An obscure rider to the Washington Gas Light Co.'s Maryland rate schedule would impose a $30-a-month surcharge on the average customer who reduces his gas consumption by adding an electric heat pump to work in conjunction with his gas furnace.

So far, the company says, nobody is having to pay the surcharge. No matter, says the heat pump industry: The utility is seeking to close off a whole new "add-on heat pump" market in the Washington area that would save $159 on the average customer's annual heat bill.

"Anybody who tries to save natural gas gets penalized - this in itself is wrong," said Robert L. Pearson. Washington area sales manager in General Electric's central air conditioning department. "It's a potential market and a potential natural resource savings that's being kicked in the rear end."

G.E. plans a series of advertisements here in the fall claiming that add-on installation of its heat pump will bring the $159 savings. WGL disputes that there will be any saving at all, but nevertheless is taking no chances on losing gas revenue.

"We're trying to protect our customers," said WGL spokesman Paul D. Young. He said that if some customers are able to use less gas by installing heat pumps, then others will have to pay a greater share of the company's fixed costs.

Young acknowledged that the utility is seeking to forestall development of the add-on heat pump market that G.E. wants to develop and said that the utility probably will try to impose similar surcharges in Virginia and the District of Columbia.

David Didion, who heads heat pump studies at the National Bureau of Standards, said he doubted that an add-on heat pump would achieve the $159 saving with a gas furnace that G.E. claims.

Didion said that gas prices are expected to rise faster than electricity charges in the future. This price spread would then lead to savings for consumers who use add-on heat pumps, he said.

WGL spokesman Young said add-on heat pumps are not an issue now, but that the company is concerned with the concept because "this type of thing is going to be out there in the future."

He said the company is now reviewing its policy on the surcharge, which in Maryland is $15 a month per therm of hourly gas input. The average house. Young said has a gas heater that use between one and two therms.

A heat pump bascially is an air conditioner that also can work in reverse, gathering what little heat there is outside a house during cold weather, concentrating it and pumping it indoors.

Buying and installing a heat pump costs several hundred dollars more than does central air conditioning. G.E. and other companies trying to open the add-onheat pump market find their cutomers among those who have no central air and want it, and those whose central air units need replacement. Heat pumps can be used by themselves to heat and cool houses - and this is where their main market has been.

For these customers, G.E. says, a few hundred extra dollars buys central air a savings in gas hills.

The heat pump market boomed in Washington five years ago following the 1972 natural gas shortage that led to a moratorium on new gas hookups. Next to gas, heat pumps are the cheapest way to heat a house. Oil furnaces became far more expensive as oil prices shot up and straight electric heating, or "resistance heating," became most expensive of all.

When a heat pump is added to work in conjunction with a gas furnace, a special switch operates the tandem setup so that the heat pump functions alone at temperatures down to about 30 degrees Farenheit - the range in which it works most efficiently. Then the gas furnace takes over - alone - for the lower temperatures, at which it is most efficient.

WGL spokesman Young said the utility won approval for the surcharge in Maryland in 1974 when it seemed that "construction people were thinking of adding heat pumps on gas furnaces."