Why Are My Upstairs Radiators Cold When Downstairs Is Hot?

If your upstairs radiators are cold while the downstairs ones are hot, the problem is almost always in the system rather than the radiators themselves. The usual causes are trapped air, a zone valve stuck shut, a pump that is too weak to push water upstairs, a badly balanced system, or, on older homes with a tank in the loft, a feed tank that has run dry. The reverse, cold downstairs and hot upstairs, has a shorter list of causes, covered further down. If it is one radiator rather than a whole floor, start with our guide to why a radiator is cold at the top, bottom or all over.

A whole floor going cold at once points to something shared by that floor, such as a zone valve, the pump or the water supply. A few scattered cold radiators upstairs usually point to air or balancing instead.

Before calling anyone, it helps to note three things. First, feel the pipes going into an upstairs radiator with the heating on: cold pipes mean no hot water is reaching that floor at all. Second, check the pressure gauge on the boiler if you have a sealed system; a low reading is a strong clue. Third, check whether your controls have separate settings for upstairs and downstairs. If they do, you have zoned heating, and the upstairs zone may simply not be calling for heat. These three checks usually narrow the cause to one or two of the faults below.

1. Air Trapped at the High Points

Air rises through a heating system and collects in the highest radiators, so upstairs radiators are the first to go cold at the top when air gets in. Bleeding them usually fixes it, but air that keeps coming back means something is letting it in.

A radiator that is warm at the bottom and cold at the top, with a hiss when you open the bleed valve, has air in it. Bleed the downstairs radiators first and the upstairs ones last, with the heating off and the radiators cool; our guide on which radiator to bleed first sets out the order. On a sealed system, check the pressure afterwards, because bleeding lets water out as well as air. If the same radiators need bleeding every few weeks, the system is drawing air in or producing gas from corrosion, and bleeding is only treating the symptom.

2. A Zone Valve Stuck Shut

If your home has separate heating zones for upstairs and downstairs, a motorised zone valve that has stuck closed will leave the upstairs cold no matter what the thermostat says. This is one of the most common causes of a whole floor going cold.

Zone valves have a small electric motor head that opens the valve when that zone's thermostat calls for heat. When the motor fails, or the valve spindle seizes, the valve stays shut and the upstairs pipes stay cold. A valve stuck open does the opposite and leaves one floor heating when it should not. If the upstairs pipes are cold right back to where they leave the airing cupboard or boiler area, and the upstairs thermostat is calling for heat, a stuck valve is the first suspect. Our article on zone valve problems covers the symptoms in more detail. Replacing the motor head or the valve is work for a heating engineer, and it involves the boiler wiring, so it is not a DIY job.

3. A Pump That Cannot Push Water Upstairs

The central heating pump has to lift water to the top floor as well as push it round the house. A pump that is failing, set too low, or blocked with sludge will keep the downstairs warm and leave the upstairs short.

Pumps wear out slowly, so this often shows as upstairs radiators that used to be hot and have become lukewarm over a season or two. A pump that hums, runs hot, or makes a grinding noise is a clue. On a combi or system boiler the pump is inside the boiler casing, and on older systems it usually sits next to the hot water cylinder. Either way, testing or adjusting it is an engineer's job: on a boiler it means working inside a gas appliance, which must be done by a Gas Safe registered engineer. Turning a pump up can mask a deeper problem, such as sludge, so a good engineer checks why the flow has dropped before changing the setting.

4. The System Is Out of Balance

Water takes the easiest route, so radiators close to the boiler, usually downstairs, take most of the flow and the upstairs ones get what is left. Balancing sets each radiator's lockshield valve so the flow is shared properly.

Imbalance tends to show as upstairs radiators that warm up slowly and never get as hot as the downstairs ones, rather than staying stone cold. It is common after a radiator has been replaced, after decorating when radiators were taken off, or after a new extension or loft room has been added to the same pipework. Balancing means adjusting the lockshield valves room by room and checking the temperature difference across each radiator, which takes time but costs little in parts. Our radiator balancing service is often the whole fix when the pump and valves turn out to be fine.

5. An Empty Feed and Expansion Tank (Older Systems)

On an open-vented system, a small tank in the loft keeps the heating topped up. If that tank runs dry, the highest part of the system empties first, so the upstairs radiators go cold while the downstairs still work.

You have this type of system if there is no pressure gauge on the boiler and there are two tanks in the loft, one large for cold water and one small for the heating. The small tank is filled by a float valve, the same kind of valve found in a toilet cistern. If that valve sticks, the tank slowly empties as the system loses water. Look in the small tank: it should have a few centimetres of water in the bottom when the heating is cold. An empty tank means the system needs refilling and the float valve needs freeing or replacing. A system that keeps losing water also has a leak somewhere, which needs finding.

6. Sludge in the Upstairs Pipework

Sludge collects in the slowest-moving parts of the system, and narrow microbore pipe to upstairs radiators blocks more easily than the wider pipe downstairs. Radiators that are cold at the bottom and dark water when bled are the signs.

If several radiators upstairs are cold at the bottom, the pipes feeding them are running warm rather than hot, and the bleed water comes out black, the system is carrying a lot of iron oxide sludge. Balancing will not fix that, and a stronger pump just moves the problem around. A power flush to clean the whole system, followed by inhibitor and ideally a magnetic filter, is the lasting fix.

What If It Is the Other Way Round: Downstairs Cold, Upstairs Hot?

Cold downstairs radiators with hot upstairs ones usually come down to a stuck zone valve, sludge settled in the lowest pipes, or a balancing problem. Air is rarely the cause, because air rises to the top floor.

Sludge is heavy and settles in the lowest parts of the system, so downstairs radiators on long pipe runs, and pipes buried in a solid ground floor, are where it collects. A ground floor radiator that is cold at the bottom on a system with dirty water points that way. If the downstairs is a separate zone, its valve may be stuck in the same way described above. If you are not sure which fault you have, an engineer can usually narrow it down on one visit by checking pipe temperatures, valve operation and the water condition.

One floor of the house staying cold? We check the valves, pump, balance and water condition, tell you plainly which one is at fault, and give you a fixed price before any work starts. Book a visit.