Chapter 4 - Boost Heating - Solar Association of New Zealand
Contents
1. Insulate pipes and weatherproof if exposed to weather Uncontrolled heat source Figure 4 1 6 Installation of slow combustion booster Control of boosting Boost heating can be controlled by thermostat only thermostat plus manual or timer based override smart electronically programmable controls Thermostats only Thermostat only systems do not allow optimisation of the solar contribution as the thermostat may switch the booster on after hot water is used and before the solar collectors have a chance to reheat the water Hot water passes through the collectors and little or no heat is added Thermostat plus manual or timer based override A manual override switch can be added to allow the user to turn the booster off before the water temperature in the tank above the booster reaches its preset temperature typically 60 C However this relies on the user remembering to switch off the booster Solar Water Heater Training Course Installer and User Manual 122 4 Boost heating A better solution is for the user to set the period of boosting with a timer This allows the user to reduce or increase the boosting time according to weather conditions expected demand in the coming hours and present tap water temperature This system works well and users quickly get a feel for the appropriate boosting time A manual override switch is still incorporated in this arrangement This adds a little to the capital costs but2. 25mm that connects the solar collector to the storage tank This particular unt has anti freeze fluid in the collectors and surrounding the storage tank so in fact there are two heat exchangers the second being the jacket around the hot water storage tank Figure 4 2 12 Roof penetration for heat exchanger Solar Water Heater Training Course Installer and User Manual 135 4 Boost heating Key points Boost heating can be done using solid fuels such as wood bagasse coal or briquette Heating using these fuels is known as an uncontrolled heat source This sort of boost heating is normally used only with vented low pressure systems since the heat source can continue to provide heat even after the water has reached its set temperature Uncontrolled heat sources can include Solid fuel heaters and cookers often using a boiler coil Boilers which might be a small rectangular tank made of copper Flue jacket boilers Water heating from solid fuels is not free since it will decrease the amount of heat available for space heating Although many solid fuel boosted solar water heaters are vented mains pressure hot water can be provided by running mains pressure water through a coil in the hot water tank or using a heat exchanger within or outside a mains pressure tank Section 4 2 questions 1 What is meant by the expression uncontrolled heat source 2 The expression comes from
3. if the storage tank is lower than the boiler but provision must be made for the steam to escape if the pump fails and a pumped system is best avoided The hot flow pipe from the boiler must have an uphill gradient all the way to the storage tank so that any air bubbles released as the water is heated can escape into the storage tank The connecting pipe work must be copper tube not plastic The storage tank must be copper or stainless steel though this is not usual not plastic or vitreous lined enamel The enamel starts to Solar Water Heater Training Course Installer and User Manual 128 4 Boost heating dissolve if the temperature of the water is too high High temperatures also set up stresses between the expanding steel and the enamel lining The whole system must be open vented This means that the boiler must not be able to develop pressure If the boiler boils the steam generated must be able to escape into the atmosphere This is the most important single safety requirement PTR pressure temperature relief valves must not be used Boilers must not be connected directly to mains pressure hot water storage tanks There must be no valves between the boiler and the storage tank When connecting a solid fuel boiler and a solar system to a hot water storage tank it is advisable to use two separate pairs of nipples This way the circulation of one system will not interfere with the other system
4. of gas all the time What are the disadvantages of an electronic ignition unit that prevents it being used on standard gas hot water storage systems 6 How is the temperature of the hot water controlled What is the name of the temperature control device that turns gas or electricity on or off Solar Water Heater Training Course Installer and User Manual 124 4 Boost heating 4 2 Connection to an uncontrolled heat source What this section is about A normal hot water service gas or electric reaches a set temperature minimum 60 C and then the electricity or gas is shut off Heating stops The source of heat is controlled An uncontrolled heat source solar firewood coal etc continues to provide heat even when the water being heated has reached a satisfactory temperature In this section we look at uncontrolled heat sources and how they can be used to boost the temperature of water in a solar hot water system By the end of this section you should have an understanding of the words solid fuel boiler and wet back open vented and what that means direct and indirect heating heat exchanger Solid fuel Solid fuel is the name given to fuels such as firewood wood chips paper sawdust pellets sugar cane residue bagasse black coal brown coal briquettes peat animal manures and straw The main solid fuels used domestically in Australia and New Zealand are fire wood and coal In Australia and New Zealan
5. saves on wasted boosting energy Smart electronically programmable controls The ideal solution with full convenience is for an intelligent controller to be programmed to only boost to preset temperatures during each hour or so of the day with the temperatures being set low or to zero during daylight hours to avoid boosting These controllers are already available learn the pattern of use of hot water in the home and to program itself using smart or fuzzy logic These controllers are currently under development An example of the first type is the Solarit controller for pump circulated systems This allows the user to pre program the temperature for each two hour period of the day Solahart s Optimiser controller can use a pre programmed algorithm Key points All solar water heaters will require boost heating at some time unless o the user accepts cold water occasionally which might be acceptable in Darwin but not Brisbane or Melbourne o the system is substantially oversized this is expensive and stresses the tank and collectors more due to more frequent overheating In Australia solar systems are generally sized to provide between 60 and 90 of the total hot water demand This means that boost heating will be used nearly all winter in cold climates or on very cloudy and or rainy days in warmer climates e Boosting of water temperature in solar systems can be done by o electricity o ga
6. tank With greater horizontal distance X pipe diameters must increase With greater vertical distance Y the smaller the diameter can be Circulation is achieved by thermosiphon flow natural convection currents due to cold water being denser than hot water The cold water falls and the hot water rises The greater the temperature difference the greater the rate of circulation Pipe friction slows the rate of flow of water through the connecting copper tube and the longer the tube the greater the pipe friction Solar Water Heater Training Course Installer and User Manual 130 4 Boost heating Pipe sizes recommended to connect a boiler to a storage tank are found in AS NZS 3500 4 The greater the distance between the boiler and the storage tank the greater the pipe diameter required The greater the heat output of the boiler the greater the pipe diameter required The smaller the vertical distance between the boiler and the storage tank the greater the pipe diameter Mains pressure hot water systems and an uncontrolled heat source It is commonly said You cannot have a mains pressure water storage and a solid fuel boiler This is only partly correct It would be better to say You must not connect a solid fuel boiler directly to a storage tank under mains pressure Most copper hot water tanks are gravity fed A small supply tank is attached to the side of the storage tank The tank must be higher
7. 4 Boost heating CHAPTER 4 Boost Heating Table of contents 4 1 The need for boost heating 4 2 Connection to an uncontrolled heat source Solar Water Heater Training Course Installer and User Manual 113 4 Boost heating 4 BOOST HEATING 4 1 The need for boost heating What this section is about This section will provide you with an understanding of the necessity for boost heating the alternative methods of boost heating commonly used booster control methods the methods to optimise the solar contribution and minimise boosting requirements The need for boost heating Figure 4 1 1 shows the anticipated solar contribution to water heating for a well designed system in various locations across Australia and New Zealand This assumes a hot water demand of around 200 litres per day and a booster temperature set at 60 C Port Headland Alice Springs J Auckland Wellington 55 65 hristchurch Map Courtesy Solahart Figure 4 1 1 Anticipated solar contribution for various Australian and New Zealand cities using modified figures from Australian Standard AS NZS 3500 As can be seen from this figure the standard recommended size of solar water heater will not provide all the energy requirements to produce hot water year round Note that in Victoria where solar water heaters are one option for new homes to comply with five star regulations zone 4 is treated as covering Solar Water Heate
8. Never use the same pair of pipes for the stove and solar collectors up to the storage tank Most domestic boilers produce about 3kW maximum many produce less The normal copper tube size to connect boiler and tank is 20mm diameter If the boiler and the storage tank are a significant distance apart perhaps 10 metres or more then 25mm tube is used It is best to use at least 25mm tube for solar connections If there is very little height difference between boiler and tank a larger pipe diameter is necessary The copper tube should be well insulated between the boiler and the storage tank Hard drawn copper tube is preferred rather than annealed tube For large boilers larger diameter copper tube is used The size is determined by the capacity of the boiler in kW the vertical difference in height between the boiler and the storage tank and the length of the pipe run As a general rule however for 3 10kW boilers use 25mm tube 10 20 kW use 32mm tube and 20kW or over use 40mm tube These figures assume a boiler at floor level a storage tank in the ceiling and a distance of no more than 10 metres between the boiler and the tank Consult the boiler manufacturer or supplier for recommended pipe sizing A valve to prevent boiling hot water reaching the bathroom hot water outlets is essential A valve that mixes cold water with the hot water may have various names a tempering valve a thermal mixing valve or a temperature modu
9. a booster at all but acts as a pre heater for the gas burner If the water entering the gas system is already at a suitable temperature the burner will not ignite This means that only the water needed is heated and that such a system is therefore more efficient Slow combustion heaters and stoves On properties where there is access to solid fuel slow combustion heaters can provide the required supplementary heating In many rural areas properties have dead timber readily available on site If timber has to be purchased at normal retail prices in urban and metropolitan areas the cost of slow combustion boosting may be significant Slow combustion boosters are deemed to be uncontrolled energy sources and must be installed to the requirements of AS NZS 3500 4 An uncontrolled energy source may produce sudden surges in the pressure of the water It must be open vented to cater for these surges and therefore will be a low Solar Water Heater Training Course Installer and User Manual 121 4 Boost heating pressure system with the header tank lower than the open vent These requirements are shown in Figure 4 1 6 4 Vent to atmosphere Primary flow and Long radius return pipes see bends 3D Cold table in AS3500 4 Niam water supply Header tank for recommended minimum diameters based on horizontal and vertical distances between heat source and storage tank Storage tank diameter D
10. amps Figure 4 1 3 Common electric boost element sizes The supply authorities have minimum requirements for both the wattage and the size of the hot water tank to qualify for off peak electricity rates These requirements vary from state to state but typically require a minimum tank of 250 litres capacity and a 2 4kW element The standard size of solar systems for an average home of three people and 170 to 200 litres of hot water per day is a 300 litre tank a 2 4kW element and two or three collectors depending on climate and system orientation This additional storage capacity of one half day to a full day s extra capacity above the daily demand gives some reserve capacity to carry over from sunny days to cloudy or rainy days This gives the solar collectors the opportunity to make their maximum contribution One limitation of using the off peak tariff is that the booster element will not able to be turned on during the day if more hot water is required i e at times of high water usage and low solar radiation In some electricity supply areas the off peak tariff is available for hot water boosters for some systems during non off peak times The booster used in many solar water heating tanks is an immersion type of element that has a sickle or curved shape This can be positioned in the storage tank to provide varying amounts of boosted hot water For example if off peak tariffs are used particularly in colder climates then the s
11. d solid fuel is used because it is cheaper than LP gas or electricity Often firewood is available to householders from their own properties and the only cost is that of collecting the wood Photos Andrew Blair and Philippa Noble Figure 4 2 1 Coal and firewood are common sources of solid fuel in New Zealand and Australia Solar Water Heater Training Course Installer and User Manual 125 4 Boost heating Heaters and cookers Solid fuel heaters and cookers may have a boiler sometimes called a wet back in the firebox so that the fire heats the water in the boiler Solid fuel cookers will provide hot water whenever the fire is alight In the summer time the sun heats the water with solar collectors and often the cooker is not used because it makes the house too hot A heater operating only in winter can heat the water at a time when the sun does not provide enough heat This cooker uses firewood to heat water in winter In summer the sun heats the water through the solar collectors Photos Andrew Blair Figure 4 2 2 Cooker providing water heating in winter The room heater provides winter heat for the house and hot water The pipes from the boiler in the heater are connected to the hot water storage tank through the brick wall behind the heater Figure 4 2 3 Wood heater with boiler coil The boiler in this heater is a simple single coil of 25mm diameter copper tube Unfortunately the coi
12. ength running through the centre The outside is well insulated paue ee SS _ _ Figure 4 2 10 Heat exchanger constructed from 40mm and 25mm tube Solar Water Heater Training Course Installer and User Manual 134 4 Boost heating The heat exchanger to conduct more heat can be made using a 100mm diameter copper tube Hot water surrounded by insulation Inside the large 4 diameter tube is a series of smaller tubes carrying one lot of water and surrounded by a different lot of water Heat is conducted between the two lots of water through the wall of the smaller tube Unlike the simpler two pipe system this unit cannot be made from standard pipe fittings The length of the tube is determined by how much heat is to be transferred Water surrounding the thin 12mm tubes is heated by hot water passing down the inside of the thin tubes If the heat exchanger is working only by convection currents thermosiphon flow hot water will enter the central connection pass through the tubes losing heat as it goes coming out the bottom central connection The water being heated enters the bottom side connection and leaves after being heated through the top side connection Cooled water Figure 4 2 11 Heat exchanger to conduct more heat In Figure 4 2 12 below the heat exchanger is in the ceiling space and the 20mm hot pipe from it comes up through a roof penetration and into the hot line
13. he tank has cooled However some people don t wish to manually limit boosting time as they find it inconvenient even though this can be done with a simple timer Merely letting the thermostatic control of the booster operate does not optimise the Solar Water Heater Training Course Installer and User Manual 116 4 Boost heating solar contribution This problem has been addressed with the smarter boost controllers now available and these are described in detail below Types of boost heating options A decision needs to be made regarding the type of booster that is used Not all the options will be available so often the choice will be made for you If available the options are electric off peak or day rate tariffs gas solid fuel heater vented systems only Electric off peak or controlled tariffs Where off peak tariffs are available an electric heating element is supplied by the electricity supply company during set off peak hours typically for about eight or nine hours For example these periods can be 9pm to 6am SE Queensland or 11pm to 7am Victoria In some states controlled tariffs are available that offer more hours per day but at a higher cost e g for up to 18 hours per day but at 50 higher cost Electric element sizes commonly used are shown in Figure 4 1 3 1 8kW at 240 volts and 7 5 2 4kW at 240 volts and 10 amps amps 3 6kW at 240 volts and 15 amps 4 8kW at 240 volts and 20
14. ickle is curved down to increase the volume of boosted water to about 85 Some manufacturers alter the orientation of the sickle element or use straight Solar Water Heater Training Course Installer and User Manual 117 4 Boost heating elements in mild to warm climates to alter the amount of boost heating This restricts boost heating to the top 50 of the tank Off peak boosting is relatively cheap typically 5 5 to 7 5 cents per kWh 2005 costs plus a minimum monthly charge However in warmer regions where solar contributions of 85 or greater are easily achievable then off peak tariffs can be no cheaper than day rate because so little electricity is being used and the off peak tariff will impose a minimum charge per month or quarter three month period that must be paid regardless of how little electricity is used For example in Brisbane where a typical solar system gives a solar fraction of 86 the minimum charge for off peak would be paid for eight months and the total yearly boosting cost would be 47 By comparison if day rate electric tariff was used the cost would be only 53 based on 2005 costs Electric day rate or continuous supply storage Day rate boosting implies that the booster is available at all times of the day and will be charged at day rate tariffs This is much higher often in the range of 14 to 15 cents per unit of electricity i e per kilowatt hour This means that the client shou
15. iler will produce enough hot water If they go for option 1 it is likely that with a large boiler the water will boil in winter Not only is the noise of the water boiling a loud banging sound annoying in time it is likely to damage the storage tank and connecting pipe work If they go for option 2 a small boiler then in overcast weather in autumn and spring the boiler will not make enough hot water Either way the heater manufacturers are likely to have dissatisfied customers so currently most heater manufacturers do not offer heaters with a boiler Boilers in heaters result in a cooler fire because the boiler takes heat away from the fire and cooler fires are more likely to smoke A smoking heater is less likely to pass the rigorous testing to which all Australian heaters are subjected Heater tests are expensive and if a heater has a boiler in its firebox a separate test must be conducted This is an expense that few heater manufacturers are prepared to pay considering that few boiler heaters would be sold Installation of a boiler The connection of a solid fuel boiler to a hot water storage tank has some similarities to connecting solar collectors to a storage tank refer to AS3500 4 section 7 3 e The heat source must be below the storage tank so that the hot water can rise to the storage tank This natural circulation by convection currents is called thermosiphon flow It is possible to use a circulating pump
16. l reduces the quantity of firewood that can be fitted into the firebox Free hot water Many owners of heaters or cookers say that they are getting free hot water because after all the heater or cooker would be running anyway To suggest that the hot water is free is not quite correct More wood or coal has to be used in a heater if it is heating water as well as the room A cooker has to burn more fuel if it has to heat water as well as do the cooking Some heater owners have found that with a boiler in their heater the room heating performance is so reduced that the heater will no longer do its job properly Cookers on the other hand are usually designed to provide enough heat for both cooking and hot water Solar Water Heater Training Course Installer and User Manual 126 4 Boost heating Boilers The boiler wet back may be a small rectangular tank made of copper stainless steel steel or cast iron The cast iron boilers are sometimes lined with vitreous enamel in order to prevent rusting The boiler may simply be a coil of copper tube as shown in the heater on the previous page In both cases the boiler is usually located within the firebox so that the flames lick up against it The boiler in this cooker left is located at the back of the firebox and appears as a flat metal surface The connecting pipes are not shown as they go out through the back of the cooker Photos Andrew Blair Figure 4 2 4 Boiler
17. lating valve In order that the valve operates properly the hot and cold pressure must be equal Any piping that is not protected by a tempering valve should be copper tube not plastic pipe Solar Water Heater Training Course Installer and User Manual 129 4 Boost heating Solar connections separate from the solid fuel boiler lines gt gt Connections to solid fuel boiler must not contain isolation valves Solid fuel boiler Drain valve Figure 4 2 6 Typical connection for solid fuel and solar system This is a typical connection that includes a solid fuel boiler and solar collectors connected to a gravity feed in ceiling copper storage tank Notice that the solar and boiler are connected at the tank independently of each other Figure 4 1 6 earlier in chapter shows how a small boiler such as in a heater or cooker is connected to a hot water storage tank AS NZS 3500 4 Section 7 3 includes a chart that shows recommended pipe diameters based on two distances X and Y This shows that the further the storage tank is e horizontally X from the boiler the greater the diameter of the connecting pipe needs to be e vertical Y above the boiler the smaller the diameter of the connecting copper tube can be It does not matter if the connecting pipes are larger diameter than specified but they must not be smaller The same applies with the connection of solar collectors to a remote storage
18. ld virtually never run out of hot water but they will pay for it The booster element will be set to come on at a predetermined lower temperature setting to give the system a chance to recover with input from solar radiation Location of the booster electric element To avoid running out of hot water the booster must be sufficiently far down from the top of the tank to heat all the water that is likely to be required before the booster is able to reheat the water above it If the booster has a low electric power rating or low gas heating rating then the amount of water it heats will need to be greater than if a higher rated heater is used If the water is heated with off peak electricity the tank must be large enough to hold an entire day s hot water above the electric element This means the booster must be able to heat one day s hot water during the off peak heating time This is the reason why the storage tanks must be larger when on off peak tariffs This larger tank size comes with a disadvantage as it has a larger surface area and heat losses are greater Figure 4 1 4 shows the ideal location of the electric boost element and the solar collector connections to the storage tank for different tariffs Solar Water Heater Training Course Installer and User Manual 118 4 Boost heating a ia Hot water J Hot water I outlet outlet I I 70 Litres p gt One day s A Booster and supply ther
19. ld water under high pressure passes through the coil and is heated by the hot water surrounding the coil so that it comes out hot If the water passes through too quickly it may not have enough time to absorb enough heat to be at a satisfactory temperature 12 litres minute is a common flow rate for a domestic system The vent allows steam to escape if the boiler heats the water to 100 C This system has the Solar Water Heater Training Course Installer and User Manual 132 4 Boost heating disadvantage of heating the water as it is used The rate of heating is limited by the rate of heat transfer from the water in the tank to the water in the coil of pipe I E F gt p o lt i SS A i bi Photo Andrew Blair Figure 4 2 8 Mains pressure coils in the Rinnai Beasley factory in Adelaide The coils are installed inside the copper tanks in the background of the photograph before the copper tops and bottoms are fitted to the tanks Mains pressure coils are just one type of heat exchanger Heat is exchanged transferred through the wall of the copper tube Notice the solar and boiler connecting nipples attached to the side of the copper tanks Heat exchangers Other combinations involve the use of a heat exchanger set within or outside a mains pressure tank The heat from the open vented water from the boiler is conducted via the heat exchanger into the mains pressure water supp
20. ly This is a more satisfactory supply than the mains pressure coil within the tank as it allows hot water to be drawn off at a faster rate and then reheated over an extended period These combinations are not common Solar Water Heater Training Course Installer and User Manual 133 4 Boost heating Open vent over header tank Q Solar storage tank mains pressure Cold in Header tank for boiler Heat exchanger allows open vented boiler to 4 heat water at mains pressure Two solar collectors The boiler is connected to the heat exchanger The hot water from the boiler loses heat to the water from the mains pressure storage tank The cooled water returns to the boiler by natural convection The mains pressure water rises to the storage tank from the heat exchanger The cold water from the bottom of the storage tank takes Boiler its place and is heated Again the circulation is by natural convection wet back thermosiphon circulation Figure 4 2 9 Boiler connection to close coupled solar water heater This shows how a boiler is connected to a close coupled solar water heater using a heat exchanger to separate the water at two different pressures It also permits water in the boiler to have additives such as corrosion inhibitors in the water This is important if the boiler is steel or cast iron A simple but perfectly adequate heat exchanger can be made from a two metre length of 40mm copper tube with a 25mm l
21. mostat 200L Booster and lt thermostat Collector y Connections Cold gt gt lt Foor inlet J inlet Off peak tank Continuous supply tank Figure 4 1 4 Ideal location of the electric boost element and the collector connections to the storage tank for different tariffs Electric day rate instantaneous Another option is to use an in line instantaneous electric heater but this can be an expensive option In this case the solar heater acts as a pre heater there is no booster in the solar system itself The water leaving the solar system then enters the instantaneous heater and is boosted if necessary The advantage of this is that only the quantity of water required to be boosted will be boosted The disadvantage is that the electricity costs 14 to 15 cents per unit kWh Effect of electric boosters on the electricity grid It should be noted that the use of any day rate i e continuous supply electric water heating may add to the peak load if it operates at peak power demand times This increases losses of the electrical transmission system and may increase greenhouse gas emissions compared with electricity supplied during off peak times Time of use electricity tariffs Time of use tariffs are where the price per unit of electricity is charged at different rates during the day to follow the different cost of generation Two or three different prices per unit may apply depending on how the supply com
22. nce between the water in the tank and the water coming out of the coil may be 10 C or 15 C but it does depend on how fast the water flows through the coil As hot water is drawn off the temperature of the water being delivered slowly falls as heat is taken from the water in the tank o It is best to use an over sized tank to provide for a greater store of hot water Mains pressure coils are well suited to a constant source of heat such as a solid fuel boiler Solar Water Heater Training Course Installer and User Manual 131 4 Boost heating These techniques are not particularly common but they demonstrate that it is possible to have a supply of high pressure hot water while still retaining an open vented boiler water system It is common to refer to these systems as mains pressure hot water systems In each case the heat exchanger separates the waters which are at two different pressures Vent Header tank Water level Hot water leaves coil in the top of the storage tank Cold water passes through the coil in the storage tank Solar connection Pipes connected to boiler in heater o cooker Boiler in cooker or heater Cooker or heater Figure 4 2 7 Heat exchanger using copper tube coil The heat exchanger in this system is a coil of copper tube sometimes called a calorifier coil It is located in the top of the hot water storage tank It is heated by the hot water surrounding it Co
23. nt of fuel consumed by the booster and running cost to a minimum Time of use of hot water lifestyle and boost heating As the sun is only available during the day system users are advised by solar manufacturers to have their showers wash their clothes etc first thing in the morning This allows the sun to reheat the replacement cold water that has entered the tank before the electrical booster comes on at night By comparison heavy usage of hot water in the evening means that the first heat source to be activated as the water temperature in the tank falls is the electric element or gas boost heater However this may not fit all people s preferred showering time A typical pattern of hot water demand is shown in Figure 4 1 2 below As can be seen from this graph more hot water is used in the evening for showers as well as kitchen and laundry needs 8 z 3 2 9 p Weekday I o Weekday i Weekend 3 w 3 g 400 I 200 im Nm i ps aY re PEPE PPPS PEPSIPPPLEELESI EP A ww OF er AP Hour Figure 4 1 2 Hot water demand in Victorian homes In warmer climates where little boosting is required or with systems that are well managed by the users this is not a problem as users who shower in the evening will leave the booster off or manually control the boosting time to a minimum before showering This means the solar collectors can do plenty of heating the next day as the water in t
24. pany divides up the day Typical options are to charge two prices one for peak periods and one for off peak periods charge three prices one for peak periods one for intermediate period called shoulder and one for off peak periods These tariffs are becoming the preferred option for electricity supply companies and governments because the full cost of electricity can be charged at any time of the day Solar Water Heater Training Course Installer and User Manual 119 4 Boost heating Metering of electricity All electricity is metered Day rate meters are already installed in all homes Off peak or time of use tariffs will require extra meters if not already on the premises This adds an additional cost for the meter and its connection Natural gas or LP gas storage Natural gas burns five times cleaner than black coal per unit of energy delivered and even more so than brown coal 6 4 times Natural gas or LPG is therefore much less harmful to the environment The gas burner can be located in the storage tank whether that is on the roof with the collectors in a close coupled system or in a conventional gas fired water storage cylinder in a pump circulated storage system in line after the solar storage tank in this case the solar heater acts as a pre heater with no booster in the solar system itself Gas burners now use electronic ignitions to minimise gas use and thermostatic control However jus
25. r Training Course Installer and User Manual 114 4 Boost heating all of Victoria For Renewable Energy Certificate allocations the above zones apply The proportion that the solar collectors supply depends on the quantity and temperature of hot water required climatic conditions of irradiation air temperature and cold water temperature the efficiency of the solar collector and storage tank the efficiency of the booster how efficiently the system is managed by the users The most likely period for boosting is during very cloudy days or those with cloud and rain but this depends on geographic location and hot water demand Table 4 1 shows the temperatures achieved in the storage tank on an average June day in Brisbane and Melbourne due to the solar energy alone and the solar plus booster input set to a maximum of 65 C In Brisbane the solar input alone achieves 61 6 C so little boosting is required This is due to the higher solar input and higher cold water temperature and lower hot water demand By contrast in Melbourne the solar achieves 32 9 C so the booster has to raise the water temperature by an additional 30 C This is because the solar input is much lower the cold water temperature is lower and the demand for hot water is higher Table 4 1 Solar input and water temperature rise in storage tank boost energy and solar input estimated from SCF program Brisbane Melbourne Solar inp
26. s natural and LPG o solid fuel heaters Boosters can be mounted o inthe storage tank to heat some proportion of the tank o in line after the storage tank to heat just the quantity of hot water that will be used This is the most efficient option Boosting is best controlled via timers manual override or smart controllers to allow the solar collectors to do as much heating as possible during the day to maximise the solar contribution Solar Water Heater Training Course Installer and User Manual 123 4 Boost heating Section 4 1 questions 1 What conditions make it necessary to boost hot water temperatures by means other than solar 2 The amount of boosting of hot water by a source of energy electricity gas or solid fuel other than solar energy will be determined by what factors 3 If the boosting is electric the time of heating and the tariff will determine the position of the heating element in the storage tank Explain Figure 4 1 4 shows details in diagrammatic form What would be the most logical position for the heating element in a tank where there is no off peak tariff and the power is available for boosting at any time during the day or night 4 Why is natural gas a preferred method of boost heating a solar hot water system rather than electricity 5 Electronic ignition for lighting gas hot water boosting units reduces the quantity of gas used compared with a pilot light that burns a small quantity
27. s wet backs This right is a boiler wet back from a room heater The wet back is purchased as an optional extra A The cast iron section conducts heat to the water in the 25mm diameter copper tube Flue Jacket boiler Another option is to use a water jacket instead of the first section of flue Heat going up the flue heats the water in the jacket These stainless steel flue jackets are placed directly on top of the heater flue spigot and then the normal flue is inserted into the top of the jacket Note the hot water connecting nipples Figure 4 2 5 Flue jacket boiler Solar Water Heater Training Course Installer and User Manual 127 4 Boost heating Pros and cons of installing wet backs in solid fuel room heaters At first glance the idea of installing a heater with a wet back to provide hot water in winter and solar collectors to provide summer hot water may sound a good system However heaters with a boiler present manufacturers with a problem 1 Do they make the boiler big enough to provide all the hot water required in the autumn and spring when the days are mild and the nights are not cold but cool enough that the heater is lit At no stage is the heater burning strongly so it needs a large boiler to make enough hot water 2 Or do they make a smaller boiler In the middle of winter when the weather is cold the heater is burning strongly for most of the day and night so a small bo
28. t like electric booster thermostats the thermostat does not take any account of the type of day and will therefore switch the burner on even if there is still the whole of the solar day in which to boost the temperature using solar radiation Smart controllers are helping to overcome this problem to maximise the solar contribution If natural gas is unavailable LP gas LPG is another option In particular people living with stand alone power supply SPS systems usually aim to reduce their overall electrical load as the cost of power is much higher and it makes little sense to use this expensive power for heating tasks when solar or gas can do it at lower cost Thus hot water boosting cooking space heating and the back up generator for the SPS systems can all be LP gas powered Solar Water Heater Training Course Installer and User Manual 120 4 Boost heating 5 Photo Courtesy Trevor Berrill Figure 4 1 5 Tank mounted gas booster LP gas is generally about two to three times dearer than natural gas Under some tariff structures the more LP gas that is used the lower the price per litre of that gas So if a fossil fuel is required a larger cylinder of LP gas filled regularly by the gas suppliers will probably provide the cheapest and cleanest energy supply for back up to the solar system and other uses Natural gas or LP gas instantaneous In instantaneous gas boosted units the solar system does not normally have
29. than all of the hot water outlets including the shower so that hot water flows downhill under gravity to each outlet An alternative is a 7 5 metre header tank This is a copper tank with a separate supply tank able to be set up to 7 5 metres above the storage tank These are fairly rare in Australia now but are commonly used in New Zealand In this case a vent pipe must run from the top of the storage tank above the supply head tank If the system boils steam can still escape from the vent pipe Storage tank with mains pressure coil The most common way to provide mains pressure hot water with a boiler connected to the storage tank is to have a mains pressure coil in the tank A coil of copper tube inside the tank is surrounded by the hot water in the tank Cold water enters the bottom of the coil and is heated by the surrounding water as it passes through the coil The pressure within the coil can be mains pressure which means that the hot water can be delivered to outlets above the storage tank The mains pressure coil has disadvantages o If the water passes through the coil too quickly it will not be adequately heated A maximum flow rate of about 12 to 15 litres minute is a common recommendation It is usual to install a valve to restrict the rate of flow on the cold water supply to the coil The water coming out of the coil will be cooler than the water in the tank once it has started to flow The temperature differe
30. the Australian Standard for plumbing installations What is the number of this standard 3 Why do we talk about solid fuel rather than just saying firewood 4 Numbers of country people whose only source of hot water was a solid fuel fired cooker have had a solar collector added to their hot water system Why did they bother when the cooker made enough hot water 5 There is an extremely important safety rule relating to solid fuel hot water installations What is it 6 What does open vented mean and why is it so important 7 If valves were installed between the boiler and the storage tank they could be closed off to do maintenance on the boiler without having to drain the storage tank Why are valves not permitted on these pipes 8 Why is it inappropriate to use plastic pipe for connecting a solid fuel cooker to a hot water storage tank 9 Why is it inappropriate to connect a solid fuel boiler to a vitreous enamel glass lined hot water tank even if the system is open vented 10 Why might an owner of a hot water system want mains pressure for his hot water supply 11 Can he she have a mains pressure hot water supply if he she has a solid fuel boiler acting as a booster 12 A person is considering installing a solid fuel room heater as a booster for a solar water heater What advice would you offer Solar Water Heater Training Course Installer and User Manual 136
31. ut 33 3 20 MJ day Booster input 2 4 27 MJ day Volume of water 167 0 200 Litres day Cold water temp 14 0 9 deg C Tank water temp 61 6 32 9 deg C due to solar only Tank water temp 65 0 65 1 deg C due to solar booster Often users of solar systems accept that the booster does not need to heat water to 60 C or greater every time and regulate boosting accordingly For example most people are comfortable showering when the water temperature is in the range 40 C to 45 C so boosting will be necessary only to enable comfortable showers For most other applications lower temperatures are often acceptable provided that the temperature is not too low too often Water should be heated to at least 60 C if it is to be stored This precaution must be taken so that legionnaire s disease does not develop in the stagnant parts of the tank Although this is an exceedingly rare occurrence in water heating systems legionnaire s may develop and multiply if the water temperature is kept below 50 C for extended periods Hence it is Solar Water Heater Training Course Installer and User Manual 115 4 Boost heating advisable to use a booster to raise the temperature above 60 C regularly e g at least once every 24 hours to keep the system safe The amount of hot water provided by the booster is generally not the full capacity of the storage tank This keeps the solar contribution to the optimum level and the amou
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