In the past, hot water was only a small percentage
of a buildings total energy demand, often around
20% for the average home. However, as the insulation
levels of buildings increases, then the space-heating
demand drops. Our hot water use for washing
and showering has if anything increased. Over
half the heat requirements can in some cases
be for domestic hot water.
Low temperature emitter systems can make room
heating very efficient, but with hot water,
there is little oportunity to reduce the working
temperatures. The fairly recent concerns about
legionella health risks have lead to increased
temperature requirements. It is therefore quite
a challenge to achieve high energy efficiencies
during hot water production when using a heat
pump.
There are two main types of hot water storage
cylinders.
(note heat pumps are never suitable for instantaneous
heating, unlike gas boilers etc)
1. The conventional hot water cylinder.
This has a heat-exchanger coil to transfer heat
from the heat pump. The cylinder holds the actual
water that comes out of the taps. Traditionally
these were fed from open-topped header tanks
in the loft. This has several drawbacks, namely:
contamination of the water in the loft tank,
risk of freezing in the loft, and low water
pressures for showers.
The UK is slowly changing towards mains-pressure
systems. These avoid the need for loft tanks,
and can give better pressures for showers,and
smaller supply pipes can be used. However, they
do come under stricter regulations since, if
not installed well, there could be safety issues
The heat-exchanger coil within the cylinder
needs to be far bigger than that designed for
a boiler. Ideally with only a 5°C difference
between the heated water from the heat pump,
and the water available to the taps. This is
achievable is a big enough coil is used. I have
found 3m² of coil surface area to be most
efficient and practical for small domestic heat
pumps.
2. The 'Thermal Store' type cylinder
is the traditional method used in Sweden. With
this system the cylinder contains the water
that circulates within in the heat pump. It
is pumped direct from heat pump to cylinder.
A large heat-exchanger coil inside the cylinder
takes cold mains water, and heats it up instantaneously
as it passes through. This system has advantages
and disadvantages.
Advantages
* Legionella risk is minimal due to low water
content within the heat-exchanger coil. Legionella
regulations do not apply.
* The hot water can be produced at high pressure
(same as cold-tap pressure)
Disadvantages
* The water temperature to tap may be flow-rate
dependant.
* Storage temperatures may need to be higher
than the conventional system.
The conventional cylinder is a bit easier
to configure and has advantages of stratification
and easier control. The thermal store will often
have inferior performance unless designed well,
and is more prone to dropping in temperature
quicker. However, developments may improve these
details.
Some heat pumps only pre-heat the water to
say 45°C. The heat pump works efficiently
at such temperatures, but the immersion heater
that is needd to top-up to a useable temperature
is very inefficient. Ideally the heat pump would
go to its highest possible limit, even if the
COP drops at such temperatures, (the maximum
possible water temperature limit varies from
model to model).
Batch heating
To improve energy efficiency, it is very important
to keep the water temperature within the heat
pump to the lowest values possible. However,
most cylinders are maintained at over 50°C
all the time, and usually heated to 60°C
daily or weekly for Legionalla protection. This
sometimes requires an direct electric immersion
heater which is very inefficient.
If the cylinder is large enough, then a considerable
delay can be arranged before the heating is
re-triggered, by which time the bottom half
of the cylinderis likely to become cold as water
is used. The heat pump can then heat the lower
section from cold right up to the required temperature.
The average water temperature is therefore considerably
lower, and the energy efficiency is significantly
improved. Two coils within the cylinder are
required for this to be done properly, however,
his method of control can be partly achieved
with a conventional cylinder if the sensor is
repositioned further up the cylinder. Careful
use of a time clock could also assist by making
the bottom of the cylinder colder at certain
times. The total volume of available hot water
will be reduced, but this may not be a problem
for much of the time.
To my knowledge, no heat pump manufacturers
use this method due to the increased cost. There
is not enough drive at present to improve the
energy efficiency of systems. Unfortunately
few people are willing to pay the extra for
such refinements.
Saving energy and water in the distribution
pipes
Whatever your method of producing hot water
for baths, sinks and showers, you should make
sure that energy and water is not wasted due
to the pipework from the storage cylinder to
the tap. Whenever a hot tap is first turned
on, there is a time-delay before water comes
through at a useable temperature. This is known
as dead-leg losses.
- Firstly, this is a waste of water.
- Secondly, it is a waste of heat since the
hot water within the pipe will slowly cool
after the tap was last used and heat will
be lost. Sometimes this heat is useful to
the building. Nonetheless, these losses are
undesirable.
One solution to save water is to use a
pumped-loop around the building so that
hot water is always close to the tap outlets.
This may save water and it reduces waiting
time, but could waste a lot of heat. This
system is common in hotels, but not necessary
for most domestic situations.
(note, return loop pipes
are often far too big, they only need be
big enough to keep the loop warm. This need
only be a tiny flow rate)
Good housekeeping
In the days of cheap energy and before
energy was a problem. The pipe runs from
a cylinder were made nice and large, usually
reducing in diameter as they got to the
last tap. If your washbasin was at the end
of a long run, then it will take ages for
the hot water to come through.
To minimise these losses there are a few
things that can be done.
- Use the smallest size pipe possible
to maintain an acceptable flow rate.
- Use multiple pipe-runs from the cylinder
so that each pipe is smaller (e.g. a separate
pipe to the bath & separate small-bore
to basins))
- Insulate pipes well. (helps most for
frequently used taps).
Experiment
1. First thing in the morning (following
no tap use for many hours), put a bowl in
your kitchen sink and turn on the hot tap
and see how long it takes before the water
is a useable hot temperature. This amount
of water is wasted frequently. This quantity
of water is left cooling in your pipework.
2. Assess the flow rate. How quickly can
the sink be filled? Is it quicker than needed?
If so, you should be able to improve matters.
Repeat this experiment for all your sinks.
Solution
- Reduce the pipe diameter from your
cylinder.
- make the pipe route more direct, hence
shorter.
- fit a small-bore pipe direct from the
cylinder to that distant wash basin.
- Insulate pipes so the heat says available
in them for a bit longer.
Note,Consult an expert if you are worried
about washing machines and dishwashers.
My experience is that lower flows simply
increase the time for things to fill, but
it could be a problem.
Noise can increase when smaller pipes are
used, but I have not found this to be a
problem.
Pipe sizes and flow rates.
The following chart gives the internal
volume of common plastic and copper pipes.
Dimensions may vary slightly with different
manufacturers . However, it shows the dramatic
difference in internal size between
copper and plastic pipes.
The following chart might be useful for
assessing the size of pipe that you may
be able to reduce to.
Note, this chart can only be used as a
rough guide. It is interesting
to note how dramatic the effects of the
internal diameter is on the flow rate. The
internal bore of plastic being much smaller
than that of copper, this may be an advantage,
or a disadvantage depending on your requirements.
Notes,
The pressure given are just a guide. The
height below a loft header tank causes the
pressure, these systems need the biggest
pipes. Mains pressure systems (e.g. thermal
store) can give the highest pressures, so
pipe sizes can be smaller. However, mains
pressure can vary considerably.
