Detectives are hunting gang members who poured boiling water over a restaurant boss to get a code to his safe.
The 48-year-old victim was tortured inside his own home after answering his door to a man claiming to have a delivery for him, Scotland Yard said.
Three attackers ransacked his property in Deptford, south east London, for valuables before telling him to open a safe.
When the victim refused, the gang beat and kicked him before dragging him into a basement where they boiled a kettle and poured it over him, police said.
Detectives leading the inquiry have been unable to trace the suspects since the attack on 22 December.
The offenders brandished a mallet and what was thought to be an imitation firearm during the raid.
The Vietnamese man was burned on the face, neck and body but his attackers fled empty-handed after failing to obtain the safe code.
"The victim stated that he didn't know it and, as a result, he was further assaulted by having hot water from a kettle poured over him," a spokesman said.
"As the victim was being assaulted, he feigned unconsciousness, and when the suspects stopped the assault he attempted to get help.
He managed to shout through a vent in the basement and alert a passing neighbour who asked if everything was OK.
"At this point the suspects fled the house empty-handed, running out through the front door before jumping into a vehicle and driving off.
"The neighbour raised the alarm and called police."
All three suspects are described as black men, about 5ft 11in and aged between 20 and 25. They had English accents.
The victim required hospital treatment but has since been discharged.
Officers in Lewisham urge anyone with information to contact them on 0208 284 7959 or Crimestoppers, anonymously, on 0800 555 111.