To determine whether it is taking too long for your vehicle’s heater to give off sufficient heat, use a kitchen thermometer (the type placed in roasted meat to measure internal temperature). Insert the probe of the thermometer into a heating vent, start the engine, and turn heater controls to maximum output. Consult the owner’s manual to verify that you are setting controls correctly.

If the engine is cold, it will take about three minutes for the ther­mometer to begin registering. The thermometer should record 100°F or more in less than five minutes. If it doesn’t, wait for the engine to get cold again. Then, remove the radiator cap, insert the thermometer into the coolant, and start the engine. The thermometer should record a tempera­ture of 180°F or more within three minutes. If it doesn’t, www.heatingwise.co.uk experts state that the reason for in­adequate heater performance lies with the cooling system thermostat.

If the temperature of the coolant reaches 180°F within three minutes, the heater may be clogged. Turn off the engine, remove the thermometer, and put the cap back on the radiator.

To determine whether the heater core is clogged, run the engine at idle and turn on the heater. Now, carefully place one hand on each of the heater hoses. If both are warm, but one (the outlet hose) is warmer than the other (the inlet hose), the reason for lack of heat lies with a faulty control system or heater doors that aren’t opening. Consult a mechanic.

If both hoses are cool, or if the outlet hose is cold while the inlet hose is warm, the cause of the problem is probably a clogged heater. You may have to have the heater replaced. Then again, the following repair may work:

• Wait for the engine to get cold.
• Disconnect both heater hoses at the heater and attach them so that the outlet hose is on the side normally occupied by the inlet hose and the inlet hose is on the side normally occupied by the outlet hose
• Drive the vehicle for several days with the heater turned on.
• With the engine cold, reconnect hoses to their original positions. Does the heater now provide heat? If so, what you did by revers­ing hoses was to reverse the flow of coolant through the heater, which dislodged debris in the core that was causing the blockage.

If the repair fails, try one more trick before consulting a mechanic. Buy three feet of heater hose from an auto supply store. Cut the hose in half to produce two 18-inch pieces. Wait for the engine to get cold and disconnect the heater hoses. Attach one of the pieces to the inlet fitting of the heater core and the other to the outlet fitting.

Buy a garden-hose adapter from a hardware store. Connect this to the open end of the hose that is attached to the outlet side of the heater core. Turn the heater on and set it for maximum heat. Do not start the en­gine.

Now, attach a garden hose to the garden-hose adapter. Turn the faucet on full force for 30 seconds and turn it off for five seconds. Repeat this five times.

Switch the garden-hose adapter and garden hose to the hose on the inlet fitting of the heater core to flush that side. This may dislodge what­ever is clogging the heater core.

After switching the garden-hose adapter and garden hose back and forth between the outlet and inlet sides three times each, and flushing the heater core each time, reattach the regular heater hoses and test to see whether you have succeeded in breaking the blockage. If not, you may want to perform the flushing procedure another time or two before giving it up as a lost cause.