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Quick answer
When the upstairs radiators stay cold while downstairs heats normally, the usual causes are air collecting at the highest point of the system, low pressure on a sealed system, a closed or faulty zone valve on a two-zone setup, a pump that is not moving enough water to reach the top floor, or a system that is badly balanced. Bleeding radiators and checking pressure are safe first steps. Zone valve, pump and diverter problems need a heating engineer.
Key takeaways
- Air rises, so upstairs radiators are the first to suffer from trapped air.
- On sealed systems, pressure below about 1 bar can starve the upper floor of water.
- Homes with separate upstairs and downstairs heating use zone valves, which can stick shut.
- A tired pump or a pump set to too low a speed may not push water to the top of the system.
- Balancing makes sure radiators nearest the boiler do not take all the flow.
Start with air and pressure
Bleed each upstairs radiator in turn, starting with the one furthest from the boiler. If you get a lot of air, check the boiler’s pressure gauge afterwards. On a combi or system boiler, pressure should typically sit between 1 and 1.5 bar when the system is cold. If it has dropped, top it up using the filling loop. Our guide to repressurising a combi boiler shows how. If pressure keeps dropping and you keep having to bleed, something is letting air in or water out; that needs investigating rather than repeated topping up.
Check your controls
Many homes have two heating zones with separate thermostats or programmer channels for upstairs and downstairs. Make sure the upstairs zone is actually calling for heat: its timer is on and its thermostat is set above room temperature. It sounds obvious, but a thermostat accidentally turned down after a bedroom redecoration is a common cause.
Zone valve faults
A zone valve is a motorised valve that opens to let heated water into one part of the system. If the motor or its internal gear fails, the valve can stay shut and the zone gets no heat. Some valves have a manual lever that shows whether they are open. Our explainer on what a zone valve is and how it works covers the basics. Replacing the motor head is usually straightforward for an engineer; replacing the valve body means draining down.
The pump
The circulating pump pushes water around the system. If it is running slowly, partly seized or set to a low speed, water may reach nearby radiators but not the upper floor. You may notice the pump is hot, noisy or silent. On combis the pump is inside the boiler; on older systems it is usually in the airing cupboard. Pump diagnosis should be left to an engineer.
Balancing and sludge
Radiators close to the boiler offer the least resistance and can take most of the flow. Balancing adjusts the lockshield valves so the furthest radiators get their share. Sludge also makes things worse by restricting pipes and radiators. If radiators are cold at the bottom, read radiators cold at the bottom and consider whether your system needs cleaning and a magnetic filter.
Gravity and open-vented systems
In older homes with a feed and expansion tank in the loft, the upstairs radiators can be affected by a blocked or incorrectly positioned vent pipe, or by the pump pulling air in. Our guide to gravity vs pumped central heating explains how these older systems differ.
Book with Gasmaster
Cold bedrooms and warm lounge? Gasmaster Plumbing & Heating, Gas Safe registered (220932), diagnoses central heating faults across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. Book through our boiler service booking page.
Frequently asked questions
Why do I have to keep bleeding my upstairs radiators?
Air getting in repeatedly points to a leak, a faulty automatic air vent, or on open-vented systems a pumping-over problem. Constant bleeding is a symptom to fix, not routine maintenance.
Can a combi boiler heat upstairs and downstairs separately?
Yes, with zone valves and separate thermostats or smart zoned controls fitted to the system.
Would a bigger boiler fix cold upstairs radiators?
Rarely. The issue is usually circulation, air or controls, not boiler output.
Sources
- Energy Saving Trust, heating controls guidance (energysavingtrust.org.uk)
- CIBSE, domestic heating design guidance (cibse.org)
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
