Monthly Archives: September 2025

Above-Ceiling MVHR Ductwork

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The supplier of the MVHR equipment (BPC Ventilation) prepared a design showing the rough locations of the inlet and extract vents, specifying that each vent should have two 75mm semi-rigid ducts attached (in order to achieve the required airflow – especially given the relatively long duct runs). The detail of the duct routing is left to the discretion of the installer. I settled on locating the vents along the centreline of the workshop then running the ducts on the East side – mostly because the access hatches are just to the West side of the central bracing planks, which form a natural walkway – at least for now, before the rest of the loft insulation gets installed. While the 75mm duct (FRÄNKISCHE profi-air classic grey NW75) specifies a minimum bend radius of only 150mm there is no harm in having much larger radius bends where space allows.

Some of the MVHR ductwork above the ceiling in the Workshop. The black plastic moulding is a plenum chamber that feeds the air vent (that will be) fitted below the ceiling.

The loft space will be ‘cold’ (deliberately ventilated to the outside) and I initially looked at pre-insulated ductwork but was advised that’s not as good as it might be and the best approach is to install uninsulated ductwork ‘in the middle’ of the loft insulation. The insulation visible in the above photo is a 100mm-thick layer between the ceiling joists and that – together with the grey ducts – will get covered by the additional 170mm of insulation installed on top.

At their other ends, the ducts connect to another plenum chamber adjacent to the MVHR unit.

The ‘inlet’ plenum chamber feeding the three inlet vents (which each have two 75mm semi-rigid duct connections). The inlet from the MVHR unit is underneath.

Validation of Passivhaus-Level Heating Performance

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The main criterion for Passivhaus-level performance is achieving a space heating requirement better than 15 kWh per m2 of floor area per year (see e.g. this page from the Passivhaus Institut) when heating to an internal temperature of 20C.

One benefit of having a heating system listed on the Heatpump Monitor website is the Heatpump + Fabric dashboard, which orders systems by kWh/m2 and thus provides a direct view of whether buildings are performing at this level.

Screenshot of the “Heatpump + Fabric” dashboard on Heatpumpmonitor.org

A few points to note:

  • By default, this list is sorted by Electricity Input (Elec kWh/m2) rather than Heat Output (Heat kWh/m2) but that’s easy to change.
  • It’s not obvious but the Elec and Heat figures relate to the Combined performance of the heat pump, when delivering Space Heating and also when heating Domestic Hot Water.
  • There is no indication of the internal temperature achieved, so some buildings might be hotter (or cooler) than the nominal 20C used for the Passivhaus assessment.

The second point tends to increase the consumption figure – potentially quite significantly, because high-performance buildings can use just as much DHW as low-performance buildings, and the higher temperatures required for stored hot water (compared with space heating) results in lower efficiency from a heat pump.

Some systems listed on heatpumpmonitor.org provide additional data on whether they are delivering Space Heating or Hot Water at a particular point in time. If this is accurate, it allows the Hot Water heating periods to be filtered out from the results, providing a more representative view of the heat required for Space Heating. However, it’s clear that many systems don’t accurately report Space Heating mode – so the overall list is unrepresentative. It can be useful to filter out Hot Water production for a single system where the data is known to be accurate.

In my case, looking only at Space Heating drops the kWh / m2 from 13.8 to 13.1, demonstrating that this is even more compliant with the 15.0 kWh / m2 Passivhaus limit (even though the target indoor temperature is 21C, rather than 20C).