Methodology

This page explains how the ChimneyCalcs calculators and the reference datasets are derived and verified — and why they need no ongoing maintenance to stay correct. It is the method behind the flue-size-by-fireplace-opening reference and its liner-size-by-appliance companion, our signature data assets.

1. Timeless flue geometry, stable conventions

Every tool computes from a closed-form identity: a fireplace opening is width × height; by the 1/10 rule a round flue must be at least 1/12 of that opening and a rectangular one at least 1/10; the area of a circle is A = π·(d/2)² and a round diameter is d = 2·√(A/π); the NFPA 211 3-2-10 chimney-height rule is a max() of two clearances (3 ft above the roof penetration, 2 ft above anything within 10 ft); a liner’s cross-section must be at least the appliance outlet; and cost = (materials + labor + access − discount) ×(1 + contingency). The only baked-in numbers are stable geometric ratios and labeled published typicals. These do not drift, so the statements stay true over time.

2. Sizing as plane geometry; cost as a quantity × unit-price sum

Chimney and flue sizing is plane geometry — an opening area, a required flue cross-section by a fixed ratio, a diameter from a circle. Draft is the theoretical stack effect (ΔP ≈ 0.0342 · B · H · (1/T_out − 1/T_in)), labeled indicative — real draft is measured with a manometer by a professional. Cost is a quantity × unit-price sum — the materials or per-foot rate plus labor and access, with a contingency. Round flue area up to the nearest standard liner or tile, never down.

3. The signature flue-size & liner references

The flue-size-by-fireplace-opening reference tabulates, for each opening, the LABELED required round flue area and diameter, the rectangular area, and the nearest standard round liner or clay tile, and the companion gives the LABELED liner diameter by appliance outlet and the standard clay-tile sizes. Both are dated snapshots (currently 2026-07-14), not live feeds: they hold only stable geometry and clearly labeled published planning ranges, so they never need maintenance. Assumptions and limits are stated on the page.

4. Where the conventions come from

The geometric constants (the 1/12 and 1/10 flue ratios, π, the 3-2-10 offsets) are definitions and long-standing masonry practice. The standard clay-tile sizes, the liner-by-appliance guidance, the creosote buildup stages, the NFPA 211 inspection Levels, the clearance-to-combustibles typicals, the cost bands and the contingency (~10%) are LABELED published planning typicals, cited in Sources and user-adjustable. The primary references are public standards and safety bodies — NFPA (NFPA 211), the Chimney Safety Institute of America, and the U.S. EPA Burn Wise program — none of which we speak for; always confirm the current text with them.

5. No prices, no rates, no feeds

There is deliberately no live material or labor rate, no regional cost index, no product catalog, no sweep or mason directory and no live rate. Every cost tool works on the prices you enter from your own quotes and bills (materials $, labor $, access $, $/linear foot, $/foot). Labeled cost bands are shown only as a sanity guide. That is why the site is correct regardless of what material, labor or service prices do.

6. Numeric self-check

Every formula is asserted against a worked example with known numbers (for instance: a 30 in × 29 in opening is 870 in², so a round flue needs at least 72.5 in² — a 10-inch round liner — or 87 in² rectangular, a 12×12 tile; a chimney with a ridge 3 ft above the penetration within 10 ft must stand 5 ft above it; a 6-inch stove collar (28.3 in²) wants a 6-inch liner; relining a 22-foot chimney with 6-inch stainless at $30/ft, $150 insulation, $600 labor and $200 access is about $1,771; a routine sweep about $220; a Level 2 inspection about $330). A release gate runs all of these and fails on any mismatch, so “verification” here is mathematical correctness plus accurate conventions — not a time-based check.

7. Estimate or sizing guide, not a design

The contingency %, per-linear-foot and per-foot rates, condition and creosote-stage multipliers, liner material rate and cost bands are labeled planning typicals — a starting point, not a spec. Every result is a planning estimate or a sizing / draft guide: get itemized written quotes from a CSIA-certified sweep and a licensed, insured mason, confirm your exact flue and opening dimensions, and follow NFPA 211, the manufacturer’s instructions and local code. Creosote, chimney fire, carbon monoxide, structural judgement and code certification are set by a certified professional and local code; they are out of scope. Nothing here is an installation procedure, a combustion or venting verdict, a medical chimney-fire / carbon-monoxide verdict, or a certified design.