Finding a wasp nest anywhere on or around your home requires immediate removal. Ones that are built in your chimney require quick attention because the insects have easy access into your home. The best time of the year to kill wasps is in June because a colony will have been established, but won't have grown large yet. Unfortunately, that also makes the nest harder to spot. Wasps have difficulty flying when the temperature drops below 10 degrees C so wait until nightfall or a cool day to kill them.
Using a flashlight, look up into your chimney to find the location of the nest. With luck, the nest will be closer to either the bottom or the top of the chimney, making later removal of the nest easier.
Put on a layer of protective clothing. Wasps become aggressive when provoked so you'll want at least a few layers of clothing between you and the wasps. Also put on a pair of protective gloves, safety glasses and a face mask that covers your mouth and nose. The last two items will help ensure that you don't inhale the poison or have any of it drop into your eyes.
Spray the nest with a projectile spray poison. The projectile spray will release a stream of poison 15 to 20 feet long so you'll be able to reach any location within your chimney. Depending on the location of the nest (top or bottom of the chimney), you might need to go onto the roof in order to reach it.
Check the nest the following day to see the level of wasp activity. If the wasps remain, spray it again two or three times. Continue applying the wasp killer daily until you no longer see any activity in the nest.
Remove the nest. Use a pole or the end of a broomstick to knock the nest loose from the side of the chimney. Dispose of the nest.
Always wear protective clothing when dealing with a wasp nest. If the nest is close to the base of your chimney or within the fireplace, you can use WD-40 in place of a wasp killer. Fumes from the WD-40 are enough to kill the wasps. A large infestation of wasps in your chimney will need to be removed by an exterminator.