Building codes, fire codes, insurance companies, and company policy all play a role in this. Plastic fill and vent pipes are against code due to the fact that they are flammable, and during a fire they will burn, causing the fuel in the tank to become exposed to flames. The fuel that is in these pipes from the delivery process, will over time weaken the glue used to join pipes. There also has been times where PVC pipes going out a wall has broken off inside the home due to the movement of foundation walls from the frost and caused spills. A vent alarm (whistle) is used by our delivery personnel to signal them when your tank is at the safe fill level. Most tanks are in basements and cannot be seen by the driver. Without a vent alarm, a spill may occur causing damage and costly cleanup to your home; they also allow for proper expansion of the fuel.