A big gas utility in Michigan is testing five devices ranging in cost from $1 to $53 that could be installed on home furnances and flues with a savings of natural gas consumption exceeding 25 per cent.

But local zoning laws will have to be changed and the gas industry's program for certifying such devices as safe will have to be speeded up if the Michigan example is to become more than a test project.

The company involved is Michigan Consolidated Gas Co., a distributor of natural gas to about 1 million customers in Michigan and generally regarded as the industry's leader in promoting conservation - as called for by President Carter in his address to the nation Wednesday night.

According to John Turko, the company's director of special projects, Detroit-based Michigan Consolidated began a program of offering better insulation for its customers in 1973, a plan since modified for use in some other parts of the country and now the foundation for a broader national effort suggested in a recent Federal Energy Administration study.

To date, the Detroit firm has added insulation to 112,000 homes for an annual savings of 3.3 billion cubic feet of gas, enough to heat 16,000 more homes per year.

A more recent program urged customers to cut their home temperatures by a few degrees, much as the President has suggested since he took office two weeks ago.

The firm decided that because natural gas consumption could be cut back so successfully, home heating systems must have excess capacity, Turko recalled during recent testimony before a House subcommittee. So in the summer of 1975, Michigan Consolidated began testing home heating systems to determine if this were true.

"We found that most furnances were oversized for the homes they heat and that a majority of chimneys are likewise oversized, thus allowing excessive amounts of warm air to escape," Turko told Rep. John D. Dingell (D-Mich.) and his subcommittee on energy and power.

Specifically, Turko said the study found that oversized chimneys were prevalent because build-codes were adopted when most homes were heating by coal-fired furnances that required large chimneys and flues for better combustion of coal and to assure strong drafts to leiminate soot build-up.

Bigger-than-neccessary furnances were installed because energy used to be cheap and plentiful and builders wanted to avoid a possibility of complaints about inadequate heating, Turko testified.

Armed with these conclusions, Michigan Consolidated set about finding how to produce more efficient heating.

Five such devices were found and tested:

A flue restrictor or "blastgate," an adjustable metal plate installed in the flue pipe to reduce its inside diameter. The device costs less than $10.

A $5 automatic damper, which electronically closes the flue when the furnance flame is off.

A two-step input valve for furnances. Attached to the gas line, it varies the furnance flame with weather conditions. It costs about $43.

A small-diameter furnance burner orifice which costs less than $1 and which permanently reduces the size of the furnance flame.

An electronic pilot light, costing $53, which replaces a constantly burning gas pilot.

These relatively inexpensive devices generate significant savings, Turko said, the flue restrictor and small-diameter burner orifice reduced gas consumption by 26 per cent; a slightly higher fuel savings can be achieved by using the audomatic damper and flue restrictor in combination, but the materials cost is higher.

In terms of cost efficiency, the flue restrictor-reduced orifice combination would make "the cost economic sense while achieving considerable fuel savings," he said.

Nevertheless the Michigan Consolidated officer said he cannot conclude that the test results are definitive. Complete reliability and safety of the devices must be established, he cautioned. Testing is continuing at 191 Michigan sites during the current heating season. Results will be verified by Battelle Memorial Institute and Environmental Research Institute of Michigan.

Turko noted that any permanent introduction of devices that alter flue sizes will require changes in building codes of virtually all Michigan communities.

Charles R. Montgomery, President of the Detroit utility, said he is convinced that the devices could cut natural gas consumption for homes by one-fourth, but he warned that state and local governments and federal agencies must help.

"Conservation needs to be added to effective efforts to increase natural gas supply and move ahead with supplemental supply projects . . . I do not know whereelse we obtain that much gas for an equivalent investment," Montgomery said.

But there is a hooker that could destroy such efforts, he added. If a utility in one region embraces conservation and ends up with significant reductions in consumption, the government could order the gas saved to be shipped elsewhere to regions where no such conservation exists.

"There is no benefit to a utility from conservation if it loses gas from other sources . . . the loss of supply will cut (the local firm's sales) and increase the customers' rates as fixed costs are spread over fewer units," he asserted.

In response to such a threat, Sen. Robert P. Griffin (R-Mich.) recently introduced what he called "finderskeepers" legislation to allow utilities to keep and reallocate on their own any gas they "find" through conservation.

Under current law, the Federal Power Commission can order supplies diverted from gas-rich states to those with shortages. Without a "finderskeepers" provision, "it is doubtful many utilities would want to participate." "The risk would to be too great that the benefits of their investment . . . would be reaped elsewhere."