FAQ
TL;DR: A 28 mm copper main moves about 1 500–1 600 L/h at 0.8 m/s [Elektroda, nikt_ważny, #9814017]; “use 16 mm to each radiator” [Elektroda, kabee84, post #9809363] Follow design temps 75/65/20 for output sizing.
Why it matters: Correct diameters keep flow noise low, pumps smaller, and the 35 kW boiler within safe ΔT limits.
Quick Facts
• 25 mm internal pipe ≈ 1 500–1 600 L/h at 0.8 m/s [Elektroda, nikt_ważny, #9814017]
• 16 mm PEX branch safely feeds ≈ 1.9 kW radiator load [Elektroda, nikt_ważny, #9814017]
• Grundfos ALPHA2 25-60: 0.05–3.3 m³/h, 6 m max head [Grundfos Datasheet, 2023]
• EA backflow valve ~PLN 40; BA valve ~PLN 900 [Elektroda, niceshoko, post #10037757]
• 75/65/20 °C = supply/return/room design temps [Elektroda, niceshoko, post #10032633]
What pipe diameter should I run from the 35 kW Defro Duo Uni boiler to a 6–8-circuit manifold?
Use 28 mm copper (≈25 mm ID). It carries up to 1 600 L/h, enough for three manifolds totalling 30 kW at ΔT = 10 K [Elektroda, nikt_ważny, #9814017].
Can I substitute PEX for copper between a solid-fuel boiler and the manifolds?
Avoid PEX near a solid-fuel boiler; high flue-gas surges can exceed 95 °C and soften the tube. If you insist, limit to 25 mm PEX after a mixing valve and thermal safety loop [Elektroda, kabee84, post #9809363]
How much flow does a 30 kW radiator load need?
At ΔT = 20 K the flow is Q = 30 kW / (4.19 kJ kg⁻¹ K⁻¹ × 20 K) ≈ 360 L/h. At ΔT = 10 K—as in the forum example—you need about 720 L/h [ASHRAE, 2017].
What size PEX should feed individual radiators?
Run 16 mm PEX to radiators up to ≈ 1.9 kW; use 20 mm PEX for emitters above 2 kW [Elektroda, nikt_ważny, #9814017]. "Oversizing branches gains nothing but cost," notes installer CEZAR67 [Elektroda, 9808259]
Is a 22 mm copper riser enough for six 600 × 1 400 mm V22 panels?
Yes. Six such panels draw roughly 14 kW. A 22 mm tube moves ≈ 1 000 L/h at 1 m/s, matching the demand [Elektroda, kabee84, post #10408537]
Which pump and speed setting suit a 240 m² single-storey house?
Grundfos ALPHA2 25-60 covers 0.05–3.3 m³/h. Start with AUTOADAPT or mid-range (stage 2); avoid max to cut noise [Elektroda, nikt_ważny, #9815267; Grundfos Datasheet, 2023].
Where do I mount temperature sensors for CH and DHW?
Clip the CH sensor on the flow pipe; insert the DHW probe in the cylinder’s mid-level well. Add outdoor and room probes if the controller supports them [Elektroda, nikt_ważny, #9814890].
How should diaphragm expansion vessels be installed?
Place the vessel on the boiler return before the pump; hang with the diaphragm down so air stays at the top. In open-vent systems add an upper vessel for safety [Elektroda, nikt_ważny, #9897117].
What does 75/65/20 °C printed in radiator tables mean?
75 °C supply, 65 °C return, 20 °C room. The quoted output only applies at those three temperatures [Elektroda, niceshoko, post #10032633]
How do I recalculate radiator output for 75/65/26 °C rooms?
Multiply catalogue output by (ΔT_new/ΔT_cat)¹·³, where ΔT = (mean water temp – room temp). For 75/65/26 °C, ΔT_new = 44 K; catalogue ΔT = 55 K, so 2000 W × (44/55)¹·³ ≈ 1 460 W [“Radiator Sizing Guide”].
What is the advantage of an EA or BA anti-pollution valve?
EA stops back-siphon of domestic water for ≈ PLN 40; BA adds pressure regulation and test ports but costs ≈ PLN 900 [Elektroda, niceshoko, post #10037757]
Edge case: what happens if the main pipe is undersized by one DN?
Velocity can exceed 1.5 m/s, causing hiss, erosion corrosion, and pump power rise by ≈ 50 % [CIBSE Guide G, 2014].
Three-step method to size a heating pipe quickly?
- Sum radiator watts on the branch.
- Convert to flow: L/h = Watts / (ΔT × 4.19).
- Choose the smallest tube giving velocity ≤ 1 m/s in sizing charts.
This matches industry practice [Elektroda, nikt_ważny, #9814017].
