MVHRDesigner

Hello. I am an MVHR Designer and I thought I would weigh in on some of the posts made, as I think they are a bit misleading. This is incorrect. Where i-joists are used, rigid ducting is the norm. The major advantage being the ease of cutting holes in i-joists, and the amount of holes that can be made (following guidelines on the data sheets). People often think of semi-rigid (flexi) ducting as space saving, but clearly with this type of joist you would require far more holes because there are far more ducts. A good designer will ensure the guidelines for holes are not breached. It is straightforward to cut duct with a hand-held grinder, even with a 1mm cutting blade. See my post above, rigid ducting is the norm where i-joists are used for the floor structure. 28m3/hr down a 75mm duct is fairly standard for a lot of companies, however, an optimal design will use two ducts for this flow rate (or higher). Bearing in mind that lower air velocity = lower pressure = lower sound. Most people would rather spend a bit more, have more ducting etc to have an inaudible system, over saving a few quid but having a system they can constantly hear. This is generally true. Rigid steel ducting tends to be lower pressure/sound. Semi-rigid does get unnecessarily harshly judged in my opinion. I have designed perfectly good systems for Passivhauses. In my experience it is due to many early systems being in semi-rigid, and were poorly designed and/or poorly installed. Even though it is straightforward to make connections airtight, I see more often semi-rigid systems with leaks due to improperly connected ducts to manifolds, plenums, etc. With rigid steel duct you just push it together once and it is airtight. And lastly the high volume of air many companies design for each duct. When I design I aim to put less than 26m3/hr down a 75mm (ID) duct, and less than 18m3/hr down a 63mm (ID) duct. To the OP, it looks like a nice system.