Three weeks ago, in the heart of the gasoline crisis of 1979, Richard and Sue Bloch of Northwest Washington decided to take a gamble.

For several years the couple had entertained the idea of converting the oil-burning heating system in their home to natural gas, but they had shoved any final decision aside because of the approximately $2,000 cost involved.

After hearing and reading recent news reports that hearing oil prices are expected to skyrocket and shortages may occur this winter, however, the Blochs committed themselves.

"it used to be an even bet, but now that's clearly not the case," said Richard Bloch, who had his gas boiler installed in his home Tuesday.

Interviews this week with area utility spokesmen and heating contractors make it clear that the Blochs have company - plenty of it - among home owners suddenly fearful that home heating oil will be either too expensive or too scarce when it turns cold in coming months.

"We've been inundated by calls in the last few weeks," said Susan Butz, a spokesman for Washington Gas Light Co., the major supplier of natural gas in Washington and its surrounding suburbs.

Butz said that since the beginning of May the company has received almost 2,500 requests from converted oil-burners to turn on the gas. In the same period a year ago, WGL got fewer than 100 calls, she said.

Private heating contractors and plumbers tell the same story. "We're swamped," said Jeff Martin, a dispatcher for Flood Plumbers Inc. in Northeast Washington. "We've got 40 or 50 estimates we haven't even looked at."

Martin said the increase in calls started as early as late May or early June, about the same time long lines began to form at area service stations. "We've added one new plumber to help out and we have an ad in the paper for another one."

The sharp rise in demand also has wholesale suppliers of gas-heating equipment scurrying.

"Our company sold more [gas] boilers during a two-week period last month than we sold all of last year," said Larry Dant of Thomas Somerville Co., a wholesaler in Northeast. Dant said his firm sold more than 200 boilers to plumbing and heating contractors last month compared to 20 in June 1978.

"To be quite frank, I've never seen anything like it in all of my 30 years in the business," said Jack Morse of Atlantic Plumbing Supply, another wholesale supplier. Atlantic sold out its 90-day summer stock of boilers in one week in June.

For homeowners betting on natural gas, projected savings on heating costs are considerable.

Richard Bloch said he expects to save between $700 and $1,000 a year on fuel bills, thus recouping the $2,000 he spent on his conversion in two or three years.

The cost of replacing an oil-burning boiler with one that burns gas ranges between $1,200 and 2,300, according to local contractors. Savings vary according to the size of individual heating bills, with the most common range mentioned between $250 and $1,000 a year.

Meanwhile, with the price of heating oil expected to reach 85 cents a gallon this winter - more than double last year's rate - the Washington area's 150,000 home heating oil users could be paying an extra $500 each to keep confortable.

For potential converts to gas, there also is word from Washington Gas Light that a moratorium on new customers is likely. The company has an improved delivery system and more storage space for its gas supply, according to spokesman Butz.

The natural gas industry also foresees plentiful supplies far into the future.

But homeowners who decide to switch to gas still may hit a snag. One area wholesale supplier said he expects substantial markups in the price of boilers over the next several months.

"A boiler costs me from $1,500 to $1,600 right now. By the end of the year that figure will probably be closer to $2,200," the wholesaler said. "I wish we had more storage space, because it would probably be smart to stock up."