Why an open-flued appliance gets a flue flow (draught) check before the spillage test, and what it proves.
The spillage test gets the attention, but on an open-flued appliance the flue flow test usually comes first, and CCN1 expects you to know why and how.
The flue flow test asks a simpler question than spillage: is the flue actually clear and able to carry products of combustion upward and out. It is checking the path itself, before the appliance is even firing.
It is normally carried out on a cold flue. A smoke source is held at the point where the appliance connects to the flue, or at the draught diverter, and you watch what the smoke does.
A good result is smoke drawn cleanly up the flue and discharging at the terminal outside, which tells you the flue is clear, continuous and developing a natural draught. Smoke that hangs, spills back or fails to reach the terminal points to a blockage, a disconnection or a flue that will not pull.
The order matters. There is little point running a spillage test if the flue is blocked, so the flue flow test proves the path first, and the spillage test then proves the appliance clears its products under real, worst-case conditions once it is firing.
Treat a failed flue flow test as a stop: find and fix the cause, whether that is debris, a collapsed liner, a disconnection or an undersized or wrongly routed flue, and confirm the method and any acceptance detail against the current standard.