Symptoms
- Bottom portion of the radiator stays cool or cold
- Top feels warm or hot normally
- Bleeding the radiator doesn't fix it
Likely causes
- CommonSludge or rust buildup settled at the bottom of the radiatorDebris accumulates at the lowest point over years and blocks water circulation there.
- PossibleClosed or partially closed lockshield valveThis valve controls flow through the radiator and can restrict circulation if set too low.
- PossibleRadiator needs a power flushSystem-wide sludge buildup often shows up first as cold spots in the coldest radiators.
What to do
- Bleed the radiator first to rule out trapped air, even though the pattern usually points to sludge.
- Feel the whole radiator to confirm the cold area is specifically at the bottom rather than a random cold spot.
- Check the lockshield valve (the capped valve, usually opposite the control knob) isn't shut most of the way.
- If it's an isolated radiator, it may need removing and flushing out separately, which is best done by a professional.
- If several radiators show the same bottom-cold pattern, the whole system likely needs a professional power flush.
- Avoid banging or forcing radiator valves that feel stuck; a seized valve can break and cause a leak.
Tools and parts
- Radiator bleed key
When to call a pro
Call a heating technician if bleeding doesn't help; sludge removal from a single radiator or a full system flush needs professional equipment.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.