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Diagram of central heating and domestic hot water installations with a buffer -

seiko.36 96960 31
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How should I correctly connect a solid-fuel boiler, buffer tank, and domestic hot water circuit so the installation will work safely and allow natural circulation in case of a power failure?

Use thick pipes between the boiler and the buffer, at least 5/4" and preferably 6/4", and keep them sloped so natural circulation can work better [#18078885] The expansion vessel connection should rise continuously without any siphon; connect the boiler-to-vessel pipe at the tee outlet, take the installation branch from the tee leg, and do not place an automatic air vent on the pump suction side because it can draw in air [#18083569][#18084596][#18085039] If you keep the open system, one properly arranged vessel connection is enough for a single boiler, and the boiler–vessel run should be as vertical as possible; if you use a bend near the boiler, the total horizontal length should stay within the standard relation to the vertical section [#18085039] It was also recommended to install shut-off valves so the buffer can be isolated from both the boiler and the heating circuit for service or repairs [#18086446] A check valve on the return to the buffer was suggested to prevent the mixing valve from pulling only from the buffer instead of the radiator return [#18097051]
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  • #31 21008551
    andrzej lukaszewicz
    Level 42  
    Posts: 7005
    Help: 622
    Rate: 2099
    @oooo123 Isn`t that a big boiler? Is the building poorly insulated? Because a 1000l buffer for it is definitely too small.
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  • #32 21009721
    oooo123
    Level 5  
    Posts: 23
    Rate: 1

    Hello. The building is freshly insulated, the temperature in the hall is no more than 15 degrees, the trestle is oversized for the future connection to a small wood drying kiln.
    We will also keep the 1000l buffer, if it is too little we will add it. Please make a simple arrangement so that my installer doesn't get too tired while deciphering "what to what"
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Topic summary

✨ The discussion revolves around a DIY installation of a central heating and domestic hot water system with a buffer tank. The user seeks feedback on their diagram, particularly regarding pipe connections and the use of valves. Key points include the recommendation for using larger pipes (minimum 5/4" or 6/4") for better natural circulation, the importance of a mixing valve for temperature regulation, and the potential issues with the proposed layout, such as the need for proper venting and the correct connection of the expansion tank. Various opinions are shared on the use of Laddomat systems versus simpler setups, with emphasis on ensuring safety and compliance with regulations. The final installation diagram is presented as field-proven, confirming that the system operates without boiling issues.
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FAQ

TL;DR: 1000 L of water absorbs a 25 kW batch without boiling; "A fully filled 25 kW boiler will never boil 1 000 L" [Elektroda, andrzej lukaszewicz, post #18097876] Size pipes ≥5/4" and add a thermostatic mixing valve for 40-70 °C control. Why it matters: Correct hydraulics prevent overheat, air entry and costly rework.

Quick Facts

• Boiler-to-buffer pipe: ≥5/4" (DN32) or 6/4" (DN40) to keep natural circulation efficient [Elektroda, andrzej lukaszewicz, post #18078885] • Adjustable return temperature: 40-70 °C using ZT or TS 1" valve + capillary sensor [Elektroda, andrzej lukaszewicz, post #18082433] • DIY Laddomat substitute parts ≈ 800 PLN (pump, TS valve, fittings) [Elektroda, andrzej lukaszewicz, post #18082433] • Expansion riser: first vertical ≥0.1 m; total horizontal ≤10× that length [Elektroda, hajtaler, post #18085039] • Heat capacity: 1 kL water × 1 °C ≈ 1.16 kWh [Engineering Toolbox].

How large should the buffer be for a 60 kW chip-fuel boiler?

Target at least 2 000 L. A 60 kW unit can raise 1 000 L by 50 °C in ≈0.8 h (60 kW ÷ (1.16 kWh · 50 °C)=0.86 h) [Engineering Toolbox]. Doubling the volume stretches burn time and lowers cycling losses.

Why do experts insist on ≥5/4" pipes between boiler and buffer?

Large pipes cut resistance and let water move under gravity during power loss. Andrzej notes 5/4–6/4" “facilitates work in natural circulation” [Elektroda, andrzej lukaszewicz, post #18078885] Smaller pipes risk boiling and pump cavitation.

Do I really need valves between the boiler and the buffer?

Yes. Gate or ball valves let you isolate the 1 000 L tank for service without draining it [Elektroda, hajtaler, post #18086446] Place them low, on both flow and return, to retain natural circulation when open.

Is a check valve required on the return to the buffer?

Install one. It stops the three-way DHW mixing valve from pulling cold water straight out of the buffer rather than the radiator return [Elektroda, hajtaler, post #18097051]

Can the system run safely during a blackout?

With open-vented buffer, big pipes and a flap (swing) valve, heat can move by gravity. "A fully filled 25 kW boiler will never boil 1 000 L" in such layout [Elektroda, andrzej lukaszewicz, post #18097876] Keep the riser unobstructed and pumps off.

What is the downside of putting an automatic air vent above the buffer?

On the pump’s suction side negative pressure forms and the vent can draw air, not expel it [Elektroda, hajtaler, post #18084596] Move the vent to the boiler top or close it after purging.

Why does the seller recommend a four-way valve for the carpentry shop?

Four-way (or three-way plus pump) blends supply and return. It keeps the boiler return above 55 °C, preventing tar and extending exchanger life on chip fuel [CIBSE Guide B].

How do I connect the open expansion vessel correctly?

  1. Run a continuously rising safety riser (RB) straight from boiler top.
  2. Maintain first vertical ≥10 cm before any bend.
  3. Limit total horizontal length to ≤10× that vertical [Elektroda, hajtaler, post #18085039]

Is a 1-function BVTS cooling valve necessary in an open system?

Not mandatory, but allowed. Open vent plus gravity return already protect the boiler; BVTS adds redundancy and opens at 95 °C [Elektroda, hajtaler, post #18086446]

What pipe sizes for radiators and air heaters?

Typical: DN25 (1") supply loop for two 25 kW heaters up to 20 m, DN20 (3/4") branches to aluminium radiators. Verify by flow rate: 0.86 m³/h needs ≤0.4 m/s for low noise [Viessmann Design Data].

How can I build a budget alternative to Laddomat?

Combine a high-Kv 1" TS mixing valve, any Class A pump, flap valve, filters and shut-offs. Andrzej priced the set at ≈800 PLN, about half a branded unit [Elektroda, andrzej lukaszewicz, post #18082433]

Quick 3-step method to size a buffer tank?

  1. Pick desired burn time (h). 2. Multiply boiler kW by hours, then divide by 1.16 to get litres per 10 °C. 3. Multiply by planned ΔT. Example: 60 kW ×1 h ÷1.16 ×50 °C ≈2 600 L.

What is a common failure when DIY owners ignore slope rules?

A horizontal safety riser longer than allowed can trap steam, leading to water hammer and vessel overflow during overheat events [Elektroda, seiko.36, post #18083914]
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