Hoping to chip away at the image of the Capitol Power Plant as an iconic symbol of the nation’s reliance on dirty fuels, Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Harry Reid (D-Nev.) on Friday announced that coal will no longer be used for heating Capitol complex buildings and water.

Instead, they said, natural gas will serve as the Capitol’s sole fuel source for steam.

Stephen Ayers, the acting Architect of the Capitol, said in a letter to Democratic leaders that coal will only be burned going forward for backup capacity. He said this may be the case if heating needs exceed the capacity of the natural gas pipeline serving the complex, if abnormally cold conditions place higher-than-normal demands on the plant, or if equipment outages on the gas boilers require a backup.

“The Congress of the United States should not only be a model for the nation, but also a good neighbor. As we green the Capitol, moving the Capitol Power Plant from the 19th century to the 21st century is a crucial step,— Pelosi said.

Reid said the switch to natural gas “shows that the House and Senate are leading by example in reducing our emissions.—

The plant is the No. 1 source of air pollution in the District of Columbia and has been the focal point for criticism from environmental and public health groups. In February, Pelosi and Reid called for switching the plant to natural gas in the lead-up to a scheduled protest against coal combustion at the plant.

Ayers previously has said the cost of switching completely to natural gas is $7 million.