Topaz Differential vs. PartFill: Same Mechanism, Opposite Goals

Look at a Topaz Differential and a Topaz PartFill side by side and they look almost identical — both use a weighted float that rides up and down a cord rather than a rigid arm, both mount through the tank wall with a Detach Long Tail, both create an adjustable gap between fill and shut-off instead of a simple full/empty cutoff. It's an easy pair to mix up. But they're built to accomplish opposite things, and picking the wrong one means the valve technically works while quietly fighting your system the entire time.

The Shared Mechanism

Both valves use the same basic idea: a weighted float travels down a cord as the tank drains until it hits a bottom stop, which opens the valve for full flow. As the tank refills, the float rises back up the cord until it hits an upper stop, which shuts the valve off. The distance between those two stops — the differential — is adjustable on both valves, which is what separates them from a standard Topaz that just goes full-open or full-closed at one level.

Differential: Built to Protect a Pump

The Topaz Differential exists for one specific problem: tanks fed by a pressure-controlled pump. Without a wide gap between fill and shut-off, a pump can end up rapid-cycling — starting and stopping over and over as the water level hovers right around the shutoff point. That's one of the leading causes of premature pump failure. The Differential's adjustable range — 50mm up to a full 2.5 meters — lets you set a wide enough gap that the pump runs in long, infrequent cycles instead of short, wearing ones. The goal here is keeping the tank reliably full while babying the equipment that fills it.

PartFill: Built to Keep a Tank Mostly Empty

The Topaz PartFill is solving the opposite problem. It's purpose-built for rainwater harvesting tanks, where the whole point is leaving capacity open to catch the next rainfall rather than topping the tank off from mains water. Instead of maintaining a full tank, PartFill only opens to maintain a low reserve level — just enough that services fed by the tank keep running through a dry spell. As soon as rain starts refilling the tank naturally, the valve simply stays closed and gets out of the way. Its differential tops out around 150mm (depending on pressure), a much narrower range than the Differential's, because the job here isn't managing a big swing — it's a small safety net at the bottom of the tank.

Where the Confusion Actually Costs You

Install a Differential on a rainwater tank and you'll end up topping the tank off with mains water far more often than intended, defeating the purpose of harvesting rain in the first place. Install a PartFill on a pump-fed tank expecting it to behave like a standard full-service valve, and you'll be running the tank chronically low — exactly what it's designed to do, just not what you wanted from a primary water tank.

One More Difference Worth Noting

PartFill includes a lock-off switch for maintenance, letting you shut the valve down without touching the main supply. The standard Differential doesn't list that feature — if serviceability without a full shutoff matters for your install, that's a point in PartFill's favor for anything within its lower flow range.

The Short Version

If a pump fills your tank and you're trying to protect it from rapid cycling, that's Differential. If your tank collects rainwater and you want a low-level backup for dry spells without defeating the purpose of harvesting, that's PartFill. Same mechanical DNA, entirely different jobs — worth double-checking before you order, since a valve that installs and runs fine is easy to mistake for a valve that's actually doing what you need.