Vent cleaning is likely classified as a service. Replacing HVAC system pipelines is likely classified as construction. For support, see DFARS 222.402-70(c), which states: "Contract construction, alteration, renovation, painting, and repair requirements (i.e., roof shingling, building structural repair, paving repairs, etc.) are subject to the Construction Wage Rate Requirements statute." HVAC replacement is likely covered in that description. The contracting officer should use his/her business judgment based on available guidance and document the contract file accordingly with the determination.