What it is
Most homes don't want the whole house heated as one block — the bedrooms, the living area, and the basement all want their own thermostat. Zoning splits a hydronic system so each area gets heat only when its thermostat calls. There are two common ways to do it:
- Zone valves — one circulator pumps the whole system, and a motorized valve on each zone opens to let flow into that loop when the zone calls.
- Zone pumps (zone circulators) — each zone has its own dedicated circulator that runs only when that zone calls.
Both end up at the same place: heat where you want it, when you want it. Knowing which approach is in front of you tells you exactly what to test when a zone won't heat.
How it works
The circulator is a centrifugal pump. An impeller spins inside a wet rotor (most residential circulators are wet-rotor — the system water actually lubricates and cools the motor), throwing water outward and creating flow around the loop. A circulator doesn't really "lift" water; in a full, sealed loop it just overcomes the friction (resistance) of the pipe, fittings, and emitters. How much flow you get depends on how much resistance the loop has versus how strong the pump is — that relationship is the pump curve.
The pump curve plots head (the resistance the pump can overcome, in feet) against flow (GPM). High resistance loop → the operating point slides up the curve → less GPM. Low resistance loop → more GPM. You size a circulator by figuring the GPM you need and the head of the loop, then picking a pump whose curve passes through that point.
Zone-valve systems: One pump, plus a zone valve per loop and a zone-valve control panel (or end switches). When a thermostat calls, its zone valve is powered open; when the valve finishes opening, its end switch closes and tells the boiler/pump to run. Multiple zones can be open at once, all fed by the one pump.
Zone-pump systems: Each zone's thermostat energizes that zone's relay, which starts that zone's circulator (and fires the boiler). Often there are flow checks (or the circulators have built-in check valves) so that when one zone's pump runs, it doesn't push water backward through the idle zones (gravity/ghost flow). Multiple pumps can run at once.
In the field
Figure out which system it is first. Count the circulators and look for zone valves on the loops:
- Several little motorized heads on the supply/return tappings, one pump → zone valves.
- A row of circulators, one per loop, no zone valves → zone pumps.
Dead-zone troubleshooting, zone-valve system:
- Confirm the thermostat is calling (24V at the valve/panel).
- Confirm the zone valve actually opens — many have a manual lever you can feel/see, and you can hear/feel the motor.
- Confirm the end switch makes — if the valve opens but the end switch doesn't close, the boiler/pump never gets told to run.
- Confirm the single system pump runs and the loop isn't air-bound.
Dead-zone troubleshooting, zone-pump system:
- Confirm the thermostat is calling and the zone relay pulls in.
- Confirm that zone's circulator actually runs (feel it, listen, check voltage at it).
- Confirm the flow check / check valve isn't stuck shut for that zone, and the loop isn't air-bound.
Either way: a zone that won't heat is almost always no flow into that loop (valve/pump/air) or no call making it through (thermostat/relay/end switch).
Normal values & targets
- Control voltage: zoning controls are 24V, same low-voltage world as forced-air. Zone-valve end switches are dry contacts that close the boiler/pump circuit.
- Typical residential circulator head: small wet-rotor circulators commonly produce on the order of single-digits to ~low-teens feet of head at usable flow — enough for a house loop. Match the curve to the loop.
- Design flow: sized from the zone's BTU load and design ΔT. As a rough rule of thumb, ~1 GPM carries roughly 10,000 BTU/h at a 20°F ΔT (a handy field approximation, not a substitute for real numbers).
- Zone-valve opening time: motorized valves take several seconds to fully open before the end switch makes — that built-in delay is normal.
Confirm against the actual pump curve and design; these are representative.
Common faults & what they mean
- One zone never heats, others fine — that zone's valve/pump/relay or an air-bound loop. Isolate to the single zone.
- Zone valve opens but no heat — end switch not making (boiler never fires), or air in that loop.
- All zones lukewarm or weak — on a zone-valve system, suspect the single shared pump or overall flow; on a zone-pump system, suspect the boiler not making temp.
- A zone heats even when its thermostat is satisfied — ghost flow: stuck zone valve, failed/missing flow check, or thermosiphoning. Heat migrates into an idle loop.
- Circulator runs but no flow develops (return stays cold) — air-bound loop, seized impeller, or a closed isolation/flow-check valve. Wet-rotor circulators can also lose their prime or get air-locked.
Tech tips & gotchas
- Most residential circulators are wet-rotor — the water cools and lubricates them. Run one dry and you'll cook the bearings. After draining/servicing, make sure the pump is full and purged before you let it run.
- Zone valves have a manual-open lever for a reason. You can force a zone open to test flow, or hold heat on a zone, without 24V. Just remember to put it back to "auto."
- The end switch is a classic phantom failure. The valve looks like it's working (it opens), the customer says "the valve's fine," but the end switch contacts are pitted and never tell the boiler to run. Meter the end switch, don't eyeball the valve.
- Flow checks earn their keep on zone-pump systems. Without them, hot water gravity-flows into zones that aren't calling, overheating rooms and wasting fuel. If a room "won't shut off," check the flow check.
- Pump on supply vs return, and pump placement relative to the expansion tank, matters for system pressure stability ("pumping away from the point of no-pressure-change"). If a system makes weird pressure swings or sucks in air when a pump starts, look at where the pump sits relative to the expansion tank.
Safety / code notes
- Zoning controls are 24V, but the pumps and boiler are line voltage — lock out/verify dead before working on a circulator or control panel.
- A wet-rotor circulator that's leaking from the flange or seal is dumping system water and dropping pressure — find and fix, don't just keep adding water.
- Relief valve and expansion provisions still govern the system regardless of zoning method — never defeat them.