Some radiators hot, others cold: balancing your radiators
Radiators near the boiler hot but upstairs or far rooms cold? Your system may need balancing. Signs, the step-by-step method and when to call an engineer.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
If radiators near the boiler get hot quickly but those further away stay cool, the system probably needs balancing. Hot water takes the easiest route, so the nearest radiators take most of it. Balancing means adjusting the lockshield valve on each radiator so water reaches every room evenly.
Quick guide: cold at the top means air (bleed it). Cold at the bottom means sludge (needs flushing). Cold all over while others are hot usually means a stuck valve.
What it could be and what to do
System needs balancing You may be able to fix this
Likely cause: Homebuilding.co.uk lists the signs: some radiators hot while others stay cold, cold spots around the house, and the radiator nearest the boiler not warming up first.
What to do: Bleed the radiators first. Open every TRV fully, then work from the radiator nearest the boiler outwards, closing each lockshield and opening it gradually until the pipes at each end differ by about 12°C. Further radiators need their lockshields opened more.
Air or sludge in the far radiators You may be able to fix this
Likely cause: Radiators at the end of the system, or upstairs, collect air. Sludge can also restrict them.
What to do: Feel each cold radiator. Cold at the top: bleed it. Cold at the bottom: it probably needs flushing by an engineer.
Weak pump or faulty zone valve Engineer needed
Likely cause: If whole floors or zones stay cold even after balancing, the pump may not be moving enough water, or a motorised zone valve may be stuck.
What to do: Check the programmer and any zone thermostats are calling for heat. If a zone still stays cold, book a heating engineer.
Safe radiator checks you can do
These are the checks we suggest without an engineer. Never open the boiler casing.
Heating off first: turn the heating off and let radiators cool before you bleed them or touch a valve. The water can scald.
Feel the radiator: with the heating on, note whether it is cold at the top, the bottom or all over. That tells you which page above to use.
Valves open: check the thermostatic valve is turned up and the lockshield at the other end is not closed.
Bleed: open the bleed valve slightly with a radiator key, catch the water and close it when water flows steadily.
Pressure: after bleeding, check the boiler gauge shows about 1 to 1.5 bar when cold, and top up as your manual shows. Low boiler pressure guide →
Radiator still cold or leaking? We can sort it.
Our callout is from £75 in standard hours (out of hours £100–£180) for attendance and diagnosis. Repair labour and any parts are agreed with you before work starts.
Our emergency line is open 24 hours a day, 7 days a week.
Every heating system is different. This page is general guidance, not a diagnosis.
YOUR QUESTIONS, ANSWERED
Helpful answers before you call.
Why are my upstairs radiators cold but downstairs are hot?
Upstairs radiators are often furthest from the boiler and collect air. Bleed them first, then balance the system. If a whole floor stays cold, a zone valve or the pump may need checking by an engineer.
How do I balance my radiators?
Bleed them, open every thermostatic valve fully, then adjust each lockshield valve, starting with the radiator nearest the boiler, so the flow and return pipes differ by about 12°C. Radiators further away need their lockshields opened more.
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