re the question on checking airflow, airflow to me means air speed, which will be different on different days, however if you mean free air, then i measure it, if the property has airbricks i use a vent measuring stick which is basically a turned aluminium stick with 5,6,7,8,9,10mm turned sections, this allows me to know the inside dimension of the air brick, then i do the usual calculation to determine what the total free air is to the solum, i then measure the floor vent and check the appliance data badge to determine whether the supplied free air meets the requirement of the appliance, i also phone the local Building Control office to check if the area i'm working in is known to have a Radon Gas Issue, if so i duct the vent to outside, (i made the last bit up)