13. Rensair Hacking the air, to bring you a cleaner, safer and more sustainable working environment. Written by Helena Young Sponsored by Sage Published on January 12, 2026 About the Startups 100 Now in its 18th year, the Startups 100 is the definitive list of the most promising new UK businesses. There's no fee for entry or for inclusion in our index. The Startups team of new business experts judge all our top 100 entrants in collaboration with specialist industry consultants. See our guide to this year’s hottest new businesses and most exceptional founders in the complete 2026 Startups 100 index. Founders: Christian Hendriksen and Frederik HendriksenYear founded: 2020Website: rensair.comBeing able to breathe freely is a subject matter near and dear to founder Christian Hendriksen’s heart. Having grown up with asthma, Christian’s father (an HVAC engineer, the system which controls indoor heating, ventilation, and air conditioning) designed and built a portable air purifier to help him breathe more easily.That tech sat dormant for a number of years until Christian Hendriksen and his brother decided that technology was too good to let drift in the wind. After updating and patenting the tech, Hendriksen had turned a childhood issue into a global solution: supporting the basic human right to clean air.It might be a shock to hear, but indoor air quality is actually two to five times worse than outside air. Why? Because of poorly looked after HVAC systems in commercial buildings. But simply tearing out and replacing these old hunks of metal isn’t a viable solution, it’s too difficult and too costly. That’s why Rensair wants to hack your HVAC.Rensair can hack old-fashioned HVAC systems with its high-quality air filtration technology. Its cloud-connected tech integrates with the existing HVAC system, rather than replacing it completely, to provide cleaner air in a more cost-efficient and simpler way.Thanks to its patented Smart Demand Controlled Ventilation (SDCV) tech, Rensair is the only company that can cut building ventilation energy use by 40% while improving indoor air quality.Rensair has already worked with the NHS (the health organisation became the startup’s biggest client during COVID), which marked a crucial step. As Hendriksen explains, “from the very beginning, the NHS was vital to our journey. It was not only an early customer, but also a powerful reference that gave other organisations the confidence to work with us.”Rensair continues to make powerful strides, having already raised $7m in a Series A round led by Hoxton Ventures, and gaining a foothold in the US. Rensair works with one of the largest senior living real estate investment trusts in the world, Welltower, with an aim of protecting senior residents who would be vulnerable to poor air conditions.We can all breathe a (clean) sigh of relief that Rensair is rocketing along in its goal of providing safe and clean air to our offices, universities, hospitals and transport, and reducing global carbon emissions by 6.5%.View the full Startups 100 Index for 20261. Omnea2. HIVED3. MAGIC AI4. OXCCU5. Lightyear6. Neuranics 7. Hormona8. Burbank9. Better Dairy 10. Lenkie Technologies11. Userled12. Hoxton Farms13. Rensair14. PulpaTronics15. TradeKart16. 32Co17. Carmoola18. cheqd19. Sightline Analytics20. Adamo Foods21. Bold Bean Co22. Beams Renovation 23. Pan Galactic24. Capi Money25. Spotted Zebra26. ANTHROTEK 27. Unravel28. Perci Health29. Untap Health30. Hair Syrup31. Connect Earth 32. spacebands 33. Unfabled34. Lightning Reach35. STOCKED36. Embeddable37. XR Therapeutics38. Literal Labs39. Scooch40. Openr41. Fibe Limited42. Inicio AI43. Peachies44. Better Nature 45. Ctrl Alt46. Resting Reef47. Nila48. ROXFIT49. Finmile50. NavLive51. Volunteero52. Inntelo AI53. Zonova54. A&B Smart Materials55. Ethicronics56. Treeapp57. kennek58. Soil Benchmark59. Vox Aeris60. Asan Cup61. Nuuri62. Mindstone63. MYNDUP64. Enbiosis Biotechnology65. JUX Food66. Danu Robotics67. Wylde Market68. Air Aware Labs69. Mimicrete Ltd70. ALYVE71. Happl72. Living Things73. Grub Club Pets74. Mealia75. PetProov76. Lylo Products77. Chest78. ZIM Connections79. Uptime Labs80. Teal81. Zero Fintech82. Micro Thermal Energy 83. Kanjo 84. Merlin Cloud85. My Skin Feels86. Eleria87. Cashblack88. RevvedUp89. Cosysense90. SuperFi91. Loom92. Legacy Compass93. Meshed94. SWURF95. Main Squeeze96. Sampl Technologies97. Over the Moo98. Pioneering People99. Bikebook100. Recondition Share this post facebook twitter linkedin Tags 2026 Written by: Helena Young Deputy Editor Helena is Deputy Editor at Startups. She oversees all news and supporting content on Startups, and is also the author of the weekly Startups email newsletter, delivering must-know SME updates straight to their inbox. From interviewing Wetherspoon's boss Tim Martin to spotting data-led working from home trends, her insight has been featured by major trade publications including the ICAEW, and news outlets like the BBC, ITV News, Daily Express, and HuffPost UK. With a background in PR and marketing, Helena is particularly passionate about giving early-stage startups a platform to boost their brands. That's one reason she manages the Startups 100 Index, our annual ranking of new UK businesses. Sponsored by: Sage Startups 100 Index Sponsor 2026 Sage exists to knock down barriers so everyone can thrive, starting with the millions of Small and Mid-Sized Businesses served by us, our partners and accountants. Customers trust our finance, HR and payroll software to make work and money flow. By digitising business processes and relationships with customers, suppliers, employees, banks and governments, our digital network connects SMBs, removing friction and delivering insights. Knocking down barriers also means we use our time, technology and experience to tackle digital inequality, economic inequality and the climate crisis.