Average monthly bills for a Virginia residential heating customer of Washington Gas Light Co. were boosted by 88 cents to $39.18, effective Wednesday.

Detailing the impact of a $4.8 million rate increase approved for WGL by the State Corporation Commission last week, the utility said residential non-heating customers will face a greater monthly rate rise of $1.29 to an average of $10.28.

The most significant change in montyly bills will not be the total cost, however. Following the lead of regulatory agencies in Maryland and the District, the SCC has approved a totally new rate structure.

Each bill now will show two charges - a monthly "system charge" or SC and a separate charge for actual gas consumption.

The system charge covers most fixed expenses of Washington Gas in serving various types of customers, regardless of how much gas they use. Previously, such charges were included as part of the price of natural gas.

Rates for gas now will be the same for all customers; previously charges allowed for rate reductions per unit of gas as consumption increased.

"The system charge promotes conservation because each additional unit of gas costs the same as the previous one," said WGL President Donald Heim. He said future WGL bills will be lower in the winter and higher in the spring and fall for Virginia gas are being spread out over a longer period.

Heating customers will be assessed a system charge of $9.40 a month for nine months of the year under the new plan. Non-heating customers will pay $4 a month all year, and all customers will pay the same rate for gas - 27.76 cents per therm.