ᐅ Sudden increase in HVAC system performance – all filters are clear

Hello everyone....

I am currently experiencing an issue with the built-in ventilation system in my Stiebel Eltro LWZ 404 SOL.
For the past two days, the system’s performance / airflow volume has increased noticeably, which has also resulted in a significantly louder noise level.

The power settings (level 2 during the day, level 1 during setback) and the associated configurations have not been changed. My first thought was that clogged filters might be causing the increase in performance, but I recently replaced all the filters and they still appeared clean upon inspection. This includes the filter behind the heat exchanger cassette.

When I went to check the filters, I noticed that even with the ventilation temporarily turned off, a lot of air was still being drawn in from outside and blown out. I had to turn the system off completely in order to inspect the filters without the fan running.

My assumption (as a complete layperson) was that the fan running despite the ventilation being off is probably used to supply the heating / hot water system with heat from the surrounding air. Since it is currently quite cold outside (around 0°C (32°F)) but the electric backup heater only activates when it reaches the bivalence point at -5°C (23°F), I can imagine that the ventilation unit has to work harder (i.e., exchange more air) than it would at above-freezing temperatures. In principle, this would be understandable. (Could someone please confirm whether this assumption is correct?)

What I don’t understand, however, is the increased performance of the central ventilation system. Normally, an increased airflow for heat recovery related to heating / hot water should not necessarily affect the ventilation system’s airflow / performance, or am I mistaken?

If these are unrelated, do you perhaps have any ideas what else could be causing the increased ventilation system performance? As I said, everything was running normally up until 2-3 days ago. Currently, we turn the ventilation completely off at night, even though the ventilation otherwise was not causing any issues.

I would really appreciate your help. Any hints or suggestions are very welcome.

Good luck, Florian

There could be two explanations for this—I’ve recently noticed a similar phenomenon with my Helios system. Since my outdoor areas are not yet finished and there is construction happening nearby, I installed an additional filter on both the fresh air intake and exhaust pipes outside.

However, when the humidity is very high and temperatures drop below 0°C (32°F) at night, water condenses and freezes on the fresh air filter, blocking the air intake. The Helios then responds by increasing the speed of the fresh air fan to maintain pressure, which also makes it louder.

The second case could be a protective function for the heat exchanger. My Helios behaves differently here: when the exhaust air temperature drops below 3°C (37°F) (factory settings), it stops the fresh air fan to allow the exhaust air stream to warm the heat exchanger back up. When the exhaust air temperature rises again to 7°C (45°F), it restarts the fan.

Perhaps the ventilation and heat recovery unit reacts here by increasing the speed of the exhaust air fan—check the manual to see if it mentions anything about this.

I am an amateur myself, but these are my experiences with my Helios over the past weeks. I also have no idea how the ventilation and heat recovery unit is exactly constructed, so this is just guessing in the dark.

Fog is exactly the right keyword. Your answer seems to hit the nail on the head. I asked the same question in another forum and received a similar response.

Especially since we’ve had quite dense fog here over the last few days...

However, the heat recovery ventilation unit (HRV) is located on the upper floor, so unfortunately I can’t just take a quick look from outside to see if the intake grille is frozen. I opened the HRV, removed the heat exchanger, and simply blew warm air from a hairdryer on medium heat into the outside air duct for 10 minutes.

After the 10 minutes, I let everything cool down briefly and restarted the HRV.

Result: the ventilation is running noticeably quieter, almost like I’m used to. Incredible! Hopefully, it stays that way.

Thanks a lot for your response anyway. I was hoping the solution would be this simple, but I already feared I’d have to call a technician. Now I can sleep peacefully again tonight.

Do you have access to the fan speeds? Logging these over time together with the outside temperature could give you another clue in this regard.

I can read the actual values of the system, but I cannot interpret them because I don’t know what the standard values are and which values are deviations...

Currently at 140m³/h (4929 cubic feet per hour)
Supply air fan speed: 36Hz
Exhaust air fan speed: 7Hz
Discharge air fan speed: 21Hz

I would have expected the supply air and exhaust air values to be at a similar level...