Defro Duo Uni 35kW Boiler: Choosing Inflow Pipe Diameter for 6-8 Circuit Manifolds in 240m² Home

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What pipe diameter should I use for the supply lines to three radiator manifolds in a 240 m² house with 6–8 circuits per manifold and a 35 kW Defro Duo Uni boiler?

Size the manifold supply by the total heat load, not by the number of circuits: the listed radiators add up to about 30.2 kW, which needs roughly 1300 l/h flow [#9814017] For that main branch, 25 mm inside-diameter pipe is only near the limit, so the recommended feed/boiler connection is 28 mm copper [#9814017] If you insist on PEX to the manifolds, one reply suggested 25 mm as the minimum, but warned that PEX is not preferred for a solid-fuel boiler manifold feed [#9809363] From each manifold to individual radiators, 16 mm tube is enough in most cases, while larger radiators around 2000 W may need 20 mm PEX [#9808259][#9812243][#9814017]

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#31 10392905 13 Jan 2012 21:25

Snoopy429 Snoopy429

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Post #31
10392905 13 Jan 2012 21:25

I will ask the question.

If I have a copper riser with a diameter of 22 mm, is it enough for 6 600x1400 v22 heaters?
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#32 10408537 17 Jan 2012 09:04

kabee84 kabee84

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Post #32
10408537 17 Jan 2012 09:04

Snoopy429 wrote:
I will ask the question.

If I have a copper riser with a diameter of 22 mm, is it enough for 6 600x1400 v22 heaters?

Yes.

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#33 10568715 19 Feb 2012 18:32

Anonymous Anonymous

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Post #33
10568715 19 Feb 2012 18:32

What do you think about 32mm pex per riser? (Actually, I mean the distributors)

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And maybe a question for someone who has experience: how many watt per meter ^ 2 3-layer walls made of limestone blocks, 10cm polystyrene, 10cm polystyrene floor. Is 100W enough, there is probably an increase, but it should be ok, I will add that there are large windows.
#34 14383832 30 Jan 2015 16:09

brzeszczot1987 brzeszczot1987

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Post #34
14383832 30 Jan 2015 16:09

Hello, I make central heating + hot water installations, the entire installation is about 15kw and now I have a small problem what diameter of the pipe to choose when reaching the manifolds from the furnace or 25 mm pp pipe is enough or better to give 32mm some ideas I thought to leave the furnace 1 1/2 "graphite protection 11/2 "and to one 25mm manifold and also 25mm separately to the other, both manifolds will support about 6 kw
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Topic summary

✨ The discussion revolves around selecting the appropriate inflow pipe diameter for a central heating system using a Defro Duo Uni 35kW boiler in a 240m² one-story home with 6-8 radiator circuits. Participants suggest that 16-20mm PEX pipes are generally sufficient for connecting radiators to manifolds, while larger diameters (25-28mm) may be necessary for the main supply lines to accommodate the total heating load. The total power requirement for the radiators is calculated to be approximately 30,189W, necessitating careful consideration of pipe sizes to ensure adequate flow rates. The use of copper pipes is recommended for connections to the boiler, and the installation of temperature sensors and expansion vessels is discussed. Various schematic diagrams are shared for clarity, and the importance of proper pump selection and configuration is emphasized.
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FAQ

TL;DR: A 28 mm copper main moves about 1 500–1 600 L/h at 0.8 m/s [Elektroda, nikt_ważny, post #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, post #9814017] • 16 mm PEX branch safely feeds ≈ 1.9 kW radiator load [Elektroda, nikt_ważny, post #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, post #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, post #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, post #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, post #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, post #9814017]