Air change rates

Bobby Gilbert · Posts: 20

I have a building set up with "excellent" crack template for infiltration, and 0.3 ac/hr from a mechanical ventilation system with 91% heat recovery, it runs 24/7.

I'm using the full compact HVAC and calculated ventilation. In my results, the total winter ventilation rate for a month (mech + nat + Inf) I get 0.25 - 0.27 ac/hr.

Any ideas why the calculated ac/hr are less than the mechanical ventilation?

Thoughts I've had
- is it something to do with unoccupied spaces being included in the calculation (roof, sunspace)
- is it something to do with wall/floor volumes in the space
- have I done something wrong.

The ac/hr dominate the results for this highly insulated building so I want to make sure I'm getting it right.

Regards

Bobby

Laurent_Support · Posts: 292 Location: Vincennes

Hello Bobby,

This smells passive house! Smile

Some ideas before digging your problem:
Check at zone level the ac/h in the HVAC tab (included any operation schedule) and compare it to your results.
If not OK, it is settings issue, if OK for every zone it could be the sum at building level which generate another ac/h ratio (as you wrote some space could change the average result).

In other cases write me back with a zone example that does not work.

Regards,
Laurent

Bobby Gilbert · Posts: 20

Laurent,

I've had a look at zone level and sure enough, the sunspace and the attic have very low air changes (0.1) so I'm guessing that this is bringing the average down. Does this sound right to you?

I thought that unoccupied spaces weren't included in the monthly sums/averages (I don't have "include unoccuppied..." ticked in the options). I have the activity as "None" in these spaces.

I would like to know what the average air changes are for all of the occupied parts of the house. Is there a way of doing this without getting answers for each zone individually.

Regards

Bobby

Bobby Gilbert · Posts: 20

Laurent,

Further to my previous reply, I realise that what I could really do with is the ability to
- have windows opening to give calculated natural ventilation
- set cracks to zero but then specify an fixed infiltration rate based on the 50mpa pressure test, e.g. 0.5ac/hr for a typical house.

It is not obvious to me how I would do this, particularly if I wanted to have a mechanical ventilation system on top of the background ac/hr. Any suggestions?

Bobby

Laurent_Support · Posts: 292 Location: Vincennes

Hello Bobby,

Laurent_Support · Posts: 292 Location: Vincennes

Hi Bobby,

Only to confirm you that ACH averages at building level include unoccupied zones.

Nest release will improve this point to follow options you choose.

Thanks pointing this out.
Laurent

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