Cold Radiators, Lost Pressure and Heating Controls
Fix Your House repairs central heating systems and radiators for homeowners, landlords and tenants across North West London. Cold radiators, panels that are hot at the top and cold at the bottom, system pressure that will not hold, leaking heating pipework, seized valves and controls that no longer match the way the house is actually used. Based in Mill Hill, we work on the wet side of the system: radiators, valves, pipework, pumps and heating controls.
Nearly every heating fault we attend comes down to one of three things — air, sludge, or no flow. Air sits at the high points. Sludge settles at the low points. No flow means something is shut, seized, blocked or pumping in the wrong direction. Working out which of the three you have is the whole job, because the wrong fix on the wrong fault is money spent for nothing.
Free call-out and assessment — you only pay for the work. The phone is answered 24 hours a day, and for urgent problems we are typically with you in under an hour.
Mill Hill, Hendon, Edgware, Finchley, Golders Green and across North West London
Is Your Heating No Longer Doing What It Used To?
A heating system rarely fails all at once. It gets slower, more uneven and more expensive to run, and by the time one radiator is stone cold the problem has usually been building for two or three winters. Water that has been round the same loop for twenty years carries the evidence of every repair, every top-up and every extension anyone has bolted onto it.
These are the symptoms we are called out to most often across North West London:
- One radiator cold while the rest of the house heats up
- Radiators hot at the top and cold along the bottom
- Radiators hot at the bottom and cold at the top
- Upstairs warm and downstairs cold, or the other way round
- The radiators furthest from the pump never getting properly hot
- System pressure dropping week after week
- Gurgling, trickling or banging noises in the pipework
- Damp patches or staining under floors, at valves or at pipe joints
- Radiator valves that turn freely but change nothing
- Black water coming out when you bleed a radiator
- Rooms that were perfectly warm before an extension was added
- A thermostat in a hallway that no longer reflects how the house is used
- Heating that takes far longer to come up to temperature than it once did
Cold Radiators
A radiator that stays cold while the others heat up is telling you something specific. Where the cold sits — the top, the bottom, or the whole panel — points to air, sludge or a valve that is not letting water through. We read the panel first and only then decide what to do to it.
System Pressure Loss
A sealed system that needs topping up every few weeks is losing water somewhere, and every top-up brings in fresh oxygenated water that speeds up corrosion. We trace where it is going rather than teaching you to live with the filling loop.
Heating Pipework Leaks
Weeping valve glands, corroded joints under floorboards, pinholes in old microbore and failed radiator tails. Small heating leaks stain ceilings and rot floors long before anyone notices a puddle.
Bleeding and Balancing
Bleeding gets the air out. Balancing decides how much hot water each radiator is allowed to take. A house where the first radiator is scalding and the last is lukewarm has never been balanced, and no amount of bleeding will fix that.
Radiator Replacement and Relocation
Replacing a corroded panel, resizing one that was never big enough for the room, or moving a radiator so a room can finally be laid out the way you want it. Including the pipework alterations that go with it.
Thermostats, TRVs and Controls
Seized thermostatic valve heads, a room thermostat in the coldest corner of the hall, and controls that were set up for a household that no longer lives the way it did. Often the cheapest comfort improvement in the house.
Air, Sludge or No Flow: What Is Actually Wrong
Central heating is a closed loop of water pushed round by a pump. When it stops working properly, the cause is almost always one of three things, and telling them apart before touching anything is what stops a repair turning into a series of guesses.
Air
Air collects at the highest point it can reach and stays there. It gets in when a system is drained and refilled, when pressure is allowed to drop far enough to draw it past a gland, and as hydrogen produced by corrosion inside the radiators themselves. Air-filled radiators are cold at the top and hot at the bottom, and they gurgle. A vent at the high point of the system lets it out. Many North West London lofts have a heating run at the highest point of the house with nothing to vent it, so the air keeps returning and no one understands why.
Sludge
Sludge is magnetite — black iron oxide produced as steel radiators corrode from the inside. It is heavier than water, so it settles wherever the water slows down: the bottom of radiator panels, the bottom of the pump, and the bends in horizontal pipe runs. Sludged radiators are hot at the top and cold along the bottom, often with a cold strip that grows year on year. Bleeding does nothing for it, because there is no air in there.
No flow
Nothing is wrong with the water; it simply is not getting there. A lockshield closed by a decorator, a thermostatic head seized shut on its pin, a pump running at the wrong speed or the wrong way, a diverter valve stuck, or a circuit so badly balanced that the water takes the short route and never reaches the far end of the house. A radiator that is cold all over, top and bottom, while the pipes feeding it are warm is nearly always this.
Three faults, three completely different repairs. This is why we would rather spend twenty minutes reading the system than an afternoon flushing something that did not need flushing.
Cold Radiators: Reading the Cold Part
Before anyone opens a valve, put a hand on the panel in three places — top centre, bottom centre, and each end. The pattern of what is warm and what is not narrows the fault down further than any description over the phone.
Hot at the top, cold at the bottom
Sludge. Iron oxide has settled into the bottom of the panel and the water is now flowing over the top of a solid layer. The cold band usually starts as a couple of inches and creeps upward over several winters. Bleeding will not touch it and neither will turning the valve up. The radiator either needs removing and flushing out, or the system needs cleaning properly and the panel replacing if it is too far gone.
Hot at the bottom, cold at the top
Air. The panel has filled from below and the trapped pocket at the top will not let the water rise into it. This one is often genuinely a five-minute bleed — but if it comes back within a few weeks, the air is being made or drawn in continuously and the reason for that is the actual fault.
Cold all over, feed pipes warm
No flow. Something between the pipe and the panel is shut. Nine times out of ten it is a thermostatic head seized on its pin after a summer of sitting in one position, and it frees with a firm push once the head is off. The rest of the time it is a lockshield that was closed and never reopened, or a valve that has failed internally.
Cold at one end only
Poor circulation through that panel — usually a partly blocked tail or a badly balanced circuit sending most of the flow somewhere easier. Common on the last radiator of a run, and very common on a radiator that was added to the end of an existing circuit when a room was extended.
Every radiator lukewarm
This one is not a radiator problem. Look at the pump, the system pressure and the controls before touching any panel.
Bleeding and Balancing: Two Different Jobs
These get talked about as if they were the same thing. They are not, and mixing them up is why a lot of houses get bled every autumn and are still cold in the back bedroom every February.
Bleeding
Bleeding removes trapped air from an individual radiator. Heating off, water cooled, key onto the vent at the top corner, quarter turn, wait for the hiss to become a dribble, close it. Start with the radiator lowest and nearest the pump and work towards the highest and furthest. Then check the pressure, because letting air out drops it — and a system that falls below its working pressure will simply draw more air in and undo what you just did.
Balancing
Balancing sets how much of the flow each radiator is allowed to take, using the lockshield valve at the opposite end from the control valve. Water, like electricity, takes the easiest route: without balancing, the radiators nearest the pump run wide open and take almost everything, and the far end of the house never gets properly hot no matter how high the thermostat goes.
Doing it properly means opening every lockshield, letting the system come up to temperature, then closing each one back down until the difference between the flow and return at that radiator sits in the right range — tighter on the near radiators, more open on the far ones. It takes a couple of hours on an average house and it is permanent. Most systems we see have never had it done, or had it done once before an extension changed the shape of the circuit.
If your heating is uneven rather than broken, balancing is usually the answer, and it costs nothing in parts.
Low System Pressure and Where the Water Goes
A sealed heating system holds a fixed volume of water under pressure, normally somewhere around one bar when cold. Topping it up once after a repair is normal. Topping it up every few weeks is not — it means water is leaving the system, and each refill brings in fresh oxygenated mains water, which is exactly what feeds the corrosion that makes sludge.
The usual causes, in the order we tend to find them:
- A leak on the pipework. Often tiny, often under a floor, often evaporating off warm pipe before it ever makes a visible mark. Staining on a ceiling below is the giveaway.
- Weeping valve glands. Old packing on a valve spindle that has been fine for years will start to seep once a system is repressurised to modern sealed-system levels. Very common on original 1970s and 1980s valves.
- A failed expansion vessel. When the vessel loses its air charge, the water has nowhere to expand into, the pressure spikes when hot and the relief valve dumps some outside. Pressure then reads low when cold. The tell is a large swing between cold and hot readings.
- A passing pressure relief valve. Once a relief valve has lifted and caught a bit of debris, it often never seats properly again and drips continuously to the outside discharge pipe.
- Air being bled out. Legitimate, but if you are bleeding often enough for it to matter, see the first item on this list.
Finding out which of these you have is a matter of watching how the pressure behaves cold versus hot, and where it falls to before it stops. We would rather do that than sell you a repair to something that was never leaking.
Heating Pipework Leaks
Heating leaks behave differently from plumbing leaks. The water is hot, so a small one evaporates off the pipe and never forms a puddle; it is also full of iron oxide, so when it does mark something it leaves a rust-brown stain that is unmistakable once you know it. By the time a heating leak is obvious it has usually been going for months.
Where we find them:
- Radiator tails and the compression joint where the valve meets the panel — the single most common point
- Valve spindle packing, which weeps only when the valve is part-open
- Soldered elbows under floorboards, particularly where a joint was made in a tight space
- Pinholes in microbore, which corrodes from the inside and fails without warning
- Joints disturbed by later work — new flooring, a kitchen refit, cables run under boards
Finding a heating leak means reading the pressure behaviour, isolating sections of the circuit, and lifting the smallest area of floor that will answer the question. We repair the joint or replace the affected run rather than patching around it, and we make good what we opened.
If water is actively running, close the valves at both ends of the nearest radiator, turn the heating off and call us. The phone is answered 24 hours a day and for an active leak we are typically with you in under an hour.
Thermostats, TRVs and Heating Controls
A surprising share of the “my heating is not working properly” calls we take are not faults at all. They are systems that were set up for a household that no longer exists, or controls that have quietly stopped doing what they appear to be doing.
Thermostatic radiator valves
A TRV head contains a pin that is pushed by a wax or liquid capsule as the room warms. Left in one position all summer, the pin corrodes into its seat and the valve stays where it was — usually shut. Pulling the head off and freeing the pin fixes it in minutes. If the pin moves freely and the radiator is still cold, the capsule has failed and the head needs replacing; the valve body normally does not.
TRVs also get fitted in the wrong place. One behind a sofa, above a lamp, or in the same room as the main thermostat will fight the system rather than help it.
Room thermostats
The classic North West London arrangement is a thermostat in the hall, next to the front door, in the coldest and draughtiest part of the house. It calls for heat the whole time the door has been opened and switches off once the hall warms, regardless of what the living room is doing. Moving it, or moving to controls that read the rooms people actually sit in, changes comfort more than any amount of work on the radiators.
Zones and timers
Houses that have been extended, converted or split into flats often still run on a single zone and a timer set years ago. Splitting upstairs from downstairs, or the extension from the original house, lets each part run to its own schedule and stops one part of the house being heated for the sake of another.
Power Flushing — and When It Will Not Help
A power flush pushes water and cleaning chemical round the system at high velocity, in both directions, to lift settled magnetite out of the radiators and pipework and pull it out through a magnetic filter. On the right system it makes a real difference: radiators that were cold along the bottom heat evenly again, and the water that comes out at the end runs clear instead of black.
When it works
When the sludge is loose and distributed — a system that has never been cleaned, water that runs dirty when you bleed it, several radiators cold along the bottom, and pipework that is otherwise sound.
When it will not help, and we will say so
- When the problem was never sludge. Air at the top of a panel, a seized TRV pin or an unbalanced circuit will be exactly as wrong afterwards as before. This is the most common wasted flush.
- When a radiator is fully blocked solid. Once a panel is packed hard, flushing water takes the open route around it. That radiator has to come off and be cleared, or be replaced.
- When the pipework is microbore and heavily corroded. High-velocity flushing through thin, pitted pipe can open up pinholes that were holding. On a system in that condition we would rather talk about replacing the affected runs.
- When there is an existing leak. Flushing first will make it worse. Find and fix the leak, then clean the system.
- When nothing is fitted to keep it clean afterwards. Without a magnetic filter and inhibitor, a flushed system starts making magnetite again from the day it is refilled, and you are back where you started in a few years.
A chemical clean followed by a proper drain and refill is often enough, and on some systems it is the safer option. We will tell you which one your system is a candidate for before anything is booked.
Radiator Replacement, Resizing and Relocation
Not every radiator problem is worth repairing, and not every radiator is in the right place or the right size for the room it is in.
Replacement
A panel that is blocked solid, pitted from the inside or weeping at a seam has reached the end of its life. Swapping it is straightforward on existing tails; where the new panel is a different height or width, the pipework is altered to suit and the floor made good.
Resizing
A radiator that has always struggled to heat its room is often simply too small for it — usually because the room changed after the radiator was fitted. Knocking two rooms together, replacing a solid wall with glazing, or converting a cold loft into a bedroom all raise what that space needs, and nobody goes back to check the radiator. Sizing is a calculation based on the room volume, the glazing and the insulation, not a guess, and getting it right often removes a problem that has been blamed on the heating system for years.
Relocation
Moving a radiator off the wall you want to put a sofa, a wardrobe or a kitchen run against. This is pipework rather than radiator work: we plan the route, alter the runs, lift and relay the minimum of flooring, and refill and rebalance the system afterwards so the change does not cost you heat somewhere else in the house.
Whichever of the three it is, the system is drained down properly, inhibitor is put back in on refill, and every radiator is rechecked before we leave — because taking one panel off a circuit changes the flow to all the others.
What North West London Housing Does to a Heating System
The housing stock here has a few recurring habits, and after enough call-outs the same handful of situations come round again and again.
Interwar semis with three generations of pipework
The Metro-land semis that make up so much of Mill Hill, Edgware, Hendon and Finchley were built long before central heating was normal. What is in them now is typically a 1970s installation, altered in the 1990s, altered again for a kitchen or a loft conversion, with each generation joining onto the last. Steel, copper and plastic all in one loop, with the original runs still buried where nobody expects them.
Loft runs with no vent at the high point
Hendon and Golders Green have a great many converted lofts where the heating was carried up into the roof space at the time of the conversion. If no automatic air vent was fitted at the top of that run, air collects there permanently. The householder bleeds the top radiator every winter, it works for a fortnight, and nobody ever explains why it keeps coming back.
Extensions hung off the end of a circuit
A rear extension or a side return gets its radiators teed off the nearest existing pipe, at the far end of a circuit that was already at its limit. Nobody rebalances the system afterwards. The extension is cold, the original front rooms are as hot as ever, and everyone assumes the new radiators are faulty.
Microbore under later flooring
A lot of 1970s and 1980s work here used microbore. It blocks easily because the bore is small, it corrodes from the inside, and it is very often now sitting under engineered wood or tiles laid decades later. That combination decides how a repair has to be approached, and it is worth knowing before work starts rather than halfway through.
Old valve packing on modern pressures
Systems originally run on an open vent and a header tank in the loft sat at very low pressure. Converted to a sealed system, the same forty-year-old valve spindles are asked to hold far more, and the packing starts to weep. It is a common reason for pressure that will not hold in a house where nothing appears to be leaking.
What to Check Before You Call Anyone
Some of this you can do in ten minutes, and any of it may save you a call-out — ours or anyone else’s. None of it involves opening anything sealed or touching an appliance.
- Read the pressure when the system is cold, then again when it is hot. Write both down. A big swing and a low cold reading point somewhere quite different from a steady reading that just sits too low.
- Feel each radiator in three places — top, bottom and both ends — and note the pattern. This is the single most useful thing you can tell us on the phone.
- Pull the head off any TRV on a cold radiator and look at the pin underneath. If it is stuck down, press it; if it springs back and the radiator warms, that was your fault.
- Check the lockshield at the other end of the radiator has not been closed. It usually hides under a small plastic cap.
- Look at the timer and the thermostat. Clocks lose an hour twice a year, batteries die, and holiday or frost settings get left on.
- Look for staining on ceilings below the heating pipework and around the base of radiators.
- Bleed one radiator and look at the water. Clear is a good sign; black tells you there is magnetite in the system.
If you have done all of that and the picture still does not make sense, that is exactly the point at which it is worth someone looking at it properly.
How the Work Goes
You describe the pattern
On the phone or by WhatsApp. Which radiators, which part of each panel is cold, what the pressure reads cold and hot. A photo of the pressure dial helps. The phone is answered 24 hours a day.
We attend and assess
Free call-out and assessment — you only pay for the work. For urgent problems we are typically with you in under an hour.
We tell you what it is
Air, sludge or no flow, which radiators are affected, what the repair involves and what it will cost, before anything is started. If we think a job is not worth doing, we say that too.
The work is carried out
Floors protected, the system drained down properly where it needs to be, inhibitor replaced on refill, and the whole system rechecked and rebalanced before we leave.
Central Heating and Radiator Repairs Across North West London
Based in Mill Hill, covering North West London and nearby areas of Hertfordshire.
Why Choose Fix Your House for Heating and Radiator Work
Based in Mill Hill
We work in these streets every week and know how the housing stock here is put together, from interwar semis to loft conversions and rear extensions.
Typically Under an Hour
For urgent heating problems and active leaks we are typically with you in under an hour across North West London.
24-Hour Phone Line
The phone is answered day and night. A heating leak at two in the morning is not a message left on an answering machine.
Free Call-Out and Assessment
Free call-out and assessment — you only pay for the work. You get told what is wrong whether or not you go ahead with us.
We Explain the Reasoning
Which of air, sludge or no flow you have, why we think so, and what each option would and would not achieve — before anything is booked.
We Will Talk You Out of Work
If a flush would not fix your problem, or a radiator is worth cleaning rather than replacing, we say so. It costs us a job and keeps a customer.
Central Heating and Radiator FAQs
Why is one radiator cold when the rest of the house is warm?
It is almost always air, sludge or no flow, and where the cold sits tells you which. Cold at the top with a hot bottom is trapped air. Hot at the top with a cold band along the bottom is sludge settled inside the panel. Cold all over while the pipes feeding it are warm means water is not getting in at all, usually a seized thermostatic valve pin or a lockshield that was closed and never reopened.
What does it mean if a radiator is hot at the top and cold at the bottom?
That is sludge. Iron oxide has settled into the bottom of the panel and hot water is now flowing over the top of it. Bleeding will not help, because there is no air in there. The panel needs removing and flushing out, or the system needs cleaning properly, and if the radiator is packed solid it may be past saving.
Why does my central heating pressure keep dropping?
Water is leaving the system somewhere. The usual causes are a small leak on the pipework, weeping packing on an old valve spindle, a failed expansion vessel, or a pressure relief valve that has lifted once and never reseated. Repeatedly topping up is not a fix: every refill brings in fresh oxygenated water, which accelerates the corrosion that produces sludge.
Will a power flush fix my heating?
Only if the problem is actually sludge. A flush will not help if the fault was air, a seized valve or an unbalanced circuit, and it cannot clear a radiator that is already blocked solid because the water simply takes the open route around it. On heavily corroded microbore it can open pinholes that were holding, and if there is an existing leak it should be fixed first. We will tell you honestly whether your system is a candidate before anything is booked.
Can you move a radiator or fit a bigger one?
Yes. We replace, resize and relocate radiators, including the pipework alterations and making good the flooring afterwards. Resizing is worth checking whenever a room has been extended, opened up, reglazed or converted, because the radiator that suited the old room is often simply too small for the new one.
Do you repair boilers or work on gas appliances?
No. We are not Gas Safe registered and do not carry out gas work. We repair and replace radiators, pipework, valves and heating controls. For anything involving the boiler or a gas appliance you will need a Gas Safe registered engineer.
How quickly can you come out?
The phone is answered 24 hours a day, and for urgent problems such as an active heating leak we are typically with you in under an hour across North West London. For non-urgent work we will agree a time that suits you.
Do you charge to come and look at it?
Free call-out and assessment - you only pay for the work. You will be told what we think is wrong and what the repair involves before anything is started, whether or not you go ahead.
Need Central Heating or Radiator Work in North West London?
Tell us which radiators are cold and where the cold part sits, and what the pressure reads. That is usually enough for us to know what we are coming to.
Free call-out and assessment — you only pay for the work.
After the job
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