Thermo Sudoku: Rules, Strategy, and Solving Tips

Thermo Sudoku is a Sudoku variant where digits must increase from the bulb to the tip of each thermometer. You still follow normal Sudoku rules for rows, columns, and 3x3 boxes, but every thermo line adds an extra constraint.

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What is Thermo Sudoku?

Thermo Sudoku keeps the same 9x9 grid and classic Sudoku objective: place digits 1 to 9 so each row, column, and 3x3 box contains every digit exactly once. The difference is the thermometer shapes drawn on the grid.

Each thermometer has a round bulb at one end and a narrow tip at the other. Digits must strictly increase as you move from bulb to tip. If a thermometer has four cells, for example, you could place 1-3-6-9, but not 1-3-3-9 and not 5-4-7-8.

Thermo Sudoku rules, clearly explained

  1. Classic Sudoku rules still apply: Every row, column, and 3x3 box contains digits 1 to 9 exactly once.
  2. Thermometer rule: Digits must increase from bulb to tip, with no repeats on a thermometer.
  3. Length creates hard limits: On a 5-cell thermometer, the bulb cannot be 6+, and the tip cannot be 4-.

Beginner strategy: where to start

1) Set min and max bounds first

For each thermo line, mark possible ranges by position. On a 4-cell thermometer: bulb 1-6, cell 2 = 2-7, cell 3 = 3-8, tip 4-9.

2) Solve bulbs and tips early

Bulbs must be relatively low and tips relatively high. Combined with row/column constraints, endpoints often collapse to singles quickly.

3) Use classic singles right after thermo pruning

After reducing candidates on thermometers, scan rows and boxes for naked singles and hidden singles.

Intermediate Thermo tactics

Thermo-box compression

If several cells of one thermometer lie inside the same 3x3 box, only certain ordered combinations are possible there. Use that order to eliminate candidates in the box.

Endpoint pressure

When a tip shares a row with many high digits already placed, its options shrink fast. Re-check endpoints after every confirmed placement.

Forward-backward checking

Check forward from bulb and backward from tip for each unsolved thermo to catch contradictions early.

Common mistakes in Thermo Sudoku

  • Treating thermo lines as optional hints (they are hard constraints).
  • Forgetting strict increase (equal digits on the same thermometer are invalid).
  • Ignoring line-length bounds and carrying too many candidates.
  • Over-focusing on one line instead of alternating thermo and grid scans.

Practical solving routine

  1. Mark min-max bounds on every thermometer.
  2. Scan bulbs and tips for immediate eliminations.
  3. Run naked/hidden singles across the grid.
  4. Revisit thermo lines with forward-backward checks.
  5. Re-scan affected rows, columns, and boxes.

Frequently Asked Questions

Is Thermo Sudoku harder than normal Sudoku?

Usually yes. Thermometers add extra constraints, but they also create strong early eliminations.

Do normal Sudoku rules still apply in Thermo Sudoku?

Yes. Rows, columns, and 3x3 boxes still require digits 1 to 9 exactly once.

Can digits repeat on the same thermometer?

No. Thermometers must strictly increase from bulb to tip.

What is the best first step in Thermo Sudoku?

Set min-max bounds on each thermometer position, then prioritize bulbs and tips.

How do I get better at Thermo Sudoku?

Use a consistent loop: thermo bounds first, then singles, then endpoint re-checks.

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