AS MORNINGS turn chilly, the question of home heating bills is rapidly descending from the theoretical to the uncomfortably practical. The surge in oil prices draws attention to disparities in the fuel markets that previously passed unnoticed. The federal Bureau of Labor Statistics has begun to publish city-by-city fuel oil prices. At the time of its most recent survey, in mid-August, the price of a gallon of heating oil was 10 cents higher in Washington than in Baltimore. Why?

The Washington area depends heavily on a couple of oil suppliers who stocked up on extremely expensive supplies at last spring's shortage prices. As that oil moves through their inventories, prices here will probably drop a little toward the national average. But that's not the whole explanation. Householders in the Washington area are, on the whole, a bit more affluent than their counterparts in Baltimore and, it seems, a bit less attentive to fuel prices. The fuel market has been less competitive here than it is there. Washington has been an extreme example of the American inclination, as long as incomes were rising and fuel was cheap, to pay little attention to heating costs. But all of that is now changing -- and fast.

Oil-heated houses in this area typically use around 1,000 gallons of fuel a year, according to our own rough estimate. That's a little less than the national average because of the mild winters. At present prices it would cost $850 a year, compared with $525 a year ago. Electric resistance heating is even more expensive. But most houses that depend on electricity now use the heat pump -- in effect, a central air conditioner running in reverse. The utilities here say that, for home heating, their customers spend an average of about $450 per winter for gas and, if they have efficient heat pumps, perhaps as little as $400 for electric heating.

One warning: these figures are inexact, and they do not apply to precisely the same kinds of houses. Houses heated by oil tend to be older, leakier and bigger than houses heated by gas and electricity. Another warning: because fuel oil prices are uncontrolled, last spring's price increases have already fully reached the consumer. Gas and electricity prices are regulated, and increases there will be fed through the system gradually and continually for months and years to come. But it's probably a fair summary to say that, at present, it costs about a third more to heat with oil than with gas or an electric-powered pump.

Further warning: those broad averages conceal the most interesting -- and most useful -- message in these numbers. One house may use only one-fourth as much fuel as another of similar size. The explanation is the type of construction and, above all, insulation. The investment of $200 and a little Saturday labor can typically cut fuel costs by one tenth -- a better return than you are likely to get on the stock market. With several thousand dollars' worth of work, the owner of the average house can cut fuel consumption in half.

This fall, the issue for many householders has been whether to stick with fuel oil or to switch to the utilities for heat. Over the longer future, everybody is going to feel the same steady upward pressure of prices, and the main question will be how best to insulate.