Jonathon Hunter Hill

Product Manager – AirMaster SMVs

In most Romance languages, the word for insulation translates directly as isolation. On the road to Net Zero, one of the UK’s primary challenges is to cut heat loss from buildings by isolating the inside from the outside. Increases in air tightness, and reductions in U-values and thermal bridging, will continue to reduce heat loss from buildings. But the increased air tightness creates a particular problem: we are aiming to eliminate natural air exchange between indoors and outdoors to reduce heat loss, cutting the primary method of ventilation that the UK has long relied upon.

Ventilation is required to maintain good indoor air quality in buildings, whether it be reducing the humidity to prevent damp and mould, or to minimise CO₂ levels to prevent inhibition of brain function. This creates a different problem: by extracting air from buildings, we also extract heat, which must then be made up from other sources. This is a vicious circle in that we have reduced heat loss through natural air exchange, but may incur heat loss through mechanical ventilation. In buildings with relatively low occupancy densities, such as domestic environments, a low rate of air change per hour (ACH^-1) is required, for example 2-4 ACH^-1 for living rooms. But in buildings with relatively high occupancy densities, such as offices and schools, the ventilation rate required to maintain good indoor air quality is 4-6 ACH^-1, so a great deal of heat can be lost.

The UK has long been in the habit of using natural ventilation for buildings, but Net Zero put paid to that. The solution is to recover the heat from the air using mechanical ventilation with heat recovery (MVHR). In Europe, this is the de facto ventilation solution for new buildings. Indeed, in European deep refurbishments and new builds this is typically a legal requirement. MVHR extracts stale air from rooms, passing it through a heat exchanger. At the same, fresh air is drawn in from outside and is passed through the heat exchanger, the two pathways being separated by a hydraulic break. Heat flows from hot to cold, so the stale air deposits its heat into the heat exchanger, which is picked up by the colder fresh air, warming it before it enters the room. This can reduce heat demand by up to 90%.

When it comes to the UK’s new build schools, in the School Output Specification (Technical Annex 2H: Energy) the Department for Education has set minimum energy intensity targets of 52 / 67 kWh/m² (primary and special educational needs / secondary schools respectively). The Output Specification indicates that heating should comprise 8 kWh/m² of this target. Heat load (heat loss), therefore, must be absolutely minimal in order to meet this criterion. Using natural, hybrid, or mixed mode ventilation solutions, this target simply will not be met. It can only be achieved using MVHR, and MVHR with a low specific fan power (SFP) at that.

Factoring in both electrical consumption, heat demand associated with ventilation, fabric losses, and internal gains, a classroom with decentralized MVHR will have a heat load of approximately 600 kWh/year. Comparatively, a classroom with the best hybrid solution would have a heat load of approximately 3,500 kWh/year. This is a factor of six different, entirely due to heat recovery, which in this case will recover approximately 84% of the classroom’s heat. (https://bit.ly/natvsmvhr)

In many cases, schools are being designed to use air source heat pumps combined with solar PV panels for generation. If we assume that the heat pump has an SCOP of 3.2 and that PV covers and average of 50% of the building’s electricity demand, the performance gap narrows. However, the outstanding heat load is still approximately a factor of five times higher when using hybrid ventilation as opposed to decentralized MVHR. The result of a reduced heat load is a reduced requirement for both heat plant and renewable energy generation, resulting in net lower lifecycle carbon emissions and may result in lower capital costs.

In specifying ventilation units of any type, I strongly encourage designers to consider not only the electrical energy, but also the heat loss associated with the type of ventilation considered; to take a holistic approach to ventilation. MVHR inevitably has a higher electrical demand, but will slash the building’s heat demand.

When we consider the building fabric to meet our Net Zero goals, it is essential that we consider minimising heat loss through ventilation as a core element of said fabric. This will only be achieved with good quality MVHR if we are to satisfy the requirements for energy intensity and the indoor air quality. With the rise in energy prices, we must reconsider CAPEX vs OPEX. We can learn a great deal from the more mature energy markets of Europe.