Refiners are maintaining normal shipments of heating oil, diesel fuel and jet fuel to their wholesale customers while rapidly increasing their own stocks to meet the Carter administration's 240-million-barrel goal in October, according to a Department of Energy survey.

A separate telephone survey of a national sample of residents of single-family homes using heating oil or kerosene found that more than four-fifths said their oil tanks were more than half full in early August, and that they had more oil in their tanks than they did a year ago.

Energy Secretary Charles Duncan, in an interview, said the two surveys were a further indication "we have done a reasonably good job of building supplies" before the winter heating season begins.

Duncan said the 240-million-barrel goal should be met by the middle of October and that he had no expectation that stocks would rise much past that point.

Expressing concern about the price of heating oil, which has risen nearly 40 cents a gallon in the last 12 months, Duncan noted, "Some people have raised the possibility there will be some moderation on prices." He said he had been urging in direct contacts with refiners that they exercise "moderation" in pricing.

"The best way to keep the price down is to keep supplies up," he added.

Some oil industry experts believe the large stock of heating oil could lead to a price decline before the winter is over.

Duncan, responding to fears that excessive supplies of heating oil and diesel fuel could lead to storage problems affecting next year's gasoline supplies, pointed out that gasoline stocks are "20 million barrels higher than they were last year."

The survey of refiners' shipments covered the four-month period from May through August. During that time another DOE official said, 342.2 million barrels of middle distillates were shipped, almost the same as last year.

"It's coming through the system." the official declared.

Last year refiners shipped 343.6 million barrels during the same four months, and only 330 million barrels in 1977.

Many members of Congress, state and local government officials and some oil jobbers have expressed concern that refiners would try to meet the 240-million-barrel goal by holding on to supplies they normally would have sold and shipped.

The National Oil Jobbers Council claimed earlier this month that as of June 1, retail dealer and customer tanks, which held an estimated 70 million barrels of heating oil in 1978, had only 54 million barrels in 1979.

"Comments by heating oil dealers indicate that the shortfall in dealer tanks has not improved since June 1 of this year," Edgar Morgan, a Council vice president told a congressional committee.

DOE officials are skeptical whether the oil jobbers' survey is statistically valid, however.

The residential survey was an outgrowth of a national study conducted by the Energy Information Administration of energy use. Nationally, 4,849 households were questioned.

Of that group, 830 used heating oil or kerosene, including some in apartment buildings and mobile homes. To get more detailed information about heating oil stocks in single-family homes, all of those within the 830 with telephones were resurveyed.

The surveyors reached 87 percent of that group, or 438 residences altogether. Despite its small size, DOE officials say it is a statistically valid sampling.

Taken together, the two surveys indicated "secondary and tertiary stocks were pretty good," Duncan said. Secondary stocks are generally those at fuel oil dealers, and tertiary those in the tanks of the ultimate users.

Duncan defended the administration's handling of the fuel oil situation as a good but not perfect job. "Who manages everything perfectly?" he asked.

"The fact is, we were tens of millions of barrels behind (on heating oil stocks) and now we've caught up," he said.