Education · Load Piping

Load Piping Hydronics

Once you understand how the loops are arranged, there's one detail left to settle: how does each load actually tap into the loop? A coil, a fan-coil unit, a radiant manifold — whatever the load is, it's a flow consumer on the other side of one short connection. That connection point has one decision to make, and that decision shapes everything around it. There are two answers.

Two-Way Valve — Variable System Flow

A two-way valve is exactly what it sounds like: two ports, in and out, sitting in the supply line to the load. The controller modulates it open or closed to vary how much flow reaches the coil. Demand high, valve opens, more water through the coil, more heat (or cooling) transferred. Demand low, valve closes, less water — and here's the part worth pausing on: when the valve throttles down, that flow doesn't go anywhere else. It just stops. The whole branch pulls less, and the system loop's total flow drops with it.

Two-way valve at the load A short section of supply main runs along the top of the diagram; a return main runs along the bottom. A single coil sits in the middle, tapped off the supply main by a vertical riser. A two-way modulating valve sits in the supply riser above the coil; the coil's return drops straight down into the return main. When the valve modulates closed, flow through the coil — and therefore through this branch of the system loop — drops; nothing diverts around it. 2-WAY VALVE COIL supply main → ← return main system flow varies with valve
supply water return water

That's the whole point — and it's why this is the modern default. If every load on the system is two-way, every coil throttling back drops the system's total flow demand. The pump doesn't need to keep pushing the same volume around all day; it can slow down with the building. Pair this layout with a variable-speed pump and you've got a system that uses meaningfully less power at part load, which is most of the day, most of the year. The trade-off: the system pressure-control problem gets more interesting (more on that in the VFDs lesson), and you have to be willing to commit to variable-flow plumbing throughout — boilers, chillers, and the rest of the plant have to be okay with that.

Three-Way Valve — Constant System Flow

A three-way valve has, as expected, three ports. What it adds to the picture is a bypass: a short loop of pipe that lets water get from the supply side to the return side without going through the coil. When the load needs less, the valve doesn't choke flow off — it sends it around the coil instead. The water still moves; it just doesn't pick up (or drop off) heat on the way through. From the system loop's point of view, the branch's total flow stays the same. The pump sees the same demand all day.

There are two common arrangements, and they look different but do the same job. A mixing valve sits at the coil's outlet and combines coil return with the bypass before sending the result back. A diverting valve sits at the coil's inlet and splits incoming supply between the coil and the bypass. Either way, system-side flow stays constant; coil-side flow varies between zero and full.

Three-way mixing valve at the load A supply main runs along the top; a return main along the bottom. A coil sits tapped off the supply main on the left side. A bypass branch runs from the supply main down to a three-way mixing valve at the bottom; the coil's outlet also feeds into that valve. The mixing valve combines coil return water with bypass supply water in variable proportions and sends the result to the return main. As the valve modulates, the coil sees more or less flow, but the total flow leaving through the return is constant. COIL 3-WAY MIXING supply main → ← return main BYPASS system flow stays constant

The diverting version flips the geometry: same loop, different placement of the valve. Supply enters the valve first, gets split between the coil branch and the bypass, and the two paths reunite at a passive tee on the return side. Functionally identical from the system's point of view; the difference shows up in valve authority (how much of the system pressure drop the valve actually controls) and in how the valve body wears under flow. Both are common; which one you'll see on a given job depends on the engineer's preference, the manufacturer's catalog, and what was on the shelf the week the system was built.

Three-way diverting valve at the load Same supply-and-return mains as before, but now a three-way diverting valve sits at the top of the supply riser, just below the supply main. The valve splits incoming supply between two outlets: one feeds the coil below it; the other feeds a bypass branch that runs over to a parallel down-pipe. The coil's outlet and the bypass meet at a passive tee on the return side, and the combined flow continues to the return main. As the valve modulates, the coil sees more or less flow, but the total flow leaving through the return is constant. 3-WAY DIVERTING COIL supply main → ← return main BYPASS system flow stays constant
supply water return water / mixed

Why pick three-way over two-way? Mostly history and pairing. Three-way valves pair naturally with constant-speed pumps and with boilers or chillers that want a steady diet of flow, so older buildings — and any new system whose plant insists on it — lean three-way. The downside is the one you can already see: the bypass is moving water around the building for no thermal reason at all. At part load, the system pump is still doing full work to push water past coils that don't want it. That's energy you'd rather not spend, which is exactly the argument that pushed the industry toward two-way + variable-speed.

One related note worth flagging: even on a constant-flow setup, each load only sees its design flow if the loop is balanced — circuit setters, automatic balancing valves, or pressure-independent control valves (PICVs) at each branch. See Hydronic Balancing for what each of those three is, when you'd reach for it, and how to tell when the loop has drifted out of balance.

Tying it back to the twin-T

If you've been through the primary-secondary twin-T, this is where the loose end gets tied off. That diagram showed two loads on the system loop as plain boxes — supply in, return out, no detail about how they connect. The detail is what we just covered, and the choice has a consequence that propagates all the way back to the loop.

Here's the same twin-T, with each load box opened up. Load A is wired two-way; Load B is wired three-way diverting. (Real systems pick one type for every load on the loop — both are drawn together here so the comparison is in one frame.)

Twin-T system loop with each load type drawn explicitly The same twin-T configuration from the Hydronic Loops lesson, redrawn with the two system-side loads opened up. The boiler primary loop sits on the left with its own pump; the bridge runs across the middle with an injection pump; the system loop runs around the right with a system pump and two loads. Load A is built around a two-way modulating valve in the supply leg; Load B is built around a three-way diverting valve at the supply inlet with a bypass branch around the coil. At the far end of the system loop, opposite the pump, a minimum-flow bypass sits in the supply-to-return crossover and keeps a guaranteed minimum flow through the system pump when the two-way loads throttle down, protecting it against dead-heading on the variable-flow (two-way) side. If all loads on the system were wired like Load A, the system's total flow would vary with demand. If all loads were wired like Load B, the system's total flow would stay constant. BOILER BOILER PUMP INJECTION PUMP SYSTEM PUMP 2-WAY LOAD A 3-WAY diverting LOAD B MIN-FLOW Same twin-T as Hydronic Loops · loads opened up to show valve type If all 2-way: system flow varies → ramp the system pump If all 3-way: system flow constant → fixed-speed pump fits
supply water return water

If the twin-T's loads are two-way (Load A above), the system loop is variable-flow. As loads modulate down, the system pump should slow down with them, and the injection pump may even need to adjust to keep the supply temperature stable as system demand swings.

The variable-flow plumbing has one easy-to-miss piece, marked MIN-FLOW at the far end of the system loop above. A minimum-flow bypass sits opposite the pump and short-circuits a controlled trickle of supply water straight back to the return main, never touching a load. With most 2-way valves throttled closed, system flow can otherwise fall low enough to dead-head the pump against its own discharge — and a VFD system pump, which can only slow down so far, still needs that floor. The guaranteed minimum flow is the pump protection a variable-flow system needs. (It's a cousin of the differential pressure bypass valve covered in the pump-control lesson — same crossover location, but a DPBV opens on a Δp setpoint and is the classic constant-speed-pump fix; under VFD pressure control the drive already caps loop Δp, so the variable-flow device is sized to hold a minimum flow instead.) Three-way systems need neither — the per-load bypasses already keep flow through the pump constant.

If the twin-T's loads are three-way (Load B above), the system loop is constant-flow. A fixed-speed system pump runs all day, and the injection pump becomes the only thing modulating in response to load. Either way, the boiler doesn't care — it's still on its own primary loop, the closely-spaced tees still decouple it — but the system loop has a personality, and the load piping is what gives it one.

That's the difference the twin-T diagram didn't mention. Pick the wrong load type for the rest of your system and you end up with a constant-speed system pump fighting a building full of throttling two-way valves, or a variable-speed pump trying to ramp down on a building full of three-ways that won't let it. The connection point at each load doesn't look like much on a riser diagram, but it determines what the rest of the loop is allowed to do.

See what the bypass does

Three knobs below: a building demand slider that throttles all three two-way loads in unison, a pump type toggle between VFD and constant-speed, and the min-flow bypass itself. Pull demand down with the bypass off and the PUMP state pill tips into WARN and then DEADHEAD — there's nowhere left for water to go. Flick the bypass on and the floor reappears.

Try it: design day · quiet night
Three loads on a hydronic loop with a min-flow bypass at the far end A simplified hydronic loop: a system pump at the bottom left pushes water up into a supply main running across the top. Three loads (A, B, C) tap off the supply main with bowtie two-way valves, drop through coils, and return into a return main running back along the bottom to the pump. At the far right end, opposite the pump, a min-flow bypass connects the supply main to the return main through a single bowtie valve. When the bypass valve is open and the building loads are throttled down, a controlled trickle of water shortcuts the loads and protects the pump from dead-heading. A B C MIN-FLOW SYS PUMP supply main → ← return main
50%
Pump type
Min-flow bypass
Pump OK
Sys flow 30GPM
Pump speed 50%
Bypass flow 0GPM
Pump speed
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