Quick answer
One heating zone not working while the others heat normally almost always means a fault that belongs to that zone alone: a failed zone-valve motor, an end switch that never tells the boiler to fire, a dead thermostat or relay, or air trapped in that loop. If the boiler gauge reads 12 to 15 psi and the other zones are hot, the boiler itself is fine.
When one heating zone is not working and the rest of the house is warm, the boiler has already told you it is healthy. A hot-water system splits the house into loops, each with its own thermostat and either its own zone valve or its own circulator pump. The fault sits in the parts that serve the cold loop only, and there are four of them worth checking in order.
The zone valve motor fails more often than anything else on the loop
A zone valve is a motorized valve on the pipe near the boiler, topped with a powerhead about the size of a fist. When the thermostat calls, 24 volts drives a small motor that opens the valve against a spring; when the call ends, the spring closes it. The motor and its gear train sit warm for hours at a time, and after enough heating seasons they stall, strip or burn out.
Most powerheads have a manual lever on the side or bottom. With the zone calling for heat, slide the lever gently: if it moves freely with no resistance, the motor has already opened the valve and the problem is elsewhere. If you feel firm spring resistance the whole way, the valve is still shut and the motor has not done its job. Latching the lever open lets hot water through whenever another zone runs the boiler, which gets some heat into the rooms until the head is replaced.
A failed end switch: the valve opens but the boiler never fires
Inside the powerhead is an end switch, a small contact that closes only when the valve reaches fully open. That contact is what tells the boiler and circulator to start. When it fails, the valve opens, the thermostat shows a call for heat, and nothing happens unless another zone happens to be running. The giveaway is a zone that heats only when a different thermostat is also calling.
The end switch is part of the head on most valves, so the repair is a new powerhead, which on common designs swaps onto the existing body without draining the system. A zone valve replacement that includes the body means isolating and draining that section of piping, then purging it afterwards.
The zone thermostat, its wiring, and the circulator relay on pump-zoned systems
A zone thermostat is a switch on two low-voltage wires. Dead batteries, a loose wire at the sub-base or a broken conductor will silence one zone without touching the others. Put fresh batteries in, raise the setpoint 3 °C above room temperature and stand at the boiler: a zone valve should hum and its lever should go slack inside a minute or so.
Some houses have no zone valves at all. Each loop has its own circulator pump, switched by a relay box on the wall beside the boiler, usually with a small light for each zone. If the thermostat calls and the light for that zone stays dark, suspect the thermostat circuit or the relay; if the light comes on and the pump stays cold and silent, the relay contacts or the circulator have failed. The signs of a dying pump are covered in how to tell if a boiler circulator is bad.
An air lock in one loop, and how a single zone is purged
Air collects at the high point of a loop and stops circulation dead: the valve is open, the pump is running, the supply pipe is hot for the first metre and the return is cold. It shows up after the system was drained for a repair, after a radiator was changed, or when the pressure has dropped far enough that the top floor has no water behind it.
Purging one zone means forcing fresh water through that loop alone, fast enough to push the air ahead of it. A technician closes the other zones, shuts the isolation valve on the return, connects a hose to the purge drain for that loop and opens the fast-fill on the feed valve until the hose runs solid with no sputter, keeping the gauge below the relief valve setting. Then the system goes back to 12 to 15 psi cold and the boiler is fired to confirm the return pipe gets hot. Radiators with bleed screws are a homeowner job, and how to bleed a radiator has the steps; baseboard loops with no bleeders need the purge.
Check this yourself before you call
- Thermostat for the cold zone: fresh batteries, set to heat, setpoint 3 °C above the room.
- Boiler pressure gauge: 12 to 15 psi cold is normal for a two-storey house. Below that, the top loop may be starved.
- Zone valve lever: slack means open, firm resistance means the motor has not opened it.
- Pipes on each side of the valve or pump for that zone, after ten minutes of calling: hot in and cold out points to air or a stuck valve.
- Relay box lights on a pump-zoned system: note which ones are lit while the cold zone is calling.
Call a technician now if
- No zone is heating and the boiler is silent. That is a different fault, covered under why a boiler will not turn on.
- Water is dripping from the valve body, the relief valve or a pump flange.
- The cold zone has pipes in an outside wall, a crawl space or a room over a garage and the forecast is well below freezing. A loop that is not circulating can freeze and split.
- The powerhead is too hot to touch or smells burnt.
A diagnostic visit runs $90–$150 and is folded into the repair if you go ahead. The repair price depends on whether the head alone or the whole valve is changed, how the piping can be isolated and whether the loop has to be purged afterwards, so it is quoted in writing before any work starts. ZK Mechanical works on hot-water heating in the pre-war brick houses of Hamilton and Burlington as well as newer homes with radiant floor zones. For boiler repair, call (647) 801-1252 or send the details through the contact page.