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A further amendment to Part L has just come into effect that will add additional capital costs to central-heating systems

Anyone intending to install a new gasfired boiler or replace an existing one (in England and Wales) will need to comply with the clauses of amendment to Approved Document L1 (2002) and use a condensing boiler as opposed to exchanging like for like. Well, actually, you might not have to, if it can be shown that it is unreasonable to do so.

I'll come back to that later.

Similar revisions are on the cards for replacing oil-fired or solid-fuel boilers, although presumably, because there is more infrastructure work required, these changes will not come into force until 1 April 2007. However, because of technical requirements for the correct installation of condensing boilers - the restrictions on the siting of flue outlets, for example - compliance with the need to install a condensing boiler may entail significant additional enabling costs.

The legislation does not, as yet, require clients to strip out and replace non-condensing boilers - although that might only be a matter of time - but the legislation has been set up to improve boiler performance and, therefore, improve the thermal as well as the financial efficiency of domestic central-heating systems.

Condensing boilers convert 86 per cent of energy into heat compared with normal boilers, which convert significantly less, maybe 78 per cent.

Older-generation boilers have a heating efficiency of around 60 per cent.

The efficiency of condensing boilers is provided by an extra heat exchanger that reclaims much of the heat from erstwhile hot exhaust gases. This is used to pre-heat the water in the boiler system. So far, so good - but there are two complications: one is that the exhaust gases are much cooler and therefore do not rise as readily. Once released from the flue pipe, the flue gases tend to settle in the air around it, meaning that flue positions in areas of stagnant air, or near windows, must be avoided. The other problem is that the water vapour that is produced in the combustion process condenses back into liquid, or condensate, which is usually slightly acidic, and so has to be discharged to a combined drain or soakaway. But the guidance document notes that the acidity level is only 'similar to tomato juice'.

The ODPM notes that the new Approved Document Part L1 will be introduced 'by the end of this year'.

Nobody seems to know the exact date, but it's a badly kept secret that the calculation methods for appraising Uvalues and SAP ratings will be altered.

Not only will the U-values be tougher, but also the target U-value and elemental U-value method of assessing a given scheme will be withdrawn. The carbon index method will then be the sole mechanism for calculating the U-value of a scheme.

However, this current amendment has been introduced at a time when it is perfectly legitimate to use the target or the elemental methods. The problem is that the need for higher-efficiency boilers will have significant effects on the results when using these two methods.

And it seems reasonable to question why the ODPM has rushed out this amendment to a regulation that is going to be void in eight months.

Tony Bryer, of Survey Design Associates, highlights the dilemma when using the target U-value method to show compliance on a scheme that has been designed but has not yet received Building Regulations approval or its equivalent. 'Don't forget that the target method was intended to allow a designer greater flexibility - it takes into account the overall insulation level of the building fabric, heating system efficiency and solar gains, etc, and is applied to the whole building. For this reason, it is a very popular tool, ' he says.

'The changes caused by this amendment mean that, typically, boiler efficiency goes up from 78 per cent to 86 per cent and the target method sets these as reference figures. The target U-value figure is modified by the actual percentage efficiency divided by the reference percentage efficiency.

'So, if your unadjusted target Uvalue was 0.5 and you were already choosing to install an 88 per cent efficient boiler to make the building comply with your adjusted target, then before 1 April 2005 it was 0.5 x 88/78 = 0.56, but now, after 1 April 2005, it becomes 0.5 x 86/88 = 0.49. In other words, you have to reduce the heat loss by 13 per cent. If you have chosen to use the carbon index method then the threshold goes up from 8.0 to 8.3 - an 8.3 per cent reduction in CO 2, ' Bryer continues.

'On our model house, if you have a standard boiler and all the solid elements have standard elemental method U-values, your door/window/rooflight limit is (with AD U-values) 25 per cent of the floor area, viz 27m 2. Pre 1 April, if you opted to install a 90 per cent efficient boiler, the target U-value went up from 0.467 to 0.539, which could be translated into an increase in the permissible opening area from 27m 2 to 37.2m 2. If, post 1 April, you want to keep the same opening area, you have to reduce the heat loss somewhere else;

for example, by reducing the wall Uvalue from 0.35 to 0.24. Alternatively, you have to reduce the opening area by 6.8m 2, ' he adds.

The National Home Energy Rating states that it is concerned that 'the ODPM chose not to inform the industry about these proposed changes.' The fact that the ODPM has introduced a form that can be filled in to show why it is unreasonable to change to a condensing boiler should be quite a common sight on building control officers'desks, until next January, when the real Part L revisions will probably be introduced.

Thanks to Tony Bryer of Survey Design Associates. Email: tonyb@sda. co. uk

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