I am looking at this product (version RK5) for both heat recovery ventilation and supplementary heating via warm air (seasonally).

It has an extra inlet for air recirculation, which it can mix with the ventilation air. This extra inlet has a low rpm, high-volume fan, and can add up to 1500 m3/h airflow through the system.

The ventilation flow is up to 440m3/h.

I am trying to have a go at putting together the ducting designs myself. I am fairly technical so I don't mind going into a bit of physics if need be.

I am confused with what seems to be a basic question. On what basis should I size the ductwork? I worked out that I only really need (according to norms) about 150m3 mandatory airflow, though I plan an extension in a few years. So at that ventilation flow rate, the unit would be working at decent efficiency with low noise/power. With up to say 440m3/h for ventilation, I would choose ducting and outlets that seek a compromise of velocity and efficiency across that range (right?).

But what if I need to heat the house with an extra 7kW of heat for a while? The airflow is suddenly 1500m3/h. With small outlets geared for 300m3/h the house would sound like a hurricane and the efficiency would be low, right?

Should I just size the ducting for 1500m3/h tops? Should I consider a 'zone' that represents a parallel ducting branch to cope with the extra volume? What should be the approach?

Many thanks for any advice.

  • Where are you on this planet? Have you looked at ACCA's Manual D? – ThreePhaseEel Jan 8 '19 at 12:36
  • @ThreePhaseEel I am in the Czech Republic. HVAC skills are currently in short supply here. Though, that is about to change dramatically as recuperative ventilation is coming in as mandatory for new builds soon. What is ACCA Man D? Ah...just googling it...looks v interesting. – Sentinel Jan 8 '19 at 12:51

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