Back in 2022 Bob gets planning to knock down and replace his home with a stunning 350sqm, 5-bed property over four levels. He’s built houses before, but this one’s to be his forever home. It’s a typical job for us and an ideal candidate for the impressive Airflow Adroit DV245 with 75mm Airflex radial ducting. Bob’s timing’s perfect too, after planning and before technical design. A guide price is issued for a turn-key project – premium spec, acoustic upgrade, fire protection, parts and labour guarantee. Estimated cost £22k all-in. Zero-rated VAT.
With more experience than most self-builders Bob thought he knew where to economise, but he still wanted quality and the best available indoor air filtration. His last project didn’t have MVHR and nobody worried about overheating back then because there were no Part-O regulations. So, to save money the first thing to go was the architect. Bob and his contractor could handle the building design, and as far as MVHR was concerned, space had been allocated in the basement plant room so ventilation could wait until later.
A timber frame supplier was hired to design the shell, a structural engineer provided technical drawings for the foundations and steel work, an independent SAP assessor produced the necessary energy calcs, the Building Control Application was passed, and the project was out of the ground. It was all looking good, everything was going smoothly and Bob was on budget with no architect fees on top.
As the shell comes together the Mechanical, Electrical, & Plumbing (MEP) first fix moves up the agenda. Now Bob thinks it’s time to finalise the MVHR plan, so two years after our initial estimate our design fee is paid and we get to see the ‘construction drawings’. Or to be precise, the planning drawings in PDF, the structural engineer’s steel frame design, the timber frame supplier’s joist plan, and the design SAP calcs. Not a single coherent set of construction drawings, but enough to stitch together to create a virtual 3D replica of the building in BIM/Revit, sufficient for a proper MVHR design.
Now we can see what they’re building and how they’re building it, the first snag is the proposed plant room. Besides being the only room in the house that gets heated during the Summer thanks to the hot water cylinder, the underfloor heating manifold, PV inverter, consumer unit and other Heat Pump plant, meaning it’s not the ideal place for air-handling equipment, it’s also unworkable for MVHR unless four 250mm ducts can run up through ceiling and into the centre of the living room. The MVHR intake and exhaust ducts must exit the building through the roof or an exterior wall. And if it’s to be through a wall, those terminals must be away from neighbours, roads, chimneys and flues to avoid acoustic and air quality issues, and they must be higher than potential snow drifts to avoid blockage. Often the position of those terminals can dictate where the rest of the system goes*.
As for the rest of the ventilation plan, nobody had given any thought to air duct distribution whatsoever. There were no suspended ceilings or web joists, no insulated eaves, no pre-cut holes through steels, and no central plant location. The MVHR became a retrofit project to a new build house. A single system would no longer fit because there was now no way to span the central stairway, so the cost immediately increased because Bob needed two systems, left and right, instead of one.
NB, it’s important to plan where you’re going to pull air from and exhaust air to before you start the MVHR design. Those exterior terminals connect to ‘mass flow’ ducting goes that may be up to 300mm diameter in a larger property, so because the mass flow duct connects to the machine, the location of the exterior terminals can dictate where the whole system goes. You can’t just stick it in a cold loft, a garage, under the stairs, or in the basement. Not if you want a quiet, efficient, fire-protected, compliant system.
The design SAP calcs included with the Building Control application are supposed to predict the ‘as-built’ energy performance. You need them to show the proposed building will meet current efficiency and environmental standards. Those calculations include a ‘simplified method’ for assessing overheating risks, but it’s only enough to tick a box to get you started. You may still need full (TM59) Thermal Modelling properly assess the risk and demonstrate Part-O compliance.
Meanwhile for Bob, everything’s on track, most of the twin MVHR system is in and we’re approaching second fix. It’s looking good until Building Control mention the National Planning Policy Framework (updated July 2021) and BS8233:2014 ‘Guidance on sound insulation and noise reduction for buildings’, which are linked to the new Part-O regs and ultimately the ventilation spec. Specifically, Approved Document O section 3 states:
Because his average outdoor noise will exceed 40 dB(A) at night, a level equivalent to ‘light rain’, Bob can’t open windows to cool the building, so that means he has no natural ‘thermal purge’ system. But purge ventilation is a requirement of Part-F, so instead of opening windows he must achieve the same result through the mechanical ventilation, at least for certain rooms. The purge rate is ten times greater than the normal rate, so that means a lot more MVHR, a separate system entirely in Bob’s case, as well as a lot more space to put it.
NB, Acoustic, air quality, overheating, and security issues can prevent you (legally) from relying on openable windows for purge. These matters therefore need to resolved before you complete the design of the house.
This oversight would normally be the responsibility of the Principal Designer, but as he no longer has one Bob automatically inherits the role of ‘Duty holder’ as defined by the Building Safety Act. Meanwhile, everything stops on site while the MVHR is re-designed and then re-approved. To retrofit this third system the first job involves paying the carpenter to move the loft hatch he only just finished. Building Control might now be satisfied, but Bob’s not happy about the revised cost or the delay, and the builders not happy because he now needs to change the programme and find alternative work for the other trades.
NB, The Building Safety Act was introduced after Grenfell. It states that a ‘Duty Holder’ is responsible for Building Regulation compliance. Ideally, it’s the ‘Principal Designer’ with the knowledge and the necessary insurance. The Duty Holder is liable should any future occupant ‘suffer a physical or financial loss’ because of something that doesn’t comply.
Considerable stress, delays, extra cost, and the second system never mind the third, could all have been avoided had the design been completed at the right time, RIBA Stage 4, and by the right designer, the original architect. Despite the best intentions, cutting corners didn’t save any money, far from it.
Unlike most MVHR providers, Solarcrest offer mechanical consultancy to the building designer as the construction drawings are coming together, ahead of your Building Control application. That doesn’t affect what it looks like, just how it works. We’ll review acoustic, air quality, overheating, and purge requirements to give you a complete ventilation solution, including a full Part-F compliance certificate that covers windows and MVHR
If you’re designing the property now, it’s time to plan your MVHR. But if you’re already on site, don’t forget Part-O can affect your choice of windows