Danfoss AVPQ (003H6539) — self-acting differential pressure and flow controller, PN 25, DN 15, for district heating and central heating systems. The AVPQ combines a control valve with an adjustable flow restrictor and an actuator with two control diaphragms; it closes on rising differential pressure or when the set maximum flow is exceeded, holding both a stable differential pressure across the controlled circuit and a limited maximum flow so connected control valves keep their authority. This AVPQ variant is mounted in the return pipe. Differential-pressure setting 0.3 – 2.0 bar adjustable via handle; kVS 1.6 m³/h; flow rate 0.03 – 0.9 m³/h. Maximum differential-pressure range 20 bar, media temperature 2–150 °C, cavitation factor z ≥ 0.6, leakage 0.02 % kVs. Valve body red bronze CuSn5ZnPb (Rg5), valve cone dezincing-free brass CuZn36Pb2As, seat stainless steel 1.4571, control diaphragms EPDM. Connection external thread G ¾ A (ISO 228/1) via tailpieces (ordered separately); supplied complete with compression fittings for Ø6 mm copper impulse tube. Media: district heating/cooling circulation water; glycolic water up to 30%. OEM P/N 003H6539. Technical data quoted verbatim from the Danfoss AVPQ/AVPQ-F data sheet and store.danfoss.com (code 003H6539). differential-pressure range 20 bar); Temperature range: 2 – 150 °C (heating/cooling water); glycolic water up to 30%; HS code: 8481.80. Used in commercial refrigeration, provision-room cooling and reefer plant rooms across HFC, HFO and CO2 (R744) refrigerants.