FAQ
TL;DR: To tame an oversized upper-combustion boiler, brick off part of the grate with firebrick; Xantix reports flue gas ~350°C and targets >200°C, noting it was “simply a tool for heating the atmosphere.” [Elektroda, Xantix, post #16741205]
Why it matters: Reducing the active hearth cuts output, stabilizes burn, and helps avoid cold, sooty operation for users stuck with an overpowered unit.
Quick Facts
- Oversizing can be extreme; one case involved a boiler installed at roughly 2× the real demand. [Elektroda, Xantix, post #16740753]
- Practical target after grate reduction: keep flue gases around or above ~200°C to maintain draft. [Elektroda, Xantix, post #16741205]
- Expect draft sensitivity; verify chimney cross‑section, cleanliness, and insulation when flue temps fall. [Elektroda, jack63, post #16741042]
- For reversible tests, seat bricks with ash rather than mortar; adjust any time. [Elektroda, Magister_123, post #16740746]
- Secondary air and a draft regulator can further clean combustion and stabilize output. [Elektroda, Magister_123, post #16740821]
Is power reduction linear with the amount of grate I brick up?
Not strictly. Boiler output depends on more than grate area; air supply and flue‑gas paths also govern burn rate and transfer. Adjust in stages and observe flue temperature and draft. As one expert put it, power depends on both air inlets and exhaust passages, not just size. [Elektroda, Sstalone, post #16741159]
How much of the grate should I block off first?
Start reversible. Dry‑set a small section of bricks with ash, fire the boiler, and check room heat, return temperature, and flue draft. Add or remove bricks until cycling and soot decrease. When stable, make it permanent. “You can add and subtract bricks at any time.” [Elektroda, Magister_123, post #16740746]
Will flue‑gas temperature drop after I reduce the hearth?
Yes, expect a drop, which can cut draft. Confirm the chimney is clean, properly sized, and, if needed, insulated to preserve pull. Poor draft is a common failure mode after heat‑exchanger or firing‑rate changes. [Elektroda, jack63, post #16741042]
What flue temperature should I aim for after the modification?
In the cited case, the boiler ran near 350°C; after reduction, the target was above ~200°C to avoid weak draft and condensation. Track with a probe thermometer during steady burn. [Elektroda, Xantix, post #16741205]
Does fireclay (chamotte) lining actually improve combustion?
Yes. Fireclay around the burner shields the flame from the cold water jacket. That raises flame temperature and improves burn‑out of volatiles and CO. “Isolation causes an increase in the combustion temperature.” [Elektroda, andrzej lukaszewicz, post #16745919]
Should I use thin or thick chamotte pieces?
Thicker pieces add thermal inertia, so the boiler stays warm longer after fuel burns out. Thin pieces heat quickly but store less heat. Choose thickness based on response time versus heat retention needs. [Elektroda, stanislaw1954, post #16745887]
How do I add secondary air or a draft regulator to clean up the burn?
Install an adjustable secondary air inlet above the fuel bed and a mechanical draft regulator on the flue. Tune both while monitoring smoke and flue temperature. These additions help stabilize an oversized unit. [Elektroda, Magister_123, post #16740821]
My oversized boiler "suffocates" at 45°C. Will the mod help me run hotter?
Yes. Reducing hearth size increases load matching, letting you run around 65°C without choking the fire. Hotter operation also raises flue temperature and reduces wet soot. This shift improves efficiency and comfort. [Elektroda, andrzej lukaszewicz, post #16742209]
Any quick, reversible method to test different hearth sizes?
Yes—use ash as the bedding material for the bricks. It grips well, tolerates heat, and lets you reconfigure the layout in minutes during trials. Mortar later if the setup proves out. [Elektroda, Magister_123, post #16740746]
Could a small convection exchanger limit my gains?
Yes. A constrained exchanger can force very high flue temperatures and waste heat despite hearth reduction. One case ran ~350°C flue gas due to poor exchange. Aim to improve heat transfer surfaces if feasible. [Elektroda, Xantix, post #16741205]
What’s an example of a well‑behaved solid‑fuel design to benchmark?
Users reported good results with the MPM DS line, noting it as a successful construction relative to garage‑made units. Use such references to set expectations for stability and cleanliness. [Elektroda, Xantix, post #16741596]
Will chimney fouling change after the modification?
Expect differences. One installer saw a thin, powdery, cream‑brown soot layer that sheds easily with certain setups. Inspect after changes and establish a new cleaning interval to maintain draft. [Elektroda, Xantix, post #16741596]
Any edge cases I should watch for after reducing boiler power?
If flue temperature falls too low, draft can collapse, causing smoke‑back and CO risk. Verify chimney cross‑section, height, and insulation, and monitor during cold starts and low fire. [Elektroda, jack63, post #16741042]
How do I trial and finalize a grate‑reduction layout?
- Dry‑fit chamotte bricks on the grate using ash as the bedding.
- Fire the boiler, monitor flue temperature and room heat; adjust brick count.
- Once stable, rebuild the layout permanently if desired.
[Elektroda, Magister_123, post #16740746]
