combining mechanical and natural ventilation

koen.govers · Posts: 80

Can I combine mechanical and natural ventilation ?

I've tried it. I've used mechanical ventilation (compact HVAC) for daytime ventilation (15 l/s per person) and natural ventilation (5 volumes) for nighttime ventilation.
I've put the nat.vent. cooling temperature on 23°C, worked with a temperature difference of 2°C, schedule only in the summer at night..

It worked fine!

But then I tried it on the same building, but with other insulation thicknesses, other infiltration rate... Suddenly I got the warning that I can't combine mechanical and natural ventilation... ??

Koen

rankoromane · Posts: 66

I did manage to do this as well but I had to use Compact HVAC to make it work. The other thing to note is that your cooling due to mech ventilation is shown as cooling delivered (as per HVAC cooling). Can't figure out why the changes you made should stop it working though.

Andy Tindale · Posts: 1196 Location: Stroud

As you have found, it is possible to mix mechanical and natural ventilation provided you use Compact HVAC... but... Cooling Design calculations don't use Compact HVAC, they use the Simple 'purchased air' and EnergyPlus' simple Ventilation option. So when you carry out your Cooling design calculations, the combined mechanical and natural ventilation in the model cause problems.

The solution may be to switch off natural ventilation before carrying out the Cooling Design calculations to size cooling equipment. Then switch natural ventilation back on and carry out the simulation. You should use the 'Manual' or 'Autosize when not set' HVAC sizing option to avoid Cooling design calculations being carried out before every simulation.

We will look at allowing Compact HVAC for cooling design calculations to get round this problem in future versions.

Andy

koen.govers · Posts: 80

Andy,

The chiller consumption in the annual fuel breakdown results... are they supposed to be correct when combining these two ways of ventilation?

I have the feeling they make sense....

Koen

Andy Tindale · Posts: 1196 Location: Stroud

Koen,

Yes chiller fuel consumption should be correct - it is simply calculated from cooling coil loads using a seasonal chiller CoP factor.

Andy

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