In a test household the “supply after HX” sensor reported 27 °C while “extract before HX” showed only 23 °C – the naming is right and the sensors sit where they should. A pure heat exchanger can never make the supply air warmer than the extract air that heats it.
Guide · ablesio Labs
Ventilation with heat recovery: four temperatures reveal how well it works
In short: A ventilation unit with a heat exchanger runs around the clock, usually unnoticed. With four temperature sensors before and after the heat exchanger and a smart plug, ablesio can work out how much heat really comes back and what the fans cost. ablesio Labs has three new levers for it – this article explains the maths behind them so you can trust (or challenge) the numbers.
4 sensors
outdoor, supply, extract, exhaust air
≥ 8 K
minimum temperature difference, otherwise ablesio does not calculate
80–90 %
efficiency good units reach
1What the four sensors measure
In the heat exchanger the warm used room air warms the cold fresh outdoor air – the two air streams never mix. Measuring happens at four points. In ablesio you add them as climate sensors and name them as they sit in the unit: “Zuluft (vor WT)”, “Zuluft (nach WT)”, “Abluft (vor WT)”, “Abluft (nach WT)” (German names: supply before/after the heat exchanger, extract before/after). That is how ablesio knows which sensor belongs where.
2How ablesio calculates
Efficiency (supply side) = (supply after HX − outdoor air) ÷ (extract before HX − outdoor air)
Efficiency (extract side) = (extract before HX − exhaust air) ÷ (extract before HX − outdoor air)
Example: 2 °C outside, 21 °C inside (extract air), the supply air leaves the exchanger at 17.5 °C. Then the unit recovers (17.5 − 2) ÷ (21 − 2) = 82 % of the heat. Both sides should give about the same value – if they differ a lot, the air flows are out of balance.
Why “at least 8 kelvin”?
If it is almost as warm outside as inside, the denominator is tiny and half a degree of measuring error shifts the result by ten percentage points. That is why ablesio only counts hours with a clear difference and takes the median – single outliers hardly matter. In summer there is often no value; that is intended.
3What is a good value?
| Efficiency | Assessment |
|---|---|
| above 80 % | very good – typical for modern counter-flow exchangers |
| 75–80 % | fine |
| below 75 % | worth a look: filters, bypass, air flows |
For the order of magnitude: a unit with about 150 m³/h, a mean temperature difference of 10 K and 5,000 heating hours a year loses, per 10 percentage points less efficiency, about 0.34 Wh/(m³·K) × 150 m³/h × 10 K × 5,000 h × 0.1 ≈ 255 kWh of heat a year – roughly €25 to €40 depending on the heating. No fortune, but a defective bypass or a clogged filter costs money for years unnoticed.
4When the numbers do not fit a pure exchanger
Then I would rather calculate nothing than claim something wrong. I report: “supply air is warmer than the extract air before it” – and tell you what may be behind it.
The most common harmless explanation: the unit heats behind the exchanger – with a reheater or a built-in supply-air heat pump. Then everything is fine, but the exchanger’s efficiency can no longer be read from “supply after HX”; that would need a sensor between the exchanger and the reheater. Other causes: a sensor on the warm casing, in the sun or in the boiler room, or swapped sensors. If your unit is built that way, hide the hint with “Not for me”.
5The fans’ electricity
A ventilation unit for a family home often draws 60 to 70 watts on stage 2 – around the clock. Example from one household: 66 W × 8,760 h = 578 kWh a year, at 31 ct/kWh about €180. That is more than many a fridge.
Fans have a helpful property: power grows roughly with the cube of the air flow. 20 % less air means in theory almost half the power (0.8³ ≈ 0.51). In practice it is less; ablesio Labs calculates cautiously with 25 to 45 % and measures after 14 days what really happened.
🛑 Never switch the ventilation off
A ventilation unit protects against humidity and mould. ablesio therefore never suggests switching it off at night or during the day. The “one stage lower” lever only appears if room humidity over the last week was between 30 and 55 % – and you decide whether the air is good enough for it.
6What ablesio Labs makes of it
🌬️Heat recovery only at …
If the efficiency is below 75 %, Labs suggests: check filters, check bypass and condensate drain, have the air flows balanced. Saving: estimate from the calculation above.
🔧Supply air warmer than the extract air before it
That does not fit a pure exchanger (e.g. reheater or heat pump behind it): a hint with three checks – before a wrong number causes confusion. Can be hidden with “Not for me”.
𖣘One ventilation stage lower?
If the unit runs at high power and the room air is dry enough, Labs works out what one stage less brings – re-measurable after 14 days via the smart plug.
7Try it yourself
- Put a smart plug in front of the ventilation unit and use “Lüftung” or “KWL” in its name.
- Fit four temperature sensors (e.g. Shelly sensors or sensors from ioBroker/Home Assistant) before and after the exchanger, adopt them in ablesio under Devices › Climate and name them as above. Role: “Ventilation”.
- Open Profile › ablesio Labs. After a few days of data the levers appear on their own.
8Honesty matters
Rules of thumb, not expert reports
The calculation assumes 150 m³/h, a 10 K mean difference and 5,000 heating hours – your unit may run differently. Temperature sensors are usually not precision instruments; one degree of deviation is normal. Balancing the air flows is a job for a specialist.
Join in 🧪
Profile → ablesio Labs. New levers appear automatically as soon as your data fits.
Yours, Ablo 🤖🌬️
Note: All amounts are rounded estimates per year, not a promise. Medical devices, animal equipment and safety technology are always excluded from saving suggestions.