If your radiators only heat up when the hot water is on, you’re dealing with a very common UK boiler problem that almost always comes down to one part of the system not doing its job properly. It’s one of those issues that seems confusing at first, but once you understand how your heating and hot water share the same boiler, the whole thing becomes much clearer.
I went through this exact issue a couple of years ago. I’d turn the heating on… nothing. But the moment I switched the hot water on, suddenly the radiators woke up. It feels like some weird “hack” in the system, but it’s actually your boiler trying—and failing—to send heat where it’s supposed to go.
How Your Boiler Decides Between Heating and Hot Water
Most UK homes with a system or conventional boiler use a part called a three-port valve (sometimes called a Y-plan valve). Its job is to direct hot water from the boiler either to:
• the radiators (heating)
* the hot water cylinder
* or both
If this valve can’t move freely, gets stuck, or loses its ability to switch positions, the boiler will only heat whichever setting still works. And in most cases, that’s the hot water side. That’s why your heating suddenly “works” only when the hot water is turned on—the valve is letting water flow to the tank but not to the radiators unless the tank is calling for heat.
The Three-Port Valve: The Usual Culprit
If you have a hot water cylinder in an airing cupboard, you almost definitely have a three-port valve next to it. Inside is a small motor that switches between heating and hot water. Over time the gears wear out, the motor weakens, or the valve itself gets stiff.
Signs your valve is failing include:
• heating only works when hot water is on
* radiators heat weakly or unevenly
* valve makes a clicking or buzzing sound
* valve feels stuck when touched
* hot water gets priority and radiators lag behind
The reason this part causes so many problems is simple—it controls the entire flow of heat. When it fails, the boiler doesn’t know where to send the hot water.
If You Have a Combi Boiler, the Issue Is Different
Combi boilers don’t use a three-port valve. Instead, they use a diverter valve, which does the same job but inside the boiler itself. A faulty diverter valve causes almost identical behaviour:
• radiators don’t heat properly
* hot water takes priority
* heating works only after running the hot tap
Combi diverter valves fail a lot because they constantly switch back and forth every time someone uses hot water.
Thermostats and Timers Can Make This Look Worse
If your thermostat isn’t calling for heat, or your wiring centre isn’t sending a proper signal, the boiler won’t activate the “heating” side of the system. But when the hot water cylinder calls for heat, the boiler fires up anyway—and the radiators might warm by accident.
This creates the illusion that the heating “only works when hot water is on,” but the real issue is the thermostat not communicating properly.
Common causes include:
• thermostat batteries dying
* thermostat in a cold or draughty hallway
* programmer/timer not sending the right signal
* loose wiring in the control box
Airlocks and Sludge Can Also Make Heating Weak
Sometimes the heating technically works, but the flow through the radiators is so poor that they barely warm up unless the hot water is also running. When the cylinder calls for heat, the boiler runs longer, pushing more water around—and that extra circulation is the only reason the radiators wake up.
If this sounds familiar, it could be:
• air trapped in the system
* sludge blocking the radiator circuits
* a weak circulation pump
* the system being badly unbalanced
This is more common in older homes or systems that haven’t been flushed for years.
How to Narrow Down the Cause (Simple Checks)
1. Check if radiators get warm at all when hot water is OFF
If they stay stone cold unless the hot water runs, it’s almost certainly the three-port valve or diverter valve.
2. Listen for clicking near the three-port valve
In airing cupboards, a faulty valve often makes a noticeable click or hum when switching modes. If it’s stuck, it may not move at all.
3. Check thermostat behaviour
If turning up the thermostat doesn’t trigger the boiler, that means the boiler isn’t receiving the signal to heat the radiators.
4. Feel the pipework around the cylinder
If the pipe leading to the hot water tank gets extremely hot while the heating pipes stay cool, the valve is stuck on the hot-water side.
Why This Problem Shouldn’t Be Ignored
Heating systems that operate “half working” are always more expensive to run. The boiler stays on longer, the radiators heat inconsistently, and the system ages faster. A stuck valve is one of those issues that slowly drains money every day without you realising it.
Plus, this issue usually gets worse. A slightly sticky valve today is a completely dead valve in a few months.
Fixing the Problem
If you’re confident with DIY, replacing the motor head on a three-port valve is manageable. It’s usually held on with two screws and doesn’t involve draining the system. But replacing the entire valve or a combi diverter valve is a job for an engineer.
Balancing radiators, bleeding air, and checking thermostats are easy wins that can improve flow—but if the heating only works with the hot water on every single time, the valve itself is almost always the real cause.
How This Fits Into Keeping the House Warm Efficiently
A system that can’t switch properly between heating and hot water wastes energy and runs longer than necessary. Fixing the root cause not only makes the house feel warmer—it lowers your heating bill over the winter.
This links directly to the wider approach I explained in the main guide: How to Keep a UK Home Warm for Cheap (Complete Guide).
