Heating won't come on but the hot water's fine

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Radiators stay cold, no fault code on the boiler, and the hot water is working away as normal.

That last bit is the useful part. If you've got hot water, the boiler is lighting, burning and heating water perfectly well, so nine times out of ten there's nothing wrong with it. Something has just stopped asking it to run the heating.

Here's how to check that yourself, before anyone charges you to come out and tell you the same thing.

A boiler in perfect working order sits there doing nothing until something tells it to run.

That something is your room thermostat, or your programmer, or both. The stat measures the room, compares it to what you've set, and if the room is colder it sends a signal. That signal is the whole job. If it never arrives, the boiler has no idea you want heating, and it behaves exactly as it should by staying off.

Which is why a boiler with nothing wrong with it can leave you in a cold house.

The single most common cause, and the one people are most embarrassed about afterwards. Don't be.

Wireless thermostats run on batteries and they get through them. When they go, plenty of them don't flash anything, don't beep and don't warn you. The screen can look completely normal. It just quietly stops sending.

Change them even if the display looks fine. Two minutes and a pound's worth of batteries against a callout charge.

Worth knowing this tends to show up at the first proper cold snap. Not because the cold killed the battery, but because the heating has been off all summer and nobody noticed the stat died back in June.

Turn the thermostat right up, five degrees above whatever the room is sitting at, and then stand there and pay attention.

A lot of stats click audibly when they call. Many have a flame symbol or a small light. Within a minute or so you should hear the boiler fire.

Whichever way it goes, you've learned something worth knowing. If it calls and the boiler fires, your stat is fine and the fault is further down the line. If nothing happens at all, no click, no symbol, no boiler, then you've narrowed it to the controls and you can stop wondering about the boiler entirely.

This one catches people out long after they've forgotten there was a power cut.

Plenty of programmers lose their schedule when the power goes, and come back on a factory default or nothing at all. The one people miss is the clock. If it's reset itself, it may think it's four in the morning, in which case it's behaving perfectly and just not at a time you're awake for.

Go and check three things: the time is right, the day is right, and there's actually a heating period set for now.

Same applies if the heating has been off all summer. Whatever was set in April probably doesn't suit October.

Worth a look, particularly if this started after you'd been away.

Most programmers and smart stats have a holiday or away setting that drops everything right down. Easy to set, very easy to forget, and it does exactly what it was told.

Also worth checking whether anyone has left it on frost protection or an eco mode, and on smart systems whether a schedule somebody set up months ago has quietly taken over.

If your thermostat is wireless, it's talking to a small box near the boiler called a receiver. They can lose each other, after new wifi kit, a router being moved, batteries changed, or just age.

The catch is that the stat on the wall carries on looking completely normal. It shows the temperature, it responds when you turn it up, it may even show it's calling. It's shouting into an empty room.

So go and look at the receiver itself. There's normally a light on it telling you whether it's paired and whether it's hearing a call. If the stat says it's calling and the receiver disagrees, that's your answer and you've found it without anyone coming out.

Re-pairing is usually a button held for a few seconds, and it'll be in the instructions for your model. If you can't find them, leave it. That's a fair thing to get someone for.

Worth knowing when to stop, because these point somewhere else entirely:

• Hot water is off as well. A room stat only controls heating. If both are dead, the stat was never the problem
• The boiler fires but the radiators stay cold. The call is arriving perfectly well, so it's circulation or a valve, not the stat
• It works sometimes. Intermittent points at a connection, a valve or a control board rather than a stat
• There's a fault code on the boiler. Then the boiler is telling you something specific and that's where to start
• Neither the stat nor the boiler has any power. That's an electrical fault rather than a heating one

Any of those and you can stop changing batteries.

• You've changed the batteries and it still won't call
• The stat calls but the boiler does nothing
• The receiver won't pair and you can't find the instructions
• It's intermittent
• There's any wiring involved

That last one particularly. Heating controls carry mains voltage, and a wrong connection doesn't just fail, it can take out a control board. That turns a five minute job into a few hundred pounds, which is the opposite of what you were trying to do.

Past that point it's testing rather than looking, and heating controls are where I see the most money wasted, because the same dead radiator comes from four or five different places and people start by replacing the cheapest one.

I'm Carlo Cennerazzo, twenty-three years in the trade. Point your phone at the thermostat and the boiler and I'll tell you what's going on, whether it's the stat, the controls or the electrics, and what it should cost if you can't sort it yourself.

£20, no callout charge, same day when there's a slot free. A written report follows afterwards, so you've got it in black and white rather than trying to remember.

And if you've already had someone out and you're holding a quote you're not sure about, that's worth a call too. I've no interest in who does the work, so I'll tell you straight whether the price is fair and whether it's even the right job.