Flue flow and spillage testing is where ventilation, combustion and unsafe-situations judgement meet. Candidates often merge the two checks into one vague "smoke test", but assessment wants a clearer chain: can the flue route discharge, does the appliance clear its products, and what action follows if it does not?
Flue flow checks the chimney route; spillage checks that products of combustion are not spilling back into the room while the appliance runs.
Flue flow and spillage decision sequence
Use this as a revision prompt for separating route checks from appliance-in-operation checks. Live testing follows the current standard and appliance instructions.
- Identify appliance type, flue type, room suitability, terminal route and permanent ventilation.
- Inspect visible appliance, flue, chimney, terminal, air vents and signs of damage or leakage.
- Treat flue flow as the route check: products must be drawn through the intended flue path and discharge correctly.
- Only run the appliance where preliminary checks allow it, under the required operating conditions.
- Treat spillage as the appliance-running check: products must not spill from the draught diverter, canopy or relief opening into the room.
- Repeat the judgement under relevant fan or air-moving conditions where the procedure requires it.
- If flow or spillage is unacceptable, follow the unsafe-situations procedure before leaving the appliance available for use.
- Separate flue-flow failure from spillage failure.
- Check air vents and extractor/fan effects.
- Do not use a CO alarm as proof of safe flueing.
- Classify and act before returning an appliance to use.
This card is not a live method statement. Exact timings, smoke sources, test points and actions come from the current procedure and manufacturer instructions.
The difference learners miss
Flue flow is about the chimney or flue route. The question is whether smoke introduced into the flue path is drawn through the route and leaves at the intended terminal, without appearing where it should not.
Spillage is about the appliance running in the room. The question is whether the appliance clears its products of combustion through the flue, or whether evidence spills back into the occupied space.
Why open-flued appliances are the focus
Open-flued appliances take combustion air from the room and rely on the flue to remove combustion products. That makes the room, air vents, extract fans, chimney condition and terminal all part of the safety picture. A room-sealed appliance has a different air/flue arrangement, so do not transfer an open-flue spillage routine blindly across appliance types.
Revision sequence
- Identify the appliance and flue type before choosing the test route.
- Check room suitability, permanent ventilation, flue route, terminal and visible condition.
- Prove the flue route before treating the appliance as safe to run.
- Run the appliance only under the conditions required by the current instructions or standard.
- Check for spillage at the correct opening for that appliance design.
- Repeat with relevant fans or air-moving equipment where the procedure requires it.
- Classify and act on failure before leaving the appliance available for use.
Fan and room-pressure traps
Extract fans, cooker hoods, tumble dryers and some warm-air arrangements can pull air from the room and disturb the draw on an open flue. In revision, do not treat the first calm-room pass as the end of the story. Ask whether any installed fan or air mover could create a worse condition that also needs checking.
How it links to CO and combustion
Good flue flow does not automatically prove good combustion, and a CO alarm is not proof that the appliance and flue are safe. Flue checks, ventilation, flame picture, gas rate, analyser readings and unsafe-situations judgement all sit together. The PlumbRevise CO/CO2 ratio tool helps with one part of that wider CPA1-style decision chain.
Common exam mistakes
- Using "smoke test" as a vague answer without saying whether you mean flue flow or spillage.
- Ignoring permanent ventilation or treating a closable window as an air-supply fix.
- Forgetting to consider extractor fans and other room-pressure effects.
- Assuming a CO alarm replaces inspection, testing or servicing.
- Leaving an appliance in use after an unacceptable flue-flow or spillage result.
Quick answers
What is the difference between flue flow and spillage?
Flue flow checks whether the flue or chimney route draws and discharges correctly. Spillage checks whether products of combustion spill back into the room while the appliance is operating.
How long is a spillage test?
There is no single public shortcut time to memorise. Follow the appliance instructions, current standard and assessment-centre method; many failures depend on warm-up, appliance design and fan conditions.
What should happen if an appliance fails a spillage test?
It should not be left in normal use. The engineer must follow the current unsafe-situations procedure, make the situation safe and rectify the cause before the appliance is returned to service.
Does a CO alarm mean the flue is safe?
No. A carbon monoxide alarm is a backup warning device. It does not replace correct installation, inspection, servicing, flue-flow checks or spillage checks.
Spotted something wrong, unclear or out of date in this guide? Email help@plumbrevise.co.uk with the guide name — content reports are treated as product defects, not support noise.