When MVHR is designed early it always costs less. The plant room can be the right size and in the right location to make the system easy to maintain, you can shorten duct runs and reduce material cost, minimise noise, improve efficiency, and reduce energy consumption. MVHR-friendly joists can be specified at the right height and in the right direction so most of the system can be hidden inside the floors instead of below them. Space can be allocated for fire-safe risers between floors for ducting, meaning you could have one big (more efficient) system instead of several, and sizable extras like NOx filters or heating/cooling coils can be put somewhere convenient without being an afterthought.
If MVHR is designed after the building structure and layout are set in stone, yours becomes a retrofit project even if no spade has hit the ground. And retrofitting MVHR is always more difficult, normally more expensive, and often involves compromises, particularly in terms of accountability. Try getting a builder to warrant the design and guarantee the performance.
To meet the minimum ventilation rate you need to supply a lot more air into the property than you think, and you have to extract the same to keep the system balanced. Two million cubic metres per year in and out for a 200m2 property, and that’s for the life of the building. This is why quality and reliability matter so much, along with acoustic and energy performance.
Forcing air around a tight bend or through a tee connector compresses the air and creates turbulence, so the more bends/tees you add the greater the air resistance on the whole system. More resistance means you need more power to push or pull the air around, and more power means more cost, more noise, and more work for the fan motors, shortening their life. Silencers might cut the noise a bit, but they also add air resistance and you’re stuck in a vicious circle.
Fewer bends is the answer, along with sweeping bends that improve laminar flow. When you minimise air resistance you make it easy for fan motors to push and pull air around the building. But that’s a lot more difficult to achieve if you’re forced to retrofit MVHR into a building that isn’t designed with air-handling in mind. The same logic applies to your airtightness, overheating, and renewable energy strategies too.
