Sunlight hitting a mirror on one of the hottest days of the year set curtains at a house in Devon alight, it is believed.
Ten crews tackled the blaze in Brownston Street in Modbury at about 18:05 on Sunday.
It is understood the blaze started in a back bedroom before spreading to the eaves. No one was injured.
The flames were so intense they started to melt a firefighter's helmet, which withstand temperatures of 300C (570F).
Modbury Fire Station said on Facebook: "So intense was the heat that on initial entry, one of the first firefighter's helmets melted.
"Our PPE (personal protective equipment) is designed to withstand extreme temperatures to tolerate our working conditions. Our helmets are temperature resistant to 300C [570F]."
Devon and Somerset Fire and Rescue Service said two fire engines stayed at the property overnight to monitor it and dampen down hotspots.
Another station involved in the call-out, Buckfastleigh, said it was also called out to a fire at the old school building on the King Edward VI Community College site on Sunday afternoon.
Firefighters said a portable toilet was set on fire and flames spread to the building it was next to.
Crews from Buckfastleigh, Totnes and Paignton brought that blaze under control, the Buckfastleigh station said on Facebook.