Heat Recovery Ventilation Unit (HRU) Efficiency

Hardware
1

Question for those who have a Heat Recovery Ventilation Unit (HRU):
What kind of efficiency are you seeing with your HRU systems, based on temperature measurements? I’d appreciate it if you could share your own values so we can compare.

I have an Alnor HRU Slim Air 500 CF unit, and the example below was measured at an airflow of 250 m³/h:

  • Indoor temperature (air extracted from the house): 22.86°C

  • Supply air temperature (fresh air entering the house after HRU): 20.16°C

  • Outdoor temperature (fresh air entering the HRU): 14.67°C

  • Exhaust air temperature (air leaving the HRU to the outside): 15.65°C

Using the standard efficiency formula:
(Supply - Outdoor) / (Indoor - Outdoor)
I get an efficiency of approximately 67%.

The manufacturer suggested I use Supply / Indoor, which gives about 88%, but to me that seems misleading — it completely ignores the outdoor air temperature.

If you can share your measurements (ideally at a known airflow), that would be really helpful to understand what’s normal and whether I should investigate further.

Thanks in advance!

2

I’d call that as company-led marketing / advertising. It looks good for their product, but it’s somewhat misleading

3

Your way of calculating it is correct. But SRE should be calculated at certain standard T/RH. For example To 0°C, RHo 75%, Ti 22°C, RHi 40%.