No Hot Water but Heating Works? (Or the Other Way Around)

When half your boiler stops working

There's a particular confusion that comes with a boiler that only half works. The taps run hot but the radiators stay cold, or every radiator in the house is warm while the shower turns icy. It feels like a contradiction. If the boiler is clearly firing, why would it heat one thing and ignore the other?

The answer is that warming your rooms and warming your water are two different jobs, and a boiler handles them through partly separate paths. When one path fails, the other often carries on as normal. Working out which side has gone quiet tells you a lot about what's likely wrong, and it helps you describe the fault clearly when you call for help.

Two jobs, one appliance

Most modern homes run a combination boiler, which makes hot water on demand and also pushes hot water around a loop of radiators. Both start with the same burner heating water inside the unit. What differs is where that heat goes next.

For central heating, the boiler sends warmed water through a circuit of pipes and radiators and back again, driven by a pump. For hot water at the tap, it either heats water instantly as it passes through a plate heat exchanger, or, in a system with a cylinder, it warms a stored tank. A part called the diverter valve decides which way the heat travels. Open a hot tap and the valve prioritises hot water. Close it and the boiler goes back to heating.

That shared start and split path is why a boiler can lose one function and keep the other. The burner is fine. Something downstream of it, on one branch only, has failed.

Radiators cold, hot water fine

If your taps still run hot but the radiators won't warm up, the problem usually sits on the heating side rather than in the boiler's core.

Start with the obvious controls. A room thermostat that has been knocked down, a programmer set to the wrong schedule, or a flat thermostat battery can all stop the heating from calling for heat even though nothing is actually broken. It sounds too simple to be the answer, and often it is exactly the answer.

Boiler pressure is the next thing to look at. Central heating relies on the water in the circuit sitting within a set pressure band, shown on a gauge on the front of the boiler. If it has dropped too low, many boilers will keep making hot water but refuse to run the heating to protect themselves. Hot water is usually fed from the mains and isn't affected the same way, which is why it keeps flowing while the radiators go cold.

Beyond those, a stuck diverter valve, a failed pump, or air trapped in the system can all leave radiators cold while the taps stay hot. Those are jobs for an engineer rather than a homeowner, but noticing that only the heating side is affected narrows things down before anyone arrives.

Hot water gone, radiators warm

The reverse pattern points the other way. If the house heats up but the shower and taps run cold, the heating circuit is clearly working, so attention shifts to how the boiler makes hot water.

In a combi, the two usual suspects are the diverter valve and the plate heat exchanger. A diverter valve stuck in the heating position never redirects heat to the tap, so the water stays cold no matter how long you run it. A heat exchanger clogged with scale or debris can't pass heat efficiently to the water moving through it, which shows up as water that never gets properly hot, or runs lukewarm and then cold.

In a system with a hot water cylinder, a cold tap can instead point to a failed motorised valve, a burnt-out immersion element, or a cylinder thermostat that has given up. The idea is the same in a different layout. The heating loop works and the water side doesn't.

What you can safely check yourself

There's a short list worth looking at before you pick up the phone, and none of it involves opening the boiler.

Confirm that the thermostat and programmer are actually calling for the function that isn't working, and that any batteries are fresh. Read the pressure gauge and note where the needle sits. Glance at the boiler display for a fault code and write down exactly what it shows, since that code often names the faulty part directly. Feel the radiators to confirm the pattern, whether they are cold at the top, cold at the bottom, or cold all the way across.

A radiator that's warm at the bottom and cold at the top usually has trapped air, and bleeding it can bring it back to life. That's within reach for most people. Anything past that, especially work on the diverter valve, the pump, the heat exchanger, or the gas side, belongs to a qualified heating engineer. Boilers combine gas, electricity, and pressurised water, which is not a mix to experiment with.

When to call a professional

Some situations shouldn't wait. A smell of gas, scorch marks around the unit, water pooling underneath it, or a boiler that locks out again and again all mean you should stop and bring in a qualified engineer rather than keep resetting it. A boiler that keeps making hot water but has quit on heating during a cold snap is also worth treating as urgent, since a cold house can lead to frozen and burst pipes.

When you do call, the details you gathered save time and often money. Telling the engineer that hot water works but heating doesn't, that the pressure gauge reads low, and that the display shows a specific code lets them turn up with a fair idea of the fault and the right parts on the van. A half-working boiler is annoying, but it's also a clue. The side that has stopped is pointing straight at what needs fixing.