Opinion
Renewing the rule
The Merton Rule for using renewables has served us well
but, Glen Irwin asks, isn’t it time to increase flexibility and
put engineers back in the driving seat?
W
hen the Merton Rule was devised a few that offset a heating load? I sympathise with the local
years ago with the aim of providing at authority planning departments – it is not easy for
least 10 per cent of a development’s them to deal with the complexities of the technologies
energy needs from on-site renewables, and the calculation methods. What we now need is
the ideas were new and fundamentally correct: reduce a new approach: throw all the emissions into one
base loads by efficient design (‘lean’), use low-carbon pot and let the engineers do their thing to save the
technologies to reduce emissions (‘clean’), and then most carbon by whatever means are cost-effective and
top it off with renewables (‘green’). But it seems this practical.
creed has been misunderstood in many quarters, We also need a system
and it now threatens to strangle a designer’s ability that is easily measured by
We should
to innovate and be creative in pursuit of lower overall calculation, that aligns with
carbon emissions. Building Regulations, is easily
throw all
For example, it could be argued that a very low- understood by planners and
emissions into one pot and
demand, passive building complies with this process makes no constraints on
in theory, but in practice it can fall foul of the renewables innovation. One solution is to
let engineers do their thing
goal – and thereby the planning process. With very extend Energy Performance
to save the most carbon
low loads there is either little point or no money left Certificates (and rename these
to spend on combined heat and power or renewables: CPCs – carbon performance
by whatever means are
the scheme has achieved a very good result overall by certificates) to include all energy
cost-effective
working hard on the lean stage alone, but some planners demands in buildings, in just
will reject it while fundamentally more energy-intensive the same way that we currently
and practical.
buildings that do have CHP, and maybe even 20 per prepare renewables studies –
cent renewables, sail through the planning process that is, regulated loads (Part L) plus non-regulated loads
because all the boxes are ticked – even though the overall (usually from benchmark documents) less the savings
emissions in Kg CO2/sq m may be double that of the from low and zero-carbon technologies. This would also
low-carbon passive building. As a result, the message make comparisons easier with future, potentially more
to the industry is: ‘It’s not what you actually achieve, widespread, Display Energy Certificates.
as long as it ticks all the boxes.’ It’s not perfect because it is theoretical – but it
Renewables may well have a place, if considered would be a flexible measure of total operational
appropriately, but an unhealthy obsession with carbon, which doesn’t constrain designers into certain
them implies that all carbon savings are equal but technologies. The planners can then set targets for all
renewable carbon savings are more equal than others. new buildings in their region to, say, meet a B energy
I have heard of some design teams employing larger rating. But how that is achieved is entirely up to a
biomass boilers than required so that they can achieve project’s architect and engineers.
a 20 per cent renewables target, while the façade If we are to look back in 2050 and congratulate
performance is allowed to be poor to save money ourselves on a job well done, then we need real net
overall. The net result is that we see little real progress carbon savings, and only full flexibility in design will
towards achieving our 2020 or 2050 emission- get us there. l
reduction targets.
Renewables are subject to targets, but they are
Glen Irwin is sustainability director with engineering
not necessarily the answer. How can solar energy consultancy Gifford. He is also involved with the MBE
absorbed by an electrically powered heat pump be a
Knowledge Transfer Network and the BCO Environmental
Sustainability Group.
higher priority than direct passive solar heat gains
18 CIBSE Journal August 2009 www.cibsejournal.com
CIBSEaug09 pp18 Opinion.indd 18 23/7/09 15:53:53
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