Can an existing forced hot water baseboard heating system be converted into an underlayment (tubing) type of system?

Thank you.

Brice Repolt
Weare, NH

Yes but it must be modified to run at a much lower temperature which if not done properly can cause condensation in the boiler section area and damage the boiler. Usually it is better to use a new style boiler that is designed for that purpose. You can also use a tempering tank design.