Are automatic timers on geysers efficient?
Simple Numerical Analysis to establish Energy Saving Guidelines
T. C. Verster
The rapidly rising cost of electricity is a high profile issue and many
householders are willing to take action (or are already doing so) to manage
their own consumption, but do not have enough factual information to choose the
best plan of action for themselves.
The purpose of this article is to provide information on the use of automatic
timers to switch the electric geyser on and off for certain periods and how this
affects the efficiency and/or wastefulness of hot water production. More and
more timers are being installed, and many of them have no or little beneficial
effect, due to misconceptions about the thermal behaviour of the geyser. On the
other hand, many consumers decide not to install timers, also due to false
assumptions. If applied intelligently, timers could play a major role in
minimising hot water energy cost.
Choice of Timers:
Either an electronic (digital) timer or an electro-mechanical timer can be
used to control a geyser, the main requirements being:
• Ability to maintain real time during power off periods, e.g. it must contain a
small backup battery to keep the electronic clock or electro-mechanical motor
running.
• Specification to switch the required amperage (e.g. 17 A for a 4 KW element).
Theory of geyser behaviour
For illustrative purposes the most common geyser size (150 L) is chosen as a
subject, but the role of smaller and bigger sized geysers will be discussed
later. This subject geyser was used in an experimental setup, filled with cold
water, heated to its practical maximum temperature (72 C) and allowed to cool
down over a period of three days. Its temperature decline was found to follow
the following equation quite closely (t = time in hours, T = Temperature in
degrees C and Tamb = ambient temperature
which is between 20 and 25 degrees C)
T = Tamb + 14e-t/17
+ 43e-t/110
….........................(1)
With some practice it becomes easy to use this equation to calculate the
exact path along which it will cool down from any specified starting temperature
over any specified time period, exactly such as required for timer effect
studies. This equation is also the basis for an exact derivation of standby
power *1 (in Kwhours/month) but it can be closely approximated by eqn. (2). This
is a non-linear curve (more or less a quadratic equation):
P = K(T – Tamb)n
.............................(2)
where K is a scaling constant (K = 0.031 for the 150 L Kwikot geyser), and n
= 2.1
The different sizes of geyser from this manufacturer, which follow the same
basic design, can be assumed to obey the same quadratic equation, but with
different scaling factors.
These two equations are shown only as evidence that the curves presented
below are based on simple but sound physical principles, and “intuitive”
arguments which often occur during discussion of this topic, can be avoided.
Case study of using a timer
One very prevalent false assumption about timers that has to be dismissed at
the beginning, is that randomly chosen short geyser switch-off periods (few
hours each) during the day will save significant energy. The important principle
to follow in setting a timer for energy-saving purposes, is to try and duplicate
the use of an electric kettle: heat up enough water for your immediate need (a
cup of tea) and refill it with cold water without reheating it, until just
before you need another cup of tea, maybe a few hours or a day later.
A single person, a couple or a small family can choose to bath or shower
within a short time span (06:00 – 7:00, say) every day. The timer should then be
set to start heating the water at 04:00 and switch off at 06:00, so that the
occupants have hot water (60 C, say) for a bath and shower, say, using 100 L of
mixed water at 45 C. The inflow of cold water at 15 C will start mixing with the
remaining hot water in the geyser, and attain a value of 40 C, which will very
slowly have cooled down to 35 C by the time the timer switches on the next
morning at 04:00 (Fig. 1 shows one such 24 hour cycle). For 22 hours no
reheating takes place, and the lukewarm contents lose much less energy, than if
the geyser had been kept at 60 C all the time. The graphs in Fig. 2 clearly
illustrate this.
It is evident that use of a timer in this example diminishes the monthly
energy account from 196 KWh to 136 KWh, for the same amount of hot water used
per day. The productive energy to produce 100 L of bath water at 45 C every day
is 105 Kwh/mo, which shows that the energy wasted in keeping the 150 L geyser
permanently at 60 C, is 91 Kwh/mo, while only 31 Kwh/mo is lost when a timer is
used as described above; this translates into a reduction of 66 % in wasted
energy.
A final question to settle is: what happens to the saving of 66 % (Fig. 2) if
the 100 L hot water usage does not take place all at once, but a 50 L bath early
in the morning and another 50 L bath 12 hours later, and the timer being reset
to preheat water twice a day? The result is that the middle curve in Fig. 2
moves up to a less efficient position (37% saving). The reason is that if a
smaller proportion of the hot water in the geyser is withdrawn, a smaller amount
of cold water enters and the resultant mix is at a temperature not much below
the thermostat setting. Therefore higher heat losses occur during the subsequent
hours when nothing further happens.
Thus, only if the demand far exceeds supply, is it advisable to set the timer
for two daily “on” periods, which should be timed such that the “electric
kettle” principle is still followed, and not letting the geyser reheat after the
rush is over. This is not optimum, but still preferable to the permanently “on”
situation without a timer. A high usage of hot water in a household can often
also be tempered by installing water saving shower heads and taps, which make a
very effective energy saving combination with a geyser timer.
Another useful fact which is relevant at this point: Eskom's request to
consumers to switch off their geysers during the early morning and late
afternoon peak demand periods, is to protect their supply reserve margin. If the
consumer tries to comply blindly, he will do his civic duty, but will not
necessarily make any significant reduction in his monthly account. It is often
possible, however, to work out his own energy saving strategy first and
subsequently accommodate the two no-go time periods into his timer settings.
Choice of geyser size and thermostat temperature setting
A small geyser controlled by a timer, can form an innovative combination, if
the objective is to allocate a specific limited amount of daily energy to hot
water production, which will not exceed the monthly free electricity allowance
for low budget households, e.g. 50 or 100KWhs. This can be achieved by an “on”
period of only two hours each day, scheduled just before the main hot water
need. Because the geyser is small, the amount of available bath/shower water is
self-limiting (30 L, say) and the budget cannot be exceeded inadvertently. This
strategy reveals an interesting insight: both the heat loss (wasteful energy)
and energy productively used for water heating are simultaneously controlled.
In practice this can be done on a higher level also, using bigger geysers. In
fact, many different custom combinations can be tailored from the four
variables: geyser size, thermostat setting, timer “on” duration, and time of day
setting, to supply just the right amount of hot water, when needed, without
wasting energy on hot water not needed. Most timers have an override function to
cater for occasional higher than usual demands, when water can be heated with
manual control, thus avoiding permanent settings aimed at maximum demand all the
time, which are energy-inefficient.
More information:
 T. C. Verster:
Geysers – How to evaluate different energy-saving strategies. WATTnow, Feb.
2010, pp 18- 27.
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