What it is
The Febco 765 (765U with union ends, 765-FZ with a freeze sensor) is a bronze pressure vacuum breaker in ½" through 2", the irrigation workhorse in Febco-spec markets alongside the Watts 800M4. Inlet from below, a spring-loaded check and air inlet in the body, side outlet, two test cocks, and a bonnet that opens to atmosphere when the system loses pressure.
Published width runs 7" at ¾" to 12¼" at 2"; overall height 6½" to 12½", a little more on union-end versions. Then the riser: the PVB must sit 12" above the highest downstream sprinkler head, so the assembly typically stands 18" to 36" above the pad.
What matters for the cover
Height is the install. Measure from the pad to the top of the bonnet. Under about 20" total, the 20 × 20 or 24 × 24 closed-top cover fits the ¾" and 1". Taller than that, or on the 1½" and 2" units, use the 30 × 24.
Leave the bonnet free. The air inlet has to open on shutdown. The cover sits over it loosely.
The FZ freeze sensor needs a lead out of the cover; route it at the base.
Drain before the siege. A charged PVB in a multi-day freeze cracks regardless of cover. The cover is for the surprise frost and the shoulder seasons; the winterization guide has the shutoff-and-drain sequence.
Stocking
Sunbelt irrigation supply branches move these by the case. Stock the 20 × 20 and 24 × 24 heavily by October, add a smaller quantity of 30 × 24 for tall risers and 2" commercial systems. The same covers fit the Watts 800M4, so the bins serve both.
The riser rule of thumb
Measure pad to bonnet. Under about 20 inches, the 20 × 20 (¾") or 24 × 24 (1") closes at the base with room. Over that, or on 1½" and 2" units, the 30 × 24 gives the height. Union-end 765U versions run about an inch taller, and the FZ freeze sensor needs its lead routed out at the base. When the riser is unusually tall or the outlet turns at an odd angle, send a photo and we will cut to it.