If Rube Goldberg were still with us, he'd probably be drawing diagrams of energy-saving contraptions for the home. And perhaps some sharp operator would be trying to sell the gismos to a gullible public.

Actually, some devices on the market can cut utilities costs, but any homeowner should ask two questions regarding any gadget that is supposed to do so: Does it really work? Is the cost worth it?

For example, is it advisable to pay $80 (plus plumber's fees if you aren't a do-it-yourselfer) to replace a perfectly good toilet with a water-conserving model? Placing a few bricks or a bleach bottle or two (filled with water) in the toilet tank will accomplish the same purpose at no cost.

On the other hand, who wouldn't argue that paying even a couple thousand dollars to convert a home heating system from oil to natural gas would pay for itself within several years -- much sooner if you merely convert your oil furnace to gas without replacing the entire heating plant. And no shutoff of Arab oil supplies or truck driver's strike would affect natural gas availability.

For example, Peoples Gas Co. estimates that the current yearly average cost of heating a six-room Chicago house with natural gas is $619. Heating with fuel oil costs about $1,200, and fuel costs have been rising far faster than natural gas costs.

The cost of replacing an oil burner with a gas burner in a furnace is about $500. Installing a new gas furnace could cost $3000.

But natural gas costs also are rising steadily, a fact that has spawned a variety of furnace flue devices designed to eliminate heat loss. Without such a device, as much as half the furnace heat may go up the chimney. With a flue heat-recovery device, the loss is not as great, but some models may not be worth installing on a particular furnace.

The devices recover some of the excessive exhaust heat headed for the chimney and convert it to usable fuel.

Consumers Union tested several of the flue heat-recovery devices and concluded that fuel savings might not be sufficient to make the $100 to $200 (plus installation in some cases) purchase price worthwhile.

The reclaimed heat -- no more than 6 percent among the devices tested -- won't benefit an area more than 20 feet from the furnace. Flue heat recovery devices work best on "incurably inefficient furnaces, if you need to heat an area within 20 feet of the basement of if you annual heating costs are so high that your saving will justify the investment," CU reported in its book, "Money-Saving Guide to Energy in the Home" (Doubleday & Co., $3.50).

One device that shows more promise, say the Consumer Report engineers, is the flue damper. Their tests showed that a flue damper, which can cost from $150 to $350, can save 10 to 30 per cent on fuel bills.

The damper is a fairly simple device. When the hermostate calls for the furnace to go on, the damper opens; when the furnace shuts off, the damper closes, thus trapping the warm air and preventing it from escaping to the outside.

Flue dampers have been used in Europe for 47 years, CU points out, but in this country experts are divided as to their safety. The American National Standards Institute warned that if the damper didn't open fast when the furnace went on, there would be a danger of gas spewing through the house.

However, last year the American Gas Association Laboratories gave its approval to certain flue damper models and set up a training program for service technicians.

One sure method of saving money on heat bills is to dial down the thermostat several degrees at night or before you leave for a trip and turn the heat back up in the morning or when you return.

If it's inconvenient to do that yourself, or if you're forgetful, purchase a timer thermostat to do the job. Prices of clock-type timers run about $40 to $80, but they could be worth it.

There has been talk lately about turning off the pilot lights on the kitchen gas range to save energy. Consumers Union says that won't save much, maybe 20 to 40 cents a month. And a pilot lights are rated as efficient sources of heat for the kitchen.

As for conserving water, Consumers Union found a massive array of water-saving gadgets and shower heads. They ranged in price from 13 cents for nylon washer inserts for the shower mounting collar to $20 for an aerating shower head.

There are gadgets galore for conserving toilet water. They include dams ($3.95 to $6.95), which block off the tank's lower portion; dual flush units ($11.99 to $14.95), which move in opposite directions to flush liquid and solid wastes, and devices to close the tank valve more quickly ($4.98 to $7.98).

The average toilet uses six to eight gallons of water with each flush, compared to 3-1/2 to six gallons used by the water-saving model. The latter range in price from $43 to $81 and Consumers Union found that only six of eight models tested did an adequate job of swirling away the waste.

Consumers Union also lists a number of gismos to avoid, including:

Transient surge supressors. A change to throw away $100 or so. The supressors are designed to hold down sudden surges of electricity in a power line, such as occurs when an appliance or power tool is switched on. Scientists say such surges occur but last only one-thousandth of a second. Governments of several states are attempting to halt sales of such suppressors.

Energy monitors. A gadget (one sells for $125) that tells you how much electricity you are using any time you look at it. It reminds you how much power you are using and what it costs but it won't save you any money.