Up until it was time for him to preside over the House during the emergency natural gas bill debate yesterday, speaker Thomas P. (Tip) O'Neill Jr. was wearing a baby blue V-neck cashmere sweater with "New England Patriots" written on it.

O'Neill was cold.

The day before, he had invited reporters to hold their daily press conference in his Capitol office because "it will warm the place up. It's freezing over there."

Actually O'Neill's shivers must have been more psychological than real - like the feeling that everybody speaks with a Southern accent in Washington these days - because the thermometer in his Capitol office read a cozy 72 degrees.

The mandate that under the current cold wave-natural gas crisis, the temperature in federal office buildings beset at 65 degrees during the daytime to conserve fuel was literally being carried out on Capitol Hill. Thermostats were set at 65 degrees.

But like much of the legislation Congress writes, between the intention and the effect there was a difference of several degrees.

In the House press gallery the thermometer said it was 74 degrees. In a first-floor corridor between the House and Senate it was 75. On a second-floor corridor outside the Senate chamber it was 68.

In the Senate chamber, an engineer's reading said it was 65. In the House chamber, during the Debate, it was 63.

In the hearing room of the Senate Commerce Committee, which produced the emergency natural gas bill, it was 72. In the House Commerce Committee room, it was impossible to tell the temperature, because the much-criticized Rayburn House Office Building, along with its many other delightful quirks ,has no thermostats.

In any case, whatever the temperature in the Capitol, no natural gas, which is in such short supply that the emergency bill was necessary, was being conserved or used. The Capitol power plant burns coal, occasionally supplemented by fuel oil.

Ray Carroll, the director of engineering in the Capitol architect's office, blamed the variances in temperature throughout the Capitol on the effect of "body heat, electrical lights and sunlight."

Since the 1973 energy crunch, there has been a policy of heating the buildings to 68 degrees, he said, so that under the new order, thermostats were only set back three degrees.

"We won't attempt to monitor each member of Congress's office," but he added that O'Neill and Senate Rules Committee Chairman Howard W. Cannon (D-Nev.) had sent out a "Dear colleague" letter asking that thermostats be turned down to 65 in the daytime and 55 at night.

But Carroll added that if the temperature does rise over 65 degrees due to natural causes, there is no effort to force it back down. One exception, however, came a few days ago, when Sen. Jennings Randolph (D-W. Va.) waved a thermometer in the Senate chamber showing it was over 70. Then, Carroll said, some cold air from the outside was allowed to flow into the air system to force the temperature foen yo 65.

If it's any consolation to consumers faced with enormous heating bills in this record cold winter, their bills can't compare with the Capitol's.

For 4,479 tons of coal at $43 a ton, and 251,940 gallons of fuel oil at 40 cents a gallon the December heating bill of the Capitol came to $293,373.