Ventilation is a big issue these days. We need good clean air to live. As you may have noted from elsewhere in this project blog, the need to manage humidity and condensation in a timber framed building is of great significance. This, above most other things, has been a guiding principle impacting most design decisions on the refurbishment project. The installation of an efficient mechanical ventilation and heat recovery (MVHR) system was anticipated at day one and carefully integrating such a system into the cottage was always going to require forethought and care.

Such systems are common place now in modern new-builds. Less so in refurbishments where retro-fitting is more complex. We opted to make use of the highly efficient Vent-Axia MVHR system supplied by BPC Ventilation

The system is simple. Fresh air is drawn in from outside and passed to the internal living rooms of the cottage, but only after it is pre-heated by exchanging heat from warm moist air drawn from the bathrooms, WCs and kitchen spaces. The extracted waste air is then exhausted to the atmosphere. The system is said to be over 90% efficient.

Our system was supplied as a ready to install kit. However, given the constraints imposed by the building’s listed structure, we needed to find ways of installing the MVHR unit, plenums and the ducting without compromising the historic fabric. The MVHR unit is sited in the small loft space above the bathrooms. This makes for easy access to the eves for the supply and extract air ducts. The MVHR unit air filters can also be accessed ready for regular cleaning and maintenance.

The supply and extract ducts radiate from a pair of stainless steel manifolds, the number of supply and extract ducts matching as far as possible to enable a “balanced” system. Whist this would be especially important in the case of a fully sealed “passive house”, it is good practice in this case notwithstanding the inevitably “leaky” nature of this historic building refurbishment. The real benefit will be minimising the potential for humidity and condensation build-up without the sacrifice of heat loss, particularly during the winter months.

The challenge of threading up to 15m of ducting to the various extremities of the cottage was overcome by making use of one of the four chimney flues that extend through the full height of the building. Three ducts were dropped down the flue through a small aperture in the loft side of the stack. The flue used to serve the lower ground floor fireplace at the bottom of the building, so it provides a ready and direct route for supply air to the most distant parts of the building.

One day soon we’ll be switching the system on for the first time, so hopefully it works as I intended!

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