South Yorkshire Fire and Rescue Service plans to introduce Close Proximity Crewing (CPC) at up to six stations over the coming years, including Aston and Maltby, to help save £10million by 2015.
The fire service said the shifts would involve working 9am-5pm for four days and include sleeping at the fire station to ensure cover for a particular area.
But the Fire Brigades Union (FBU) says the scheme is outside firefighters terms and conditions.
The announcement comes as the union has accused managers of multiple breaches of health and safety legislation over the imposed introduction of small vans to replace properly equipped and crewed fire engines.
The Fire Brigades Union has served a Safety Critical Notice warning them of their liability if a member of the public or a firefighter is injured or killed.
But the fire service says Small Incident Units have been in service for two years, with no reported accidents or near misses.
The units are commercial vans painted fire service red with blue lights and high-visibility stripes. They have limited water capacity and are operated by a crew of three firefighters.
Firefighters are unhappy about plans to introduce a unit at a Sheffield station.
John Gilliver FBU brigade secretary said: “Fire professionals know that every large fire starts as a small fire and we don’t always know what we will find when we arrive. Firefighters will be put at intolerable risk at incidents if they are deployed in these vans.”
The fire service said: “We are extremely disappointed the FBU will still not allow its members to carry out the necessary training for the Small Incidents Unit, which has passed a series of rigorous and independent safety examinations and has responded to more than 700 fires successfully since August 2009.”
“We have always stated the SIU will not be mobilised to any type of premise fire or vehicle fire.”
Meanwhile, deputy chief fire officer Mark Shaw said the introduction of CPC will provide the same level emergency cover to the public while making a saving of £375,000 a year, per station.
He added: “We appreciate that this system of working isn’t for everyone but for those who do choose to volunteer there are significant benefits in relation to pay and flexibility.”
Firefighters at CPC stations are provided with accommodation and recreational facilities, and are allowed visitors during stand-down time. They are also able to self-roster shifts, working an average of 38 hours a week and on close-proximity stand-by for around 43 hours a week.
Mr Gilliver said the scheme was outside of firefighters’ terms and conditions.
He added: “It’s doubling the hours and decreasing the staff by half.”
“Twenty nine per cent might sound brilliant, especially in these austere times, but it comes at a price and that is less fighters.”