SuperHeat 5 has been approved by the BRE as a suitable
program for producing SAP (Standard Assessment Procedure) home energy ratings
and Carbon Index values for dwellings. The SAP-2001 rating is a whole number
between 1 (very bad) and 120 (very good) which indicates the
cost of providing energy heat, light and domestic hot water
per m² of floor space. The Carbon Index is a number (expressed to one
decimal place) between 0.0 (bad) and 10.0 (very good) which measures the CO2
emissions per m² of floor space
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The original house
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The DEMOH02.H5B file that comes with SuperHeat 5 contains data for a
2-storey 9m x 6m detached house built to the 2002 L1 Approved Document
standards with a condensing boiler. The U-values of the walls, floor and roof
are 0.35, 0.221 and 0.16 respectively. This dwelling has a SAP-2001 rating of
98 and estimated CO2 emissions of 2.0 tonnes/year giving a Carbon Index of 7.7.
The figures below are obtained by changing the indicated item only; obviously
changing a number of items would have a cumulative effect.
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Changing the heating system
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In general changing the heating system and/or fuel has more effect on
the SAP rating than anything else. When producing SAP ratings you
MUST identify the system type, controls and fuel and enter
them correctly. Here are the SAP ratings that you get when you change the
heating system and/or fuel, leaving the structure unchanged
| Change
|
SAP-2001 rating
|
Carbon Index
|
| Default system: condensing gas boiler
|
98
|
7.7
|
| Baxi 100HE condensing boiler - SEDBUK efficiency 90.9%
|
101
|
8.0
|
| Non condensing boiler - 78% efficiency
|
95
|
7.5
|
| Non condensing combi (no h.w. cylinder of course) - 78% efficiency
|
97
|
7.6
|
| Condensing combi - generic 83% efficiency
|
100
|
7.9
|
| Vaillant Ecomax condesing combi - SEDBUK efficiency 91.1%
|
103
|
8.2
|
| Fan flue gas condensing boiler bulk LPG
|
62
|
6.8
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| Fan flue gas boiler: bottled gas (CO2 emissions are the same for bulk
LPG and bottled gas - about 28% higher than for mains gas - so CI does not
change)
|
57
|
6.8
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| Standard oil boiler
|
95
|
6.3
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| Condensing oil boiler 83% efficiency - the additional cost of the
fuel (£4.53/GJ v. £3.74 for gas) is compensated for by there being
no standing charge so the SAP rating is virtually identical to that for the
default gas-fired system. The CI is significantly lower though
|
97
|
6.5
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| Slimline storage heaters (off peak) dual immersion 160 litre 50mm
foam lagged cylinder. Electrical panel heaters as secondary heating
|
70
|
5.5
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| Electric fires (on peak) [not a serious suggestion!] - note that
the CI is higher than that for the storage heater system because less
electricity is used
|
39
|
5.8
|
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Ventilation
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The various ventilation parameters have a relatively small effect on
the SAP rating, though you should still assess them accurately:
| Change
|
SAP-2001 rating
|
Carbon Index
|
| Default: 2 sheltered sides, solid floor, no
chimneys
|
98
|
7.7
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| No sheltered sides
|
97
|
7.6
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| Four sheltered sides
|
98
|
7.8
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| Unsealed wooden floor
|
95
|
7.5
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| No draught stripping to windows and doors: (not allowed for new
dwellings)
|
95
|
7.5
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| Add two chimneys
|
94
|
74.
|
|
Structure
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This dwelling is already constructed to a reasonable standard so
improving the wall insulation only has a limited effect on the SAP rating:
| Change
|
SAP-2001 rating
|
Carbon Index
|
Default: All external walls 0.35
|
98
|
7.7
|
| Improve wall U-value from 0.35 to 0.30
|
99
|
7.8
|
|
Mis-measuring the floor area
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The SAP rating is a measure of notional energy cost per square metre of
floor space. Getting the floor area wrong may result in a serious error:
| Floor area entered as
|
SAP-2001 rating
|
Carbon Index
|
| 54 m² (ground floor area entered
instead of total floor area)
|
77
|
5.8
|
| 86.4 m² (-20%)
|
90
|
7.1
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| 97.2 m² (-10%)
|
94
|
7.4
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| 108 m² (corrrect value)
|
98
|
7.7
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| 118.8 m² (+10%)
|
101
|
8.0
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| 129.6 m² (+20%)
|
104
|
8.2
|
|
Conclusion
When doing SAP ratings you MUST get the heating system, fuel and
floor area right. Use of a high-efficiency boiler can significantly improve SAP
ratings, but if you are designing for a third party you must stress that any
change in the heating system may have a significant effect on the SAP
rating.
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