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 15.5oC.

The values of Degree Days are based on an internal temperature of 18.3oC.

The number of degree days for a region is;

The number of degrees of mean outside temperature less than 15.5oC in a day….. times the number of heating days in the year.

So, if the mean outside temperature on a day is 8.5 oC, then the number of degree days is 15.5oC - 8.5oC = 7 degree days.

‘Degree-days 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 18.5oC and well insulated building with high internal gains 10oC.

The CIBSE guide publishes data for degree days for various regions.

CIBSE Guide A (2006) section 2.5 gives typical 20-year 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 Northern Ireland (base temperature 15.5oC) is 2360.

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 15.5oC).

 

 

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

Thames Valley

340

309

261

197

111

49

20

23

53

128

234

308

2033

1941

1777

Northern Ireland

362

321

304

234

158

88

47

56

102

189

269

330

2360

2169

1909

 

In northern UK, the 39 week heating season is often used, therefore 2169 Degree Days can be used in calculations.

 

 

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  Dd 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.

Dd        =          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 18.3oC 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   ( Dd  x  Fg  x  Fm   x  Fo )     /   h

 

Where;

            E          =          Annual Heating Energy Demand (kW.h)

            W        =          Total heat loss (kW)

Dd        =          Number of annual Degree Days for the region

h         =          Product of plant efficiencies i.e. Part Load efficiency x Thermal efficiency.

            Fg         =          Factor for building heat gain (see table below).

            Fm        =          Factor for mode of plant operation (see table below).

            Fo         =          Factor for building occupation (see table below).

 

          (Dd  x  Fg  x  Fm   x  Fo ) 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

(oC)

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 (Fg)

for Inside Design Temperature 21oC

Temperature increment

(oC)

from above Table

Outside  Design temp.

(-2 oC)

Outside  Design temp.

(-3 oC)

Outside  Design temp.

(-4 oC)

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 (Fm)

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

Multi-storey buildings with a lightweight façade and solid partitions and floors

1.0

0.85

0.95

0.81

0.85

0.71

Medium

Single-storey buildings of masonry or concrete with solid partitions

1.0

0.80

0.85

0.68

0.70

0.56

Light

Single-storey factory type buildings with little partitioning

1.0

0.75

0.70

0.53

0.55

0.41

 

Building Occupation Factors (Fo)

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 m2 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  =     21oC – (- 2oC)  =  23 deg.C.

Region  = Northern Ireland, 39 week heating season.

Plant Thermal efficiency  =  0.80

Part Load efficiency = 0.80

Building is Medium weight, average occupation period  = 9 hours,  Building Occupation Factor (Fo) by extrapolation = 1.0325

Heat Gain temperature increment  = say 6oC.

Heating system = fast response with intermittent operation, 7 days / week.

 

 

 

Answer

E          =          W  x   ( Dd  x  Fg  x  Fm   x  Fo )  /  h

                  

Degree days for region for 39 week operation = 2169.

From Tables above;  Fg  = 0.99,  Fm  = 0.70,  Fo  = 1.0325.

Equivalent full load hours per annum (Dd  x  Fg  x  Fm   x  Fo )  =  (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 Single-storey Office building given the following data;

DATA

Total heat loss from heat loss calculation sheet       =          65 kW

Temperature difference between inside and outside  =     21oC – (- 2oC)  =  23 deg.C.

Region  = Northern Ireland, 39 week heating season.

Plant Thermal efficiency  =  0.80

Part Load efficiency = 0.80

Building is Light weight according to the above Table Mode of Plant Operation Factors (Fm).

Heat Gain temperature increment  = 3oC.

Heating system = fast response with intermittent operation, 5 days / week & 8 hours occupation.

 

 

 

Answer

E          =          W  x   ( Dd  x  Fg  x  Fm   x  Fo )  /  h

                  

Degree days for region for 39 week operation = 2169.

From Tables above;  Fg  = 1.34,  Fm  = 0.56,  Fo  = 1.00.

Equivalent full load hours per annum (Dd  x  Fg  x  Fm   x  Fo )  =  (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  =     19oC – (- 2oC)  =  21 deg.C.

Region  = Northern Ireland, 39 week heating season.

Plant Thermal efficiency  =  0.80

Part Load efficiency = 0.80

Building is Light weight.

Heat Gain temperature increment  = 3oC.

Heating system = fast response with intermittent operation, 7 days / week & 12 hours occupation.

 

 

 

Answer

E          =          W  x   ( Dd  x  Fg  x  Fm   x  Fo )  /  h

                  

Degree days for region for 39 week operation = 2169.

From Tables above;  Fg  = 1.34,  Fm  = 0.55,  Fo  = 1.23.

Equivalent full load hours per annum (Dd  x  Fg  x  Fm   x  Fo )  =  (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

 

 


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