How to choose a hot water system

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
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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.
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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
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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.
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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.
http://www.rinnai.com.au/hotwater/home/safer.asp?whs=home&pg=2
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
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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
Litres/day
16
Litres/day
30
Litres/day
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.
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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.
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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
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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
Tank
Location
Booster
Cost $
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
Cost – REC’s $
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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
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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
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