A 100ft fire took over five hours to be extinguished by firefighters at a barn in Talachddu, Brecon.
The incident happened at around 5.30am on Thursday (August 4) after a bale of hay set alight due to the recent warm weather.
Mid and West Wales Fire and Rescue Service says a similar fire was caused in the area only a week ago.
With recent high temperatures, hay bales are said to have retained much of the heat and have much higher moisture content than usual, causing them to get extremely hot.
Last month, a farmer was caught on camera racing to save his straw bales as a raging fire ripped across his fields.
No one was hurt in the Talachddu fire and the service says no significant damage was caused, but with warmer conditions, these incidents could become more frequent.
Crews from Brecon, Llandrindod Wells, Haye On Wye, and Builth Wells tackled the blaze, which contained around 150 bales of hay.
Welsh Water says around 60,000 litres of water was used to put out the fire. The service also had the help of the National Grid, which isolated the electric supply to barn.
Crews continued to extinguish and dampen down and left the incident with the owner to monitor at around 10.53am.
Richie Vaughan Williams, the Station Manager for Mid & West Wales Fire and Rescue Service, said: "This morning’s incident demonstrates the time and resources required to effectively deal with these types of fires safely and effectively, often requiring the help and expertise of other agencies.
"This is why we maintain focus on preventing these incidents by working within our rural communities to provide free assessments, guidance, and advice via our Farm Liaison Officers and other partners.
"These services are readily available and we strongly encourage our farming community to contact us should they be unsure about stack testing or any other farm fire safety issues."