Using a 240V dryer plug for a tankless water heater is not recommended and could be dangerous. The reason is that the electrical requirements for a dryer and a tankless water heater are significantly different, and using the wrong voltage can lead to equipment damage, electrical hazards, and potential fire risks.
Dryers typically operate on 240V to provide the necessary power for their heating elements. On the other hand, tankless water heaters also require 240V but often need a higher current capacity due to their higher power demands. They are typically rated in kilowatts (kW), and their power requirements can vary depending on the model and the flow rate of water they need to heat.
Swapping out a 240V dryer for a 120V dryer is not as straightforward as it may seem. Dryers that operate on 120V are generally much less powerful and have much smaller heating elements. These dryers are designed for smaller loads and may not adequately dry larger loads of laundry.
Additionally, you mention that you don't know how much kW your circuit takes for the plug used for your current dryer. This lack of information is concerning because it's essential to know the electrical capacity of the circuit to ensure it can safely handle the power requirements of any new appliance, such as a tankless water heater.
If you are considering installing a tankless water heater, it's crucial to have a qualified electrician assess your home's electrical system to determine if it can handle the increased load. They will be able to check the circuit's capacity and make any necessary upgrades to ensure safe and proper operation.
In summary, never attempt to use a 240V dryer plug for a tankless water heater or swap a 240V dryer for a 120V dryer without proper professional guidance. Electrical work should always be carried out by licensed professionals who can ensure the safety and compliance of the installation.