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🍁 Built for Canadian homes — written to Canadian codes and provincial rules. Not intended for use outside Canada.
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Some radiators are cold while others are hot

When some radiators are hot and others are cold, the boiler is usually fine — the heat just isn't reaching every rad, and the most common reason is trapped air you can bleed out yourself in a few minutes.

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Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.
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In Canadian homes, start with the cold radiators: feel them top and bottom. A rad that is warm at the bottom but cold at the top has trapped air, and bleeding it with a radiator key (a couple of dollars at any hardware store) is the single most common fix. Have a cloth ready for the trickle of water when the hissing stops.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order
Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

  1. With the heating on and the rad calling for heat, run your hand from the bottom of the cold radiator up to the top.
  2. Warm at the bottom and cold across the top is the signature of trapped air — the air pocket is blocking the top channels.
  3. Cold top AND bottom instead points elsewhere — check the radiator's valves (a closed or stuck valve) rather than bleeding.
buildradiator-keybuildcloth schedule5 min

handymanHow to fix it

With a radiator key and a cloth, open the bleed valve at the top of the cold radiator a quarter-turn until the hissing air stops and water trickles out, then close it — and re-check the boiler pressure afterward, topping up if bleeding dropped it.

  1. Turn the heating off and let the radiators cool a little so you are not working with scalding water, and so the pump is not pushing more air around.
  2. Fit the radiator key onto the square bleed valve at the top corner of the rad, hold a cloth under it, and turn a quarter-turn counter-clockwise until you hear a steady hiss of escaping air.
  3. When the hiss stops and a steady trickle of water appears, close the valve snugly (do not overtighten). Wipe up and move to the next cold rad.
  4. Turn the heating back on and check the boiler pressure gauge — bleeding releases water, so top up with the filling loop if it dropped below the normal band.
schedule10 min savings$2–10
Radiator bleed keyA cheap square-socket key sized to your bleed valve; the clock/dumbbell style fits most Canadian rads
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Bleeding radiators and topping up boiler pressure are generally owner-permitted across Canada; work on the gas appliance itself is licensed.

visibilityHow to check

  1. Find the pressure gauge on the front or underside of the boiler. Read it with the system cold.
  2. Compare it to your boiler's marked normal band (commonly around 1 to 1.5 bar cold — check the label or manual for your unit).
  3. A reading well below the band, with upstairs rads cold and downstairs fine, points to low pressure. If it keeps dropping after topping up, stop and route to a pro — that means a leak or a failed expansion vessel.
buildNo tools schedule3 min

handymanHow to fix it

With the boiler off and cool, open the filling loop slowly while watching the gauge, close it the moment the needle reaches the marked pressure (often ~1 to 1.5 bar cold), and make sure the loop is fully closed afterward. If pressure keeps falling, stop and call a pro — that is a leak or a failed expansion vessel, not a top-up.

  1. Switch the boiler off and let it cool — pressure is read and set cold. Locate the filling loop (usually a braided hose with one or two valves under the boiler).
  2. Open the valve(s) slowly; you will hear water flowing in. Watch the pressure gauge climb.
  3. Close the valve(s) firmly the instant the needle reaches your boiler's marked cold pressure — overfilling can lift the safety valve.
  4. Confirm the filling loop is fully closed, then restore power. If the pressure drops again over the next days, do not keep topping up — book a licensed technician to find the leak or check the expansion vessel.
schedule10 min savingsusually free

verifiedWhy this one goes to a pro

A stuck circulator pump sits on a gas boiler and a sealed, pressurized hot-water loop. Freeing or replacing it means working around live gas and hot pressurized water, and getting either wrong is dangerous. Leave the boiler off and have a licensed HVAC technician or gas-fitter diagnose it — the visit is modest next to the cost of a gas or scald incident.

gavel Check your province first AB: licensed professionals only · BC: licensed professionals only · ON: licensed professionals only · PE: licensed professionals only · QC: licensed professionals only
Work on a gas boiler and its circulator pump is licensed-only across Canada (gas-fitter / HVAC); a homeowner should not attempt it.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.