Clarkey

Thanks @JohnMo for the response. I'm definitely not sure on any of my working assumptions or calculations - this is definitely all new to me! The figures I quoted for system capacities per floor were based on Passihaus supply recommendations that are higher than regs, include occupancy and factor the HRU at only 70% loading - so probably way over the top. Meanwhile, BPC calculated the house airflow rate based solely on UK regs and for the floor area they measured at 129m2, as 38.7 l/s, which I believe is 139.32m3/h. Against this, the Vent Axia Econiq S that they recommended has 350m3/h capacity, so a 40% loading, (possibly because it's the smallest model in that range). Thanks too regarding cascade systems, I did investigate this after reading your comments in other threads. My understanding from some of the papers referenced suggested that this approach works where the path of least resistance can be forced to cascade through targeted rooms and not be short-circuited by easier routes from supply to the nearest extract, Unfortunately, both floors of our 'classic' 3-bedroom semi have all supply & extract rooms directly off the hall, stairs & landing area, so every potential cascade route would be shorted out. Our ducting challenges lie less with the horizontal aspect of getting to the room vents duct so much as the need for vertical 'risers' on the first floor. I did look at using floor vents for bedrooms to avoid the risers, but I'd worry that they'd get covered, draughty and/or with, no chance of Coanda effect, fail to diffuse, and just ventilate the route straight across the floor to the door. Regarding dMEV/MEV, I will look at this if I can't get MVHR to work, but I'd prefer to deliberately supply fresh, filtered & pre-warmed air to living rooms and bedrooms alongside extraction. I'm sure our house is miles from airtight but I take heart in the conclusion of the 2020 Passivhaus Case for MVHR paper that concludes with the following...

Hi folks, We're looking to retrofit MVHR into our 130m2 (326m3) 30s semi, to improve air quality & introduce controlled ventilation, as other improvements gradually reduce air leakage. I've built out a spreadsheet and various ducting models, and whilst a Vent Axia Econiq S, with radial semi-rigid ducts looks good on paper, wherever I site the HRU, the vertical ducting needs for a single system are creating blocking issues. Reading about from others' successes in deploying multiple HRUs, it seems this need for vertical ducts in our two-story retrofit could be avoided by physically zoning the MVHR solution by floor, i.e.: Multi-room MVHR system per floor (85m2 & 50m2), each with a dedicated HRU that has control inputs for... Home Assistant to coordinate 'whole house' MVHR actions and/or balance between floor HRU systems, as needed. I calculate needing MVHR units of 150m3/h (ground floor) & 120 m3/h (upper floor) for this application. A pair of Vent Axia Econiq S HRU seems overkill (and physically too deep to get into one of the lofts, which has a 650mm x 360mm access restriction). So, I've started researching apartment/flat installations and units such as the Zehnder 155 variants. I'd welcome any recommendations/insights.

Ha, too true @Iceverge - then again, why dump or hoard stuff in the attic when it's far more amusing to keep those weird and wonderful white elephant purchases visibly in the way! Incidentally, the access isn't for storage - I would have first surfaced in the MVHR forum, but there's no way I can get aVent Axia Econiq S unit through a 🤬 360mm inspection hatch! Thanks @Redbeard for the response - hopefully some additional timber can do the trick, but if it needs to be steel, there's an excuse to buy a welder!

Hi folks, First post after lurking for a number of weeks reading up on various potential improvement projects for our 30s semi. I would like to install loft hatches to improve access to a couple of 4.5m x 4m lofts above rooms in a single-story side extension. Unlike the house's main roof, which has purlins, struts and other bracing that made it easy to identify suitable 'safe' joists to cut, for a hatch, these smaller (4.5m wide) hipped roofs have no bracing. Instead, they use simple, fairly chunky triangular trusses, formed from pairs of 7x2 rafters tied with 7x2 joists, and set at 400mm (max) centres. The attached image shows one side of the kitchen roof. Working from right to left, from the gable end abutment with the house outside wall, six of these triangular trusses support the ridge, and the highlighted rafter, third from the right indicates the truss central to this set that would be weakened by its joist being cut to frame a hatch. My understanding is that the usual loft hatch framing for a single cut joist, of double headers in matching 7x2, is to support the ceiling rather than prevent roof spread. I may be worrying unnecessarily about that risk, as these roof frames seem nicely over-engineered to my layman's eye (they're only supporting plain clay tiles). I'm wondering if I may need to (open the face of the roof to add) sister joists to those either side of the opening? Alternatively, do I just have to accept that I'd need work with narrow access panels between joists? I'd welcome any insights.