Ventilation

Fresh air has to be heated in winter and, sometimes, cooled in summer. The model counts two sources of air exchange: air that leaks through the building envelope (the shell of walls, roof, floor and windows), and air you bring in on purpose. Both are counted in air changes per hour, where 1.0 means the whole volume of air in the house is replaced once an hour.

Infiltration and airtightness

Infiltration is air that leaks through gaps in the building when no window is open, and airtightness says how much. A blower-door test measures it: a fan in a doorway holds the house at 50 Pa (far more than a normal breeze) and records the air changes per hour the leaks then allow (EN ISO 9972). That result is n50, and a lower number means a tighter house. At normal pressures the leakage is much smaller, and the model uses the common rule of thumb that it is one twentieth of n50. An n50 of 3.5 gives 0.175 air changes per hour.

The wizard fills n50 from the construction year, and you can replace it with a measured value. The defaults run from 0.8 for the newest houses to 10 for those built before 1960. They currently come from a Luxembourg-based table. An unheated cellar, attic or garage is given twice the n50 of the rest of the house.

Infiltration is a constant rate for each thermal zone (a floor, treated as one body of air), all year. It does not rise with wind or with the temperature difference between inside and out, and the divide-by-20 rule is only an approximation, so measured leakage in real houses can differ noticeably.

Natural ventilation

Natural ventilation means no fan: air comes in through the gaps in the building (the infiltration above) and when you open a window or a trickle vent, a small slot in a window frame. The model represents the opening part as a background rate of 0.3 air changes per hour. It runs only when the house is warmer than the outdoors, the indoor temperature is between 18 and 32 °C and the outdoor temperature is between 0 and 32 °C. In cold weather, between minus 15 and 0 °C outdoors, 0.1 air changes per hour stands in for short bursts of window airing and door openings. Below minus 15 °C there is none. The 0.3 figure is a modelling default for the daily average of occupant behaviour, not a regulatory requirement.

In everyday terms, 0.3 air changes per hour is roughly half an hour a day with the windows wide open. The arithmetic: 0.3 per hour is 7.2 air changes a day, and German indoor-air guidance puts a cold-weather airing with opposite windows open at three to five minutes. If each of those exchanges most of a room’s air, which is what the advice is for, half an hour a day gets you to the same place. Treat it as an order of magnitude, not a measurement. The model spreads the air evenly through the day rather than in a few bursts, so it misses the cold draught of the real thing.

Leakage comes on top, and it depends on the age of the house. For a 400 m³ house (roughly 150 m² of floor) the airing is 120 m³/h either way, while the building itself leaks about 16 m³/h when it is new and airtight (n50 0.8), 70 m³/h for a house from the 2000s (n50 3.5), 120 m³/h for one from the 1970s (n50 6) and 200 m³/h for one built before 1960 (n50 10). In the oldest case the house ventilates itself more than the airing does. In the newest it hardly ventilates at all on its own. On a mild day a house with an n50 of 3.5 sees about 0.475 air changes per hour, the 0.175 from infiltration plus the 0.3 background. That is arithmetic from the defaults, not a result for your house.

Mechanical ventilation with heat recovery

Mechanical ventilation with heat recovery is a pair of fans and a heat exchanger. One fan brings fresh air in, the other pushes stale air out, and the exchanger passes the outgoing air’s heat to the incoming air without mixing the two. The model adds a balanced unit to each heated zone: supply and extract flows are equal, so the house is not pressurised. The total flow is split equally between the heated zones and runs at a constant rate all year.

Sensible heat recovery effectiveness is the share of the outgoing air’s heat that reaches the incoming air (“sensible” means the warmth a thermometer reads, not moisture). The model uses 0.87 by default, and no moisture recovery. Warming 200 m³ of fresh air an hour costs about 67 W for every kelvin (degree) the outdoor air is colder than the indoor air. With 87 % recovery that falls to about 9 W. These are illustrative figures from the formula, not outputs.

The wizard proposes a heat-recovery unit for houses built from 2015 and natural ventilation for older ones, and you can choose differently. The default flow is half an air change of the heated volume, rounded to the nearest 50 m³/h and kept between 150 and 400 m³/h. The fans use electricity at every hour of the year: the flow times a specific fan power (electricity per m³ of air moved), 0.25 Wh/m³ by default. At 200 m³/h that is 50 W, about 440 kWh a year, and it is counted in the results.

When the unit is on, the 0.3 natural rate is replaced by a residual of 0.1 air changes per hour. It stands for the doors, brief window openings and stack effect (warm air rising through the house, drawing air in low and out high) that remain. Infiltration still applies.

In summer the unit has a bypass, a flap that sends air around the heat exchanger so cool night air can enter without being warmed by the outgoing air. For each heated floor the model opens it when three things hold at once: the room is warm, the outdoor air is above 12 °C, and the outdoor air is cooler than the room. “Warm” switches on above your daytime heating temperature plus 2 K and switches off again below that temperature plus 1 K. With daytime heating at 21 °C, the bypass opens above 23 °C and closes below 22 °C. While it is open the fans keep their flow and the exchanger stops exchanging.

Limits of the model

Extract-only ventilation (fans pull stale air out of kitchen and bathrooms, fresh air enters through vents) is listed in the wizard but not modelled as a separate system. It behaves like natural ventilation, with no heat recovery and no fan electricity.
Infiltration and natural ventilation are fixed rates. Wind-driven and stack-driven flows, and the opening of windows in a heatwave, are not represented.
The unit has no demand control (flow that follows CO2 or humidity), boost mode (more flow while cooking), frost protection (against the exchanger icing up) or moisture recovery, and its flow does not vary with occupancy.
Air quality is not simulated. The model does not check CO2, humidity or whether the flow meets a ventilation standard.
Duct heat losses and the effect of filters on fan power are not modelled.