Quick answer
No heat upstairs with a boiler usually means air or low pressure. Air rises to the top-floor radiators first, and a standard 12 psi fill lifts water only about 8.5 m (28 ft), which is marginal in a tall three-storey house. Bleed the top-floor radiators, then check that the gauge reads 12 to 15 psi cold. If both are right, suspect the circulator or the upstairs zone valve.
When there is no heat upstairs with your boiler and the main floor is warm, the boiler itself is rarely the fault: it is making hot water, and the water is not reaching the top of the house. Three things stop it. Air sits in the highest radiators, the system pressure is too low to hold water at that height, or the pump, zone valve or thermostat serving the upper floor has quit. Check them in that order, because the first two cost nothing to check.
Air collects in the highest radiators first
Air in a hot-water heating system rises until it can go no higher, and the highest point is a top-floor radiator. A radiator that is hot at the bottom and cold across the top, or cold end to end while its supply pipe is warm, is holding an air pocket that water cannot push past. Bleed the top-floor radiators with the circulator off, starting with the one farthest from the boiler, until water runs steady. How to bleed a radiator has the step-by-step.
Bleeding lets air out and pressure with it. Look at the boiler gauge afterward: if it has dropped, the automatic fill valve should bring it back, and if it does not, the fill valve is stuck or shut off. Air that returns every few weeks is a symptom of something else, since it is getting in through a leak, a failed air vent or a waterlogged expansion tank.
Why 12 psi is not enough for a tall three-storey house
Pressure is what holds water up at the top of the system. One psi supports a column of water about 0.7 m (2.31 ft) high, so the standard 12 psi cold fill lifts water only about 8.5 m (28 ft) above the gauge. In a two-storey house that leaves a comfortable margin. In a tall three-storey home with the boiler in the basement, the top radiators can sit 9 m or more above the gauge, and at 12 psi the water never reaches them: they stay cold, and opening the bleeder draws air in instead of letting it out.
The working rule is the height in feet from the gauge to the top of the highest radiator, multiplied by 0.433, plus 4 psi so there is positive pressure at the top for bleeding. A 30 ft rise needs about 17 psi cold. The fill valve is adjustable, and the expansion tank's air charge has to be raised to match, or the relief valve (set at 30 psi on most residential boilers) will drip when the system heats up. We look for this in tall pre-war brick houses in Hamilton and the west side of Toronto, especially where a radiator was added in a finished attic.
Weak circulator, zone valve or upstairs thermostat
If the radiators are full of water and the pressure is right, look at what moves and directs the flow.
- Weak circulator. A worn pump still turns and moves too little water. The nearest radiators take the heat and the water reaching the top floor arrives lukewarm, with the return pipe at the boiler much cooler than the supply. Signs of a bad circulator pump covers the tests.
- Failed zone valve. Where the upper floor is its own zone, a motorized valve near the boiler opens when that thermostat calls. A burnt-out valve head stays closed. Most have a manual lever that opens the valve by hand, and heat arriving upstairs with the lever held open confirms the head has failed. Why one heating zone is not working goes further.
- Upstairs thermostat. Dead batteries, a broken wire or a failed thermostat means the zone never calls at all. Turn it up 3 °C and listen at the boiler for the valve or relay to click.
- Balance. In a one-zone house with no fault at all, the main floor can simply take too much of the flow. Partly closing the valves on the nearest radiators pushes more water upstairs.
Check this yourself first
- Read the boiler pressure gauge with the system cold. Below 12 psi, or at zero, is the answer before you touch a radiator.
- Feel each top-floor radiator top and bottom, carefully, and bleed any that are cold across the top.
- Confirm every radiator valve upstairs is open, including the ones behind furniture.
- Turn up the upstairs thermostat and feel for a warm supply pipe on that zone after ten minutes.
Call a technician when
- The gauge reads low and does not recover, or you have to add water repeatedly: there is a leak or a failed fill valve.
- The top radiators pull air inward or give nothing when the bleeder is opened: the pressure is below what the height needs, and the fill valve and expansion tank must be reset together.
- The relief valve drips after the pressure is raised.
- Pipes to the top floor run through cold spaces and the heat has been off in freezing weather. Frozen heating pipes split, so do not wait.
Raising fill pressure, recharging an expansion tank and replacing a zone valve all involve the boiler's safety components, and are a service visit, not a weekend project. ZK Mechanical quotes boiler repair and radiator bleeding and balancing in writing before work starts, after a diagnostic visit at $90–$150 that goes toward the repair. Call (647) 801-1252 or describe the problem here.