Modelling of Transfer Air to WC's (supply and return to MVHR

[DesignCO2]

Hi all, as per usual, any comments on the following would be much appreciated.

I have a WC that receives transfer air from a neighbouring circulation space. The circulation spaces is served with supply air via a MVHR unit whilst the WC exhaust returns the stale air to the MVHR also (rather than direct exhaust to outside the building).

Would it be deemed acceptable to model both the WC and circulation space as having both supply and return for compliance purposes? Within IES, this would take the form of only using the supply SFP box at the room level for both the WC and circulation spaces whilst ignoring the exhaust SFP and ACH boxes. My interpretation is that the exhaust SFP box and the exhaust ACH inputs should only be used if the exhaust from the WC does not go via an MVHR unit (or larger AHU for that matter). The implications of this approach would be that IES will use the non-editable NCM auxiliary ventilation rates rather than a user exhaust ventilation rate.

Thanks in advance for any thoughts.