Upstairs Radiators Cold, Downstairs Hot

What's happening?

When the upstairs radiators stay cold or warm slowly while the downstairs ones are hot, the water is going to the easier route. Air and weak flow are the reasons. Trapped air collects at the highest points, which are the upstairs radiators and the pipework above them. Low pressure lets more air in and gives the pump less to push against upstairs. A system that has never been balanced sends most of the water to the radiators nearest the boiler, because it is the shortest and least resistive path, and leaves the far ones starved. On an older open-vented system with a feed and expansion tank in the loft, a low tank level starves the top of the system. The opposite pattern, downstairs cold and upstairs hot, points more to sludge and is covered on the cold-at-the-bottom page.

What usually causes it?

• Air trapped in the upstairs radiators and pipework, often after draining or a pressure loss
• System pressure below about 1 bar, which stops water reaching the highest radiators
• A system that has never been balanced, so downstairs radiators take most of the flow
• A pump set to a low speed or ageing and unable to lift the water
• A low or empty feed and expansion tank in the loft on an open-vented system
• Sludge restricting the upstairs pipework or radiators

Is it dangerous?

No. A cold upstairs is a comfort issue.

Can I fix it myself?

Switch the heating off and let it cool. Bleed the upstairs radiators one at a time, starting with the one nearest the top of the house, until water comes without a hiss. Then check the pressure gauge and top up to about 1.2 bar. If you have a tank in the loft, check it has water and the float valve is working. Restart the heating and see whether upstairs warms.

When should you call a heating engineer?

Call a heating engineer if bleeding and topping up do not help, if the pressure keeps falling, or if the system has never been balanced. Balancing takes a thermometer and time, and an engineer will set the lockshield valves so every radiator gets its share.

What will the heating engineer actually do?

• Bleed the system from the top down and check the pressure or tank level
• Set the pump speed for the system and check it moves water to the top
• Balance the radiators with the lockshield valves using measured temperatures
• Flush the system and fit a magnetic filter where sludge is a cause
• Check for pipe runs that trap air and add vents at high points

Who fixes this

• Heating not working
• Power flush

Questions people ask

Sources & References

1. Vaillant: how to repressurise a boiler
2. Vaillant: why is my boiler losing pressure?
3. Vaillant: what is a magnetic boiler filter?
4. Vaillant: heating controls