When MVHR is designed early it always costs less. The plantroom can be the right size and in the right location to make the system easier to maintain, you can shorten duct runs and reduce material cost, 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 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 later when the internal layout, floor construction, plantroom size and location are all finalised, yours becomes an MVHR retrofit project, even if you haven’t put a spade in the ground yet. Retrofitting is always more difficult; things get squashed in places they shouldn’t; installers have to deviate from the design just to get it in, adding more materials, delays, acoustic and performance risks; and in larger properties you can end up with two or more systems instead of one.
If you’re paying for it, get it designed early and get it designed properly.
To meet even the minimum ventilation rate you need to move a lot air around the property through ducting. Every time you add a bend to the ducting the air inside it gets compressed a bit and that creates air turbulence, especially if it’s a 90 degree bend or a tee connector. Turbulance adds air resistance so more energy is needed to push or pull the air around. More energy means more running cost and more work for the fan motors, shortening their life. Thrashing motors and air turbulence creates more noise, which means more attenuation is needed. Silencers add more resistance and you’re in a vicious circle.
The best bet is to design it early so the ducting fits where it really needs to be instead of where someone happens to be able to put it. Aim for sweeping bends that promote laminar air flow, fewer bends, less turbulance, minimal air resistance, and ideally no perceptable noise. That’s what we do.
Don’t put it off. We need to support your architect from the outset.