A Virginia regulatory agency said yesterday that a Reston water utility's request for a $42,000 rate increase didn't hold water. Neither do some of the company's pipes.

After listening to pleas from Reston Air Conditioning Corp. that it needed more income to repair leaky equipment, the State Corporation Commission voted in Richmond to give the firm a one-year increase amounting to only $38,000. If that amount proves excessive, Reston Air Conditioning will have to make refunds plus 8 per cent interest, the three-member panel ruled.

According to state officials, Reston Air Conditioning, which serves 400 customers in the Lake Anne section, was losing 800 gallons of chilled water every hour. Some of the loss was due to leaks, but company officials also said some resourceful residents had figured out how to get free service.

Company officials said cheaters were opening a sealed valve that permits chilled water to enter their houses for cooling, then diverting the used water to a drain instead of letting it circulate back to the system for re-cooling. Only company workers are authorized to open the valves, but an SCC spokesman said any resident with a monkey wrench could do the job.

Reston Air Conditioning pledged to fix leaks and try to halt cheating, but its detective work won't be easy. The valve are inside the house, and the company can't inspect them without permission.

Ordinarily the state agency which is in charge of approving utility rate increases deals with giant companies like Virginia Electric and Power Co., which has 1.2 million customers in the state. But all day Wednesday and for half of yesterday's session, the SCC dealt with the tiny Reston utility, which has never expanded beyond its limited base in the 15-year-old town.

Under the temporary increase, the five-month cooling season charge for residents will go from $14.01 to $16.67 monthly and for commercial customers from $24.52 to $26.61.