Throwing the window open at 34 °C is not ventilating, it is heating. This guide explains why, what a decentralised heat recovery ventilation system actually achieves in summer – with figures – and where its limits are.
The short version: during the day the ceramic heat exchanger acts as a thermal barrier. At 34 °C outside and 26 °C indoors, air enters at around 27.8 °C instead of 34 °C — calculated with the TÜV-certified minimum efficiency of 78%. At night the system switches to free cooling, turns heat recovery off and draws in the cool outdoor air unimpeded. It does not replace air conditioning, but it stops the home heating up through ventilation in the first place.
Opening the windows on a hot afternoon does not exchange air, it exchanges heat. The incoming air warms walls, screed and furniture. That thermal mass releases the heat again for hours — long after it has cooled down outside. A single badly timed airing can ruin a whole evening.
Simply keeping the windows shut does not solve the problem either. A household produces several litres of moisture every day through cooking, showering and simply breathing. If it stays in the room, humidity, CO₂ levels and the risk of mould all rise. Doing both at once — keeping the heat out and still ventilating — is not possible with windows.
Summer heat protection is essentially a question of timing:
With windows this is barely practical. Airing at night means open windows, street noise, insects — and on the ground floor or in a holiday home a security problem too. This is exactly where decentralised ventilation comes in: it does both without a window being opened.
A decentralised ventilation unit with heat recovery works in alternating cycles. For around 70 seconds indoor air flows outwards and charges the ceramic core of the heat exchanger. Then the fan reverses and outdoor air flows back in through the same core.
In winter the effect is familiar: the outgoing warm air heats the ceramic, and the incoming cold air is pre-warmed by it. In summer exactly the same happens in reverse. The cooler indoor air cools the ceramic core down, and the hot outdoor air is cooled by that core as it enters. In summer, heat recovery is simply coolness recovery — at the same efficiency.
What that means can be calculated. The table applies to a room temperature of 26 °C and is deliberately calculated with the minimum efficiency of 78% verified by TÜV SÜD, not with the up to 93 % the ceramic exchanger reaches at best.
| Outdoor temperature | Window open | With Ambientika (78%) | Difference |
|---|---|---|---|
| 30 °C | 30.0 °C | 26.9 °C | −3.1 K |
| 32 °C | 32.0 °C | 27.3 °C | −4.7 K |
| 34 °C | 34.0 °C | 27.8 °C | −6.2 K |
| 36 °C | 36.0 °C | 28.2 °C | −7.8 K |
| 38 °C | 38.0 °C | 28.6 °C | −9.4 K |
Even at 38 °C outside, the incoming air stays below 29 °C. The hotter it gets, the bigger the difference. The home does not get cooler as a result — but it barely heats up through ventilation, and in high summer that is the decisive point.
As soon as the outdoor air is cooler than the indoor air, the logic reverses. Now heat recovery would be a hindrance, because it would warm the cool night air again on its way into the room. So it is switched off — that is free cooling.
How it is triggered depends on the unit:
For scale: four units together move around 240 m³ of air per hour. If the night air is 6 °C below room temperature, that corresponds to roughly 500 watts of cooling capacity — for around 28 watts of power consumption (4 × 6.9 W). Over one night that is about 4 kWh of cooling for a good 0.2 kWh of electricity. No refrigerant, no outdoor unit, no open window.
Honest advice states both sides:
What decentralised ventilation delivers in summer:
What it does not do:
In summer the largest heat gain comes through the glazing. External shading — roller shutters, external blinds, awnings — stops most of the solar energy before it even reaches the glass. Internal curtains help far less, because by then the heat is already in the room.
Shading and ventilation solve different parts of the same problem: shading blocks the radiant heat, ventilation ensures fresh air behind closed windows and makes the night-time coolness usable. Only together do they add up to a summer-proof concept.
| Model | Free cooling | Automatic summer mode | Typical application |
|---|---|---|---|
| solo+ | – | no | Single room, basement, rented properties |
| advanced+ | by remote control | no | Living spaces, wired, up to 16 units |
| wireless+ | by remote control | no | Retrofit in existing buildings, no cabling |
| advanced B+ | by remote control | no | Bathrooms and wet rooms |
| smart | yes | yes | Living spaces, app and smart home control |
| Office | yes | yes | Offices, practices, schools, 100 m³/h |
If summer is the main argument, smart or Office is the better choice: these units detect summer mode themselves and do not depend on someone pressing a button on the right evening.
Ambientika is developed by the South Tyrolean family business Südwind, which has been working in ventilation technology for over 60 years. The units work without ducting, achieve heat recovery of up to 93 per cent and are whisper-quiet from 20 dB. They are TÜV SÜD tested, sound-tested by IFT Rosenheim and hold the German national technical approval of the Deutsches Institut für Bautechnik (Z-51.3-482). The ceramic heat exchanger achieves fire classification A1 — non-combustible.
No. A ventilation system does not lower the room temperature below the outdoor temperature. It does prevent the home from heating up further through ventilation, and it uses the cool night air without an open window. In well-insulated and shaded buildings in central Europe that is often enough.
At 26 °C room temperature and a certified efficiency of 78%, the supply air is around 28 °C — roughly 7 K below the outdoor air. At the peak efficiency of 93 % it would be around 26.6 °C.
Free cooling means deliberately switching off heat recovery so that cool outdoor air enters the room unimpeded. It makes sense whenever it is cooler outside than in — above all in the evening and at night.
With Ambientika smart and Office, no — the units detect summer mode themselves once it is above 24 °C indoors, above 20 °C outdoors and cooler outside than in. With advanced+, wireless+ and advanced B+ one press on the remote control is enough when it cools down in the evening.
Only indirectly. The ceramic core transfers heat, barely any moisture — there is no active dehumidification. If the outdoor air is drier than the indoor air, the air exchange lowers the humidity in the room; in muggy weather it does not.
One unit draws around 6.9 watts in operation. Four units running through an entire summer night consume around 0.2 kWh together — around 1.5 kWh over a week.
Yes – especially there. The units operate from 20 dB, and once it is dark the twilight sensor reduces them to whisper-quiet night mode. A tilted window is louder by comparison and less effective, because without a fan hardly any air is moved.
Yes, and the effect is greatest there because loft spaces heat up the most. What matters is the combination: external shading against radiant heat, ventilation for the air exchange and for night-time cooling.
How many units a building needs and where they are best placed depends on the floor plan, the room sizes and the building envelope. We calculate the air volumes for your rooms to DIN 1946-6 and produce a free ventilation plan from it — without obligation, as a sound basis for your decision.