The first properly cold night in October is when most people find out. The heat comes on for the first time since March, and one room never warms up while the rest of the house is fine.
Where the radiator is cold tells you most of what you need to know. A hot water heating system moves heat by circulating water through a loop, so a radiator that isn't heating is a radiator that isn't getting hot water through it — and there are only a handful of reasons for that. Work through them in order and you can usually name the problem before anyone comes out.
Cold at the top, warm at the bottom
Air. Almost always air, and it's the easiest one to fix yourself.
Air rises to the highest point in the radiator and sits there, occupying the space hot water should fill. The bottom still heats because water still flows through it; the top is an air pocket. It builds up gradually — some from fresh water introduced during a top-up, some from corrosion inside the system slowly producing gas.
To bleed it: turn the heating off and let things cool for a half hour or so, then find the bleed valve at the top of one end of the radiator. Hold a cloth under it, turn the key or the screwdriver slot a quarter turn anticlockwise, and let the hiss run until water dribbles out steadily instead of spitting. Close it. That's it.
Start with the radiator furthest from the boiler and work back, and do the upstairs ones after the downstairs ones.
The part people skip: bleeding lets water out, so your system pressure drops. Check the gauge on the boiler afterwards and top back up if it's fallen below the normal band — most systems sit somewhere around 1 to 1.5 bar cold, and your boiler's label or manual will give the right figure for yours. Bleeding several radiators without repressurising is a common way to end up with a boiler that locks out on low pressure the same evening.
If you're bleeding radiators every few weeks, the air is coming from somewhere and that's a different problem — usually a small leak drawing air in, or corrosion inside the system. Our post on why a boiler keeps losing pressure covers where that water is going.
Cold at the bottom, warm at the top
The opposite pattern, and a more serious one. This is usually sludge.
Over years, steel radiators corrode internally and produce magnetite — a black, heavy iron oxide sediment. It settles in the bottom of each radiator where flow is slowest, and it builds into a layer that hot water simply can't get through. The top of the radiator still works; the bottom is packed.
You can often feel the line where it changes. Sometimes you can hear it, a faint gritty sound in the pipework.
Bleeding will not fix this and neither will turning the valve. The sludge has to come out, which means either flushing the system or, in bad cases, removing the affected radiators to flush individually. If several radiators show it, the system as a whole is due — and it's worth doing, because that same sediment is going through your circulator pump and your heat exchanger.
A magnetic filter fitted on the return catches the material before it circulates, and it's the standard preventative on any system we flush. Worth asking about if yours doesn't have one.
The whole radiator is cold
Nothing is reaching it. Three things to check, cheapest first.
A closed or stuck valve. There's a valve at each end. One sets the flow — often a thermostatic head with numbers on it — and the other balances the return. The thermostatic ones seize regularly, and specifically they seize shut after a summer of sitting in one position.
Pull the head off and look at the pin underneath. It should move freely and spring back. If it's stuck down, that radiator is closed no matter what the dial says. Free it with a gentle tap and some easing, or replace the head — they're inexpensive.
This is the single most common cause of one cold radiator in October, precisely because the system has been idle since spring.
It's last on the loop and the system needs balancing. If the radiators nearest the boiler are roasting and the far ones are lukewarm, water is taking the path of least resistance and short-circuiting back before it reaches the end. The fix is balancing — throttling the return valves on the nearest radiators slightly so more flow is pushed to the far ones. It's fiddly and it's done with a thermometer, but it's a proper fix rather than a workaround.
A genuine blockage. Less common, but possible after work has been done on the system, or where sludge has bridged a pipe rather than settling in a radiator.
Upstairs cold, downstairs hot — or the reverse
A pattern by floor rather than by room usually points at circulation rather than any individual radiator.
If upstairs is cold, suspect air first, since air collects at the highest points. Bleed the upstairs radiators. If they were already clear, look at pump speed — many circulators have a three-speed selector, and one set too low may not carry water to the top of the house against the resistance of a fully open system.
If downstairs is cold while upstairs is fine, that's more unusual and more often a balancing or valve problem than a pump one.
If everything is lukewarm rather than some hot and some cold, the problem isn't distribution at all — it's the boiler not reaching temperature, or the pump not running properly. Different diagnosis. Boiler repair is the right starting point there, and if it's cutting out rather than under-heating, why a boiler keeps turning off covers that pattern.
Some Calgary-specific notes
Everything shows up in October. A hydronic system sits idle from roughly April to October here. Valves seize in position, air accumulates, the pump sits still for six months. The first heating call of the season isn't a coincidence — it's five months of dormancy revealing itself in one evening.
Older inner-city homes often have original cast iron. Those radiators are heavy, slow to warm, and genuinely take longer to come up to temperature than a modern panel. A cast iron radiator that's cool twenty minutes after the heat comes on may be entirely normal. Give it an hour before diagnosing it.
Converted and extended systems get unbalanced. If a previous owner added a radiator, finished a basement, or ran a loop out to a garage without rebalancing afterwards, the whole system's flow pattern changed and nobody adjusted for it. That's a very common cause of one room that never quite worked since the renovation.
In-floor zones behave differently. If part of your house is in-floor heating and part is radiators, they run at different water temperatures and often on separate zones. A cold floor is a zone or manifold question, not a radiator question, and it's diagnosed differently.
What's worth doing yourself, and what isn't
Reasonable to do yourself:
- Bleeding radiators, and repressurising afterwards
- Pulling a thermostatic head off to check the pin isn't stuck
- Checking every valve is actually open
- Confirming the boiler pressure is in its normal band
Worth calling for:
- Cold at the bottom across several radiators — that's a flush, not a fix
- Balancing a whole system, which needs a thermometer and patience
- Anything involving the pump, the expansion vessel or the heat exchanger
- Repeated pressure loss, because water is escaping somewhere
Before the heating season really starts
If one radiator was cold last winter and you lived with it, now is the time — not January. A seized head or an air pocket takes minutes in September and becomes an evening call-out when it's −25°C and the phones are busy.
The same is true of a system that's never been flushed. Sludge doesn't improve on its own, and it's steadily working on the most expensive part of the system.
If your radiators aren't heating evenly and you've worked through the list, call 587-834-3668. We'll check the pressure, the pump, the balance and the state of the water itself, and tell you honestly whether it's a bleed, a balance or a flush. See boiler service for what an annual visit covers, or boiler repair if something has actually failed.
