When that happens, it’s usually your heating system showing you that hot water is reaching part of the circuit easily, but struggling to move upwards. Once you understand why, the whole situation becomes much less confusing.
1. The First Sign Something Was Off
The thing that stood out to me was the timing. Every time the heating turned on, the downstairs radiators reacted straight away. The upstairs ones either took ages to warm or didn’t heat properly at all. It wasn’t random — it happened the same way every day.
That’s usually the clearest hint that circulation isn’t reaching the upper floor properly.
2. Low Pressure Affects Upstairs Before Anywhere Else
One of the biggest reasons this problem shows up is boiler pressure dropping slightly below where it needs to be. When the pressure is low, hot water struggles to climb the pipework to the upper floor. The ground floor radiators still heat because they require less effort from the system.
When this happened to me, the gauge was just under 1 bar. After topping it up to around 1.3–1.4 bar, the upstairs radiators finally started heating like they should.
3. Air in the Upstairs Radiators Can Block Heat Completely
Air naturally rises in a heating system, which means upstairs radiators are usually the first victims when air builds up. Even a small amount of trapped air can stop hot water from filling the radiator properly.
I bled all the upstairs radiators — and even though only one of them hissed noticeably, all of them heated better afterward. Sometimes the air isn’t trapped at the very top; it can sit just deep enough to disrupt circulation.
If your issue is specifically a radiator that’s cold at the top, you might want to check this article:
Radiator Warm at Bottom but Cold at Top
4. Downstairs Radiators Might Be Taking Too Much Flow
I didn’t realise this at first, but the downstairs radiators were simply getting more than their fair share of hot water. Their lockshields were set too open, so all the flow was being taken before it ever reached upstairs.
Once I turned the lockshields downstairs slightly towards closed, the balance changed almost immediately. The upstairs radiators finally woke up because the system wasn’t being drained by the downstairs ones.
If you haven’t balanced your radiators before, this is where it really makes a difference.
5. The Pump Might Not Be Pushing Hard Enough Upstairs
A weak or incorrectly set pump can also cause this exact pattern. When the pump isn’t circulating strongly, the easiest path (downstairs) gets the flow first. The upstairs radiators end up with whatever’s left, which isn’t enough to keep them warm.
I’ve had this happen in a previous house. The pump wasn’t broken — it was just set too low. After adjusting it to a higher setting, the upstairs radiators suddenly behaved like part of the system again.
6. Pipe Layout Plays a Big Factor
Some homes naturally favour downstairs radiators because of how the pipes were routed. The boiler might be on the ground floor, and the flow paths might reach downstairs long before they reach upstairs. It’s not a design flaw — just how many older heating systems were built.
Balancing and proper pressure usually overcome that limitation, but it explains why the issue shows up consistently in certain houses.
7. What Finally Fixed Mine
In my case, it took a combination of topping up the pressure and slightly tightening the downstairs lockshields. Once the flow was more evenly shared, the upstairs radiators warmed in sync with the ground floor. The difference in comfort was instant.
This problem almost never means your boiler is failing — it’s almost always circulation.
8. If You Want the Whole House to Heat Properly
This issue is just one piece of the bigger puzzle of getting your home warm efficiently. If you want to understand how to keep every room comfortable without wasting energy, here’s the guide that ties everything together:
