produced. The current can be made be made to do useful work by exciting an electro-magnet, which will hold a valve open while ever the junction between the two metals is hot. When the junction cools the current ceases to flow and the magnet allows the valve to close. This is the principle used by gas control valves to determine whether the pilot burner is alight and hence if it is safe to supply gas to the main burner. If the pilot is not alight a dangerous situation could arise if large quantities of gas are supplied to the main burner but there is no source of ignition to light the gas-perhaps until someone walks past and lit a cigarette. By this time there could be many litres of unburnt gas in the vicinity. A further application of the thermocouple is in testing of appliances. By measuring the current produced, the temperature of the hot junction can be determined. This is used as a means of determining temperatures of floors, appliance jackets, spot temperatures etc. without the use of a thermometer.


A device which automatically maintains a pre-determined temperature in an appliance. Thermostats can be of the “immersion” pattern where the actuating components are immersed in fluid whose temperature is to be controlled or of the “contact” pattern where the actuating components sense the fluids temperature through the wall of the container. The thermostats used in Dux electric units are of the “contact” type. Thermostats can be “snap acting”, i.e. operate from full-on to full-off and vice versa, or “modulating”, operating gradually from full-on to a minimum by-pass flow and vice versa. Some thermostats combine a modulating function from full onto low-fire, then snap action to off. The “differential” of snap-action thermostats is the temperature difference between the opening and closing functions. In the case of the thermostats used in Dux electric units the “differential” value is marked on the thermostat as “DIFF..”.


A device that mixes two streams of hot and cold water to achieve the desired temperature with a greater degree of accuracy than a tempering valve. These devices are used in hospitals and aged care facilities in place of tempering valves due to a more reliable fail safe mechanism however they require regular maintenance.


A device that measures temperature by monitoring changes in the electrical resistance of a probe immersed in hot water. Thermistors perform the temperature measurements in the Endurance range.


The sum of all the dissolved minerals in the water expressed in milligrams per litre (mg/L).



As applied specifically to water heaters, this means that there is no open exhaust or vent pipe in the hot water system open to atmosphere at all times. All Dux storage type water heaters, both gas and electric are of the unvented type. The advantage of unvented water heaters is that they can provide all the full-flow benefits of mains pressure water at the hot water outlet.



When referring to water heaters, this is fundamentally a coating of vitreous enamel bonded to a steel surface by firing at high temperature (approx 850°C). The enamel is a special formulation resistant to dissolving in hot water and is not the same type of glass as that used in windows and glassware. Glass-lined heaters may be referred to as “glass enamel lined”, “glass enamelled”, “vitreous enamelled”, “bonded vitreous lined”, or “porcelain enamelled”. Dux “Duronamel” is a form of low solubility (in hot water) vitreous enamel.


An appliance designed with a provision for a vent permanently open to the atmosphere. These types of systems are often marketed as “low” or “medium” pressure units and their installation is characterised by “risers” or vent pipes in place of a PTR Valve. These vents ensure that the units supply hot water at satisfactory pressure without excessive pressurisation of the water heater vessel. Water heaters of this type may be connected to mains pressure cold water but this installation involves special valves to ensure that the vessel is not subject to full mains pressure. This reduced pressure of operation may not be sufficient to supply more than one outlet at a time and may not provide sufficient flow to shower roses etc.



WaterMark approval of a product indicates that it has been tested and has passed a number of requirements regarding its suitability for supplying water for household use and human consumption. These tests encompass areas such as taste, smell, toxins and corrosive effects on plumbing connections and pipework. WaterMark approval is not proof that a product will operate correctly or that it complies with the relevant Australian Standards regarding product quality or performance. WaterMark approval is mandatory for plumbing products intended to be connected to municipal water mains in most parts of Australia.


Your Dux water heater has been manufactured to suit water conditions of most Australian metropolitan supplies. Please note that harsh water supplies can have a detrimental effect on the water heater and its life expectancy. If you are unsure about your water quality you can obtain information from your local water supply authority.

The water heater is designed for use in areas where the Total Dissolved Solids (TDS) content of the water supply is less than 2500 mg/L. In areas where the TDS exceeds 600mg/L it is possible that the magnesium alloy anode (supplied in the heater) may become over reactive. To alleviate this, the magnesium alloy anode should be replaced with an aluminium alloy anode, available from your local Dux supplier.

Water can also be very corrosive, the measure of this is the saturation index, if the water saturation index is greater than 0.40 a expansion control valve should be fitted and where the index is greater than 0.80 the water heater should have an Aluminium Anode fitted. Electric units should also have the copper-sheathed element replaced with an Incalloy-type element. Please consult our Customer Service Department for advice if required.