23 October 2026: The heating’s back on, the lounge is toasty, the kitchen’s warm, and then you go upstairs and it’s like nobody told the radiators. It’s the same call every October around Guildford, and it’s nearly always one radiator, or the first one on the run getting hot while the rest of the house sits there lukewarm.
Good news is it doesn’t automatically mean the boiler’s on its way out. Usually it’s air, a stuck valve, dirty water, or a system that’s never been balanced properly since it went in. Here’s the order we work through it in, starting with the things you can safely check yourself.
1. Check for air, then bleed carefully
Air trapped in a radiator can stop hot water filling it properly. If the top is cold while the bottom is warm, air is a likely culprit. (A radiator that’s cold at the bottom may point to sludge instead. More on that below.)
Turn the heating off and let the radiators cool before bleeding. Put a cloth beneath the bleed valve, fit a radiator key and open the valve slowly, just enough to hear the air escape. When it changes to a steady trickle of water, close the valve gently. Don’t unscrew it all the way.
Do one radiator at a time, working from downstairs to upstairs. Then check the boiler pressure against the instructions for your model. Bleeding can lower the pressure, so you may need to top it up using the filling loop if the gauge is below the recommended range. If you’re unsure how, check the boiler manual rather than guessing. And if pressure keeps dropping or you need to top it up regularly, there may be another issue to investigate.
We do this ourselves all the time, and honestly the giveaway is the noise. A radiator with air in it tends to sound like it’s gargling when the heating first comes on. If you bleed one and nothing but water comes out straight away, air probably isn’t your problem, so it’s worth looking further down this list rather than bleeding it again.
2. See if a TRV pin has stuck
A thermostatic radiator valve, or TRV, is the numbered control on many radiators. After sitting unused over summer, the small pin beneath its head can stick. That can leave one radiator cold even when the rest of the house warms up.
If you’re comfortable doing so, turn the heating off, let the radiator cool and remove the TRV head according to its instructions. The little pin underneath should move down slightly and spring back up. If it looks stuck, a gentle tap on the side of the valve body may free it. Don’t pull the pin out or hammer it. If it still won’t move, it’s time to have the valve checked.
3. Balance the radiators, don’t just turn the boiler up
If the radiators nearest the boiler heat quickly but those farther away stay cool, water may be taking the easiest route through the system. That’s where balancing comes in.
We adjust the lockshield valves, usually beneath small caps at the other end of each radiator, so the nearest radiators don’t take more than their share of the flow. That gives hot water a better chance of reaching the colder, farther rooms.
Turning the boiler temperature up isn’t the same thing. It makes the water hotter, but doesn’t correct uneven flow; the nearest radiators can get hotter while the far end of the house stays behind. Proper balancing is done gradually, checking how each radiator heats rather than shutting valves at random.
4. Look for sludge and magnetite
Older heating systems can collect black iron-oxide debris, often called sludge or magnetite. It can restrict flow through a radiator or pipework, and a radiator that’s warm at the top but cold across the bottom is a common clue. It’s not a perfect diagnosis, but it tells us where to look next.
Depending on how dirty the system is, a chemical clean may be enough, or it may need a powerflush. A powerflush uses flushing equipment to clean the system, working through radiators individually. A magnetic filter can also help catch circulating debris and protect the system, although fitting one won’t clear a radiator that’s already badly blocked.
We’re honest about the work involved: flushing can mean black water in buckets, so we put sheets down and protect carpets and nearby surfaces. A proper balance and flush is a half-day job in many houses, not a five-minute tweak. We book a visit at a time that suits the customer and allow enough time to do it carefully.
For more background, Fernox explains how powerflushing works. Whether it’s right for a particular system depends on its condition. A flush won’t repair a stuck valve, failing pump or unsuitable radiator.
5. If you have hot water but no heating
The controls matter too. A combi boiler may have a diverter valve that directs heat either to the taps or to the radiators. In a system with a hot-water cylinder, a three-port valve may direct flow between the cylinder and heating circuit.
If you have hot water but the radiators won’t heat despite a proper call for heating, a valve fault is one possibility. It’s not the first thing we’d assume when downstairs radiators are hot and upstairs ones are cold, though. First we’d check air, valves, balance and circulation. Diagnosing or replacing these valves is a job for a heating engineer; Viessmann’s guide to diverter valves explains some typical symptoms.
6. Consider the pump, radiator size and the rest of the system
The circulation pump moves water around the heating system. Its operation and setting can affect how well the heat reaches the far end, but simply turning it to maximum isn’t a reliable fix. The right setting depends on the system. A pump can also be noisy, airlocked or failing, which needs proper diagnosis.
Sometimes the pipework or radiator sizes are part of the picture. A radiator that’s too small for its room may struggle to make it comfortable, even when it’s working as it should. And a radiator in a conservatory can draw flow away from other rooms if the system isn’t set up or controlled well. We check the whole layout before recommending changes.
What if the boiler shows a fault code?
A fault code is different from a simple radiator-balancing problem. Note the code and check the boiler’s own manual for what it means. Don’t repeatedly reset the boiler or remove its casing to investigate. If there’s a fault, or you’re unsure what the code is telling you, have it checked by an appropriately qualified engineer.
Stop DIY if a valve is leaking, a bleed screw won’t close, the boiler pressure keeps falling, or you’d need to work on wiring or inside the boiler. For gas appliances, use a Gas Safe registered engineer.
Typical costs we see in the area
As a rough guide from our own experience, a visit for bleeding and balancing might be around £90–£180. A powerflush can be about £350–£650, a magnetic filter around £150–£300 fitted, and a replacement valve roughly £180–£350. These are approximate ranges, not fixed quotes: radiator count, access, system condition and the parts needed all affect the price. We’ll talk through what we find before recommending work.
If some rooms are warm and others aren’t, there’s usually a reason, and it’s usually findable in an afternoon rather than by turning everything up and hoping for the best. Most of the time it comes back to air, a stuck pin, a system that’s never been balanced or a radiator half full of black sludge, and we’d much rather tell you which of those it is than sell you a new boiler you don’t need. You can read more about our plumbing and heating services, or book a visit through our contact page at a time that suits. We serve Aldershot, Farnborough, Farnham, Guildford and surrounding areas.