Ämne: Nibe S735 Low airflow reports

[Raymond67]

Hi, I'm sorry for posting in English but I'm from The Netherlands and, unfortunately, don't speak Swedish. I found this interesting forum as I'm looking for a solution for the issue I have with our Nibe S735-7
Our house is about 150 m2 and we do have floor hoating on ground level and radiators on 2nd level. We are with 2 persons and do live near to the coast.

We do have the the pump installed since Oct 2024 and since the start we had several times the following alarms:
Low airflow - EB100-EP14. When this happens I do restart the pump and then it goes well for some days and the it happens again.
The settings for airflows are
1. 30%
2. 45%
3. 60%
4. 75%
Normal 57%

60% is about 212 m3/h in our house

Installation has been checked by a Nibe (dutch) engineer but the errors are still occuring. I already replaced the filter to exclude this as a root cause. Does anyone has an dea where I need to look at?


Attached 2 pictures from today. Airpressure Airflow (BP15) and Airflow (BP16)

[Carl N]

Do you have any idea why the airflow varies so much?

[Raymond67]

Hi, I really don't know. And that's exactly I want to try to find out.

I cleaned the filter last Friday after it happened again. Now I get 250m3/h with 60% ventilation. Then, because of the the low temperatures outside and the high humidity in The Netherlands the pump is going to defrost every 45-52 minutes. So the airflow drops from 250 to 225/220 and then starts the defrost. But sometimes it doesn't start the defrost and then the airflow is going down till 50m3/h and then I get the alarms.
In the picture you can see the relation between the airflow and the average outside temperature.

Sorry for Dutch text, see the strange value at "afname vanaf referentiewaarde" it says 0% where the airflow is 163.4 and the reference airflow is 214m3/h. So the 0% is incorrect (it should be around 30%). I don't understand what's happening

[Carl N]

How is the inlet air designed to the house?
Can the inlet of air vary?
Because it looks like somthing throttling the air inlet to the house?

[Raymond67]

I have 5 air inlets at home and no special valves in the tubes. See picture how it looks like on the heat pump itself.
At the moment the outside temperatures are high where I live
The airflow from the heatpump to the outside is going via a pipe on the roof

[Erik Renberg]

Could it be that the defrosting isn't really complete, so that there is a big block of ice in it when it thinks it is done with defrosting, and that it quickly builds up blocking the air once the compressor starts going again? Have you looked inside? (I'm not sure how/where you do that)

[Raymond67]

Thanks for the answer, Erik. That's also one of my thoughts. Today, I will contact the company who installed the heat pump and will tell them this. I also want to know from them why sometimes the "Reduction from reference value" is incorrect. Maybe that's related to some sensor (?)

[Raymond67]

20250121 update
After several restarts and low airflow reports, yesterday, a Nibe Engineer visited us. As I had the CDS10 and THS10 as sensors at home, he told us they suspect them to be the root cause of the problems as there are know issues with them together with the S735. It's known at Nibe and they are working on a software update. It seems that there are conflicting assignements from the sensors to the pump which are sometimes over-rulling each other. The engineer stopped in 7.1.4.4 the requested ventialtion mode and now it seems to be more stable behavior. See picture. I will keep you informed in 2 weeks how things are going.