QD3 vs QD4 Quick Disconnects: Choosing the Right Size for Your Loop

Liquid Loop Lab · Practical liquid-cooling engineering

Quick disconnects are the most under-budgeted pressure drop in a liquid loop. A pair of no-spill QDs can cost as much pressure as metres of tubing — and the wrong size class can quietly eat a third of your pump's head before the coolant ever reaches a cold plate. "QD3 or QD4?" is really a sizing question, and it has a method. This guide gives you the method, the reference numbers for the standard size classes, and the one distinction — branch flow vs loop flow — that decides most real installations.

What the names mean

Koolance QD3 and QD4 are the two no-spill quick-disconnect families most bench and PC-scale builders meet first. The QD3 is the compact class, built around smaller tubing (10 mm / 3/8 in class, with 13 mm variants); the QD4 is the larger-bore class (13 mm / 1/2 in and up), trading size and cost for substantially lower pressure drop at higher flow. Both seal on both halves when disconnected, so a pump or a load comes out of the loop without draining it.

OCP UQD is the data-center standard (Open Compute Project Universal Quick Disconnect) used at server and rack level. It comes in four sizes — UQD02, 04, 06, and 08 — with defined hose-barb terminations, rated flows, and maximum spill volumes, so parts from different vendors mate and behave the same. If you are building toward rack hardware, the UQD table below is your reference frame; if you are building a bench loop, QD3/QD4 are the buyable equivalents and the same sizing logic applies.

The reference numbers (OCP UQD, from the public specification)

All UQD sizes: maximum operating pressure 100 psi(g) / 6.89 bar(g); minimum burst 300 psi(g). Pressure drop follows ΔP = A·Q² + B·Q per the specification — which is the mathematical way of saying it climbs fast. Source: OCP Universal Quick Disconnect Specification (public draft, v2 series). For Koolance QD3/QD4 pressure-drop curves, use Koolance's published data for the exact part number — the class logic below tells you which family to start from.

The selection method

1. Start from the hose, not the fitting bin. The QD's barb or compression size must match your hose ID. An adapter stack to force a smaller QD onto a bigger hose just moves the restriction into the open — the smallest bore in the path sets the pressure drop.
2. Keep design flow at or below the class rated flow. Rated flow is the class's working neighbourhood. A 6 L/min loop is at home in the UQD04 / QD3 class; at 12 L/min you are past UQD06's rating and should be in the largest class or above.
3. Budget the drop — per mated pair, then multiply by pairs. A connection point is a plug and a socket. A typical bench loop has two connection points (pump and load) — that's two pairs in series, twice the single-pair drop. And because ΔP rises with roughly the square of flow through a fixed bore, doubling flow through the same QD costs about four times the pressure. This is why "one size smaller, it'll be fine" is the most expensive sentence in loop building.
4. Check spill volume against how often you'll disconnect. Even the largest UQD class spills at most 0.07 mL per disconnect — a fraction of a drop. For service-friendly builds, spill is a solved problem at every size; pick on flow and drop, not on spill.

The distinction that decides real installations: branch flow vs loop flow

A CDU moving 150–300 L/min does not push that flow through one pair of quick disconnects. The big flow stays in the manifold; each server or cold-plate branch sees only its own share — typically a few litres per minute — which is exactly the UQD02–UQD06 neighbourhood. Size the QD for the flow at that connection, not for the nameplate flow of the system it belongs to.

The reverse trap catches bench builders: a single test loop at 28.8 L/min (our 10 kW worked example) exceeds even UQD08's rated flow. At that point the honest options are the largest QD class run knowingly above rating with the ΔP budgeted from the curve, parallel QD pairs, or a larger industrial coupling class — not a small QD and hope. The Loop Builder maps each hose size to its QD class so the grocery list never mixes them up.

So, QD3 or QD4?

• Choose the QD3 class when branch/loop flow is up to roughly 6 L/min and hose is 3/8 in class — compact, cheaper, and the drop is affordable because the flow is small.
• Choose the QD4 class when flow runs past roughly 10 L/min per connection, when the pump's head budget is tight (small circulators have little to spare), or when the hose is already 1/2 in or larger — the bigger bore pays for itself in recovered pressure.
• Choose UQD when the loop must mate with data-center rack hardware — it's the interoperability standard, in blind-mate (UQDB) form for slide-in servers and manual form for manifolds and bench connections.

Where to buy

Supplier searches for the current families (links open the supplier's search — confirm the exact barb/thread variant before ordering): Koolance QD3 on Amazon · Koolance QD4 on Amazon · No-spill couplings on Grainger · No-spill couplings on Zoro