How to choose a hot water system Several factors need to be considered when determining the specification of a new hot water heating system. They include, but are not limited to, the type of system, the amount of hot water required, the demand pattern of its use, the availability of natural gas if desired for boosting, the importance of conserving water versus reducing greenhouse gas emissions, viable locations for the installation of a system and upfront costs versus operating costs.
Solar Hot Water Systems While considering the use of a solar hot water system it is important to remember that while they offer many advantages, they only produce hot water when the sun shines, this makes the choice of booster system critical. The dependence on boosting will also be greatly influenced by usage patterns, high morning use increases booster reliance, as the sun has not had time to heat the water. Another weather related issue for solar hot water systems is hail, although an infrequent occurance, large hail stones can be fatal for solar panels. When damage occurs the system will cease to function so Solar powered hot water systems consist of a flat plate solar collector that is generally installed on the roof and an associated tank either directly above the collector panel (passive system) or separately on the ground (split or active system). The solar collector consists of an airtight box with a transparent cover, housing dark metallic tubes for the water to attract solar gain and high levels of insulation to minimise heat loss. Traditional flat plate collectors circulate water through the panels (open circuit systems), as opposed to closed circuit systems which use a liquid with a lower freezing point than water. This use of water as the heat conductor has potential problems in extreme conditions. In frost areas, in order to prevent freezing, hot water is released to the panels, this ‘dumping’ of hot water decreases the efficiency of the system as a whole by wasting precious heated water, especially when this occurs over night with no solar recharge available for many hours. The water may find its way into a rainwater tank although it’s more likely to be wasted via a first flush device. Closed circuit systems are necessary for frost prone areas although some still release hot water to prevent freezing. In times of excessive heat gain many systems release hot water to prevent pressure build up in the tank and panels. Again, this ‘dumping’ of water is undesirable and different systems deal with the issue in slightly different ways. In some instances blinds have been installed over the panels in summer time to reduce solar gain. In non-frost areas water is circulated from the tank into the collector panel where it is heated directly by the sun and then flows back into the tank. In a close © [email protected]
0406 75 45 33 Close coupled passive solar hot water system http://www.greenhouse.gov.au/yourhome/technical/fs43.htm
coupled passive system the movement of the water is controlled by the thermosiphon effect whereby the hottest water flows to the top of the horizontal tank with the coldest at the bottom. As the lowest part of the system is the solar collector this ensures a continual heating of the coldest water, as long as the sun shines. The passive system has the advantage of not requiring a pump to circulate the water during the heating process, however it may call for the reinforcement of the roof structure to carry the weight of the tank. Many people do not like the aesthetics of the roof-mounted tank and it is harder to maintain the tank when located on the roof although this location may be suitable if ground space is limited. These systems are often prone to more heat loss due to the exposure of the tank, also they are often less well insulated due to different tank material choices being based on mass i.e. the use of stainless steel instead of vitreous enamel tanks. For split (active) solar systems a small pump is used to move the coldest water from the base of the vertical tank up to the collector panel. The pump is activated by a change in temperature so it will only pump cool water up to the Split (active) solar hot water system http://www.greenhouse.gov.au/yourhome/technical/fs43.htm
panels when the water in the panels has increased by a certain temperature. The energy consumption associated with the pump seems to be mitigated by the efficiencies achieved having a vertical tank (where the increased water stratification (layering) leads to more efficient water heating) and the increased insulation from the use of vitreous enamel tanks. Obviously the insulation of the hot water return pipe from the panels is vital to the systems efficiency. Evacuated tube solar hot water systems operate in an identical way to split solar systems however their advantage lies in the solar collector. Instead of being a flat plate containing tubes to carry water, it is a collection of tubes. The tubes consist of an inner and outer tube with a vacuum created in between. This functions in a similar way to a thermos flask keeping the liquid within highly insulated. The liquid within the tubes transfers the heat it gains from the sun to the water, through a simple copper header at the top of the tube array. The circular section of the tubes allows for an increased surface area relative to roof area, this translate to more surface exposed to the sun, in addition, as the sun angle drops in the sky the tubes receive more sun than their flat plate cousins. Independent engineering tests show the evacuated tubes to be up to 40% more efficient than flat plate collectors. Due to the tubular shape the panels can be orientated up to 90° from solar north with only a 10% reduction in efficiency, a big improvement over traditional solar hot water heaters. © [email protected]
0406 75 45 33
Location of Solar Hot Water Systems The installation of a solar collector and its efficiency will be governed by its relationship to the sun. Ideal orientation is facing due North with panels inclined at 45° being the latitude of the location i.e. Sydney (35°) plus 10°. This can be varied and depending on the system the decrease in efficiency may be insignificant. The additional 10° improves performance during the winter months when the sun angle is lower and hot water demand is generally higher. Evacuated tubes have a distinct advantage over the traditional flat plate collectors in that their increased surface area and tubular section enables them to receive a greater portion of direct sun especially from the lower angles experienced in early morning and late afternoon. They will experience a loss of only 10% if orientated due east or west, whereas flat plate collectors rapidly lose efficiency when orientated 45° or more from north. Evacuated tube systems must be installed at a minimum 20° pitch but will perform much better at 30°. Independent engineering tests show the evacuated tubes to be up to 40% more efficient than flat plate collectors. Unfortunately, few homes have roof pitches of 45° therefore a choice needs to be made to either install a framing system to orientate the panels at the desired angle or to attach them flush to the roof and accept the loss in efficiency, generally this will be made on the basis of aesthetics. The relationships between the tank, panels and outlets (taps etc.) are crucial to the efficient operation of the system. Many warranties specify a maximum distance between tank and panels', ignoring these, voids warranty and decreases efficiency. The greater the distance between the tank and panel, the less efficient the system, more power will be required to circulate the water between the two and there is an increase in heat loss from the hot return pipe. Hot water return pipes should always be insulated although the effectiveness of the insulation is proportionate to its size, i.e. the better the insulation, the bigger and uglier it will be. The relationship between tank and hot water outlets needs to be given careful consideration. Bathrooms use more hot water than kitchens or laundries, however kitchen taps are used a greater number of times per day, the issue to be considered is how much hot water wasted in the pipe run. The amount of water wasted can be minimised by installing a reticulation device but this will not recover the energy consumed heating the water. Therefore it is desirable to have the tank located closest to the kitchen, then the bathroom, the final consideration being the laundry, in reality there are often few options especially if gas boosting is being used as there are regulatory demands on the placement of gas burning devices.
Booster systems Both thermosiphon and split solar hot water systems will require some form of boosting system as the sun is not always able to provide sufficient heat gain for the hot water demand, this may be due to unusually high demand or prolonged periods of © [email protected]
0406 75 45 33
cloud cover. In the winter months, the boosting device may often become the main source of hot water therefore its selection is critical. There are two predominant forms of boosting available; electric and instantaneous gas. Electric boosting is the less efficient of the two. It consists of an electric element in the tank that will activate when the water temperature falls below the thermostat level (minimum 60°C for health reasons). This methodology means there is always a tank full of ready to use water, overnight for example the element would be activated several times to maintain the temperature even though there would likely be no demand for hot water at all. It is this unnecessary heating of water, and its electric power source, that lead to inefficiencies. It is also the case that after a large draw of hot water there will be a lag time of possibly several hours before adequate hot water becomes available again. This can thought of as a ‘solar boosted electric water storage heater’. There is an instantaneous electric heating system available (Wilson Hot Water 03 9720 2888) although it can only produce 4.5 litre/min (gas systems can produce up to 32 litre/min) and also requires 3-phase power making it impractical for residential purposes. Gas boosting of solar hot water systems is currently the preferred approach. Firstly, it uses natural gas, which produces less carbon dioxide emissions per energy unit than coal-fired electricity. Secondly, instantaneous gas systems only consume power to heat water when the water is needed. An instantaneous gas hot water system is installed near the hot water outlet of the tank, when a hot water outlet is turned on the water begins to flow from the tank, a sensor will detect the water temperature and if it is not adequate the gas system will turn on and begin heating the water as it flows.
Heat Pump Systems Heat pump hot water systems work on the principle of a refrigeration circuit, drawing heat out of one space and discharging it into another. A Solar Heat Pump consists basically of three major components; a compressor plus two heat exchangers (evaporator & condenser). In operation, the evaporator absorbs whatever heat energy is available to it from the atmosphere (air) to vaporise the refrigerant. The vapour is then compressed raising its pressure and temperature. This high temperature vapour is passed through special pipes permanently bonded around the outside of the water storage tank, forming the condenser. As the refrigerant vapour condenses back to its liquid form, it gives off its heat to the stored water. As this happens, the condensed refrigerant liquid passes back to the evaporator panels through an expansion device, is vaporised, and the cycle is then repeated. The refrigerant material is unusual in that it has a very low boiling (or vaporising) point well below 0°C at atmospheric pressure and a freezing point more than 100°C below zero. It is liquid when cold but easily becomes a vapour when heated and vice versa. In operation within the heat pump the refrigerant can be vaporising at a temperature of around minus 20°C. An ambient temperature of +5°C is HOT compared with such a low figure. The heat pump system does not generally require a booster system but does experience a lag time after large amounts of hot water are drawn off.
© [email protected]
0406 75 45 33
Instantaneous Gas Hot Water Systems Instantaneous gas hot water systems do not require a storage tank, they comprise of a small wall mounted box with a water inlet and outlet and a gas connection. When a hot water outlet (tap) is opened the water flows into the heater, the ignition system begins to heat the water until the tap is turned off. This system has many advantages, the lack of wasteful water heating for storage, it’s ability to continually produce hot water at rates of up to 32 litres/minute, lack of space required to house the system. The main negative is that even though natural gas is less polluting than coal-fired electricity, it is still a finite, non-renewable energy source. Most instantaneous gas systems come with the option of ‘water controllers’, devices that are placed near hot water outlets that can be programmed to specific temperatures. This means the water will only be heated to the desired temperature as opposed to a traditional approach of heating water to 60°C, then adding cold water.
Water Storage Tanks Tank life will be dependent upon the water quality. There is also a difference in maintenance requirements between vitreous enamel tanks, which require the installation and replacement of sacrificial anodes approximately every 5 years, and stainless steel tanks, which do not. The vitreous enamel tanks can be constructed with either one or two bonded linings (this can usually be deduced from the length of warranty offered) as well as the outer casing, these appear to be more highly insulated than the stainless steel tanks whose preference seems to be reserved for roof top installation due to their lesser weight. The use of an instantaneous gas water heating system will eliminate the need for a tank entirely, helping to reduce the upfront cost.
Load Calculations Number of people Showers per person per day
4 2 © [email protected]
0406 75 45 33
Total number of showers per day Hot water per shower Total hot water for showers @ 60°C Hand washes per person per day Total number of hand washes per day Hot water per hand wash Total hot water for hand wash @ 60°C Number of meals per day Hot water per dish wash Total hot water for dish wash @ 60°C Hot water for laundry per person per day Total hot water for laundry @ 60°C Total hot water requirements @ 60°C Ring Main Losses
8 18 144
2 8 2 3 10 10 40 230 Less than Add 10% 100 metres
Litres/day Litres/day To total demand
Usage Patterns A careful analysis of usage patterns will affect the choices made. Solar systems are more suited to usage later in the day allowing time for them to harness the suns heat. High early morning usage will utilise the booster system to compensate for the temperature drop in water overnight. Heat pump systems are suited to less frequent usage, large volumes can be drawn off but a suitable lag time must be allowed for the system to recharge before the next major draw. Instantaneous gas system performance has little to do with the timing of the usage as heating is on demand but obviously the associated running costs and CO2 generation are a direct function of the volume used.
Running costs There are two primary factors associated with the running costs of a hot water heating system, the financial and the environmental. © [email protected]
0406 75 45 33
The following tables can be found at http://www1.sedo.energy.wa.gov.au/pages/emissions.asp where there is a list of the assumptions behind the results. If the price of power were to increase as predicted then obviously these costs would rise.
Water Usage The primary method of reducing water consumption in relation to water heating is to minimise the flow of cooled water exiting the outlet before the arrival of the hot. © [email protected]
0406 75 45 33
Some instantaneous gas systems can be fitted with controllers at the outlet which set the temperature of the water, this allows the system to heat it to the exact temperature as opposed to a storage system which must, by law, heat the water to 60°C for the user to then reduce it’s temperature using cold water. The Rinnai now incorporates a ‘pre heat’ function, which operates as a ring main, whereby a button is pressed on the controller but the water will not flow until the desired temperature is reached; the cool water is reticulated back to the inflow on the heating system. Unfortunately, this precise control of water temperature is only available on instantaneous systems, as the storage systems have already achieved a temperature above any desired human use. Some products now available do perform in a similar way to the ‘pre heat’ function mentioned above. The Dux ReadyHot (http://www.dux.com.au/recirculation.htm), the EcoSmart Water Guardian (http://www.ecosmart.com.au/water_guardian.html) and the Everwater ChilliPepper (http://www.everwater.com/index.php?View=1_3). All utilise existing pipe work and therefore suited to retrofitting. A button is activated at the outlet, a pump will turn on and remain on until the water in the hot pipe has increased by 3°C, it will then switch off, the tap should now be turned on with the hot water flowing immediately. Only one unit is required per hot water line and several buttons can be installed at various outlets along the line. These products are all compatible with any type and brand of hot water system making them particularly useful in conjunction with solar systems.
Renewable Energy Certificates (REC’s) REC’s is a Federal government scheme, which allocates points for households installing a hot water system that will reduce the consumption of fossil fuel based power. The scheme allocates one REC for approximately every one-megawatt hour of energy saved; these can be cashed in or exchanged for an upfront discount on the purchase of a new hot water system. See http://www.orer.gov.au/ for details and follow the links to find the full list of heating systems and their REC’s points, alternatively see manufacturers’ websites (the government site is 157 pages long). Theoretically, the more REC’s points a system attracts the more efficient it is, while this may not be 100% true, especially in relation to evacuated tube technology, it is a reasonable indicator of performance.
Comparisons Solar Systems
© [email protected]
0406 75 45 33
Brand Dux Solahart Solahart Rinnai Rinnai EcoSmart EcoSmart EcoSmart
Tank Size (litres) 315 270 300 250 315 250 315 400
EcoSmart Endless Solar* Endless Solar*
330 250 315
REC’s Cost – REC’s $
Ground Ground Roof Ground Ground Ground Ground Ground 3 roof panels Roof Ground Ground
Inst. Gas Inst. Gas Inst. Gas Inst. Gas Inst. Gas Inst. Gas Inst. Gas Inst. Gas
5500 5015 5735
4642 4001 4643
4150 4350 5250
33 39 42 37 42 40 40 47
Inst. Gas Inst. Gas Inst. Gas
4350 5562 6217
38 30 37
3362 4782 5255
3110 3310 4028
REC’s assumed price of $26 each. 2 roof panels per system unless specified. * Evacuated tube system, manufacturers dispute the accuracy of the REC’s system as a measuring tool for efficiency. They have engineering statistics showing evacuated tubes being 40% more efficient than flat plate collectors.
Instantaneous Gas Systems Brand Dux Endurance Rinnai Rinnai Rinnai Rinnai Rheem Rheem Bosch Bosch
Flow Rate (ltr/min) 26 16 20 26 32 18 26 17 26
Cost $ 1200 925 1200 1400 2040 1000 1400 1049 1279
Ind. Controller available No Yes Yes Yes Yes Yes Yes Yes Yes
Integrated preheat No Yes Yes Yes Yes No No No No
Heat Pump Systems Brand
Tank Size (litres)
Cost – REC’s $
© [email protected]
0406 75 45 33
Solahart Dux Quantum Quantum Ecosmart Rheem
310 250 270 340 250 310
3905 4000 2750 2875 3750 3500
28 28 26 26 28 28
3177 3272 2074 2199 3022 2772
REC’s assumed price of $26 each. Reticulation devices Brand EverWater Chili Pepper Dux ReadyHot EcoSmart Water Guardian
Cost $470 one size fits all $1000 larger size $700 larger size
Conclusions It is not possible to give a definitive answer as to which system to install, however some differences between similar sounding systems have become apparent. In solar systems, the split system where the tank is at ground level seems to be more efficient (as well as reducing the roof load and visual impact) than the roof mounted tank. This efficiency is partly due to the tank construction (see above) and also the increased water stratification enabled by the vertical orientation. The evacuated tube system is technologically superior although this is not reflected by it REC’s rating. It is hard to know whether this is a fault in the REC’s system or due to shortcomings in other areas of the Endless Solar system. The Rinnai instantaneous gas systems are the only one with a pre-heat function, although a reticulation device can be fitted to any system the additional cost should be considered. The difference between specifications for reticulation systems is insignificant and it should be noted that EcoSmart and Dux are ultimately the same company; therefore it is hard to explain the difference in prices. The Bosch instantaneous gas system has a unique water flow activated ignition system, eliminating the need for a power connection or batteries, as other systems no longer use a pilot light the saving in gas would be minimal, but a saving none the less. There would be a reduced capital cost associated with the elimination of an electrician for the installation process.
Resources http://www.greenhouse.gov.au/yourhome/technical/fs40.htm guide to hot water systems © [email protected]
0406 75 45 33
http://www.solahart.com.au/default.asp?V_DOC_ID=909 for water usage statistics http://www1.sedo.energy.wa.gov.au/pages/emissions.asp Running costs and greenhouse gas emissions http://www.apricus.com/ & http://www.endless-solar.com/ Evacuated tube solar hot water systems http://www.dux.com.au/ Solar, gas, heat pump, water controller and water reticulation devices http://www.ecosmart.com.au/ Solar, gas, heat pump, water controller and water reticulation devices http://www.quantumenergy.com.au/ Quantum heat pump http://www.bosch.com.au/content/language1/html/4188.htm Bosch HydroFlow instantaneous gas systems http://www.orer.gov.au/ Renewable Energy Certificate information, official Government site http://www.everwater.com/index.php?View=1_3 EverWater ChiliPepper water reticulation device http://www.rinnai.com.au/hotwater/home/defaultB.asp Instantaneous gas and solar systems http://www.rheem.com.au/home.asp Instantaneous gas, solar and heat pump systems
© [email protected]
0406 75 45 33