Quick answer
Radiators that are cold with the heating on come down to one test: touch the boiler. A hot boiler with every radiator cold means the water is not circulating, usually a failed or air-locked circulator pump or a closed zone valve. A cold boiler means it is not firing. Read the gauge too: under about 12 psi cold, water cannot reach the upstairs radiators.
Cold radiators when the heating is on mean heat is either not being made or not being moved, and one hand near the boiler tells you which. Feel the boiler jacket and the first metre of supply pipe leaving it, carefully, since a working supply pipe is too hot to hold. Then read the pressure and temperature gauge on the front. Those two observations split the problem in half before anyone opens a toolbox.
Boiler hot but every radiator cold: the water is not circulating
A boiler that is hot while all the radiators are cold is making heat that has no way to leave. The burner fires, the water in the boiler reaches its limit, the burner shuts off, and the cycle repeats every few minutes while the house cools. The part that should be moving the water is the circulator pump, and there are three common reasons the water stays put.
- Failed circulator: the motor is seized, its capacitor has failed or the relay that powers it has burned out. A dead pump is silent and cold, or hums and is too hot to touch. The signs are laid out in how to tell if a boiler circulator pump is bad.
- Air-locked circulator: a pocket of air sits in the pump body, and the impeller spins in it without moving water. This follows a drain-down, a repair or a summer of slow pressure loss, and you can often hear gurgling at the boiler.
- Closed zone valve or isolation valve: a zone valve whose motor has failed stays shut with the thermostat calling, and a manual valve left closed after service work does the same thing to the whole loop.
Many pre-war houses in Hamilton and the west side of Toronto still heat with their original cast-iron radiators on what began as a gravity system, with wide pipes and a circulator added decades later. Those wide pipes will drift a little heat upstairs by gravity even with the pump dead. That is why, in such a house, a second-floor radiator can stay faintly warm while the ground floor goes cold.
Low system pressure: the water never reaches the radiators
Water pressure is what lifts water to the top of the house, and the gauge should sit at about 12 to 15 psi with the system cold. Each psi raises water roughly 0.7 metres, so 12 psi fills a two-storey house with a few psi to spare at the highest radiator. Below that, the upstairs radiators empty first and go cold, and a gauge near zero leaves the ground floor cold as well.
Low pressure always has a cause. The automatic fill valve may be shut off or stuck, the expansion tank may have failed and pushed water out through the relief valve, or there is a leak. Topping up restores heat for now, but a system that keeps dropping needs the leak found; why a boiler loses pressure walks through where the water goes.
Boiler cold too: a no-fire fault, and what to report on the phone
If the boiler itself is cold, the radiators are innocent and so is the pump. The boiler is not being asked to run, is being held off by a safety, or has locked out on ignition. That is a different diagnosis with different parts, set out under why a boiler is not turning on.
A steam system follows the same split. All radiators cold on steam means the boiler is not making steam: look at the water level in the sight glass, which should sit about halfway up, and at whether the burner is lit. Steam radiators are never bled.
Whichever way the split falls, five facts given on the phone shorten the visit and help the right parts get loaded on the truck.
- Boiler hot or cold to the touch, and the temperature shown on the gauge.
- Pressure reading, whether it was taken cold or hot, and whether it has been dropping.
- Circulator: silent and cold, humming and hot, or running normally.
- Scope: every radiator, one floor or one zone, and whether domestic hot water still works on a combi or indirect tank.
- Any code or flashing light on the boiler control.
Check this yourself first, and when to call now
- Thermostat calling for heat and the boiler switch on.
- Gauge between 12 and 15 psi cold; if lower, see that the shut-off feeding the fill valve is open.
- Valves beside the circulator and on the main loop: handles in line with the pipe.
- Zone valves: on many models the manual lever moves freely when the valve is open and resists when it is closed.
- Call now if the boiler is hot, the radiators are cold and the relief valve is dripping: turn the boiler off, because it is overheating with nowhere to send the heat.
- Call now if the pressure reads zero, water is on the floor, or it is below freezing outside and the house is cooling.
What a circulation repair costs
The diagnostic visit is $90–$150 and comes off the repair if you go ahead. A circulator, a zone-valve head and an expansion tank each have their own price depending on the size and brand fitted to your system, so the repair is quoted in writing once the fault is confirmed. If the pump is the culprit, circulator pump replacement is often completed on the first visit when the pump is a common size.
ZK Mechanical repairs hot-water heating systems from its Burlington shop across Hamilton, Oakville, Mississauga and Etobicoke. Book boiler repair at (647) 801-1252, day or night, or send us the gauge readings and we will tell you what we expect to find.