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29 Jun 2022 < Back

New Approved Document F - A good thing or a catalyst for some negative unintended consequences?

Much has been written about the new Approved Document F (ADF), including a recent blog from PCA's Technical Manager James Berry where he highlighted the key changes to this revised document. John Bradley, Managing Director at Homevent and PCA Chairman shares his thoughts on Approved Document F and the dilemma this brings to many within the sector - is this a good thing, or a catalyst for some negative unintended consequences? Scroll down to read more….

Approved Document F - Volume 1: Dwellings

To try and prevent disease spread during the Great Plague of London (1665-1666), a killing of dogs and cats was ordered by those in authority. If left untouched, they would have made a significant reduction in the rat population that carried the fleas which transmitted the disease. There are many other examples through history where best intentions have resulted in unexpected and unwanted results. There is no doubt that those responsible for the new Approved Document F (ADF) have the best intentions in mind. Every day we hear about climate change and increased energy bills. We simply have to reduce the energy consumption of the homes we live in by better insulation and draughtproofing and those behind the new Approved Document F undoubtedly want to ensure this is not done at the expense of ensuring an “…adequate means of ventilation provided for people in the building”.

Whole Dwelling Ventilation Rate

Many consider the most important constituent of any ventilation system to be the “Whole Dwelling Ventilation”. Getting this general, continuous ventilation right has been proven to make a major contribution to good overall ventilation in a dwelling, while getting it wrong often leads to condensation and mould and a build-up of undesirable indoor air pollutants. The Whole Dwelling Ventilation Rate also has a very big impact on the energy consumption of a home and the running costs for its occupants. Unnecessarily high ventilation rates, lead to unnecessarily high carbon emissions to the environment and energy bills for the occupants. In the previous edition of Approved Document F, guidance was given on the Whole Dwelling Ventilation Rate for mechanical ventilation which considered assumed or actual occupancy levels in homes. The new Approved Document F replaces the previous guidance with guidance based on the number of bedrooms, regardless of how many occupants there are or will be in a home. This has surprised many of us involved in assessing and providing ventilation systems for existing homes, as it is occupants who produce pollutants that need to be controlled, not bedrooms.

The old Approved Doc F versus the new version

If the new Approved Document F is followed, depending on floor area, you may be required to provide the same Whole Dwelling Ventilation Rate in a 4-bedroom property occupied by 2 people as you would in the same house occupied by 8 people. To many of us, this does not make sense. There is also the fact to consider that if we do assume a similar occupancy per bedroom as the previous ADF, the new ADF suggest that we now typically need nearly 50% more air to be supplied to a home to provide the level of Whole Dwelling Ventilation now required. This raises two points:

  • Firstly, where is the evidence that the previous rates were not adequate? There is lots of evidence that the previous rates were not achieved in practice and compliance was not enforced, however, there is little evidence to support such a large increase in ventilation.
  • The second point is that providing ventilation systems with these increased airflows are likely to offset a large proportion, if not all of the energy savings that increased insulation and airtightness measures are intended to bring.

An increase in carbon emissions and running costs

In existing homes, not subject to any building fabric improvement measures, there will be a very large increase in carbon emissions and running costs for occupants if the new ADF is followed, when the ventilation only is being upgraded. Whether or not the existing heating system is capable of actually heating the extra air supplied to a home to the required temperature is another story altogether. To put this into some kind of perspective, if we assume a similar occupancy level in the previous and new ADF for a 3 bedroom house and are considering a continuous running mechanical extract ventilation system, the new ADF may require the home heating system to heat an additional 864,000 litres of air per day from outside to inside air temperature compared to that required under the previous ADF. When it comes to Mechanical Extract Ventilation with Heat Recovery, the increased airflows are likely to be even higher for reasons that are not able to be covered within the limits of this article.

Residential Ventilation Specialists face a dilemma...

The Whole Dwelling Ventilation Rates in the new ADF are presenting some Residential Ventilation Specialists, especially those working in existing dwellings, with a dilemma. Do they stick to the guidance in the previous ADF knowing from experience that if it is achieved and enforced, has been proven to do what is required in practice and contribute well to “Adequate Ventilation” in a dwelling? Or do they advise their clients to go for what they know will often be oversized ventilation systems that will cost more to buy, maintain and run so that they can comply with the new ADF? It is a dilemma some of us are still struggling with. It is already well documented that many occupants do not use ventilation systems as they should do, or allow them to operate as intended, because of perceptions of high running costs. There is a strong possibility that the new ADF will lead to an increase in this. Conversely, ventilation systems allowed to work at much higher flow rates may end up off-setting most, if not all, of the energy savings building fabric improvements hoped to bring.

Unintended negative consequences?

The new ADF may not have the same level of unintended negative consequences as happened during the Great Plague of London referred to at the start of this blog. But, a potential condensation, mould and poor indoor air quality plague affecting those living in homes throughout the UK and increased carbon emissions and energy bills, are certainly not what those behind the new ADF were intending... 


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