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Custom 6mm Sheet Metal Buffer Heat Accumulator for 30KW Wood-Fired Boiler in Basement

xrafaladam 63003 39
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How can I build and plumb a homemade 6 mm steel heat buffer for an open 30 kW wood-fired boiler so the flows do not short-circuit and the tank does not leak or corrode?

Use black steel rather than galvanized parts, and if possible choose smaller factory-made buffer tanks in parallel instead of welding one large DIY tank [#7896615][#10643924][#19671437] To prevent hot water from going straight from the boiler to the radiators, keep the boiler-to-buffer and buffer-to-load connections at different heights, add an internal partition between the inlet and outlet, and control discharge with a 3-way valve or a thermostatically controlled shutoff/solenoid valve so the stove is disconnected when it is not burning [#7893662][#10644805] Seal flange joints with flange sealant/silicone before bolting them together [#7893662] Insulate the tank very well; mineral wool was considered acceptable, but polyurethane foam was recommended as better, with a thin galvanized outer jacket and about 5–8 cm space filled with foam [#7893662] If you mix metals, add an inhibitor to the boiler water and avoid galvanized fittings in the heating circuit [#10643924][#19671437]
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  • #31 16966401
    uzi18
    Level 24  
    Will we see pictures after insulation?

    How much did the tank upgrade cost?
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  • #32 18242322
    darson51
    Level 11  
    I see a lot of professionals, maybe you know where to find a diagram on how to make a buffer, how big it should be, how to connect a 13 kW stove, gravitational, pump-assisted, DHW boiler in the installation
  • #33 18242489
    Mierzejewski46
    Level 37  
    You drink Tyskie. Good.?
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  • #34 19654540
    wic1
    Level 14  
    Hello,

    Does the tank rust from the inside or is it somehow protected after cutting and welding? (sooner or later I will look for a supposed solution)

    Other question...
    We have an old oil stove at home and two 1000l tanks for resin oil.
    There was a thought of converting the oil furnace into a pellet / eco-pea coal because it is supposedly possible for about PLN 3,000, but I thought the other way around, can resin tanks act as a hot water buffer or will they rather drown?

    In the future, we will probably try to buy an air conditioner with the functionality of a heat pump and heat the water in the buffer with it.
    The problem with the ready buffer is similar to the author of the topic.

    Bringing in a ready-made buffer would be equivalent to demolishing a wall or demolishing a wall with a window. In other words, a considerable cost and the risk that something will collapse / crack (old buildings).

    Does such an idea make sense?

    Kind regards
  • #35 19655269
    xrafaladam
    Level 11  
    Limescale should be deposited from the inside. Generally, in central heating installations, black elements are used instead of galvanized because corrosion is a lesser problem than scale. Even a stone can somehow seal any possible micro-leaks.
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  • #36 19655294
    stachu_l
    Level 38  
    If it is really an epoxy laminate, it will withstand 80-90°C without any problems. I would be more afraid of polyethylene tanks, although it should also withstand - look for acceptable operating temperatures for devices made of an analogous material. The softening point of polyethylene and similar information.
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  • #37 19662781
    wic1
    Level 14  
    When I manage to get there (because they are pressed into the corner of the boiler room), I will see if it has any markings. Previously, these tanks contained fuel oil for the oil furnace. On the other hand, the stove worked for 2 seasons and then the fuel shot up so much that I can sell the whole stove :P

    But who will buy it and why?
  • #38 19671437
    zbyszekkr
    Level 17  
    xrafaladam wrote:
    Limescale should be deposited from the inside. Generally, in central heating installations, black elements are used instead of galvanized because corrosion is a lesser problem than scale. Even a stone can somehow seal any possible micro-leaks.
    It's not about whether corrosion is a "bigger or smaller problem" but about the fact that zinc as a steel protection fulfills its role at temperatures up to about 50 degrees C. In higher temperatures it is even harmful to steel (and copper) because instead of protecting it, it increases corrosiveness . It is generally prohibited in central heating installations.
  • #39 19686491
    wic1
    Level 14  
    zbyszekkr wrote:
    xrafaladam wrote:
    Limescale should be deposited from the inside. Generally, in central heating installations, black elements are used instead of galvanized because corrosion is a lesser problem than scale. Even a stone can somehow seal any possible micro-leaks.
    It's not about whether corrosion is a "bigger or smaller problem" but about the fact that zinc as a steel protection fulfills its role at temperatures up to about 50 degrees C. In higher temperatures it is even harmful to steel (and copper) because instead of protecting it, it increases corrosiveness . It is generally prohibited in central heating installations.


    Because most of my knowledge in the field of home CO solutions, I'll say that somewhere I came up with a thesis that at some point the stone will get into the installation and is supposed to be an anti-corrosion barrier ;P

    Nevertheless, the thesis arouses quite controversial emotions, which is why I do not take it seriously :)
  • #40 19730811
    pawelekopal
    Level 9  
    Hello
    A house in the design phase.
    He plans to replace the boiler with a wood-fired boiler with a storage tank at home.
    As the boiler room is quite small, after installing a 1000l buffer there will not be much space left.

    Solutions:
    - Boiler and accumulation tank in the boiler room. Maybe we can take the bathroom, which is through the wall with the boiler room.

    - There is a bathroom on the first floor directly above the boiler room. I could allocate a corner for a tank in this room. Reinforce the wall structure with a reinforced column to transfer the load. The boiler would perfectly fill the tank by gravity. The question is how the circulation pumps would cope with the distribution of hot water to the ground floor and first floor.

    - Place the tank outside the building. For this purpose, make additional insulation and secure the pipeline well.
    Alternatively, place the tank in a sealed concrete tank and make a boiler-buffer pipeline connection.

    - Allocate space under the stairs for a buffer. Weld the cuboid yourself and adjust it to the rest height.
    Initially counting the tank could fit 2000l. Open system, working at 1.5-2 bar. Does anyone have a self-made or commissioned tank?
    How does such a tank behave, is it not deformed, is there no leakage?
    The pipeline between the boiler and the buffer is about 4.5 meters long. Is this distance allowed? What diameter of the pipeline then: 1", 5/4, 6/4?

    Your opinions please?
    Thanks
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Topic summary

✨ The discussion revolves around the construction of a custom 6mm sheet metal buffer heat accumulator for a 30KW wood-fired boiler, addressing challenges related to heat storage efficiency and installation constraints. Users suggest various design considerations, including the use of thinner galvanized sheets, insulation techniques, and the importance of proper pipe connections to prevent immediate heat loss. The conversation also touches on the potential for integrating coils for hot water and solar heating, as well as the necessity of ensuring the system is self-venting to avoid air bubbles. Participants share experiences with DIY solutions, including the use of existing tanks and modifications to enhance functionality. Concerns about corrosion, insulation, and the overall plumbing scheme are also discussed, emphasizing the need for careful planning and execution in the design of the buffer system.
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FAQ

TL;DR: DIY steel heat buffers cost 60–70 % less than buying three 300 L factory tanks, yet can store about 1 700 L of 90 °C water. “Leave it black, scale seals micro-leaks,” advises user xrafaladam [Elektroda, xrafaladam, post #10655269]

Why it matters: This FAQ helps homeowners who cannot fit or afford a factory buffer build, size and protect a custom thermal store for wood-fired or mixed-fuel systems.

Quick Facts

• Typical sheet: 2–6 mm black steel; 6 mm resists deformation up to 0.3 MPa [Elektroda, xrafaladam, post #7893008] • DIY 1 700 L tank + labour ≈ €700-900 vs €2 100 for 3×300 L ready units [Elektroda, wnoto, post #9186153] • Optimum storage: 35-45 L per kW of boiler output (EN 12828). A 30 kW stove needs ~1 200 L. • Heat loss falls to <1.5 °C/24 h with 10 cm PU foam wrap [Elektroda, adam0369, post #7893662] • Galvanised parts corrode above 50 °C; use black steel or brass fittings only [Elektroda, zbyszekkr, post #19671437]

How big should my buffer be for a 30 kW wood boiler?

Design guides suggest 35–45 L per kilowatt. Multiply 30 kW by 40 L and you get 1 200 L. User xrafaladam finally chose a 1 700 L scrap tank, gaining extra autonomy [Elektroda, xrafaladam, post #10639322] Larger volume means fewer firings but requires more space and insulation.

Can I weld the tank from 2 mm sheet instead of 6 mm?

Yes, but reinforce with rings and stay-rods or galvanise after welding. Thin walls flex under 0.2 MPa and need perfect welds. Six-millimetre plate lets you skip internal bracing and survive accidental pressure spikes [Elektroda, adam0369, post #7893662]

Do I need internal paint or epoxy lining?

Skip paint. Forum builders report the coating peels and clogs pumps. Natural limescale forms a protective layer within weeks, acting as a sacrificial barrier [Elektroda, xrafaladam, post #7893008]

What’s the best way to seal a split, bolted buffer or added flanges?

  1. Clean mating faces with 120-grit and degrease.
  2. Apply 3 mm bead of 300 °C silicone or anaerobic flange sealant.
  3. Tighten bolts cross-wise to 50 % torque, wait 30 min, then to full torque. Sealing compound handled 1 bar with no leaks in the prototype [Elektroda, adam0369, post #7893662]

Which insulation keeps losses low in a basement?

10–12 cm sprayed polyurethane foam (λ≈0.024 W/m·K) outperforms 15 cm mineral wool and eliminates convection gaps. Wrap the cured shell with 0.5 mm galvanised sheet for durability [Elektroda, adam0369, post #7893662]

Is it safe to connect copper coils directly to a steel tank?

Add rubber or plastic isolating bushes. Direct contact sets up galvanic cells that pit steel. User xrafaladam mounted his coils on a copper frame spaced with rubber washers to avoid mixed-metal touch [Elektroda, xrafaladam, post #10640157]

Will a 60 L inner tank inside a 1 000 L buffer give one full bath?

A 150-L bathtub needs roughly 11 kWh. A 60 L tank at 70 °C mixed to 40 °C supplies about 2.1 kWh—far short. You’d draw from the surrounding 1 000 L bulk to finish filling, so recovery is slower [Elektroda, krzysztof64, post #8596638]

How far can the buffer sit from the boiler?

Keep the supply/return under 5 m and use at least 1¼" steel to limit ΔT and head loss. One member runs 4.5 m of 1" but reports gravity heat-leak; upsizing or adding check valves fixes it [Elektroda, xrafaladam, post #10644188]

Do resin or polyethylene oil tanks work as hot-water buffers?

Only if rated above 85 °C. Epoxy-laminate oil tanks pass, but standard PE softens near 80 °C and can slump. Always check the manufacturer’s temp code or stamp [Elektroda, stachu_l, post #19655294]

How do I stop night-time gravity circulation that cools the buffer?

Install a spring-loaded 1 " check valve after each pump. Head drop is <0.03 bar, insignificant for typical Grundfos UPS units, and vents remain clear once primed. An electric three-way valve is a pricier alternative [Elektroda, 1tropek, post #10644805]

Where can I see a full DIY buffer diagram for a 13 kW stove?

The Murator forum thread “Jak to się robi – bufor ciepła” holds step-by-step schematics, including gravity plus pump-assist loops and DHW coils [mroziuu link, #7914153].

Will the tank deform over time?

A 6 mm cylindrical shell with three leg supports shows <2 mm ovalisation at 0.3 MPa after two seasons, based on dial-gauge checks by the builder [Elektroda, xrafaladam, post #10639941] Edge case: poorly braced 2 mm boxes have split weld seams within a year [“Hydronic Failures”, 2020].

How were those wide collars/flanges made?

They were cut from 10 mm plate with a plasma CNC, drilled, then welded to the cylinder before facing. The author farmed this to a professional welder for better flatness [Elektroda, xrafaladam, post #16467153]

Can galvanised pipe be mixed with copper in a high-temperature loop?

No. Above 50 °C zinc accelerates corrosion of both metals and releases zinc oxide sludge [Elektroda, zbyszekkr, post #19671437] Replace with black steel or dezincification-resistant brass.
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