What temperature are condensing boiler flue gases?

Condensing boilers are designed to recover latent heat by condensing water vapour in the flue gases. When operating in condensing mode (return water temperature below approximately 55°C), flue gas temperatures are typically 40-60°C. At higher return temperatures the boiler may not fully condense and flue gas temperatures can rise to 80-100°C. Modern condensing boilers use plastic flue systems rated for these temperatures.

Can I use plastic flue on any boiler?

Plastic (polypropylene) flue systems are only suitable for condensing boilers where the flue gas temperature stays below the plastic rating — typically 120°C maximum. Non-condensing boilers, oil boilers operating in non-condensing mode, and solid fuel appliances produce much higher flue gas temperatures and require metal (stainless steel or aluminium) flue systems. Always check the boiler manufacturer's flue specification.

Why do condensing boilers produce a white plume?

The white plume (sometimes mistaken for smoke) is water vapour condensing as it exits the flue terminal into cold outdoor air. Condensing boilers extract so much heat that the flue gases are close to dewpoint temperature. When they hit cold air, the water vapour condenses into visible droplets — the same process as seeing your breath in winter. Plume management kits are available but are rarely required for domestic installations.

What is the condensate from a condensing boiler?

Condensing boilers produce acidic condensate (pH 3-5) at a rate of approximately 1-2 litres per hour during normal operation. This must be drained via a condensate pipe to an internal drain (preferred), external drain with trace heating, or soakaway. The condensate pipe must be at least 22mm internal diameter and protected from freezing if it runs externally. Frozen condensate pipes are the most common cause of condensing boiler breakdowns in winter.

Boiler Flue Temperature Ranges — Material Selection

Flue gas temperatures by boiler type and their implications for flue material selection. Condensing vs non-condensing boilers, plastic vs metal flue systems.

Flue Gas Temperature by Boiler Type

Boiler Type	Normal Flue Temp (°C)	Peak Flue Temp (°C)	Flue Material
Gas condensing (condensing mode)	40 – 60	80	Plastic (PP) or aluminium
Gas condensing (non-condensing mode)	80 – 120	120	Plastic (PP) rated to 120°C
Gas non-condensing (conventional)	120 – 180	250	Stainless steel or aluminium
Oil condensing	50 – 80	120	Stainless steel (acid-resistant)
Oil non-condensing	180 – 260	350	Stainless steel (316 grade)
Wood pellet boiler	120 – 200	300	Stainless steel (316 grade)
Log burner / multi-fuel stove	150 – 350	600+	Stainless steel twin-wall or clay liner
Open fire	200 – 500	800+	Clay/concrete liner or stainless steel

Condensing Temperature Thresholds

Fuel	Dewpoint (°C)	Return Temp for Condensing	Notes
Natural gas	~57	< 55°C	Most condensing occurs below 45°C return
LPG	~52	< 50°C	Lower hydrogen content, less condensate
Oil (kerosene)	~47	< 45°C	Sulphur content makes condensate more acidic

Flue Material Comparison

Material	Max Temp Rating	Acid Resistance	Typical Use
Polypropylene (PP)	120°C	Excellent	Gas condensing boilers
Aluminium	200°C	Good	Gas boilers (condensing and some conventional)
Stainless steel 316	600°C+	Excellent	Oil boilers, solid fuel, multi-fuel
Stainless steel 904L	450°C	Superior	Oil condensing — highly acidic condensate
Vitreous enamel	600°C	Moderate	Solid fuel stoves (single-wall connections)
Clay/concrete liner	800°C+	Good	Traditional chimneys, open fires