We have a gas fireplace and that we think that CO2 is coming in through the vent pipe outside from our neighbor's building. We had a tester for CO2 that had readings of CO2 around the fireplace inside our house. Will the test pellets work to see if CO2 is coming in through the gaskets in the fireplace?
These test pellets can indeed be used inside a sealed appliance to determine if airflow is pushing past a gasket or seal. This is the best way to troubleshoot for the CO2 spillage that you suspect.
Can this product be used effectively in a free standing pellet stove? Can you briefly describe how they would be used?
These test pellets have a variety of uses. These can be placed within any free standing or built in appliance when checking for chimney/venting system leaks. The pellets release a dense smoke that will penetrate any gaps in the venting system and allow you to check for leakage.
For a pellet stove, I would advise inserting and burning the pellet at the exit of the venting system on the back of the stove. You will of course need to have the combustion fan operating in order to propel the smoke and create positive pressure in the vent system. Depending on your stove, you may be able to run the fan independently, however you may also need to have the stove burning for this check.