Residential and conference centre

A new building is planned as a residential and conference centre. The building will be three-storey, but part of the ground floor will be two-storey in height with vehicle unloading bay doors. The building will be heated by two identical natural gas fired boilers of efficiency 78% and of combined rated heat output of 120 kW. The main characteristics of the building are:

Dimensions: rectangular, 40 m x 15 m on plan, 10.5 m high, flat roof

Windows: fitted with 12 mm air gap low-E (sn = 0.05) argon filled double glazing units in metal frames linear run of 75 m on ground and first floors linear run of 90 m on second floor total linear run 240 m height from sill to head 1.5 m total area 240 x 1.5 = 360 m2 Personnel doors: 2 double @ 3.8 m2, 3 single @ 1.9 m2

total area = 13.3 m2 Vehicle door 1 roller door @ 25 m2

From this we may calculate the remaining areas:

Total area of windows and personnel doors is thus 373.3 m2

Total perimeter wall area = (40 + 40 + 15 + 15) x 10.5 = 1155.0m2

Area of exposed wall = 1155 — 373.3 — 25 = 756.7m2

The area of windows and personnel doors as a percentage of the perimeter wall area is:

373.3

This exceeds the allowance of30% given in Table 3.2, and so either the window/ door area must be reduced or compensating measures must be taken. This can be attempted in several ways. For example, some of the U-values for the proposed building may be lower than the maximum values specified by the elemental method, and up to half of the allowable rooflight area can be converted into an increase in window area. Consider first the U-values, as in Table 10.1.

Table 10.1 Selected U-values for proposed building.

Element

Elemental method

Selected U

Max U, W/m2K

W/m2K

Roof

0.25

0.25

Walls - extra insulation added

0.35

0.30

Windows - U-value determined by selected window

2.20

2.30

and glazing design

2.20

2.30

Personnel door - same design as windows

Vehicle door - insulated to elemental standard

0.70

0.70

Ground floor - insulated with 75 mm EPS

0.25

0.20

In attempting to allow for the fact that both the window area and the window U-value exceed the requirements of the elemental method, the designers have decided to add insulation to the walls and the floor. The U-value of the floor may be determined from Table 6.1. The perimeter to area ratio is 110/600 = 0.183 m/m2, which is nearest to 0.20 in the table, and which gives an uninsulated U-value of 0.37 W/m2K. The expanded polystyrene insulation has a thermal conductivity of 0.040 W/mK, and so the thermal resistance of the extra insulation is 0.075/0.040 = 1.875 m2K/W. Interpolating between the 1.5 and 2.0 columns of the table gives a U-value of 0.20W/m2K.

The allowable area for rooflights is 20% of the roof area, but as there are no rooflights in the proposed building, half of this allowance (i.e. 10% of the roof area) can be used to increase the allowable window area. However, this cannot be done by simply adding 10% of the roof area to the allowance for windows. This is because the maximum allowable U-values of the roof and the walls are different. This difficulty can be overcome by including rooflights in the notional building, as shown below.

The rate of heat loss from the proposed building is then compared with a notional building of the same size and shape which satisfies the elemental method. Step 1 is to prepare a heat loss table of the proposed building, incorporating the actual U-values of the proposed construction elements, as shown in Table 10.2.

Table 10.2 Proposed building heat loss table.

Element

Area

U-value

Heat loss

m2

W/m2K

W/K

Roof

600.0

0.25

150.00

Exposed walls

756.7

0.30

227.01

Windows

360.0

2.30

828.00

Personnel doors

13.3

2.30

30.59

Vehicle loading bay doors

25.0

0.70

17.50

Ground floor

600.0

0.20

120.00

Totals

2355.0

1373.10

The notional building must have the same dimensions as the proposed building, and the combined area of windows and personnel doors must account for a maximum of 30% of the perimeter wall area. However, half of the allowable rooflight area (i.e. 10% of the roof area) can be included in the notional building to help cater for the increased window area in the proposed building. The relevant areas for the notional building are thus:

Total perimeter wall area = (40 + 40

+ 15 + 15) x 10.5

= 1155.0 m-

Area of windows and personnel doors

= 0.30 x 1155

= 346.5 m:

Area of exposed wall = 1155 — 346.5

- 25

= 783.5m:

Area of personnel doors

= 13.3 m:

Area of vehicle doors

= 25.0 m:

Area of windows = 346.5 — 13.3

= 333.2m

Area of rooflights = 0.10 x 600

= 60.0 m:

Area of roof = 600 — 60

= 540.0 m:

Area of ground floor = 40 x 15

= 600.0 m:

Combining these with the maximum U-values allowed by the elemental method yields the heat loss table (Table 10.3). Note that the U-value of the ground floor in the notional building is 0.25 (the elemental standard) and not 0.20. This is because the value of 0.20 in the proposed building was achieved with added insulation. If it had been 0.20 without added insulation then it would have to be 0.20 in the notional building as well.

The results show that the proposed building has a lower heat loss rate than

Table 10.3 Notional building heat loss table.

Element

Area

U-value

Heat loss

m2

W/m2K

W/K

Roof

540.0

0.25

135.00

Rooflights

60.0

2.20

132.00

Exposed walls

783.5

0.35

274.23

Windows

333.2

2.20

733.04

Personnel doors

13.3

2.20

29.26

Vehicle loading bay doors

25.0

0.70

17.50

Ground floor

600.0

0.25

150.00

Totals

2355.0

1471.03

the notional building, and so the proposed building complies with regard to U-values and areas.

It is now necessary to consider the efficiency of the heating system. This can be done by calculating the carbon intensity of the heating system at 100% load and at 30% load, as follows:

At 100% output:

„ , 1 /RCA 1 /60 x 0.053 60 x 0.053 Carbon intensity, sc =-X - =- - 1

sc = 0.0679 kg/kWh At 30% output, with only the lead boiler operating:

Table 3.4 specifies maxima of 0.068 kg/kWh at 100% output and 0.065 kg/kWh at 30% output. The results show that the heating system is satisfactory at 100% output, but fails at 30% output. There are two ways of addressing this problem:

• Choose a more efficient lead boiler

• Check to see if boiler efficiency can be traded-off against fabric performance.

Considering the first of these options, assume that a more efficient lead boiler is available, say a condensing boiler, of efficiency 85%. The calculation at 30% load is now:

This is below the maximum of 0.065 kg/kWh and therefore the heating system complies.

Before considering taking action to implement the second option, it should be noted that the proposed building has a lower average U-value than the notional building. This lower value may already be sufficient. Thus:

Required average U-value, Ureq = Uref= 0.625 x 0 0650

From Table 10.2, the proposed building has an average U-value of 0.583 W/ m2K, and as this is less than Ureq the heating system is satisfactory without changing either the fabric or the heating system.

0.0679

0.598 W/m2K

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