CENWAT Flue Requirements & Pressure Relief 4 — Questions and Answers
Question 1: What is a balanced flue boiler?
- A room-sealed boiler where the flue terminal faces the outside, drawing in combustion air and expelling flue gases in a balanced arrangement (Correct answer)
- A boiler with equal pressure in all flue sections
- A boiler using two separate flues
- A boiler with a fan to balance flue draught
Correct answer: A room-sealed boiler where the flue terminal faces the outside, drawing in combustion air and expelling flue gases in a balanced arrangement
A balanced flue boiler is room-sealed with a concentric terminal positioned on an outside wall; air is drawn in through the outer annulus and flue gases expelled through the inner tube, with wind effects balanced on both.
Question 2: What is a fan dilution flue system (also known as a 'Potterton Performa' type fan flue)?
- A system where a fan dilutes flue gases with large amounts of air before discharge, allowing flexible terminal positions (Correct answer)
- A flue with an additional secondary fan
- A flue using only natural draught with a fan for backup
- A fan that draws combustion air from outside only
Correct answer: A system where a fan dilutes flue gases with large amounts of air before discharge, allowing flexible terminal positions
Fan dilution systems use a high-volume dilution fan to mix large quantities of air with flue gases, reducing their temperature and concentration, which allows more flexible terminal siting.
Question 3: What is the minimum flue gas temperature that a flue must be designed to withstand on a condensing boiler?
- The condensing boiler flue must withstand flue gas temperatures specified by the manufacturer, typically up to 120°C (Correct answer)
- 500°C
- 1000°C
- 200°C minimum
Correct answer: The condensing boiler flue must withstand flue gas temperatures specified by the manufacturer, typically up to 120°C
Condensing boiler flues operate at much lower temperatures than non-condensing boilers (typically 50–120°C) but must still be rated for the maximum temperature specified by the appliance manufacturer.
Question 4: What is a flue liner and when is it required?
- A flexible or rigid duct inserted into an existing chimney to provide a safe gas-tight flue passage (Correct answer)
- A decorative lining inside the boiler casing
- An insulating layer around the flue
- A liner fitted to a new single-wall flue
Correct answer: A flexible or rigid duct inserted into an existing chimney to provide a safe gas-tight flue passage
A flue liner is installed inside an existing chimney or flue duct to provide a correctly sized, gas-tight and heat-resistant passage for flue gases, required when using an existing chimney for a new appliance.
Question 5: What is 'spillage' in the context of an open-flued gas appliance?
- Flue gases spilling back into the room rather than being drawn up the flue (Correct answer)
- Water leaking from the heat exchanger
- Gas leaking at the meter connection
- Condensate overflowing from the trap
Correct answer: Flue gases spilling back into the room rather than being drawn up the flue
Spillage occurs when flue gases escape into the habitable space rather than discharging through the flue; it can be caused by blocked flues, down-draughts or insufficient ventilation.
Question 6: What test is used to check for flue spillage on an open-flued gas appliance?
- A smoke match or smoke pellet test near the draught diverter when the appliance is running (Correct answer)
- A flue gas analyser test at the terminal
- A CO meter test at the floor level
- A pressure test on the flue duct
Correct answer: A smoke match or smoke pellet test near the draught diverter when the appliance is running
A smoke match or smoke pellet is held near the draught diverter while the appliance is at operating temperature; smoke should be drawn into the flue. If it spills into the room, a spillage problem is confirmed.
CENWAT Flue Requirements & Pressure Relief 4
What is a balanced flue boiler?