Heating – Running Costs  Page 1 2 3 4 5
Degree
Days
Introduction
Degree days are used to ascertain the energy used to heat
a building.
This does not include energy to heat hot water in a hot water
cylinder.
It is assumed that a building does not need to be
heated when the outside temperature is more than the base
temperature of 15.5^{o}C.
The values of Degree Days are based on an internal
temperature of 18.3^{o}C.
The number of degree days for a region is;
The number of degrees of mean
outside temperature less than 15.5^{o}C
in a day….. times the number of heating days in the year.
So, if the mean outside temperature on a day is 8.5^{
o}C, then the number of degree days is 15.5^{o}C  8.5^{o}C
= 7 degree days.
‘Degreedays are essentially the
summation of temperature differences over time, and hence they capture both extremity
and duration of outdoor temperatures.’  from TM41 (2006)
The following diagram shows a typical graph of daily
outside temperatures and how the number of Degree Days is calculated.
The base temperature
may change for some building types as recommended in CIBSE Guide J (2002) Table 4.16 –
health services buildings 18.5^{o}C and well insulated building with
high internal gains 10^{o}C.
The CIBSE guide publishes data for degree days for
various regions.
CIBSE
Guide A (2006) section 2.5 gives typical 20year average monthly and annual
degree day values and Table 4.17 of CIBSE Guide J (CIBSE 2002) gives similar
data.
From
Table 4.17 (Guide J) the annual degree days total for
The
same figure is given in Table 2.17 (Guide A).
The Table below gives some typical Degree Day
monthly and yearly values ( base temperature 15.5^{o}C).
Region 
Degree Days 

Jan 
Feb 
Mar 
Apr 
May 
Jun 
Jul 
Aug 
Sep 
Oct 
Nov 
Dec 
Annual 
Sept to May 39 week Heating season 
Oct to Apr 30 week Heating season 


340 
309 
261 
197 
111 
49 
20 
23 
53 
128 
234 
308 
2033 
1941 
1777 

362 
321 
304 
234 
158 
88 
47 
56 
102 
189 
269 
330 
2360 
2169 
1909 
In
northern
Annual Central Heating
Energy Demand
The
following formula is used to calculate the annual central heating energy demand
for a building.
E = S (AU) x D_{d}
x 24
Where;
E = Annual
Heating Energy Demand (kW.h)
S(AU) = Heat loss coefficient or Maximum Heat loss for the building /
Temperature difference
from heat loss calculation (kW/degC)  includes ventilation heat loss.
D_{d} = Number
of annual Degree Days for the region
More Accurate
Assessment of Annual Heating Energy Demand
Some factors can be considered to
make this calculation more accurate;
1.
Buildings
may be only heated for 39 weeks per year or other periods.
2.
The
inside temperature may be higher than 18.3^{o}C on which degree days
are based.
3.
Buildings
with high heat gains have less requirement for winter heating.
4.
Light
and heavy weight buildings respond differently to being heated.
5.
Some
buildings are used intermittently, some are continuous use.
CIBSE TM41 (2006) outlines methods
of determining more accurate annual energy usage but the following is a simpler
approach.
The following formula can be used;
E = W x ( D_{d} x F_{g} x F_{m
} x
F_{o }) / h
Where;
E = Annual
Heating Energy Demand (kW.h)
W = Total heat loss
(kW)
D_{d} = Number
of annual Degree Days for the region
h = Product of plant efficiencies i.e.
Part Load efficiency x Thermal efficiency.
F_{g} = Factor
for building heat gain (see table below).
F_{m} = Factor
for mode of plant operation (see table below).
F_{o} = Factor
for building occupation (see table below).
(D_{d} x F_{g} x F_{m
} x
F_{o }) is also known as equivalent full load hours per annum.
The
following Tables give factors for use in the above formulae.
Building
characteristics 
Temperature
increment due to Heat Gains (^{o}C) 
Large areas of external glazing, dense occupancy 
5
 6 
One or two of the above characteristics 
4
 5 
Traditional with normal levels of glazing,
equipment and occupancy. 
3
 4 
Small glazed area, little or no heat producing equipment,
sparse occupancy 
2
 3 
Residential and dwellings 
5
 8 
Building Heat Gain Factors (F_{g}) for Inside Design Temperature 21^{o}C 

Temperature
increment (^{o}C)
from above Table 
Outside Design temp. (2^{
o}C) 
Outside Design temp. (3^{
o}C) 
Outside Design temp. (4^{
o}C) 
2 
1.45 
1.39 
1.33 
3 
1.34 
1.28 
1.23 
4 
1.22 
1.17 
1.12 
5 
1.11 
1.06 
1.02 
6 
0.99 
0.95 
0.91 
7 
0.88 
0.84 
0.81 
Mode of Plant Operation Factors (F_{m}) 

Building
Structure 
Continuous
Plant Operation 24
hrs. 
Intermittent
Plant Operation 

Slow Response system 
Fast Response system 

7 days per week 
5 days per week 
7 days per week 
5 days per week 
7 days per week 
5 days per week 

Heavy Multistorey buildings with a lightweight façade
and solid partitions and floors 
1.0 
0.85 
0.95 
0.81 
0.85 
0.71 
Medium Singlestorey buildings of masonry or concrete with
solid partitions 
1.0 
0.80 
0.85 
0.68 
0.70 
0.56 
Light Singlestorey factory type buildings with little
partitioning 
1.0 
0.75 
0.70 
0.53 
0.55 
0.41 
Building Occupation Factors (F_{o}) 

Building
Structure 
Occupation
period 

4 hours 
8 hours 
12 hours 
16 hours 

Heavy 
0.96 
1.0 
1.03 
1.05 
Medium 
0.82 
1.0 
1.13 
1.23 
Light 
0.68 
1.0 
1.23 
1.40 
Example 1
Calculate the annual total
energy demand for central heating and hot water for a 160 m^{2} floor
area house.
DATA
Use data
from; Heating –
Running Costs – page 4, Example 6.
Total
heat loss = 11.52 kW
Annual energy requirement to heat a 120 litre hot water
cylinder = 3849 kWh/ year.
Temperature
difference between inside and outside
= 21^{o}C – ( 2^{o}C) = 23
deg.C.
Region =
Plant
Thermal efficiency = 0.80
Part Load
efficiency = 0.80
Building
is Medium weight, average occupation period
= 9 hours, Building Occupation
Factor (F_{o}) by extrapolation = 1.0325
Heat Gain
temperature increment = say 6^{o}C.
Heating
system = fast response with intermittent operation, 7 days / week.
Answer
E = W
x ( D_{d} x F_{g} x F_{m
} x
F_{o }) / h
Degree
days for region for 39 week operation = 2169.
From
Tables above; F_{g} = 0.99,
F_{m} = 0.70, F_{o}
= 1.0325.
Equivalent full load hours per annum (D_{d} x F_{g} x F_{m } x F_{o }) = (2169 x 0.99 x 0.70 x 1.0325) = 1552
E = 11.52
x 1552 / 0.80
x 0.80
E = 17,879 kW.h / 0.64
E = 27,936
kW.h
Total
annual energy demand (kWh) = Total
annual heating energy demand (kWh) (E) + Total
annual hot water energy demand (kWh)
Total
annual energy demand (kWh) = 27,936
kWh + 3849 kWh
Total
annual energy demand (kWh) = 31,785 kWh.
The annual heating energy
demand of 27,936 kWh compares
with the value calculated in; Heating – Running Costs – page 4, Example 6 of 26,536 kWh.
Example
2
Calculate
the annual heating energy demand for a traditionally built Singlestorey Office
building given the following data;
DATA
Total
heat loss from heat loss calculation sheet = 65 kW
Temperature
difference between inside and outside
= 21^{o}C – ( 2^{o}C) = 23
deg.C.
Region =
Plant
Thermal efficiency = 0.80
Part Load
efficiency = 0.80
Building
is Light weight according to the above Table Mode of Plant Operation Factors (F_{m}).
Heat Gain
temperature increment = 3^{o}C.
Heating
system = fast response with intermittent operation, 5 days / week & 8 hours
occupation.
Answer
E = W
x ( D_{d} x F_{g} x F_{m
} x
F_{o }) / h
Degree
days for region for 39 week operation = 2169.
From
Tables above; F_{g} = 1.34,
F_{m} = 0.56, F_{o}
= 1.00.
Equivalent full load hours per annum (D_{d} x F_{g} x F_{m } x F_{o }) = (2169 x 1.34 x 0.56 x 1.00) = 1627.6
E = 65
x 1627.6 / 0.80
x 0.80
E = 105,794 kW.h / 0.64
E = 165,303 kW.h
Example
3
Calculate
the annual heating energy demand for a Factory building.
The data
for the factory is given in the Heating section
 Example – Factory.
DATA
Total
heat loss given in the example including 20% margin = 167 kW.
Temperature
difference between inside and outside
= 19^{o}C – ( 2^{o}C) = 21
deg.C.
Region =
Plant
Thermal efficiency = 0.80
Part Load
efficiency = 0.80
Building
is Light weight.
Heat Gain
temperature increment = 3^{o}C.
Heating
system = fast response with intermittent operation, 7 days / week & 12
hours occupation.
Answer
E = W
x ( D_{d} x F_{g} x F_{m
} x
F_{o }) / h
Degree
days for region for 39 week operation = 2169.
From
Tables above; F_{g} = 1.34,
F_{m} = 0.55, F_{o}
= 1.23.
Equivalent
full load hours per annum (D_{d}
x F_{g} x F_{m
} x
F_{o }) = (2169 x 1.34 x 0.55 x 1.23) = 1966
E = 167
x 1966 / 0.80
x 0.80
E = 328,322 kW.h / 0.64
E = 513,003 kW.h
Heating – Running Costs  Page 1 2 3 4 5