Figures 8.2a and 8.2b are plan views of the ground and first floors of a detached dwelling. The dwelling lies approximately on a north-south axis. The windows have metal frames, with 24.8 m2 on the southern face, 11.4 m2 on the northern face, and 1.8 m2 to the side. It is proposed to fit a mains gas fired boiler with a SEDBUK rating of 76%. The areas and U-values of the elements of the dwelling are given in Table 8.2.

The total area of openings is 43.70m2, and the total floor area is 156.00m2. The openings are therefore 28.0% of the floor area. The dwelling is therefore below the requirements of the elemental method, but can be assessed using the target U-value method.

(a) Ground floor

Fig. 8.2 Detached dwelling.

(b) First floor

Fig. 8.2 Detached dwelling.

Element |
Area |
U-value |
Heat loss per degree |

m2 |
W/m2K |
W/K | |

Wall |
133.80 |
0.35 |
46.83 |

Roof |
92.00 |
0.20 |
18.40 |

Ground floor |
64.00 |
0.25 |
16.00 |

First floor to garage |
28.00 |
0.15 |
4.20 |

Windows |
38.00 |
2.30 |
87.40 |

Doors (3 No.) |
5.70 |
1.70 |
9.69 |

Totals |
At = 361.50 |
ZAU = 182.52 |

Step 1 The initial target U-value U1

0.413

156.00'

361.50

0.462 W/m2K

361.50 361.50 Step 2 Adjust for boiler efficiency

SEDBUK of proposed boiler = 76% SEDBUK of reference boiler = 78%

U2 = 0.974 X 0.462 = 0.450 W/m2K Step 3 Allowance for additional solar gain due to window frame material The window frames are metal, and so:

U3 = 1.03 U2 = 0.464 W/m2K Step 4 Allowance for additional solar gain due to orientation of windows

South facing area North facing area Total window area

Solar adjustment factor AS — 0.04 Step 5 Convert to final target U-value

38.0

0.014

Step 6 Find UAVg

For compliance, UAVG must be less than or equal to UT . However in this case UAVG is greater than UT, and so the dwelling does not comply by the target U-value method. There are several ways, applied either singly or in combination, in which the design of the dwelling could be altered to improve the possibility of compliance. These may either increase the target U-value, or reduce UAVG, or affect both. The following are examples.

(1) Raise boiler efficiency - and hence raise Uj

A boiler with a higher SEDBUK rating, say 81%, could be used. The boiler efficiency factor is then 81/78 = 1.04, and this would alter UT as follows:

This has resulted in UAVG being less than the target U-value, and so the dwelling now complies.

(2) Reduce the U-values of some of the elements - and hence reduce Uavg This may be attempted by trial and error by choosing lower U-values and repeating the calculation of UAVG. Alternatively, the required reduction can be estimated by setting UAVG equal to UT (which would satisfy the target U-value requirement) and calculating a new value for EAU.

Reduced EAU = ATUAVG = 361.5 x 0.476 = 172.07 Required reduction = 182.52 - 172.07 = 10.45

This reduction may be obtained from any one of the exposed elements, or from a combination of several. If only one element is altered, the effect on its U-value would be:

Walls

Roof

Ground floor

Windows

Doors

Reduction = New U-value Reduction = New U-value Reduction = New U-value Reduction =

New U-value = 2.30 - 0.28 = 2.02 W/m2K Reduction = 10.45/5.7 = 1.83

New U-value = 1.70 — 1.83 negative, therefore not possible

Most of these reduced U-values may be difficult to achieve. In the case of the windows, it would be possible to achieve a U-value of 2.0 or less by specifying triple glazing. In order to retain double glazing it would be necessary to change to non-metal frames. This would reduce UT because the factor of 1.03 for metal frames would no longer apply. Nevertheless, in this particular case, the adoption of non-metal frames may be the most practical option. Otherwise it would be necessary to take a combination of measures.

(3) Reduce the window area - and hence reduce UAyG

From (2) above, the required reduction in EAU is 10.45. However, if the window area is reduced, the wall area is increased by the same amount. Consequently the gain from reducing the window area is offset by a corresponding increase in wall area. If 8A is the reduction in window area, then we may write the following equation:

10.45 = 2.30 x 8A - 0.35 x 8A = 2.65 x 8A 8A = 10.45/2.65 = 3.94m2

Therefore:

New window area = 38.00 - 3.94 = 34.06 m2 New wall area = 133.80 + 3.94 = 137.74m2

In this case, adopting a window area of 34.06 m2 or less would be sufficient to reduce UAVG to 0.476 W/m2K, and hence ensure compliance.

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