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 £24k 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 possible indoor air to breathe. The last project didn’t have MVHR and, unlike now, nobody worried about overheating back then. There was no Part-O last time. 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 take a back burner.
A timber frame supplier was hired to design the shell, a structural engineer provided technical drawings for the foundations and the 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 as planned, and they were still on budget with no architect fees on top.
As the place starts coming together the MEP first fix moves up the agenda. Now he thought it was time to plan the MVHR, so two years after providing budget costs 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 one coherent set of construction drawings, but enough to stitch together to create a virtual 3D replica of the building in BIM/Revit. Something sufficient to produce an ‘installable’ MVHR design, i.e. one you know will fit before expensive pallets start landing on the drive.
Now we can see what they’re building and how they’re building it, the first snag is the proposed plant room. The hottest room in the house that’s full of heat pump equipment, the hot water cylinder, and the underfloor heating manifold is not the best place to put air handling. It’s also unworkable for MVHR unless four 250mm ducts can run up through ceiling and out of the living room wall at reasonably high level. Higher than a snow drift so the terminals don’t get blocked in Winter. Not many basements have exterior walls.
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 plasma cut holes through steels, and no central plant location. The MVHR was now a retrofit project, and the bill went up immediately because Bob now needed two systems, left and right, instead of one. There was no way to span the central stairwell.
NB, quite often it’s the most appropriate location for the exterior terminals that dictates where the large ‘mass flow’ ducting goes. You can’t suck in fresh air from a busy road, through a wall very close to a neighbour, downwind from a chimney or boiler flue, and ideally not from a South facing wall because that can also contribute to overheating. The mass flow ducting can then dictate where the machinery goes along with the smaller 75mm pipes. 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 machine itself is less than 20% of the total, so it’s the 80% you need to consider first.
The ‘design’ SAP calcs included with the BC application are supposed to predict the ‘as-built’ energy performance. They’re required so you can show the proposed building will meet current efficiency and environmental standards. The calculations include a ‘simplified method’ for assessing overheating risks, but it’s only enough to tick a box. If there’s any doubt or potential complication you may need full (TM59) Thermal Modelling to properly assess the risks and demonstrate Part-O compliance.
Meanwhile for Bob, everything’s on track and most of the twin MVHR system is in and we’re approaching second fix. All good, but then Building Control raise a concern. They want evidence that the external noise will not exceed 40 decibels between 11pm and 7am, a similar sound level to ‘light rain’ and nothing horrendous like overhead aircraft. It’s another requirement of Part-O. If the average exceeds 40 dB(A) Bob can’t open windows at night to cool the house down during the summer, so that means he has no ‘thermal purge’ system. But purge ventilation is a requirement of Part-F, it’s not optional.
NB, a regular MVHR commissioning certificate alone does not mean your property satisfies the ventilation regulations. You need the right windows too. As well as noise, air pollution can also stop you opening windows too (legally).
Without openable windows, Bob needs ‘mechanical purge’ to compensate. The purge ventilation rate is ten times greater than the normal rate, i.e. 4 ‘Air changes per Hour’ instead of 0.4, meaning Bob now needs a lot more MVHR and a lot more available space. You can’t just flick a button and turn a regular MVHR up to a higher setting. Even if it could deliver enough air to the right places, it would fail Part-F on acoustics and it would fail Part-L on energy consumption. Mechanical ventilation must be ‘whisper’ quiet and not exceed 30dB(A), or consume more than 1.5Watts per litre per second.
For mechanical purge you need a much bigger system. In Bob’s case, a separate system entirely. And that needs retrofitting on top of the first system. So, everything stops on site while the MVHR is re-designed and then re-approved. To get the new purge system in, the first job is paying the carpenter to remove the loft hatch he just finished. Building Control might be satisfied, but Bob’s not happy and neither is the carpenter or the main contractor, who now must find alternative work for the other trades while the oversight is rectified.
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 but it did lose accountability. If you want MVHR and the sealed, low energy property it allows you to create, just remember:
Besides Part-O, the other thing that didn’t exist last time is the Building Safety Act, introduced after Grenfell. It states that a ‘Duty Holder’ is responsible for Building Regulation compliance. Ideally, it’s the ‘Principal Designer’ or the ‘Principal Contractor’, but in this case it’s Bob, because he’s now officially both. Under the BSA, Bob is now liable should any future occupant ‘suffer a physical or financial loss‘ because of something that doesn’t comply. It’s a big responsibility with potentially very heavy penalties, and it lasts for up to 15 years.
Solarcrest provide mechanical consultancy to the building designer as the construction drawings are coming together, ahead of your Building Control application. Not 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. Properly planning MVHR early avoids constraints and compromises that occur when MVHR is shoehorned in later. It’s much better to make the building MVHR compatible from the outset.
If you’re at Stage 4 now, it’s time to plan your MVHR. But if you’re already on site, just remember Part-O can affect your choice of windows so feel free to call..
Click here for more MVHR regulations