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20 November 2009
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Introduction
One of the commonest questions is how best to heat a house or apartment
in winter and keep it cool in summer. This is a loaded question because
there are various factors involved, some of which may be contradictory. Are
you interested in maximum comfort, minimum capital cost, minimum running
costs or, above all, minimum impact on the environment?
The most important factor is the quality of insulation in the house or
apartment. This cannot be stressed too highly, because many constructions in
Cyprus leave a lot to be desired in this department. Personally, I bought a
"standard" type of villa in 1997 and, as I became familiar with its quirks,
I was appalled by the lack of insulation. You can read how I tried to
improve this in this essay.
Let it be stated loud and clear that if you insist on heating or cooling the
outside of your house, then you will never be comfortable in winter or
summer. Unfortunately, it is hardly an exaggeration to say that most houses
and apartment waste enormous quantities of energy by heating and cooling the
outside or, at least, allowing the heat to pass through the roof, the
windows, the walls or the floor. Theoretically, modern houses are required
to be better insulated but only lip-service is made to the regulations. For
example, I have seen a 2009 reconstruction made with thin single-glazed,
aluminium-framed, ill-fitting windows, which almost might as well not be
there!
For most owners, they wish to be comfortable throughout the year. Comfort
is not incompatible with minimising energy consumption in a well-insulated
house, but best results are obtained if one understands a few basic facts
towards being comfortable:
- The first point is that comfort depends on
humidity, possibly even more than temperature. For example, a summer
temperature of 40°C with a relative humidity of 15% is more comfortable than
30°C with a relative humidity of 85% (relative humidity or RH is the
percentage of moisture in the air at a given temperature compared to what
the air can contain before it forms a mist).
- A room
is most comfortable when it is entirely at as close to a suitable uniform temperature as possible. This includes all the interior
surfaces, such as walls, windows, ceiling, floor, furnishings as well as the
air. The moment you have warmer and colder areas, so the comfort becomes
less.
- A flame always consumes oxygen, so
requires air to come in from outside to replace that used by the combustion.
If you have a room that is well closed with a paraffin or gas heater burning
and no chimney to the exterior, you may be in mortal danger and this is very
real; there are several deaths each year from this cause. What happens is
that the flame consumes the oxygen and, initially, replaces it with carbon
dioxide, which can suffocate anyone inside the room. Worse, as the oxygen
diminishes, there is insufficient to maintain full combustion and, instead
of suffocating carbon dioxide, the flame produces carbon monoxide, which is
a very deadly toxic gas which may be dangerous at concentrations as low as
100 parts per million or 0.01%. At 0.1%, death will occur in a normally
healthy person in about 2-3 hours, much faster for those with respiratory
problems. Such forms of heating are incompatible with well insulated houses.
In the event that a flame has its combustion gases evacuated by a chimney,
including open or closed log fires, it is essential that sufficient fresh
air be available from outside the house.
- A fourth point is that
the ideal temperature for a room varies with the season. For living rooms,
try to maintain a constant 19-21°C in winter (at 19°, you may prefer to wear
a sweater!) and 26-28°C in summer. For bedrooms, I recommend 17-19°C and
25-27°C respectively. It is a big mistake to set thermostats too high in
winter or too low in summer; not only do you waste energy, your body has a
great difficulty to adapt to temperature differences when you go outside and
your health may suffer.
- For maximum comfort and minimum heating or
cooling costs and carbon emissions, the way rooms are aired is important.
With good insulation, there should be as little interchange between inside
and outside air as possible, but this leads to "stuffiness" in the rooms,
especially if there is tobacco smoke or other indoor pollutants. This make
the airing of rooms important. A window should be opened a minimal length of
time to ensure a good exchange of the air, usually once per day. However, it
is important to restrict airing so that the temperature of the walls is not
significantly altered; this limits the opening of the window to a maximum of
about 10 minutes for most rooms, often less. In addition, the time of day to
open the window is also important. In summer, the best time is the early
morning, when the outside temperature is at a minimum. Conversely, in
winter, it is better to air the room in the early afternoon, when the
outside temperature is at a maximum. This way, the outside air has the least
temperature differential with the inside temperature.
A little bit off-topic, but it is not a good idea to smoke in rooms
with air conditioners running. The smoke is drawn in and tarry
condensates are deposited on the heat exchanger fins and the filter.
These also evaporate over time and give the room an unpleasant smell
(and maybe affect the
health of the occupants!)
This essay is essentially to help those with existing houses; planning to
build a new house requires a great deal of thought as to how best to heat
and cool it. For example, reversible central heat pumps spring to mind to
provide both heat and "coolth", as a complement to solar water heating, but
this solution, attractive though it is, would be very difficult and
expensive to adapt to an existing house.
There is a popular myth that it is cheaper to keep a house constantly
warm than to switch the heating off when the house is empty. This
supposes that it take more energy to re-heat the house than to keep it
at a constant temperature. If the temperature drops to a very low value,
there may be an element of truth in the myth; to heat a room from 0° to
20°C each time would certainly consume a lot of energy but the
temperature drop in practice would rarely exceed 5°C in 12 hours in a
reasonably insulated house. You can do the mathematics; as the
temperature decays exponentially and asymptotic to the outside
temperature, the energy saved by not heating to a constant 20°C would be
greater than that needed to bring the air up from 15°C to 20°C, mainly
because of the higher differential between the inside and outside
temperatures. Do not hesitate to switch off the heating if it is not
required for 2 or 3 hours or longer. The same applies to
air-conditioning. It is a false economy to keep it running 24/7.
Heating
For the sake of attempting to work out the comparative costs,
effectiveness and pollution, I'll take a hypothetical example of a house
requiring an average of 4.17 kW over 24 hours (100 kWh of thermal energy) to keep warm on
a very cold winter's day. The costs are calculated as average for
late 2009, as well as can be determined. Of course, prices may vary from
day-to-day, so the calculations shown are only indicative.
For electric normal tariff heating, the >500 kWh tariff in 2 months is
used for the basic cost. The Comfort index is a relative subjective
appreciation. The figures presume constant usage 24/7, which may not be
realistic, but will be proportional to individuals' preferred heating
regime.
Electricity
Normal tariff resistive
This is the cheapest system to install, but the most expensive to run and
is amongst the worst causes of pollution, including carbon dioxide
emissions. It is characterised by a wide choice of heating sources, but they
all have the same efficiency. The choice depends as much on personal
preference as anything. For a steady background heat, the oil-filled
radiators are good. For rapid air heating of a cold room, preference may be
given to a fan heater. Radiant heaters, such as quartz tube types, provide a
very localised effect, but the energy is mainly emitted as an
electromagnetic wave which is translated into heat only when it is absorbed
by a surface; its effectiveness at heating the air is limited. Panel heaters
usually work by convection heating of the air. For new or rebuilt
constructions, screed underfloor heating is possible, on condition that very
good insulation is placed between the screed and the slab.
Our hypothetical house requires 100 kWh of heat and 100 kWh of
electricity to heat.
| Capital cost € |
Running cost € |
CO2
emissions kg |
Comfort index |
| 500 |
Basic cost 8.80
Fuel adjustment 5.00
VAT 2.07
Total 15.87 |
98 |
Oil filled/convection 5-8
Fan 3-6
Radiant 2-4
Underfloor 6-8 |
Reduced tariff resistive
There are two ways to use Tariff 55: storage heaters and a special
type of underfloor heater. With this tariff, the user has electricity
from a 3-phase supply for 9½ hours per day, switched by the EAC. Most of
this is for 8 hours during the night, switching on between 2030 and 2330
h; the starting time is set by the EAC and varies periodically. The
other 1½ hour period starts between 1200 and 1330.
Modern storage heaters contain special 'bricks' which are heated by a
resistive element. These have the characteristic of keeping hot for many
hours, once they are heated up, letting out the accumulated heat slowly.
In our hypothetical case, the 100 kW must be consumed in the 9½ hours
that they are switched on, and not over 24 hours. Their power rating
must therefore be almost three times higher than a normal resistive
heater, to compensate for the short 'on' time. The heaters are fitted
with two thermostats. The first is to regulate the temperature which the
bricks are heated to and may be considered as 'energy in'. The second is
set to the temperature of the room and may be considered as 'energy
out'. The user sets the 'in' thermostat so that there is sufficient
energy stored to keep the room warm through to the end of the 'power
off' period at the given setting of the 'out' thermostat. The 'out'
thermostat is set to keep the room at the desired temperature. Some
users prefer to have storage heaters to maintain a low background
temperature throughout the house, say 15-17°C, and to use a
complementary heating system, such as a log fire, for the evenings while
eating dinner or watching TV.
The other system is to have underfloor heaters in the middle of the
slab, using the mass of concrete as the heat storage medium. The under
side of the slab must be well insulated, so that the heat is directed
upwards through the screed. This method has a very high thermal inertia
and, by this token, is less easily controlled. The thermostat element
should be placed in the top half of the slab and will control only the
energy needed to keep the slab at a more or less constant temperature.
There can be no control over the air temperature other than by adjusting
the slab temperature so that altering the thermostat setting may have
little effect in comfort level in under 24 hours or longer. Obviously,
this method cannot be installed in existing floors.
Our hypothetical house requires 100 kWh of heat and 100 kWh of
electricity to heat.
| Capital cost € |
Running cost € |
CO2
emissions kg |
Comfort index |
| 2000 |
Basic cost 2.80
Fuel adjustment 5.00
VAT 1.17
Total 8.97 |
98 |
Storage 7-9
Underfloor 4-7 |
Reversed aircon
Using air conditioning units to heat a house is extremely economical.
This is because the energy that is exploited does not come from the
electricity mains, but from the heat in the outside air, which is
transformed to a higher temperature by means of a heat pump. The latter
is what consumes the electricity. It is an apparent paradox that one can
obtain more energy than it seems to use, but extracting heat from the
outside air is more efficient than converting electricity to heat. The
apparent energy efficiency (heat obtained * 100/electricity consumed) is
generally between 250% and 350% depending on the units and the
refrigerant gas employed. Because this energy efficiency defies physical
laws, because only the electricity consumed is considered, a better term
used is Coefficient of Performance (COP) defined as heat
obtained/electricity consumed and is thus generally between 2.5 and 3.5.
(For the pedants, the overall efficiency, including the heat absorbed
from outside, is considerably less than 100%, probably around 60% in
some cases).
To some, this apparent exploitation of free energy is little short of
miraculous but it is not without its disadvantages, particularly in
terms of comfort. The main problem is because the air recirculating
throughout the room must absorb heat instantaneously from the heat
exchanger. This has virtually no thermal mass, so the air must pass at a
high velocity. This cannot be done without generating noise and large
units can, indeed, be very noisy. In addition, the warm air comes out
from a unit at about 2-3 m above floor level and it must be forced down
using the louvres. This creates a draught that may be unpleasant if not
managed correctly. Because the thermostat cycles regularly, the draught
alternates between heated air and room-temperature air, which is very
unpleasant for anyone sitting directly in the air stream. It is
therefore very important that the installation of the internal
air-conditioning units be very carefully planned to avoid this
inconvenience.
The outside units also produce noise. Having several on
simultaneously, to heat a house, may disturb neighbours.
There is one disadvantage that is not always evident: from the fact
that the heat that is transferred is extracted from the outside air, the
higher the temperature of the latter, the better is the efficiency. At
low air temperatures, the COP drops considerably. Depending on the type
of gas, heating may become insufficient. This is not likely to cause a
problem at, say, -5°C, so is not a real issue in Cyprus, but a drop in
effectiveness may possibly be noticed on very cold nights in the Troodos
area, especially with older units.
Even if a whole house is not heated by air-conditioning units, they
can still be economically used to complement other background heating,
to provide occasional heating etc. Some householders put a bedroom unit
on for half-an-hour before retiring, where there is no other heating, to
take the chill off the room.
The COP of a unit depends largely on the refrigerant, which may be
designated R-22, R-134a, R-407, R-410a or c etc. R-22 is no longer
current but was largely used in older units but it had a low COP,
typically around 2.5. For new installations, it is recommended to
purchase units with R-410, which will give a COP substantially more than
3, with consequently better economy.
For our example, we assume the whole house is heated by
air-conditioning units with a worst-case COP of 2.5.
Our hypothetical house requires 100 kWh of heat and 40 kWh of
electricity to heat.
| Capital cost € |
Running cost € |
CO2
emissions kg |
Comfort index |
| 3000 |
Basic cost 3.52
Fuel adjustment 2.00
VAT 0.83
Total 6.35 |
39.2 |
Split level 1-4 |
Central heating
Central heating, of any type, is not easy to install in an existing
house, if no provision has been made for it. Apart from the hassle of
putting in bulky pipework throughout the house, you have to make
provision for a safe fuel supply and a boiler room. We are therefore
talking about higher capital costs.
Oil
The oil used for domestic heating is light fuel oil (LFO). This is
similar to kerosene and diesel fuel, but is less highly refined and may
contain a wider range of hydrocarbon components. It is consequently
somewhat cheaper. It is also called mazout. At the time of writing, it
is delivered at about €0.57/l. Each litre will theoretically yield 11.2
kWh of chemical energy. In a typical conventional, well maintained,
modern domestic boiler and pipework system, taking into account the
electricity used for the burner, control circuits and circulation pump,
as well as heat losses, the overall average efficiency is about 7.0
kWh/l. This may rise to 8.0 kWh/l with a condensing boiler, but this
type is notoriously difficult to set up to obtain optimal performance.
Because of the higher efficiency of a heating boiler, compared with
an electric power station, carbon dioxide emissions are lower than
resistive heating but higher than aircon heating. However, LFO contains
up to 0.1% sulfur (2008 EU limit). This burns into the highly toxic
sulfur dioxide gas which is an added pollutant. Each litre of LFO burnt
produces about 0.62 l of sulfur dioxide, when cooled down to 20°C at 760
mm mercury barometric pressure, which is considerable.
Provision must be made for an approved oil tank, which may be metal
or plastic. What is essential is that a mandatory catch tray must be
installed under the tank and it must have a capacity at least equal to
that of the tank itself. This is to ensure that fuel spills from
overfilling or a leak are caught before they can cause any damage. LFO
will easily penetrate concrete and spills may otherwise cause severe
ground water pollution or they may run into your swimming pool (or kill
your plants!).
One point that may be of interest is that the simplest (and
most common) installation incorporates a single indoor control
box with a thermostat. It is obvious that this single thermostat
switches the burner on and off according to the room
temperature. However, the heat throughout the rest of the house
is almost uncontrolled, as it is pro rata to the heat in the
controlled room. The only way of adjusting the average
temperatures of the other rooms is by partially closing the
valves on the individual radiators. It is therefore advisable to
put the control box in the room where you want the highest
temperature, usually the living room and reducing the flow
through the other radiators (this presupposes that the radiator
size is correct for each room). Nevertheless, the temperatures
of the uncontrolled rooms are difficult to adjust because
changing weather conditions may upset the balance between the
controlled and uncontrolled rooms. This problem can be partially
overcome by fitting thermostatted valves to all the radiators,
except the one in the controlled room (if that room has one,
this thermostat should be set to a higher temperature than the
control box one). This will partially (the radiator thermostats
are not perfect because they are too close to the radiators
themselves, unless you choose the rather ugly ones with a remote
bulb and capillary) ensure the best compromise of the room
temperatures that you wish.
Our hypothetical house requires 100 kWh of heat and 14.3 l of oil to heat.
| Capital cost € |
Running cost € |
CO2
emissions kg |
Comfort index |
| 7500 |
14.3 l @ €0.57
Total 8.15 |
50 |
7-9 |
Gas
Other than the above paragraphs concerning the fuel, there is
almost no difference between oil and gas central heating and the
same generalities apply.
The only gas used in Cyprus is liquid petroleum gas (LPG),
which is an indeterminate mixture of propane and butane. This is
sold in small 10 kg "bombs" for cooking applications and gas
heaters, but refillable bulk tanks are a better proposition for
central heating boilers, although some use two or three 50 kg
cylinders. LPG is highly explosive with just about 2% in air.
For this reason, the installation must
be done by certified professionals who follow strict rules. This
includes floor level ventilation in the boiler room, because the
gas is heavier than air and may otherwise accumulate to
dangerous levels.
The tank and pressure reducer must be placed where direct sunlight
cannot reach it, to avoid pressure build-up in summer. If in a
construction, it should be very well ventilated, preferably open on one
side, at least.
We make the same assumptions of efficiency as for oil-fired CH. The
price varies according to the size of the delivery. We assume a current
price of €0.92/kg, based on a bulk delivery of about 200 kg or 350 l; if
delivered in 50 kg cylinders, the price is about €1.16/kg.
Our hypothetical house requires 100 kWh of heat and 10.2 kg of LPG to heat.
| Capital cost € |
Running cost € |
CO2
emissions kg |
Comfort index |
| 7500 |
10.2 kg @ €0.92
Total 9.41 |
33 |
7-9 |
Micro-CHP
In temperate climates, some efforts have been made to introduce
micro-CHP (combined heat and power). This is a special type of boiler
where the excess heat is used to drive a Stirling engine, in turn
driving an electric generator. In Cyprus, this is not economically
viable because it would not be working throughout the year; in summer,
most people would heat their water with a solar panel, so there would be
no call for heat and the efficiency as an electric generator alone would
be abysmal. It is therefore not considered as an option here.
Wood chip or pellets
In some countries, automatic wood pellet boilers are popular. These
burn waste wood from forestry operations which has been made into pellets. These have a disadvantage that, although they
burn biomass, the chimney gases are not purified and, in fact, are very
polluting. I have not found a source of pellets in Cyprus, so I'll
assume that it cannot be used in this country. Nevertheless, home
chipping and pelleting machines (very expensive and laborious) are available, for those with a large
source of wood and don't mind handling and disposing of a lot of ash, as well as the
pellets.
The Institute of Agricultural Research of Cyprus carries out research
in order to determine the energy plants that can be cultivated in Cyprus
for biofuel production, including mixed biomass pellet production.
Pellets are small oval-shaped particles of compressed wood and they
flow freely, almost like a liquid, and can be delivered by tanker. They
are usually transferred to a hopper. As a rough guideline, 100 litres of
pellets have the equivalent chemical heat as about 50 litres of LFO, so
considerable space is required for storage.
As pellets are essentially a biofuel, they have zero intrinsic carbon
footprint. They do have an extrinsic footprint, because of transport
from the forest to the boiler and the manufacture. This would be small
if the pellets were made here but would be considerable if shipped from
the continent.
Pellet boilers are available in a number of countries.
Radiator or underfloor?
This is an everlasting subject of dissension and each school will not
give in by an iota to the other. Central underfloor heating has the
pipes laid under the screed on top of a good layer of insulation.
Despite the ugly radiators and the concomitant dust streaking above them
(there are many designs available to suit all tastes), I favour them
because of the more rapid response time and the easier individual
control in each room. Notwithstanding, underfloor heating gives a more
uniform and constant heat distribution. A correctly designed system
avoids "hot spots" which can be uncomfortable for those with
heat-sensitive feet.
Chimney types
It is not proposed to discuss unventilated flame inside a
well-insulated house. As stated above, this type of heating can
be mortal. The use of carbon monoxide detectors is no guaranteed
way to avoid problems, especially the cheaper ones as they do
not react to excessive carbon dioxide. In addition, such flames
also produce water vapour which can produce mould on walls or
condensation on windows.
Whether wood or gas, it is not sufficient to connect the flue
of a fireplace or stove to a chimney; it is also necessary to
have an air intake into the room. This means that cold air has
to be drawn into the house from outside and this will reduce the
efficiency of the heating. For our hypothetical house, this
reduction, to produce 100 kWh of net heat, we increase it
arbitrarily to 120 kWh to compensate for the cooling.
Modern houses do not have chimneys in every room, as a rule.
This means that heating a house by direct wood or gas flame is
rather theoretical. We do our calculations accordingly. In
practice, on the other hand, such fires are usually used as a
secondary heat source, usually in the living room and for
aesthetic, rather than energetic, reasons..
Wood logs
In this country, hardwood logs are usually from olive,
eucalyptus or fruit trees. Some people use softwood, such as
pine, but this is less recommended because they burn fast,
producing tarry resin smoke which is more polluting and may
leave deposits in the chimney which may cause fires.
Although wood is a biofuel and thus has a quasi-zero carbon
footprint, this does not mean that wood fires do not pollute the
air - they do! Wood is a complex mixture of chemicals, albeit
largely cellulose, and, when burnt, produces raw carbon
particles, fly ash, nitrous oxide, tars, even dioxins. That
distinctive smell of a wood fire is chemical!
A wood fire needs more time and labour than most other forms
of heating. Assuming you buy dry hardwood logs, already cut and
split, you have the handling of them at the woodpile, taking
them in and removing the ash (then cleaning up the mess!).
A stere of dry wood weighs about 600 kg and is the typical
load of a single-cabin pick-up, selling typically for €150. At
4.0 kWh/kg of wood with 15% moisture content, the stere can
produce 2400 kWh, but about half the heat will go up the
chimney, so the effective heat will be 1200 kWh/stere.
Our hypothetical house requires 120 kWh of heat and 0.10 stere to
heat.
| Capital cost € |
Running cost € |
CO2
emissions kg |
Comfort index |
| 500-2500 |
0.1 st @ €150
Total €15.00 |
~0 |
1-6 |
Gas
This kind of heater is about 50% efficient, because of heat
lost up the chimney, as against 80% for a good central-heating
boiler. It is more likely that each heater will run from a 10 kg
cylinder and the price per kg delivered is adjusted accordingly.
Much of the energy produced in such appliances is radiant
(i.e., electromagnetic radiation) and conversion of this to heat
requires surface absorption. This is not the best answer for
comfort, because the heat appears subjectively to be unevenly
distributed (sitting in front of a radiant heater in a cold room
gives the sensation of roasting the front while the back
freezes!).
Our hypothetical house requires 120 kWh of heat and 19 kg of LPG to
heat.
| Capital cost € |
Running cost € |
CO2
emissions kg |
Comfort index |
| 200-1000 |
19 kg @ €1.00
Total €19.00 |
60 |
1-6 |
Solar house
This is not relevant on this page, because it requires a new (and
expensive) construction to use the sun's heat to keep a whole house warm
in winter, while keeping it acceptably cool in summer. An example of a
solar house with minimal energy use in a climate that is hot in summer,
cold in winter and humid all the year round can be found
here.
Cooling
Strange as it may seem, the houses in this country have some features
that favour comfort in hot weather, despite the lack of insulation.
Firstly, the white exterior paint of most of them tends to reflect solar
radiation. High ceilings allow for layering hot air. Tiled floors make
it more comfortable to bare feet.
Fans, whether portable or ceiling types, give an illusion of comfort,
as they enable more cooling evaporation of perspiration but, of course,
they cannot lower the air temperature. Ceiling fans may actually
decrease comfort unless you are in the direct draught from them, because
they can pull down layered hot air from the ceiling; it is better to
keep the air as still as possible, if you want the hot air to rise.
Reflective windows help in summer but hinder in winter and are not a
real answer to a holistic energy saving. Having the outside pane of
double glazing in photochromic glass (glass which becomes darker with
more light) may be an expensive partial help but it will reduce solar
heating even in winter, as well. The best answer is the simple shutter
outside the window, preferably light coloured. Louvred types may be
adjusted to let in some light. It is important to keep windows and, if
fitted, shutters tightly closed during the day and open them only after
sundown to profit from the cooler night air, assuming you are not
running air-conditioning.
This brings us down to air-conditioning, which is a sine qua non
in modern houses of the Cypriot summer. Again, maximum insulation is the
best way to keep your electricity bills reasonable, along with sensible
use, of course.
As with using air conditioning units for heating (see above), there
are variations between units. It is important that you fit units that
are adapted to the size of room. It is a false economy to fit units that
are either too large or too small. Analogically to the COP for heating,
different units may have a rating called the Energy Efficiency Ratio or
EER. Unfortunately, in the USA, this is defined as the BTU rating/over
its energy consumption; this is a crazy mixture of units and generally
gives figures >10. In Europe, the EER is more usually defined as the
heat energy pumped to the exterior in kWh/over its energy consumption
and is usually <4. In fact, the EER is usually slightly less than the
COP, in the range 2 to 3.5. As for heating, using the aircon units, the
EER is dependent most on the refrigerant gas used and, again, the R-410
types give the highest figures and are the most cost-effective.
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