Two circulators, two very different answers
Zones piped in parallel off a common supply and return all see the same pressure difference across them. Flow through each one is set by that pressure difference and by how much resistance the zone presents, and a good working approximation is that flow is proportional to the square root of the pressure difference for a given path. So the share each zone takes is fixed by its own resistance relative to the others, and the total is whatever the circulator produces against the combined resistance.
Now close three of four zone valves. The combined resistance goes up sharply, because you have removed three of the four parallel paths. What happens next depends entirely on the circulator. A fixed-speed pump responds by riding up its curve: it produces more head and somewhat less total flow, and every bit of that flow goes through the one open zone. A pressure-controlled circulator does the opposite, slowing down to hold the pressure difference roughly constant, so the open zone keeps close to the flow it had before.
Those two behaviours are the bounds this page reports, and they are far apart. With the defaults — three zones totalling 4.50 GPM — the living zone is designed for 2.00 GPM, and a fixed-speed circulator holding its flow would push the whole 4.50 through it. That is 2.3 times the design flow through a single circuit, and in a 0.681 inch bore it takes the velocity from 1.76 to 3.97 feet per second. The basement zone, designed for 1.00 GPM, would see four and a half times its share.
What the excess actually does
It is not a catastrophe and it is worth being precise about that. Extra flow through a zone does not overheat the room, because the room is controlled by the thermostat and the valve. What it does is raise velocity, which is heard as water noise in the one room that happens to be calling on its own, usually late at night when the rest of the house has satisfied. It also cuts the temperature drop across that zone, so the return water comes back hotter, which matters if the boiler was chosen to condense and its efficiency depends on return temperature.
The reverse case is worth as much attention. If the circulator was sized for the whole house and a small zone calls alone, the small zone is the one taking the full flow, and the smallest zone is usually the one with the smallest tube. Velocity goes as flow divided by the square of the bore, so the small zone gets hit twice.
Reading the numbers this page gives you
| Column | What it assumes | When it is the realistic one |
|---|---|---|
| Design flow per zone | Every zone calling, correctly balanced | Design day, all thermostats satisfied together |
| Share of total | Flow split in proportion to design flow | Sets the balancing valve positions |
| One zone alone | Circulator holds its full flow | Fixed-speed pump, no bypass, no balancing |
| Constant pressure bound | Circulator backs off as valves close | Pressure-controlled circulator on a suitable curve |
The per-zone flow itself comes from the same relation as everywhere else in hydronics: load divided by the fluid constant times the temperature drop. That constant is a field on this page rather than a hard-coded 500, because a glycol system has a different one — the reasoning is on the hydronic flow calculator, which takes the fluid properties apart.
The air-side version of the same problem
A ducted system with zone dampers has exactly this argument in a different medium, and the consequences there are worse because a blower starved of airflow can ice a coil or trip a limit. The zoning and bypass airflow calculator works that side. Water is more forgiving: the failure mode is noise and a warm return rather than equipment damage. That difference is why bypass arrangements are near-universal in air zoning and a matter of judgement in water zoning.
For the loads that feed this page, the heat loss calculator. For what the water temperature does to each zone emitter, the baseboard length calculator and the radiator output calculator. For how a small zone calling alone interacts with a boiler that cannot turn down that far, the buffer tank calculator.
Questions people ask
Zone valves or a circulator per zone?
Both work and the difference shows up at the edges rather than in the middle. Zone valves put every zone on one pump, so the pump must suit the whole range from one small zone calling to everything calling, which is the spread this page quantifies. A circulator per zone gives each zone its own flow regardless of what the others do, at the cost of more pumps, more electricity when several run together, and more things that can fail. Valves are generally cheaper to install and quieter to live with when the pump is pressure-controlled; individual circulators are simpler to reason about when zones differ wildly in size. Neither answer is universal and the piping arrangement usually decides it.
Do I need a bypass on a zone valve system?
It depends on what the circulator does when valves close, which is why this page reports two bounds instead of one answer. A pressure-controlled circulator that reduces speed as zones close is addressing the same problem the bypass addresses, and adding both can produce a bypass that never opens or one that short-circuits flow past the zones. A fixed-speed pump on a system with a large spread between the smallest zone and the total is the case where something has to give. What that something is — bypass, balancing, a different pump, or different zone divisions — is a design decision for whoever is responsible for the system, made with the pump curve in hand.
Why is one room noisy only when it is the only room calling?
Because that is exactly when it receives the most flow. With the other zone valves shut, a fixed-speed circulator sends nearly everything it can move through the single open path, and velocity in that circuit can be several times the design figure. Water noise scales sharply with velocity, so a circuit that is silent when the whole house is calling can whistle or rush when it is alone. Enter your zone loads and bores above and the single-zone velocity for each zone is the number that predicts it.
How do I balance zones that are very different in size?
Balancing valves on each zone, set so that each takes its design share when all are open, are the standard mechanism, and the shares in the table above are what you set them to. The complication is that balancing is done for the all-calling condition and the system spends most of the season in some other condition. A large spread between zones makes that worse, because the balancing restriction that suits a small zone at full call is a large restriction the pump has to overcome. Where the spread is extreme, redividing the zones is usually a better answer than trying to balance around it.
Does this tell me which circulator to buy?
No, and it deliberately stops short of that. Pump selection needs the head your piping presents at each flow condition, which comes from the length of the runs, every fitting in them, the tube size and the fluid, none of which this page knows. Work the head out on the pressure drop calculator, plot your flow and head requirement against the published pump curves, and check what the pump does at the reduced-flow conditions this page describes as well as at the design point. That last step is the one that gets skipped, and it is where the single-zone behaviour is decided.