As the cost of home heating rises, more homeowners think of firewood as a way to cut costs. They also think of a chainsaw, a way to cut that firewood. But most folks know little about buying and using one.

Most firewood work involves trees less than a foot in diameter. That's great, because it means you can get by with a small saw Probably the best all-around tool for cutting cordwood is a lightweight saw with a 14-inch bar (see sketch). Power enough to zip through the hardwoods you'll want to use as fuel, yet so light (around 10 pounds) that it won't wear you down.

When you shop for saws you'll see a variety of convention and safety features. Most are worth the extra money if you cut a lot of wood.

One is automatic chain oiling, which sees to it that your chain always gets enough lubrication. Without it you have to oil the chain manually by pushing a plunger every half-minute or so. If you forget - and that's easy to do - the chain may be ruined. At $15 a crack, chains are worth protecting.

Two good safety features are the chain brake and the tip guard, both designed to prevent injury due to kickback when you touch the tip of the cutting bar to tree or log. The saw jumps up and back toward your head. The chain brake instantly stops the chain. The tip guard deters kickback by, simply covering the tip of the bar so you can't accidentally stab it against a log. But the tip guard also gets prevents you from cutting logs as thick as the length of the bar or thicker. This forces you to remove the guard and leaves you without protection.

Another worthwhile feature is a vibration isolation system. Without it you may feel tired and dizzy after a long cutting session, and your hands may itch, tingle and shake.

What can you expect to pay? About $120 for a basic saw, maybe below $100 during a clearance sale or close to $200 for a saw loaded with features. If you pay $120, the saw will pay for itself when you've used it to to cut three cords of wood. You can easily recover the cost of a saw in a single year.

A chain saw is a remarkable efficient tool, but it is just about the most dangerous tool you can lay hands on. Be sure to read your owner's manual carefully.

A reputable dealer will probably hel p you get the feel of the saw before you take it home. He may have logs on hand for demonstration purposes. He should also show you how to use a sharpening file and guide, a vital accessory in cutting hardwood. Don't expect this kind of service from a discount house.