Is it possible to fix a leak in PP-R piping by injecting some type of substance?
As Steven mentioned how do you know you've fixed it? Also, do you want to circulate sealing compound through your boiler?And damp under a house can cause a lot of problems. If you want to know the exact location of the leak and under the floor is totally inaccessible you could consider using a thermal imaging contractor to find the leak to localize damage to the flooring and [this site] (http://www.ekoplastik.com/?download=ekoplastik_ppr_mont_en.pdf) has information on repairing of PP-R pipe in page 27.