Hot Water But No Heating? Here's What to Check

The heating goes on for the first time in months, the radiators stay stone cold, and yet the taps are running as hot as ever. It's one of the most common calls we take from the middle of September onwards. The good news is hiding in the symptom itself: if you've still got hot water, the boiler is lighting and heating water, which makes the expensive faults a good deal less likely and narrows the search right down. This guide covers what's actually going on, the checks worth doing before you phone anyone, and what the repair costs if you do.

What hot water tells you about the fault

A boiler making hot water is a boiler that's getting gas, igniting and heating water properly. So the fault rarely sits in the parts people worry about most, the burner or the main heat exchanger. It sits somewhere between the boiler and your radiators: in the controls telling the boiler to run the heating, in the valve deciding where the heat goes, or in the pump meant to be pushing it round the house.

That's a much cheaper list of possibilities than “the boiler's broken”, and a fair few of them you can rule out yourself in ten minutes.

The ten-minute checks, before you call anyone

Work through these in order. Between them they account for a surprising share of the no-heating calls we get in September and October, and not one of them needs an engineer:

• The programmer is still set for summer. The most common one by a mile at this time of year. Plenty of people knock the heating off in May and leave the hot water running on a timer, then forget. Check the programmer is set to heating on, or both, rather than hot water only.
• The room thermostat batteries are flat. A wireless thermostat with dying batteries quietly stops calling for heat, often without showing anything obvious. Fit fresh batteries and turn it well above room temperature, around 25°C, to force it to call.
• The thermostat is set below room temperature. It sounds too simple to be worth checking. It has caught out plenty of people, particularly where somebody else in the house turned it down.
• The boiler pressure has dropped. Look at the gauge on the front of the boiler. Cold, it should sit somewhere around 1 to 1.5 bar. Down near zero and many boilers will refuse to run the heating at all.
• Every radiator valve is shut. Thermostatic valves that sat untouched all summer can seize with the pin stuck down. Take the head off one and see whether the pin underneath springs back when you press it.
• A smart system has lost its connection. Smart thermostats that have dropped off the wifi or lost their pairing with the hub will sit there looking perfectly normal and never actually call for heat.

The most likely culprit: a stuck diverter valve

If those checks come up clean and you have a combi boiler, the diverter valve is the first thing we look at.

A combi has one heat source and two jobs to do with it. The diverter valve decides which job gets the heat: open a tap and it swings across to send hot water to the taps, close the tap and it sits back over to the heating side. Over the years that valve can stick, or its motor and internal seals simply wear out.

When it sticks in the hot water position you get exactly what you're seeing, endless hot water at the taps and radiators that never warm up. It's a well-known failure on combis and it is emphatically not a boiler replacement job. Depending on the make it's either a new valve cartridge or a new actuator head, and it's a routine boiler repair rather than a big one.

The other usual suspects

• A seized or failed circulating pump. The pump pushes hot water round the radiator circuit. Seized after a summer of standing still, or clogged with sludge, and the boiler will fire while nothing moves. A working pump usually feels warm and hums gently. Silent and stone cold is a suspect.
• A failed motorised valve. On systems with a hot water cylinder rather than a combi, a motorised valve, or a mid-position valve on a Y-plan, routes heat to the cylinder or to the radiators. When the actuator burns out on the heating side, hot water carries on and the heating stops.
• Sludge in the system. Years of rust and debris settling into radiators and pipework restrict the flow until heat can't get round the house. This one arrives gradually, with radiators cold along the bottom, rather than everything dying overnight. That's a power flush rather than a repair.
• An airlock. Air trapped at a high point stops circulation. It usually affects some radiators rather than all of them, and bleeding them is the first move.
• A wiring or PCB fault. Less common, but a failed relay on the boiler's circuit board, or a broken wire between the thermostat and the boiler, will stop the heating ever being called for while the hot water side carries on happily.

The other way round: heating works but no hot water

The same handful of parts, failing in the opposite direction. On a combi it's most often the diverter valve stuck on the heating side, or a furred-up plate heat exchanger, the small secondary exchanger that passes heat into your tap water. In a hard water area it scales up steadily, so the hot water usually goes lukewarm for a while before it goes altogether.

On a system with a cylinder, look at the cylinder thermostat and the motorised valve feeding it, and check the programmer is genuinely set to bring the hot water on. If you have an immersion heater as backup, it's worth finding out whether it works before the day you need it.

What does it cost to put right?

Cost follows the part, and the part follows the diagnosis, which is why nobody honest will quote you a repair over the phone. As a guide to what's normal:

Those are market ranges rather than our quote. One honest bit worth saying: if you're facing a PCB or a heat exchanger on a boiler already past twelve or thirteen years old, price a new boiler alongside the repair before you commit. Sometimes the repair is still the right call. Sometimes it's good money after bad, and we'll tell you which one you're looking at.

When to stop and call someone

Most of what's above is safe enough to check yourself. These are not:

• You can smell gas. Turn the gas off at the meter, open the windows and get everyone out. Don't smoke or light anything, and don't touch electrical switches, on or off. Then call the National Gas Emergency Service on 0800 111 999 before you call anyone else.
• The boiler is leaking water, or there's water anywhere near the electrics.
• There's staining, sooting or scorching around the boiler casing.
• The boiler keeps locking out and needs resetting again and again. Repeated resets just mask a fault that is trying to tell you something.
• Anything inside the boiler's casing. Don't take the case off or touch the parts inside it. That's work for a Gas Safe registered engineer, legally as well as practically.

Heating repairs in Kirkcaldy and across Fife

We're a Gas Safe registered plumbing and heating team based in Kirkcaldy, covering Fife, Edinburgh and the Lothians. We'll diagnose a heating fault on any make of boiler, whoever fitted it, and give you a fixed price before we start, so there are no surprises on the bill. If it's the middle of the night, the house is cold and there are people in it, our 24hr emergency line is a real one rather than an answerphone.

And if it turns out the system needs more than a repair, we'll price the heating work properly rather than guessing at it. And if you'd rather not have this conversation again next autumn, our care plans cover boiler and heating breakdowns from £15 a month, with no call-out fee and no excess.