Hello. We do not use flow simulation frequently, but when we do intermittently we run into "One of the selected faces does not belong to the fluid subdomain." Sometimes we get passed it after multiple tries without really being able to tell what difference got it to work. On the model we are working on now we have tried both the automatic lids and making our own lids, and can't get passed the problem. After multiple tries we were able to get it to work on the water side (circular entrance and exit), but we can't get it to work on what will be the flue gas side which is sheet metal at the entrance and exit. We're making sure we have overlap. Thanks for any help you can provide.