Rural fire stations have been out of action for as much as five months of the year, figures obtained by the Western Morning News reveal.
Fire chiefs admit there is a problem with daytime cover and say they are struggling to recruit part-time firefighters in at least half a dozen towns in Devon and Somerset.
The Fire Brigades Union (FBU) said part-time staff – who are to replace six full-time crews under "devastating" cuts made last month – cannot provide the "resilience" needed.
The 83 fire stations in the two counties were "off the run" for a total of 2,085 hours, or 86 days, a snapshot of down-time for three weeks last month has revealed.
Stations recorded as many as 37 separate incidents where appliances were not available to attend an emergency call – Modbury was out of action for a third of the 21 days and 41% of last year.
Porlock, on Exmoor, was down for eight days, and last year could not raise a crew for 3,266 hours – equal to more than four months.
The FBU said the figures reveal the dangers of relying on "on-call" crews, and councillors in Plymouth fear a major incident could stretch the service beyond its limit.
Assistant fire officer Trevor Stratford said the breaks were "intermittent", adding that managers had a "great deal of experience" managing what is the largest fleet of appliances in England.
He said 34 stations were never down and the overall availability was 95%.