Cold radiator. Boiler light on. No idea why. It’s a problem where you hope it’ll fix itself over time, unfortunately it won’t but it was worth trying.
The good news is that central heating not working is rarely as serious as it first seems. A surprising number of call-outs turn out to be low pressure, a stuck valve, or air in the system. Things you can check and often fix yourself in under half an hour. Throughout this blog, I’ll talk about the most common problems and how you can go about checking them, so you’re not wasting money on a service you didn’t even need.
Why Is My Central Heating Not Working?
The first thing worth doing when central heating is not working is figuring out what’s actually failing. Is the boiler completely dead? Is it running but nothing is getting warm? Are some radiators fine and others stone cold? Each of those points to something different, so it’s worth taking a minute to observe before diving in.
The Boiler Isn’t Firing Up
Before anything else, check the obvious stuff. Is the boiler actually switched on at the wall? Has the timer been knocked or reset after a power cut? It feels almost too simple to mention, but a reset clock or a tripped switch is behind more engineer call-outs than the industry would probably like to admit. Rule it out first.
If the boiler has power but won’t ignite, look at the pressure gauge before anything else. Most combis and system boilers have one on the front panel or underneath the casing. Cold system, it should read between 1 and 1.5 bar. Crept below 0.5 and the boiler has cut itself off not because it’s broken, but because low pressure triggers an automatic lockout. It’s doing its job. Central heating not working for this reason is actually about as straightforward as it gets. Get the pressure back up to around 1.2 bar and you should be back in business.
To repressurise, find the filling loop underneath the boiler. It’s the silver braided hose with a valve at each end. Open both valves slowly and watch the gauge. Once it climbs to around 1.2 bar, close both valves and try the boiler again. Job done, hopefully. If the pressure starts creeping back down over the next few days though, there’s a leak in the system somewhere and it’ll need a proper look.
Still nothing? Check the display for a fault code. Most modern boilers throw one up when something’s gone wrong. Search the model number alongside the code and you’ll usually find out quickly what you’re dealing with. Frozen condensate pipes, failed ignition, blocked flues these are the usual suspects in cold weather.
The Boiler Is Running But No Radiators Are Getting Warm
This one tends to cause more confusion because the boiler sounds like it’s working fine. And it might be. The problem is often further down the line.
The circulation pump is the first thing worth checking. Its job is to push hot water around the system, and if it’s seized or given up entirely, the boiler can heat the water all it likes but nothing goes anywhere. You might hear the boiler firing and feel warmth right near the unit, but the radiators stay stone cold.
A stuck zone valve can do the same thing. These are the components that control which circuits get hot water, and they can seize up in older systems, especially ones that haven’t been touched in a while. If the boiler is clearly running but nothing is getting warm anywhere in the house, that’s often where to look.
One more thing before picking up the phone: check the thermostat. If it’s sitting near a heat source, or the batteries are fading, it might be telling the system the room is warmer than it actually is. The heating never gets asked to come on. Try turning it well above the actual room temperature and give it a few minutes to see if anything kicks in.
Some Radiators Cold, Some Fine
Honestly, this is the one we hear about most. And nine times out of ten it’s something you can sort yourself.
Cold upstairs, warm downstairs. Or one side of the house fine, the other not. The most likely culprit is air in the system. Air works its way in over time and rises to the top of radiators, where it sits and stops hot water from filling that space properly. The tell is a radiator that’s warm at the bottom but cold across the top. Bleeding it lets the air out and usually fixes it within minutes.
All you need is a bleed key (a couple of pounds at any hardware shop) and an old cloth. Turn the key anti-clockwise on the bleed valve at the top corner of the radiator, wait for the hiss of air to turn into a trickle of water, then close it back up. Work through all the affected radiators and check your boiler pressure afterwards, because bleeding does bring it down a touch.
If bleeding sorts some radiators but not all, the system probably needs balancing. Central heating not working evenly across the house often comes down to hot water taking the path of least resistance and flooding the nearest radiators while the ones further away barely get a look in. Balancing means adjusting the lock shield valves on each radiator to even things out. It takes a bit of patience but the difference is noticeable. Our heating vessels and valves page covers the components involved if you want to understand how it all fits together.
What About Sludge?
Warm at the top, cold at the bottom. Flip that pattern around from trapped air and you’re usually looking at sludge and it’s one of those causes of central heating not working that people don’t think to check until things have got pretty bad.
The technical name is magnetite. It’s a black, gritty residue that forms as the metal inside your system gradually corrodes. Over time it settles at the base of your radiators and builds up. A little bit reduces efficiency. A lot of it and whole radiators stop heating. If it gets as far as the pump or boiler, you’re looking at a much bigger problem.
The solution is a power flush or a chemical flush, depending on the age and condition of the system. A power flush pushes water and cleaning chemicals through at high pressure to shift the build-up. A chemical flush is less aggressive and better suited to newer systems. Neither is a DIY job, but they make a significant difference and can extend the life of a system noticeably.
Prevention is the smarter route if you want to avoid central heating not working due to sludge further down the line. A good inhibitor in the system water stops the corrosion that creates it in the first place. Pair that with a magnetic boiler filter and you’re catching particles before they get a chance to settle anywhere. We stock the heating system chemicals that engineers actually specify for this: inhibitors, cleaners, descales. Not the diluted stuff you find on a supermarket shelf.
Check the TRV on the Cold Radiator
If it’s just one or two specific radiators not heating up while everything else is fine, a seized TRV is a surprisingly common cause of central heating not working in isolated areas of the house. TRVs have a small pin inside that lifts and drops to control water flow. Leave the heating off over summer and that pin can seize in the closed position, which is central heating not working in one room despite the rest of the house being fine.
The fix takes about two minutes. Twist or unclip the TRV head and you’ll see the pin sitting in the top of the valve body. Push it down and let it spring back up a few times, or wiggle it gently with a pair of pliers if it’s really stuck. That’s usually enough to free it. Pop the head back on and the radiator should start warming up within ten minutes.
If the valve itself is old, corroded, or beyond freeing, replacing it is the right call. Our fittings range includes TRVs and the parts that go with them.
When the Problem Really Is the Boiler
Sometimes central heating not working does come back to the boiler itself, and it’s worth knowing what the warning signs look like.
Regular pressure drops mean a leak, simple as that. A boiler that fires then cuts out shortly after usually points to a pump issue, a blockage, or a failing component. That grumbly boiling noise is limescale on the heat exchanger, rife in London and hard water areas. Central heating not working for any of these reasons won’t get better with time, and the longer it’s left, the more it tends to cost.
If your boiler is fault-coding regularly, cutting out without explanation, or getting on for 12 to 15 years old, central heating not working reliably is only going to become more frequent. Get a Gas Safe engineer to assess it properly rather than keep patching it. We stock a wide range of boilers and spares at Mirage, and our spares hotline at 0208 947 6105 is specifically for situations where an engineer needs a part quickly. Give us a call and we’ll tell you straight whether we have it.
Don’t Overlook the Smart Thermostat
If a smart thermostat was fitted recently, or updated itself quietly in the background, it may not be set up the way you think. Heating schedules get reset. Away modes stay on. Minimum temperature settings end up lower than they should be. Any of those will stop the heating from coming on, and central heating not working because of a settings issue is more common than most people realise.
Check the app. Look at the schedule, the current mode, and the target temperature. If it all looks right but you’re still getting nothing, our smart heating controls page goes into more detail on how these systems behave and where they commonly go wrong.
Still No Joy?
If you’ve tried everything above and still got nowhere, central heating not working might be a frustrating thing to deal with and it might be time to hire an expert to have a look at it, some problems are just really technical and require diagnostic equipment to find
What we can help with is parts. When central heating is not working and your engineer needs a component quickly, Mirage Heating & Plumbing Supplies is a trade counter in Tooting that supplies heating professionals across London, often on the same day. Pumps, zone valves, filters, inhibitors, boiler spares. If your engineer is looking for something, there’s a reasonable chance we have it on the shelf.
Get in touch with our team if you have a question, need to check stock, or just want to talk to someone who actually knows heating.
