CPD PROGRAMME | VENTILATION Figure 1: A generic MVHR air handling unit1 Optional: defrost coil Outside (cold) fresh supply air Supply filter(s) Exhaust filter Warm and polluted exhaust indoor air Optional (AHU) Heat recovery unit Supply fan Exhaust fan Heat exchanger coil(s) Preheated fresh supply air Cooled-down exhaust air Control CPU and sensors User/timer/ bus interface External room sensors (occupancy/CO2) Condensate drain To/from chiller/boiler generated by normal wind and stack forces (see Air permeability box). A permeability of 5m3h-1m-2 at 50Pa is highlighted in AD F as a limit value that is typically used as the threshold2 below which MVHR is commonly applied as a dwelling ventilation system. The default AD F position is that a dwelling would have zero air permeability and so assumes no infiltration and reliance solely on a specific installed ventilation system. Industry guidance has various values of recommended maximum permeability to enable successful deployment of MVHR, typically ranging from 1 to 3m3h-1m-2 at 50Pa, although Crawleys recent research2 (see page 30) indicates that the vast majority of the UKs MVHR installations are in dwellings with air permeability of 5m3h-1m-2 at 50Pa. (The Passivhaus standard requires a maximum dwelling air leakage of 0.60 air changes per hour at 50Pa, that for the example house in the Air permeability box (below), would imply an approximate air permeability of 0.67m3 h-1m-2.) AIR PERMEABILITY Figure 2: Example of a commercially available MVHR air handling unit (Source: Zehnder) systems should be able to maintain improved dwelling humidity ranges, as well as providing enhanced annual energy performance compared with sensible heat exchangers. MVHR systems such as that shown in Figure 2 are capable of recovering 96% of sensible energy, and can automatically adjust to changing demands within the dwelling. Typically, an MVHR system will be designed to operate continuously throughout the year, and systems increasingly make use of variable speed fans to improve control flexibility to meet extreme and normal internal loads without unnecessary noise. For MVHR to be successful in recovering energy from the extract air, the dwelling must be appropriately airtight so that there is a balance in the supply and extract flowrates through the heat exchanger. This is established as a design parameter for new dwellings and may be measured in completed buildings and is typically specified in terms of air permeability, evaluated at pressures that exceed those 72 February 2019 www.cibsejournal.com CIBSE Feb19 pp71-74 CPD v2.indd 72 Air permeability is the volume airflow rate per hour per square metre of envelope area at a test reference pressure differential across the building envelope of 50Pa,3 and is typically quoted in terms of m3h-1m-2. Specific local regulations and air permeability standards provide varying guidance relating to the building airtightness and the necessity for purpose-provided ventilation. So, for example, England Approved Document L1A Conservation of fuel and power has a high limit to air permeability of 10m3h-1m-2. This is a high value for a modern dwelling and should be seen as an absolute backstop for an 8m x 8m x 5m house it would equate approximately to nine air changes per hour at 50Pa. (The air permeability measured at 50Pa can be approximated to airflow through all external surfaces under typical ambient pressures by dividing by 20 so, in this case, it would be 9/20 = 0.45 air changes per hour.) The recommendations of England Approved Document F Ventilation (AD F) indicate that new buildings can readily achieve air permeability down to around 2to 4m3h-1m-2 at 50Pa pressure difference and some buildings constructed are tighter than this. Summer bypass method Effect on ventilation performance Impact on indoor air quality Shutdown: Turning it off: manual controls to switch off supply insummer. Creates negative pressurisation because of the supply motor being turned off. If open-flue appliances are present, this negative pressurisation could pose a danger to occupants. The supply air comes directly through inlets, such as trickle vents, which are typically unfiltered. Summer slowdown: Reducing the supply air fan speed during the warmer months. As an MVHR unit needs balanced supply and extract, slowdown of supply air creates negative pressure, leading to uncontrolled and unfiltered replacement air. Negative pressurisation means the air coming into a home isnt filtered, potentially bringing in harmful pollutants, such as NOx. Modulating bypass: The summer bypass can be open, closed, or anywhere in between, with enough heat recovery to achieve comfort. During a cooler summer night, the bypass can remain active, keeping the supply temperature above the dew point, while keeping internal temperatures bearable. The supply air will continuously be filtered and the supply and extract air will remain balanced even if the bypass is open, closed or modulating. Table 1: Methods of summer bypass functionality4 25/01/2019 16:00