Zone Valves vs Zone Circulators for Hydronic Heat

In Caleffi's four-zone example, one ECM circulator with four zone valves draws an estimated 51 W at design load, while four ECM zone circulators draw 96 W to deliver the same 20,000 BTU/h. With zone valves, the open question is the pump: on a fixed-speed Taco 007 in our example below, the last zone left open runs 3.86 GPM, against 3.07 GPM with all four open and 2.5 GPM by design.

So the choice comes down to what the single pump does as zones drop out, and what each zone pump needs if you go the other way. Below, one four-zone baseboard system goes through both layouts with a fixed-speed pump, a pressure-regulated ECM pump and a pump on every zone. Every version starts from each zone's design GPM and head, and for copper zones our free circulator pump size calculator gives both from the zone load and the pipe run.

Should you zone with valves or with circulators?

In Caleffi's idronics 36 four-zone example, zone valves on one ECM circulator draw less power at design load than four ECM zone pumps. Most zone valves run on 24 VAC, and valves suit low-flow circuits of 0.5 to 2 GPM. Zone circulators keep the other zones heating when one pump fails, and each pump can be picked for its own zone's flow and head.

Everything in the table comes from sections 2 and 3 of idronics 36, which Caleffi published in fall 2025. Caleffi makes zone valves and the relay controls for both layouts, and the issue covers each layout in turn.

The question comes from an r/Plumbing thread. An owner building a house in northern Alberta planned in-floor heat in the basement and garage, a coil in an air handler and maybe a water heater, all on an outdoor wood boiler about 100 ft from the house with electric backup. The owner's father, a plumber, had said a pump on every zone was a waste. The replies split three ways: one ECM pump with zone valves, a pump per zone so one failure doesn't take out the house, or several 1/2" radiant zones on one pump with the air handler on its own. We come back to that house at the end.

What happens to zone flow when zone valves close?

With a fixed-speed pump, each valve that closes raises the pressure across the pump, and the zones still open take more water. In our four-zone example, the last open zone on a Taco 007 ends up 26% above its all-open flow. On the steeper Taco 008 the jump is 65%, and that zone runs at 1.9 times its design flow.

Here's the example, our own estimate:

1. Four identical baseboard zones, each 25,000 BTU/h at a 20°F drop. That's 2.5 GPM per zone and 10 GPM in total.
2. Each zone is 200 ft equivalent length of 3/4" Type M copper, the same zone length Caleffi uses in idronics 36, with one zone valve rated at a Cv of 3.5.
3. The headers and near-boiler piping add up to 40 ft equivalent of 1-1/2" Type M. We left out the head loss of the cast-iron boiler, which is low.
4. Tube losses come from Caleffi's Formula 4-2 for smooth tube with 140°F water (idronics 16). Valve losses come from the Cv formula in idronics 36, which gives 1.18 ft for this valve at 2.5 GPM.
5. Taco prints its 007 and 008 curves only as a chart (submittal 101-029, June 2015). We read the points off it.

At design flow, one zone loses 2.93 ft in the tube and 1.18 ft in the valve. Add 0.33 ft for the common piping at 10 GPM and the system needs 4.43 ft at 10 GPM. Read off Taco's chart, the 007 makes about 7.35 ft at 10 GPM, more than the system asks for. Even with all four valves open, it settles above design flow.

Header pressure is the difference between the supply and return headers, the pressure every open zone sees. On the 007 it climbs 53% from four zones to one. On the 008, whose chart starts at about 15.3 ft of head at no flow against 9.8 ft for the 007, it climbs to 2.5 times its starting value, and the last zone runs at 1.9 times its design flow.

Caleffi idronics 36 § Controlling differential pressure in systems with zone valves

"In an ideal system, there would be no change in differential pressure across the circulator as the number of active zone circuits changes." - Caleffi, caleffi.com

Caleffi lists what that pressure rise can cause: flow noise and metal erosion from high velocity, idle zone valves whose springs can't hold the paddle shut, and cavitation in some systems. In our example even 4.74 GPM in 3/4" Type M is 2.94 ft/s, under the 4 ft/s that idronics 16 gives as the ASHRAE-suggested noise limit for tube 2" and smaller. Smaller tube hits the limit much sooner: idronics 16 puts 4 ft/s in 1/2" copper at 3.2 GPM.

How do you hold differential pressure steady with zone valves?

Use a pressure-regulated ECM circulator set for constant differential pressure, or proportional mode when the supply and return mains lose real head. With a fixed-speed pump, pick one with a flat curve or add a differential pressure bypass valve across the headers. In Caleffi's five-zone example the bypass starts to open 0.5 to 1 psi above the all-open pressure, 3.1 to 3.6 psi in our 007 example.

Caleffi gives four ways to keep zones from overrunning:

1. Constant differential pressure. The ECM circulator slows down as valves close. In Caleffi's example the pump holds 6.5 ft while flow drops from about 11 GPM to 7.8 GPM when one of four valves closes. Caleffi calls this mode ideal when the common piping loses little compared with the zone circuits, which is why our ECM column above still drifts from 2.50 to 2.60 GPM.
2. Proportional differential pressure. The pump's head target falls with flow, to half of the full-speed setting at zero flow. Caleffi points it at systems with real head loss in the supply and return mains, such as two-pipe direct return.
3. A fixed-speed pump with a flat curve. Operating points still climb as valves close, but less. Caleffi's wording is blunt: fixed-speed circulators with steep pump curves "should not be used in systems with zone valves."
4. Differential pressure bypass valve. With a fixed-speed pump, the valve sits across the headers. In Caleffi's five-zone example it's set to start opening at 0.5 to 1 psi above the all-zones-open pressure. Size it where the pump curve crosses the valve's curve with every zone closed. Caleffi's 519 series sets from 2 to 10 psi, so the 3.1 to 3.6 psi we get by applying that setting to our 007 example sits inside that range.

The zone valve has two ratings to check. They pull against each other. A higher Cv loses less head, and the close-off rating drops as Cv goes up. Caleffi's 3/4" Z-one valves show the trade:

Caleffi's conservative check is the pump's head at zero flow converted to psi. That's about 4.2 psi for the 007 and 6.6 psi for the 008 from Taco's chart, well under the 20 psi of even the largest valve.

How much power does each layout draw?

Caleffi estimates four ECM zone circulators at 24 W each, 96 W with every zone on, against 51 W for one ECM circulator of the same model at 39 W plus four zone valves at 3 W. Per watt of pump and valve power, that's 208 against 392 BTU/h of heat delivered, so the valve layout moves about 88% more heat per watt.

Caleffi's example zones are 200 ft equivalent of 3/4" copper delivering 5,000 BTU/h each with 130°F supply water, so these are design-load estimates, not metered numbers. For the PSC zone circulators in the 26-pump system shown in the issue, Caleffi estimates 75 W each. Taco's sheet for the PSC 007 lists 0.74 A at 115 V and no wattage. At Caleffi's 75 W, four PSC zone pumps come to about 300 W with every zone calling, by our arithmetic.

Fixed-speed PSC pumps also have a federal deadline. DOE's circulator pump rule, published in May 2024, sets a maximum circulator energy index (CEI) of 1.00 for clean-water circulator pumps, other than submersible and header pumps, manufactured in or imported into the U.S. starting May 22, 2028, under 10 CFR 431.465(i). DOE expects manufacturers to reach that level with ECMs, and notes that some induction-motor models with automatic variable-speed controls stay on the market. The rule goes by date of manufacture or import. Pumps already in service aren't touched.

What does a zone circulator system need to work?

Every zone needs a spring-loaded check valve, which Caleffi puts at about 0.5 psi of opening resistance, so hot water can't thermosiphon into idle zones or flow backward through them. Keep the headers short and size them for 2 ft/s with every zone on, and pick each pump for its own zone. In our example a Taco 007 on each 2.5 GPM zone runs about 4.5 GPM before the check valve's loss.

Here's what each of those means on the job:

• Check valves. Caleffi says a typical swing check can't stop thermosiphon flow, and many wet-rotor zoning circulators come with a spring-loaded check in the discharge port. If a zone uses an older weighted-plug flow-control valve, count it in the zone's head: idronics 16 lists a 3/4" B&G flow-control valve at 83 ft of equivalent length, 42% of our 200 ft zone.
• Headers. Caleffi sizes them for a maximum of 2 ft/s with all zones running, so size them for the flow the pumps actually move. At our 10 GPM design, the hydronic pipe size calculator with 100,000 BTU/h, a 20°F drop, Type M and a 2 ft/s limit picks 1-1/2" at 1.75 ft/s. The page also notes that 1.75 ft/s is under the 2 ft/s Caleffi gives for carrying air bubbles to the separator, and idronics 36 draws the air separator on the headers. Four 007 zone pumps move about 17.9 GPM before check valve losses, which is 3.1 ft/s in 1-1/2" and 1.8 ft/s in 2" Type M, by our arithmetic. Even in 1-1/2" the common piping loses 0.91 ft, and each zone drops only from 4.71 GPM alone to 4.47 GPM.
• Pump size. Those 4.47 to 4.71 GPM, before check valve losses, are 179% to 188% of the 2.5 GPM design, because a 007 is far more pump than one 2.5 GPM zone needs. Caleffi notes that zone circulators don't have to be the same make or model, and multi-speed pumps can be set lower for small zones.
• High-resistance heat sources. Caleffi gives certain modulating condensing boilers and hydronic heat pumps their own circulator and a hydraulic separator, with either zoning layout.

Which layout fits the house in the thread?

It depends on each circuit's GPM and head, and the post doesn't give them yet. If the floor zones share one supply temperature and similar head, one pressure-regulated pump with zone valves can carry them. A coil or water heater that needs much more flow or head can get its own circulator, since idronics 36 says zone pumps don't have to match.

A second thread shows the failure to avoid. In a Northeast Illinois house, one pump fed two radiant zones and an unused third through zone valves, a second pump fed the indirect water heater, and the controller ran both pumps on any call. With only the water heater calling, the radiant pump was pushing against closed valves. The owner measured 228°F on that pump with an infrared thermometer and heard it screeching. Both pumps had already been replaced after failing in February, and the owner guesses the water was boiling inside the pump. Caleffi notes that most valve systems run the circulator only when at least one zone is open.

For the house in the first thread, here's the order we'd work in:

1. Get each circuit's load, design temperature drop and GPM. Enter copper circuits in the circulator pump size calculator. It covers water in Type L and M copper only, so take the head of PEX floor loops, or of any glycol loop, from the tubing and fluid makers' data. For one of our example zones, enter 25,000 BTU/h, a 20°F drop and 200 ft of straight 3/4" Type M, and set Check valves, the other fitting counts and Other equivalent length to 0. The page gives 2.50 GPM and 2.97 ft of head. It uses the Copper Tube Handbook's cold-water friction, which is why it reads a little above the 2.93 ft from Caleffi's 140°F formula.
2. For a zone valve layout, put the valve's loss in the field "Head loss of boiler, emitters and valves at design flow". Our Cv 3.5 valve adds 1.18 ft, and the zone comes to 4.15 ft.
3. Group circuits that share a supply temperature and similar head on one pump with zone valves. Give a circuit with much higher flow or head its own circulator.
4. With a fixed-speed pump, check two points on its curve, all zones open and the smallest zone alone. If the smallest zone alone runs over 4 ft/s, or the header pressure gets near the valves' close-off rating, pick a flatter curve, an ECM pump in constant or proportional mode, or a differential pressure bypass valve set just above the all-open header pressure.
5. Check that each zone valve's close-off rating is above the pump's head at zero flow, converted to psi.
6. With zone pumps, fit a spring check in every zone and size the headers for 2 ft/s at the flow the pumps actually move.
7. Wire it so no pump runs while all of its valves are shut.

Start with the zone numbers. The circulator pump size calculator gives each copper zone's GPM and head, and the hydronic pipe size calculator sizes the headers at 2 ft/s.

Sources

• Caleffi, idronics 36, Hydronic Zoning Options, Fall 2025, caleffi.com
• Caleffi, idronics 16, Circulation in Hydronic Systems, January 2015, caleffi.com
• Caleffi, 519 series differential pressure bypass valve instruction sheet, consolidated-supply
• U.S. Department of Energy, Energy Conservation Standards for Circulator Pumps, final rule, 89 FR 44464, May 20, 2024, govinfo.gov
• 10 CFR 431.465(i), circulator pump standard, ecfr.gov
• Taco, Submittal Data 101-029, Model 007 Cartridge Circulator, June 10, 2015, neuco.com
• r/Plumbing thread with the question, "Zone valves or pumps on each zone", reddit.com
• r/hvacadvice thread, "How do I get my zone pumps to only run when there is a heat call from that zone?", reddit.com