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?
- Clean mating faces with 120-grit and degrease.
- Apply 3 mm bead of 300 °C silicone or anaerobic flange sealant.
- 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.
