OFTEC Flue Systems for Oil 4 — Questions and Answers
Question 1: What is a twin-wall insulated flue system and when is it typically used?
- A double-walled flue with insulation between the walls, used for external or exposed runs (Correct answer)
- A system using two separate flue pipes
- A flue with two terminals
- A backup flue in case the primary one fails
Correct answer: A double-walled flue with insulation between the walls, used for external or exposed runs
Twin-wall insulated flue comprises an inner and outer pipe with insulation between them. It is used for external installations or internal runs through unheated spaces. The insulation maintains flue gas temperature, preventing excessive condensation and maintaining draught.
Question 2: What is the minimum terminal height above ground level for a horizontal balanced flue on an oil appliance?
- 600mm above ground level (Correct answer)
- 100mm above ground level
- 2 metres above ground level
- No minimum height is specified
Correct answer: 600mm above ground level
A balanced flue terminal must be at least 600mm above ground level or any surface where a person might stand. This prevents interference with the air intake, contamination from ground-level debris, and prevents people from being directly exposed to flue gases.
Question 3: Why must an oil flue pipe slope upward away from the appliance?
- To ensure condensation flows back toward the appliance's condensate trap rather than pooling (Correct answer)
- To make the flue look better
- To increase the cost of installation
- Downward slope is actually preferred
Correct answer: To ensure condensation flows back toward the appliance's condensate trap rather than pooling
Sloping the flue upward (minimum 1 in 12) away from the appliance ensures condensation drains back to the appliance where it can be safely dealt with by the condensate trap. Pooling condensate in the flue causes corrosion and blockage.
Question 4: What is the maximum number of 90-degree bends typically allowed in a domestic oil flue system?
- Two 90-degree bends (or equivalent 45-degree bends) (Correct answer)
- Five 90-degree bends
- No limit on bends
- One 90-degree bend only
Correct answer: Two 90-degree bends (or equivalent 45-degree bends)
Most manufacturer instructions and Building Regulations limit the flue to two 90-degree bends (or their equivalent in 45-degree bends). Each bend reduces available draught and increases resistance to gas flow. Two 45-degree bends equal one 90-degree bend.
Question 5: What happens if a conventional oil flue does not have adequate ventilation air supply to the room?
- The burner starves for air, causing incomplete combustion and potential CO production (Correct answer)
- The room gets too cold
- The burner runs more efficiently
- There is no effect on the burner
Correct answer: The burner starves for air, causing incomplete combustion and potential CO production
Conventional (open) flue oil appliances draw combustion air from the room. Without adequate ventilation (permanent air vents), the room becomes depleted of oxygen, causing incomplete combustion, CO production, and potential flue gas spillage.
Question 6: What size permanent air vent is typically required for a conventional flue oil boiler?
- 550mm² per kW of rated input over 5kW (Correct answer)
- No ventilation is required
- A single 100mm² vent for any size boiler
- 5,000mm² regardless of boiler size
Correct answer: 550mm² per kW of rated input over 5kW
For conventional flue oil appliances, a permanent air vent of 550mm² per kW of rated input above 5kW net is typically required. This ensures adequate combustion air supply. The vent must be non-closable and positioned to avoid draughts.
OFTEC Flue Systems for Oil 4
What is a twin-wall insulated flue system and when is it typically used?