The ranked leader in this 2,510-record dataset holds 60 records, with the fifth-place assignee at 26. However, the top 5 combined account for only 7.0% of all records in scope, so no single company holds a controlling position. A freedom-to-operate review needs to look well past the top few names, since a meaningful share of filings sits with entities outside the ranked leaders.

Filings rose to a peak of 163 in 2019, then eased to 120 by 2024, a 10% decline over the 2021-2024 span. That reads as a plateau after a growth phase rather than a collapse. Figures for 2025 and 2026 are still incomplete because publication typically lags the actual filing date by roughly 18 months, so recent-year totals will rise before they can be judged.

F27D, covering furnace details and accessories, is the largest single class, touching 36.2% of all 2,510 records. F23G incinerators, F23J flue and residue handling, and F22B steam generation follow at roughly 9 to 13% each. Because a single patent can be classified under several IPC subclasses, these percentages overlap rather than summing to 100%, and any competitive search should query multiple classes together.

The smaller classes in this dataset — B01D separation processes at 8.4% and F01K steam and thermal power plants at 8.1% — carry meaningfully less claim density than F27D furnace hardware at 36.2%. That does not make them easy, but it does mean fewer prior filings occupy the space around separation-integrated or power-cycle-integrated heat recovery designs, which is where a narrower first claim is more likely to clear existing art.

China is the largest receiving office by a wide margin at 1,349 records, followed by Japan at 352 and the United States at 168. South Korea, the EPO and WIPO PCT filings trail further behind. A portfolio or clearance strategy limited to US and European filings will miss most of the documented activity, particularly the China-originated furnace and boiler hardware filings.