No Heating or No Hot Water — Diagnostics

A boiler firing but the house staying cold, or hot water that's vanished entirely, has a shorter list of likely causes than it feels like from the outside — but which one it is changes the fix completely. Low system pressure, a failed diverter valve, an ignition fault, a broken thermostat or programmer, or a stuck pump can all produce 'no heat' or 'no hot water', and they're not interchangeable diagnoses.

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Our engineers work through the likely causes in order, starting with the checks that take minutes — pressure, thermostat settings, whether the boiler is actually firing — before moving to component-level diagnosis. That order matters: a five-minute pressure top-up shouldn't turn into a part being replaced unnecessarily.

Because "no heating" and "no hot water" can be the same underlying fault or two different ones depending on whether you have a combi, system or regular boiler, we ask about your setup before we arrive, so the visit starts with a shortlist rather than a blank page.

Diagnostics are quoted as part of the visit; the repair itself is quoted once the cause is confirmed — you approve the fix before it happens, not after.

What you get

Structured fault-finding — pressure and settings checked before components are suspected.
Covers both symptoms: no heating, no hot water, or both together.
Explains whether the cause is the boiler, the programmer/thermostat, or elsewhere in the system.
Gas Safe registered engineers for any gas-side diagnosis or work.
Clear next step if the fix is beyond a same-visit repair — parts on order, or a replacement conversation.
Advice on preventing repeat failures, not just restoring heat for now.

How it works

A method, not a guess

Describe what stopped

Heating, hot water, or both — and whether it happened suddenly or has been getting worse. This narrows the likely cause before the engineer even arrives.

Quick checks first

Pressure, thermostat and programmer settings, and whether the boiler ignites at all — the fastest-to-rule-out causes are checked before anything is assumed to be a component failure.

Component-level diagnosis

If the quick checks don't explain it, the engineer moves to the diverter valve, ignition system, pump or PCB — testing rather than swapping parts speculatively.

Explain, quote, fix

You're told what's actually wrong and what it costs to fix before work starts, then the repair is carried out and the system checked for both heating and hot water before the engineer leaves.

Questions

No Heating / No Hot Water FAQs for London homeowners

My boiler is on but the radiators are cold — what does that usually mean?

It often points to low system pressure, a stuck diverter valve, or a pump issue rather than the boiler itself having failed outright. It can also be trapped air needing radiators bled, which is a much smaller job. Diagnosis is what tells the two apart.

I have hot water but no heating (or heating but no hot water) — is that normal?

It's a very common combi-boiler symptom, and it usually points at the diverter valve, which switches the boiler between the two functions. It's a well-understood fault, not a mystery, but it does need an engineer to confirm before parts are ordered.

Can low pressure really cause a total loss of heating?

Yes — most modern boilers will refuse to fire, or fire and then lock out, if pressure drops below the safe operating range. It's one of the first things worth checking, and sometimes the whole fix is a pressure top-up plus finding out why it dropped in the first place.

How fast can a no-heating fault actually be fixed?

Some causes — low pressure, a reset lockout, a thermostat setting — are same-visit fixes. Others, like a failed PCB or diverter valve, may need a part ordered. We'll tell you honestly which category your fault falls into once it's diagnosed, rather than promising a same-day fix before we've seen it.

Is a total heating failure ever a gas safety issue?

Usually not — most no-heating faults are mechanical or electrical. But if you smell gas, hear unusual noises from the boiler, or see scorch marks or a yellow/orange flame instead of blue, treat that as urgent and get a Gas Safe engineer to look at it rather than continuing to use the appliance.

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