Radiators not working upstairs or downstairs: why one floor is cold
When every radiator on one floor stays cold while the other floor heats normally, the radiators themselves are rarely the problem. Something is stopping hot water reaching that floor. The usual causes are a zone valve that controls that floor not opening, air collected in the upstairs pipes, system pressure that has dropped too low to push water to the top of the house, radiators that are badly balanced so the water takes the shortest route, or a pump that is too weak to reach the far end of the system. As a rule of thumb, cold upstairs radiators point to air, pressure or the pump, and cold downstairs radiators point to balancing, a sludged pipe or a zone valve. Five minutes of checks will usually tell you which.
Loyal Plumbers · Updated
Discuss your plumbing workQuick checks before anything else
Start with the things that are easy to miss. Walk round the cold floor and make sure the thermostatic radiator valves are turned up, not left on the frost setting from the summer. If your house has two room thermostats or a programmer with two heating channels, often labelled Heating 1 and Heating 2 or Upstairs and Downstairs, you have a zoned system, and the cold floor's thermostat or timer channel may simply be off.
Next, look at the boiler's pressure gauge if it has one. On a sealed system, the pressure usually sits somewhere around 1 to 1.5 bar when the heating is cold, but check the boiler's manual for the figure it gives. Then feel the pipes going into a cold radiator. If the pipes are hot and the radiator is cold, the problem is at that radiator. If the pipes are cold too, hot water is not reaching that part of the house.
What the pattern tells you
The floor that is cold, and what else is going on, narrows it down.
| What you notice | Most likely cause | What to check |
|---|---|---|
| Upstairs cold, downstairs warm, gurgling or hissing in the pipes | Air in the upstairs circuit | Bleed the upstairs radiators, then check the pressure |
| Upstairs cold, boiler pressure low or keeps falling | Not enough pressure to lift water to the top floor | Top up the pressure and look for a leak |
| One floor cold on a house with two thermostats or two heating channels | Zone valve for that floor not opening | Check that zone's thermostat and timer, then the valve |
| Downstairs cold, upstairs hot | Poor balancing, a sludged pipe or a zone valve | Check lockshield valves and whether radiators are cold at the bottom |
| Radiators near the boiler hot, far ones cold on every floor | System badly balanced or pump too weak | Balance the radiators; have the pump checked |
| Floor went cold after work on the heating | Valves left shut, or air let in when the system was drained | Check isolating and lockshield valves; bleed |
Zoned systems and zone valves
Larger houses often have the heating split into zones, typically upstairs and downstairs, each with its own room thermostat and a motorised zone valve on the pipework near the boiler or cylinder. When a zone calls for heat, its valve opens, a switch on the valve tells the boiler to fire, and hot water flows to that floor only. If the valve's motor fails or the valve sticks shut, that zone stays cold while the other works normally.
Some valve heads have a small manual lever that shows whether the valve is open. It can be a useful clue, but do not take the valve head off: it is usually wired at mains voltage. A failing motor head is a common and fairly quick repair. Our guide to zone-valve faults describes the symptoms in more detail, and if the thermostat for the cold zone seems dead, read our page on a thermostat not working first.
Air and pressure: when upstairs is cold
Air always rises to the highest point, which in most houses means the upstairs radiators. A radiator full of air is cold at the top and warm at the bottom, and if several upstairs radiators are full of air, the whole floor can feel cold. Our guide on how to bleed a radiator explains how to let it out safely, and the page on a radiator cold at the top covers the symptom in detail.
On a sealed system, bleeding lets water out as well as air, so the pressure drops. If it falls too far, the pump cannot lift water to the top floor at all, and the boiler may lock out. Top it back up after bleeding. If you find yourself bleeding the same radiators or topping up the pressure every week or two, air or water is getting in or out somewhere. Our page on a boiler losing pressure explains where to look, and our guide to central heating inhibitor explains why corrosion inside the system can keep producing gas.
Older open-vented systems, with a small feed and expansion tank in the loft, can suck air in or pump water over into that tank if the pump or pipework is not set up right. That also shows up as upstairs radiators that keep needing to be bled.
Balancing: when downstairs is cold
Hot water takes the easiest route round the system. If the radiators on one floor have their lockshield valves wide open and the others are nearly shut, one floor gets most of the flow and the other gets whatever is left. This often happens after radiators have been replaced or added, or after someone has adjusted valves while decorating.
The fix is to balance the system: adjusting each radiator's lockshield valve so the ones nearest the boiler are held back and the far ones get their share. Our guide on how to balance radiators goes through it step by step, and the page on a lockshield valve explains what that valve does and how to adjust it.
Pump and sludge problems
The heating pump has to push water round every radiator on every floor. A pump set to too low a speed, or one that is wearing out, may manage the ground floor but not the top of the house. A pump that is noisy, very hot to touch or that stops and starts is worth having checked. Our guide to heating pump faults explains how they are tested and when they are replaced.
Sludge is the other culprit. If radiators on the cold floor are warm at the top but cold across the bottom, sludge has settled in them. If a whole floor is fed through one narrow pipe, or through microbore pipes, a build-up in that pipe can starve every radiator after it. The pages on a radiator cold at the bottom and on microbore pipes explain those cases, and when sludge is widespread, power flushing cleans the whole system.
When to call an engineer
Bleeding, topping up the pressure and checking the settings are all reasonable to try yourself. Call an engineer if:
- The floor is still cold after bleeding and checking the thermostats and timer
- The boiler pressure keeps dropping, or keeps rising when the heating is on
- You have a zoned system and one zone never comes on
- The pump is noisy, hot or keeps stopping
- Radiators on the cold floor are cold across the bottom, or the water you bleed is black
- The boiler shows a fault code or has locked out
What an engineer will do
An engineer will check the pressure and the controls for each zone, test the zone valves and their switches, check the pump speed and operation, and look for air, leaks and sludge. From there the fix may be a new valve motor, a pump adjustment or replacement, balancing the radiators, or cleaning the system. Any work on the boiler itself is carried out by a Gas Safe registered engineer.
Get heat back on every floor
Loyal Plumbers covers London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. We repair zone valves, pumps, radiators and boilers, and balance and clean heating systems. Call 020 4652 8585 and tell us which floor is cold, what the boiler pressure reads, and whether you have one thermostat or two.
Our call-out charge is £60, and the engineer explains what they have found and what the work involves before starting. You can read more about our central heating repairs and our radiator repair and replacement work, or about the individual case of a radiator not heating up in one room.
Common questions
- Why are my upstairs radiators cold but downstairs hot?
- Usually air trapped in the upstairs radiators, pressure too low to lift water to the top floor, or a zone valve for the upstairs zone not opening. Bleed the upstairs radiators, check the boiler pressure and check the upstairs thermostat and timer.
- Why are my downstairs radiators not working when upstairs is fine?
- The most common reasons are poor balancing, where the upstairs radiators take most of the flow, a zone valve for the downstairs zone that is stuck, or sludge in the pipe feeding the ground floor.
- Should I bleed upstairs or downstairs radiators first?
- Work from the ground floor up, finishing with the highest radiators, since air collects at the top of the system. On a sealed system, check and top up the boiler pressure when you have finished.
- How do I know if my heating has zones?
- Look for two room thermostats, or a programmer with two heating channels. Near the boiler or cylinder you will usually find two or more motorised valves on the heating pipework, each with a small box-shaped head.
- Do I need a power flush if one floor is cold?
- Not always. Air, pressure, a zone valve or balancing are more common causes. A power flush is worth considering when radiators on that floor are cold across the bottom or the water from them is dark with sludge.
Sources and further reading
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