Explainer - Heat Recovery - Sensible v Latent


  • Office in London with Part L Envelope,
  • Fan coil units w/ central OA default values,
  • Core/perimeter automatic zoning


  • To see the effect on peak heating load when improving heat recovery from 50% efficient to 70% efficient


 With heat recovery as default (sensible 50%, latent 0%);


  With heat recovery 'improved' (sensible 70%, latent 0%);

Why does my peak cooling increase when I improve my heat recovery efficiency? I thought it would only reduce my peak heating. 


In this particular case, my heating peak load comes down as expected but something 'unexpected' is seen - my cooling peak load has gone up by improving heat recovery efficiency. What is happening?



London has quite mild summer temperatures in our weather file. In order for sensible HR to be effective, outside air temperatures have to be higher than the space setpoint. Outside air in the weather file is ~22oC peak and our space setpoint in Sefaira is 24oC. In this case, the more efficient/effective your heat recovery the more heat you bring back into the intake air and hence the more cooling you need.

Changing the latent effectiveness from 50% to 70% will show a more 'expected' difference in cooling sizing.

Here is with heat recovery as default (sensible 50%, latent 0%);


Here is with heat recovery improved (sensible 50%, latent 50%);


Here is with heat recovery improved (sensible 50%, latent 70%);


This is because the humidity of outside air is much higher than the humidity of the air inside the building so the heat recovery with higher latent effectiveness shows a return, in this case. This may be inverse in hot, dry climates.

Please note that the default heat recovery in Sefaira systems is sensible only. 


In Northern Europe and temperate climates pay particular attention to the latent heat recovery element of Sefaira's heat recovery. The heat recovery results will always show much higher relative changes in the heating season when outside air is much lower than inside air temperatures. 



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