This kind of behaviour usually means the radiator is getting initial flow, but something is stopping that flow from continuing properly. Once you catch the signs, the pattern becomes obvious.
1. When a Radiator Heats Then Cools, It’s Almost Always a Flow Balance Issue
The first clue for me was the timing. Every single cycle was the same: warm → hot → slowly cooling → barely warm → then back to warming when the boiler kicked in again. That tells you the radiator isn’t getting a consistent supply of hot water.
The system is doing its job — but not evenly. Radiators with weaker flow tend to “lose the fight” once the system settles into its rhythm.
2. The Lockshield Being Too Tight Can Cause Exactly This Pattern
When my radiator did this, the lockshield was the first thing I checked. It was set so tight that the radiator only got a good burst of water at the start, when the boiler first fired and everything was pushing at full strength. Once the boiler stabilised, the radiator’s flow dropped too low to keep it hot.
Opening the lockshield just a touch made a huge difference. The radiator suddenly stopped fading out mid-cycle and stayed warm the whole time.
3. TRV Heads That Half-Close After a While
TRVs can behave strangely when the room warms up. If the TRV thinks the room is warm enough, it’ll shut the radiator off even though the rest of the house is still heating. In my case, the TRV wasn’t broken — it was just reacting too quickly.
After taking the TRV head off and testing the pin underneath, it was clear the pin wasn’t opening fully. The radiator would get that first rush of heat but couldn’t keep it up. Freeing the pin made the whole problem disappear.
If you’ve ever had a radiator that only works sometimes, this is worth checking.
4. Air Pockets Can Cause the Radiator to “Drop Off” After the Initial Warm-Up
Air doesn’t always sit neatly at the top of the radiator. Sometimes it collects in a way that lets hot water in at first, but as the flow settles, the air shifts and blocks circulation. That’s why the radiator warms, then fades out.
I bled mine even though it didn’t sound like much air was inside, and the difference was immediate. The radiator held its temperature instead of dying off halfway through the heating cycle.
5. Low Pressure Causes Radiators to Compete — Some Lose
If the boiler pressure drops below a certain point, the radiators closest to the boiler or on more favourable pipe runs get most of the flow. The weaker ones heat at first (when the boiler is pushing harder) and then cool when the flow evens out.
When I topped my system back up to around 1.2–1.5 bar, this exact problem disappeared in another room. It’s surprising how easily pressure can slip down without you noticing.
6. Sludge Can Cause a “Heat Then Cool” Behaviour
Sludge doesn’t always block the entire radiator. Sometimes it affects certain channels so that hot water reaches the radiator at first, but can’t keep circulating once everything stabilises. The radiator warms early, cools later, and never fully recovers until the next heating cycle.
If this sounds familiar and the radiator also feels uneven in temperature, sludge is worth considering.
If your radiator only heats when the boiler is on full, that’s a related issue — here’s the article on that:
Radiator Only Heats When the Boiler Is On Full
7. Making Sense of the Pattern
A radiator that heats then goes cold again is basically telling you: “I’m getting flow, but not enough of it.” The fixes that usually sort it are simple — open the lockshield slightly, make sure the TRV pin is moving freely, bleed the radiator properly, and check boiler pressure.
Once the flow is steady, the radiator stays warm the whole cycle instead of fading halfway through. That’s exactly what happened with mine.
8. If You Want Your Whole Home to Heat More Evenly
One misbehaving radiator always throws off the room it’s in, but it also affects how efficiently the entire system runs. Fixing these smaller issues makes the house feel warmer and the boiler work less.
If you’re working through your heating system and want the full breakdown of keeping a UK home warm without overspending, here’s the guide everything links back to:
