If you’re staring at a radiator that’s hot on one part and stubbornly cold on the other, you’re not alone. The good news: most fixes are simple and well within DIY territory. This guide walks you through how to fix a half cold radiator step by step, whether it’s cold at the top, bottom, or just one side, with practical checks, proper bleeding technique, valve tweaks, and when to escalate. You’ll finish with warmer rooms, a happier boiler, and lower bills.
Before You Start: Safety, Tools, And Quick Checks
What You’ll Need
- Radiator bleed key (or a flat bleed tool, depending on your valve)
- Old towel or cloths and a small container
- Flat-head screwdriver (for some lockshields and TRVs)
- Adjustable spanner
- PTFE tape (handy if you’re removing a radiator)
- Gloves and a torch
- Filling loop access for your boiler (to top up pressure)
A quick note on timing: bleeding and balancing are best done when the heating’s off or cooling, unless the step specifically says otherwise.
Safety, System Type, And Boiler Pressure
- Switch off the heating before bleeding or removing a radiator. Hot water can scald, and pumps can draw in air if left running.
- System type: If you have a combi or sealed system, you’ll re-pressurise via a filling loop. If you have an older open-vented system (tank in the loft), the header tank keeps it topped up, still proceed cautiously.
- Pressure: On sealed systems, check the boiler pressure when cold. Aim around 1.0–1.5 bar (follow your manufacturer’s range). If it’s already low, fix that before any bleeding. After bleeding, you’ll likely need to top up again.
Identify Your Valves: TRV And Lockshield
- TRV (thermostatic radiator valve): the numbered/marked valve that controls room temperature. It should be fully open when diagnosing heat issues.
- Lockshield valve: usually capped and adjusted with a screwdriver or spanner. This sets the flow balance in your system. Note its current position (count turns) before you change anything so you can revert if needed.
Diagnose The Pattern: Top Cold, Bottom Cold, Or One Side Cold
Understanding the cold pattern is half the battle. Put your hand across the radiator from top to bottom and side to side.
Top Cold: Trapped Air
Air rises, so the top going cold while the bottom is warm usually screams “bleed me.” You’ll typically hear hissing during bleeding and then water.
Bottom Cold: Sludge Or Poor Circulation
If the bottom is cold but the top is hot, you’re likely dealing with sludge (iron oxide) restricting flow. Expect brown/black water if you remove and flush the radiator. Sometimes a weak pump or closed lockshield can mimic this, but sludge is the classic culprit.
One Side Cold: Valve Or Balancing Issue
Hot on one side and cold on the other often points to a stuck TRV, a closed or overly throttled lockshield, or a system that’s out of balance, where nearby radiators hog the flow. Freeing a valve and balancing usually fixes it.
Fix Top-Cold Radiators: Bleeding Done Properly
Step-By-Step Bleeding Procedure
- Turn the heating off and let the system cool for 20–30 minutes. You want calm water, not a circulating pump pulling air.
- Fully open the TRV on the radiator you’re bleeding. Place a cloth and small container under the bleed valve at the top corner.
- Insert the bleed key and turn anticlockwise just a quarter to half turn. You’ll hear air hiss. Keep the valve slightly open until water flows steadily, no spluttering.
- Close the bleed valve firmly but gently (don’t overtighten). Wipe any drips.
- Repeat on other radiators that feel spongy or top-cold, starting with the highest floor and working down.
Pro tip: If air keeps returning, check for a micro-leak or a faulty automatic air vent elsewhere in the system.
Re-Pressurise A Combi/System After Bleeding
- Check the boiler gauge. If pressure has dropped below the recommended cold range, open the filling loop slowly until the needle reaches around 1.2–1.5 bar. Then close the loop fully.
- Reset any fault codes if your boiler locked out on low pressure. Keep an eye out for pressure drifting, persistent drops suggest a leak that needs investigation.
Confirm Heat-Up And Check Again
Turn the heating back on. The radiator should now heat evenly from the bottom up, with the top warming shortly after. If the top still goes cold: bleed again once more, verify pump operation, and check nearby radiators aren’t stealing flow. Occasionally, an airlock in pipework may need an engineer to purge at a higher point.
Fix One-Side-Cold Radiators: Free TRVs And Balance The System
Freeing A Stuck TRV Pin
A stuck TRV is common after summer when valves sit closed. To free it:
- Turn the TRV to max (fully open) and remove the plastic head, usually a knurled ring or screw holds it.
- You’ll see a small metal pin. It should spring up and down 2–3 mm. If it’s stuck, press it gently with pliers and release repeatedly until it moves freely. Don’t pull the pin.
- If it won’t budge, a tiny dab of penetrating oil around the pin’s collar can help. Wipe excess and re-fit the head.
If the TRV body is faulty or leaking, replace it. Isolate the radiator first, or call a professional if you’re not comfortable.
Open The Lockshield And Set TRVs Correctly
Make sure the lockshield on the opposite side is open at least a turn from fully closed. Count turns so you can return if needed. Set TRVs fully open for all radiators while you troubleshoot, this removes variables so you can balance properly later.
Balance Radiators For Even Heat
Balancing ensures every radiator gets its fair share of hot water:
- Turn the heating on with all TRVs fully open. Start with the radiator closest to the boiler.
- Close that radiator’s lockshield, then open it a quarter-turn. Feel the pipes: you want the radiator to heat but not rocket ahead of others.
- Move to the next radiator in the circuit, opening its lockshield slightly more if it’s slow to warm. Farther radiators generally need more opening.
- Work around the house, adjusting in small increments, giving each change 5–10 minutes to settle. Aim for all radiators to warm at a similar rate.
Balancing sounds fiddly, but once done, you’ll fix most “one side cold” complaints and stop rooms competing for heat.
Fix Bottom-Cold Radiators: Flush And Improve Flow
Isolate, Remove, And Flush The Radiator
If the bottom stays cold, flush out sludge:
- Turn off the heating and allow to cool. Close both the TRV and lockshield (note positions). Place towels and a tray under the valves.
- Loosen one valve union with a spanner and drain into a container. Then loosen the other. Lift the radiator off its brackets, get help: they’re heavy.
- Take it outside and flush with a hose until water runs clear. Gentle taps can dislodge stubborn sludge. If it’s really bad, a radiator cleaning chemical (follow instructions) can help.
- Refit with fresh PTFE on unions, reopen valves to the previous settings, and bleed. Top up boiler pressure as needed.
Use Inhibitor And Consider A Magnetic Filter
- Add a quality corrosion inhibitor to your system after flushing. It slows future sludge build-up and protects pumps and heat exchangers.
- A magnetic filter on the return pipe captures circulating debris. It’s an easy win during a service or boiler upgrade and can be cleaned annually.
When A Powerflush Or Pipework Check Is Needed
If several radiators are bottom-cold, or the water is consistently inky black, a professional powerflush may be worth it. They’ll use chemicals and high-flow pumps to clear sludge from the entire system. Also consider:
- Pump performance: an underpowered or failing pump starves distant radiators.
- Pipe design: microbore (8–10 mm) systems are more prone to blockages and need careful balancing.
- Partially closed isolation valves or kinked flexible connectors near the boiler can quietly throttle flow.
Prevention, Extra Troubleshooting, And When To Call A Professional
Smart TRVs, Schedules, And Thermostat Placement
Smart TRVs let you zone rooms without over-throttling the system. Avoid setting many rooms to “off” for long periods: it can unbalance flow. Keep the main wall thermostat away from direct sunlight or radiators, false readings lead to short cycling and uneven heat.
Pump Speed, Airlocks, And Microbore Considerations
- Pump speed: Too low, and far radiators lag: too high, and you can create noise or draw air in. Try the next speed setting and reassess heat-up times.
- Airlocks: High points in pipework can trap air. If bleeding radiators isn’t enough, you may need an automatic air vent fitted at a high point.
- Microbore systems: Be gentle when balancing, open lockshields incrementally, and keep inhibitor levels healthy to reduce sludge risk.
Routine Maintenance And Red Flags For An Engineer
- Annual service: Have your boiler and system filter cleaned yearly and inhibitor checked or topped up.
- Quick seasonal check: Spin TRVs to full and back at the end of summer to prevent sticking.
- Call a pro if you notice: persistent pressure loss, frequent need to bleed the same radiator, leaks, banging/booming in the boiler, repeatedly cold radiators after balancing, or if you suspect a blocked pipe or faulty pump.
Knowing how to fix a half cold radiator is one thing: preventing the next one is what keeps your home cosy and bills sensible.