Kronmiller started to work at Time-O-Stat Controls Company in October of 1929, which merged with the former Minneapolis-Honeywell Regulator Company in 1931, and is now Honeywell, Inc. He was granted 71 U.S. patents in many different fields, either as the sole or a joint inventor. Early in his career, he designed the multi-wall cement bag that is still universally used, and the machinery to produce it.

The safety device is for gas burners using standing pilots where the thermocouple must be heated by the pilot flame before the main burner valve can be manually opened. In July, 1949, a patent was issued for a safety device he developed for gas burners. The gas valve could be safely opened to furnish fuel to a pilot burner and a bimetal or flame-responsive device such as a thermocouple would then be heated. When the pilot flame ignited, the main gas value could be opened to allow gas to flow to the main burner. If the pilot flame went out, the flame-responsive device would cool off and unlatch the mail gas value to close off the gas supply. This invention became a standard in the industry and was recognized as important in making gas furnaces and gas waters safe. Nearly all constant gas burners for furnaces and waters heaters today make use of this invention.

In 1940, he started work on a new thermostat that would use sealed contacts, so household dust and grease would not foul them. The round thermostat was developed over 12 years, interrupted by World War II. Kronmiller encountered high cost and reliability problems and the project was shelved until solutions were found. After its perfection and introduction, the round thermostat was an immediate success. The T86 Thermostat was introduced to the market in 1952 and is substantially the same as the Honeywell Round today. By the time it was introduced in 1953, a total of $633,500 had been spend on development. In was an expensive gamble, but the Round became a winner. By 1987, over 55 million Round Thermostats had been sold.