Bristol's Floating Harbour was an industrial inferno waiting to happen. Wooden ships loaded with valuable cargo lay packed alongside warehouses storing timber, sugar, tobacco, and tar. A single fire could leap from vessel to vessel, then up the ropes onto a quayside warehouse, and from there across an entire dock. Land-based fire engines could not reach the water side of a moored ship. So Bristol, like London before it, took to the water. The first recorded British fire-float was rowed out of London in 1765. James Hilhouse built Bristol its first in the 1780s. By 1884 the Fire Queen, built by Shand Mason of London, was steaming around the City Docks with a three-cylinder steam pump. The Salamander followed in 1905 at Avonmouth. Steam fire-floats were powerful, but slow to start: the Salamander once arrived at a fire at 8:36 a.m. for a 7:30 a.m. callout, having spent the intervening hour raising steam while the shore appliances did the work.

In 1931 the Bristol Fire Brigade decided steam was no longer good enough. Diesel engines started immediately, took no firemen to stoke them, and could be controlled remotely. Two new diesel fire-floats were ordered. Charles Hill & Sons of Albion Dockyard, Bristol, built both. The first, Bristol Phoenix II, was launched in 1934. She had two Petter Atomic diesels delivering 55 brake horsepower each, driving twin Merryweather reciprocating pumps that could throw 500 imperial gallons of water per minute. Her crew of three worked from a Prince Street Bridge river police station. The engineer stood below in the noisy engine room, taking telegraph orders from the wheel, controlling each engine's speed and direction by hand. In 1938 a second Bristol vessel was found to be called Phoenix, which was not allowed on the ship register, so the fire-float was renamed Pyronaut. The new name was thought up by the teenage son of the chairman of the Watch Committee.

Then came the war. Beginning in November 1940, Luftwaffe raids targeted Bristol relentlessly. The docks, the aircraft works at Filton, the city centre itself were repeatedly hit. The Bristol Blitz killed roughly 1,300 people and destroyed huge stretches of medieval Bristol. Pyronaut was manned twenty-four hours a day, working through the worst raids alongside two motor launches fitted with fire-fighting gear. The Floating Harbour burned in patches almost continuously. Warehouses collapsed. Bonded stores of spirits went up in white-hot pillars. Pyronaut pumped salt water from the harbour onto whatever the call directed her at, often returning to base soaked through and her crew exhausted, ready to be called out again before they had dried. She survived the war intact. Many of the buildings she protected did not.

Postwar work was less frequent but still serious. In February 1948, during pantomime season, the Bristol Hippodrome caught fire. Pyronaut pumped water from St Augustine's Reach while firefighters waded through the wreckage rescuing soaked costumes, including the comedian Sid Phasey's dress suit. A reporter described 'clusters of men soaked to the skin, their eyes red-rimmed with the smoke and fumes.' She fought oil fires at Avonmouth in 1951, pumping continuously for two days. By the late 1960s her equipment was obsolete and she was re-engined in 1968 with Ruston & Hornsby diesels and Coventry Climax centrifugal pumps that doubled her output. But Bristol City Docks closed to commercial traffic in 1975, and by then very few quayside buildings were beyond the reach of land-based engines. Pyronaut was sold in 1973. A private owner began converting her to a divers' boat, then a saloon for an Irish coastal cruise, before selling her back to Bristol City Museum in 1989. Restored by 1995, she now lives outside M Shed on Prince's Wharf, listed on the National Historic Fleet, performing at harbour events. In June 2012 she travelled to London by road to take part in Queen Elizabeth II's Diamond Jubilee Pageant on the Thames, alongside a thousand other historic vessels.

Pyronaut is normally moored outside M Shed at 51.4481°N, 2.5981°W on Prince's Wharf, on the south side of Bristol's Floating Harbour just east of Prince Street Bridge. From the air she is a small black-and-red vessel berthed alongside the museum's larger tugs Mayflower and John King and the replica caravel Matthew. Recommended viewing altitude 1,500-3,000 feet. Bristol Airport (EGGD/BRS) lies 7 nautical miles south. The Floating Harbour stretches west toward Hotwells and east toward Temple Meads station; the SS Great Britain lies a short distance west on the same harbour.