Metro commuters know that when summer arrives, they may face a ride even hotter than a D.C. sidewalk.
Now they have proof.
The air conditioning units on Metro's trains failed on average more than once per rail car last year, according to a new Metro report slated to be presented to the board of directors Thursday.
In 2011, Metro had 1,830 air conditioning failures out of 1,104 rail cars in its active fleet, which means some cars' units failed again even after they were repaired.
But the transit agency says that's an improvement from the past. The year before, the agency had 2,390 failures, which translates to an average of two failures per car, according to the data.
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"Last year, we saw a 23 percent reduction in HVAC issues across our fleet, but we are not resting on that success," Metro spokeswoman Cathy Asato said in an email. "We will continue to work on improving the reliability of railcar systems to ensure that our customers enjoy a comfortable ride."
The issue is all-too-familiar for regular riders during the Washington region's hot months when the cars can feel like saunas.
The problem even prompted one rider to create a blog tracking the hot cars at FixWMATA.com.
"There's a large problem facing all the rail cars," said Chris Barnes, who is tracking the hot cars for a third season. "They definitely were not intended to handle heat."
Barnes' catalog is not scientific because riders report the outages via Twitter. Metro is not involved. And riders could be noticing a car that has repeat failures -- or they may be reporting the same car repeatedly before a fix is made.
Still, his findings reflect Metro's own numbers. Barnes received more reports last year on the CAF 5000 series than any of the other five models of rail cars, with 36 percent of the total complaints even though the cars represent just 17 percent of the fleet. Last year, he said riders reported one car from that series 11 separate times.
Metro acknowledges those cars have the worst record, despite being the system's second newest cars. Last year, those cars averaged more than three failures each. The oldest and second-worst performing cars, the Rohr 1000, averaged two failures each.
Metro is replacing all the components in the 1000 and 5000 series air conditioning units, Asato said. The agency also plans to start work on the BREDA 2000 and 3000 series cars next week. It is nearly two-thirds of the way through a separate seasonal tune-up of all the cars, as well.
Metro's rail fleet varies in age and make but are all set to regulate temperature between 68 and 72 degrees through automatic sensors.
However, the agency historically has had a tough time keeping the temperatures constant. The trains let in the outdoor heat and cold each time the six sets of doors per car open at a station. Sun on the above-ground sections of the system also heats up the cars.
Then there's the perennial problem for the 36-year-old rail system: age. "The age of the various components of the HVAC systems is the main cause of the failures," Asato said.