The D.C. Public Service Commission yesterday granted Washington Gas Light Co. a $7.2 million, 8 percent rate increase that could raise the average monthly gas bill of a residential heating customer in the District of Columbia by more than $3.

At the same time, the commission reduced the monthly "system charge" -- the charge that a customer pays whether or not he uses any gas -- from $8 to $5 a month for residential heating customers and from $3.75 to $2.50 a month for residential customers who use gas only for cooking and water heating.

The company had requested a $10.9 million, 12.7 percent increase for its 150,000 customers in the District of Columbia.

"The need for the increased rates... is for the most part beyond management's control," the commission said in the 105-page decision issued yesterday. "The company's legitimate expenses have continued to spiral with unabated inflation."

The new rates go into effect Feb. 23 unless the commission orders otherwise.

Yesterday's decision was not final. Melvin J. Washington, the commission's special counsel, said yesterday that the commission has 30 days to change its mind on any part of the decision. He said, however, that this was not likely.

The system charge was reduced "to soften the adverse impact that a fixed charge each month has on smallvolume customers," the written decision said.

The commission pared down Washington Gas' request by disallowing company-claimed expenses for advertising, trade association dues, business and civic memberships and community affairs programs.

The commission ordered that WGL profits from some propane sales and settlement of a lawsuit be passed on to ratepayers, rather than to stockholders as the company had requested.

Yesterday's decision held WGL's overall rate of return on investment to 9.25 percent. The company had requested a rate of return close to 10 percent. The rate of return is what the company pays in dividends and interest to holders of its stocks and bonds.

Although the systems charge was lowered by yesterday's decision, the charge for a therm of gas for residential and most commercial classes of customers was raised from 21 cents to 32.6 cents.

In December 1978 an average District of Columbia WGL customer with gas heating used 172.4 therms -- a measure of gas -- and paid a bill of $60.37, according to WGL. This average bill would have increased by $4.60 to $64.97 under the new rates, according to calculations by The Washington Post using WGL statistics.

The average bill for a gas heating customer in D.C. during 1978 as a whole was $41.17, according to WGL. That would be raised by slightly more than $3 a month under the new rates, according to calculations performed by The Washington Post using WGL statistics.

WGL had not received a copy of the decision as of the close of business yesterday and a spokesman said the company was not immediately able to calculate the impact of the rate change on an average customer.

Any party to the case adversely affected by yesterday's decision may file opposition to the decision and a hearing will be held Feb. 22 to consider any such opposition, the decision said.

Yesterday's decision was signed by commission chairman Elizabeth Hayes Patterson. Commissioner William R. Stratton will file a dissent if yesterday's decision is made final, the decision said.