THE COVERT COST OF "HIDDEN MOISTURE"
Domestic heating systems
usually work on a control system whereby the air within the dwelling is
heated up to a temperature in excess of the target temperature, the
heating appliance is then switched off and the temperature will drop
slowly over a period of time.
Once the temperature drops
below the target temperature the heating appliance will be switched on
again until the higher temperature is again obtained and this process
continues until the control mechanism is switched off.
Typically the desired
temperature will be 20oC therefore the higher temperature will
be set at 22oC and the lowest temperature setting will be 18oC.
How do we calculate the
approximate heating requirements to achieve this typical heating cycle?
Most domestic houses are
likely to have their heating system on for approximately seven months of
the year and are likely to set the controls to operate for two hours in
the morning and say five hours in the evening (a total of seven hours per
day).
An educated guestimate is that
the heating system operates for five minutes and then switches off for
three minutes. Hence over
seven hours there will have been two main heating periods and 52.5 minor
heating periods.
Each of these minor heating
periods will cost approximately 0.00138 pence and therefore over the day
will cost a total of 0.072 pence. The two main heating periods will cost
1.6 pence each giving a daily total of ,0.03272
(i.e. 3.272 pence).
In practice heating costs will
be considerably higher due to heat losses and a lack of efficiency in the
type of heating but these guestimates give an approximate weekly heating
cost of ,0.229 with a total cost for the seven
months of ,6.94. This cost may appear to be,
indeed is insignificant and this coupled with the fact that for every
percentage drop in humidity the cost of maintaining the temperature at 20oC
will also drop, means that in dry homes the cost of heating is
affordable.
Based on the premise of 20oC
at 80% RH, the saving if the air were to be maintained predominantly at 20oC
and at 50%RH, would be approx 2%, i.e. a total saving for the heating
period of approx ,0.14. It can clearly be seen that the
energy saving for the air alone is not that significant, however, when the
water vapour has settled on a surface and has become condensation or water
droplets/puddles or has become absorbed into the building fabric as "Hidden Moisture", a far more significant energy
waste occurs.
The cost of heating the air
pales into insignificance when compared with the cost of trying to
maintain comfort temperatures, if simultaneously you have to heat ever
increasing amounts of "Hidden
Moisture".
If moisture is able to soak
into the fabric of a building in ever-increasing amounts, over a prolonged
period of time, the costs associated with heating the building fabric and
the "Hidden" water
it contains will be considerably higher than in a dry building and will
greatly affect the efficiency of any heating system. e.g. during the
absorption process the surface areas will always be damp and as a
consequence the U values of the walls and ceilings will plummet,
regardless of whether they are dry lined or insulated because the water on
the surface dissipates all the heat.
Research has proven, that at
the time of constructing a building, as much as 4,000 litres of water is
used for mixing concrete and plaster and this may take up to a year to dry
out, proof positive, that huge quantities of "Hidden
Moisture"
can easily be absorbed and contained within the fabric.
An average family creates 15
litres of water per day, 105 litres per week, and again, under
our original premise, they will create 3193.75 litres over the seven
months. Most of this water is continuously being absorbed into the
building fabric, the carpets, the curtains, the plaster, the ceilings
etc..
This vapour creation does not
just occur over the seven months of course, it goes on week in week out
for 52 weeks of the year and therefore the potential quantity of water
absorbed into the dwelling could be as much as 5,460 litres, a
phenomenal amount.
There will of course be
evaporation and leakage through the fabric, BRE research suggests the
equivalent of between 0.2 and 0.7 air changes an hour occur from natural
leakage, but any such leakage will be confined to specific areas and is
unlikely to affect a complete air change throughout the dwelling, leaving
pockets of damp stagnant air in cupboards, wardrobes, behind furniture
etc..
Natural leakage is like a slow
puncture in a tyre, so long as you keep blowing air in, faster than it
escapes, it will stay inflated and, similarly, the continuous vapour
creation of an average family, more than keeps pace with natural leakage
or air exhausted by conventional extractor fans, the severe mould growth
problems in up to 30% of buildings being a testament to this.
There may also be water
ingress through the brickwork as a result of rainfall, which will swell
the reservoir of hidden moisture, and if, for example, the masonry
is painted with a non porous coating, it will prevent moisture from drying
out, in effect sealing all the water in.
This can create interstitial
condensation or cold bridging problems whenever and wherever there are
temperature differentials.
All this water held within the
fabric, drains the heat from the dwelling remorselessly, making the whole
house colder than it need be, particularly in winter and
additional heating will be needed, over and above that which would
normally be required to raise the air temperature in a dry room.
As with boiling a kettle,
there is a cost attached to heating up all this hidden water, a cost that
has to be paid day in day out if the occupant wants to heat the internal
air supply to a comfort level of say 20oC.
It is of course a cost far
higher than anyone can afford to pay, certainly anyone in social housing,
or on benefit, or living on a state pension, but nevertheless it is the
cost one would have to pay to achieve adequate heating comfort levels and
it is a major factor contributing to fuel poverty.
Landlords in social housing or
the public sector, who force their tenants to live in damp, condensation
affected dwellings, are condemning them not just to the misery and ill
health that breathing in damp polluted air brings, they are condemning
people, usually on low incomes, to squander their precious resources by
having to pay a far, far greater proportion of their income on heating
than they need to, just to stay warm.
Specific
Heat Capacity of water is 4.12kJ/kgoC i.e. it requires 4,120
Joules per kilogram per degree and one litre has a mass of one kilogram
ˆ
Cost to heat 1litre by 1oC = Specific Heat Capacity/1kWhr x
Cost for one kWhr
= 4,120/3,600,000
x 1.52 pence = 0.00174 pence
= ,0.0000174
If it costs in excess of
0.00174 pence to raise 1 Litre of water by 1oC using current
British Gas costs it follows therefore that it would cost 20 times that
amount to heat 1 litre of water by 20oC
(This again takes no account of the losses for the time whilst the
heating process is occurring, so the actual figure will be considerably
higher than these calculated).
For the purposes of this
example, assume that when the air within a dwelling is heated to a target
temperature of 20oC, the water/moisture within the walls is
likely to be heated to say 7oC and the heating system will
perform its heating cycles between 6oC and 8oC, it
should be noted that this figure is very hard to calculate as there are
numerous materials which make up the fabric of any one given building.
We have already shown that it
is possible for the building fabric of a dwelling to hold all the water
created by the occupants and that the maximum 5460 litres is possible,
and, even in the unlikely event that the maximum 70% Air change, by
natural leakage, occurs throughout the whole house, the minimum 1365
litres will still be contained within the fabric.
To heat
5,460 litres of water within the fabric of the building, the cost
in pounds sterling will be in excess of:
(212
x ((2 x 5,460 x 8 x 0.00178) + (52.5 x 5,460 x 2 x 0.00178))) / 100
=
(212 x (155.501 + 1020.47)) / 100 =
,2,493.06
If there were only half of
this amount i.e. 2,730 litres of water within the fabric of the building
the cost in pounds sterling would be approximately:
(212
x ((2 x 2,730 x 8 x 0.00178) + (52.5 x 2,730 x 2 x 0.00178))) / 100
=
(212 x (77.750 + 510.24)) / 100 =
,1,246.53
If there were only a quarter
of this amount i.e. 1,365 litres of water within the fabric of the
building the cost in pounds sterling would be approximately:
(212
x ((2 x 1,365 x 8 x 0.00178) + (52.5 x 1,365 x 2 x 0.00178))) / 100
=
(212 x (38.875 + 255.12)) / 100 =
,623.265
The moisture content of the
fabric of the building will be closely related to the typical humidity
levels and will vary continuously depending upon the use of the dwelling
and the materials from which they are made.
There will always be some
moisture contained within the building fabric, the furniture etc and it is
desirable to reduce these levels to the optimum, in effect to A dry out@ the dwelling so that the heating
system becomes more efficient.
The most cost effective,
user-friendly way to do this is with heat recovery ventilators.
Other costs which will be a
factor in typical Domestic Gas bills are likely to be heating the water
desired for the cooker (if it is a gas appliance), or the bath or sinks
etc..
This again is likely to vary
for all dwellings depending upon the number of occupants and their
respective lifestyles and in any event we are concerned about the energy
lost through the fabric, over which we have no control, not with energy
lost due to an individuals lifestyle, this being very much a
personal choice
Energy consumed for cooking,
washing and bathing therefore is a factor that can be ignored, although
the varying quantities of water vapour created will undoubtedly have a
bearing on the cost of heating the hidden moisture.
Logically therefore, if you
dry out the hidden moisture, simultaneously reduce the humidity to the
optimum level and keep it there, you will:
1)
Save, if you are a tenant or householder, the cost of heating up excessive
levels of hidden water held in the fabric, because it will have dried out.
2)
Save, if you are a tenant or householder, the cost of heating up
the excess moisture contained in the humid air, because there will be
significantly less when optimum humidity levels are maintained.
3)
Save, if you are an Owner Occupier or a Landlord, the cost of the
damage to the fabric, to the decorations, the legal costs and compensation
awards, all consequences of hidden
water, condensation and mould growth.
4)
Save, if you are an authority landlord, a great deal of money, by
not having to administer or deal with complaints of dampness, condensation
and mould growth. (See "How To Avoid a Void"), often as high as ,30, 000.
5)
Save, if you are the government, local or central, the huge amounts
of money now being spent on energy waste, health care costs, absenteeism
through ill health, building repairs and redecoration, increases in fuel
poverty, failing in your duty of care to your electorate.
6)
Save, if you and your family are living in damp, mould-ridden
dwellings, the misery, the stress, the damage to your health, caused by
breathing in polluted air day after day and that saving is worth
everything.
© Kiltox – July
1999 – May not be reproduced without our express written permission
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