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Total System Failure
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.
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
01
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.
02
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.
03
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.
04
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.
Fully Gas Safe registered
Every engineer is registered to work on gas boilers — ask to see ID on the day.
Diagnosed before it's quoted
The actual fault is confirmed before you're asked to approve a fix.
Written fault explanation
What was wrong and what was done, useful for landlords and your own records.
Fixed price before we start
Call-out fee confirmed on the phone, never announced once the engineer is on site; any repair is quoted after diagnosis, before work starts.
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.
Explore the cluster
Related boiler & heating services
Boiler repair
Diagnosis first, then a confirmed fix
Losing pressure
Finds where the water is actually going
Radiator not heating
Trapped air, sludge or a stuck valve
Pump repair
Circulation faults diagnosed and fixed
Emergency repair
Genuinely urgent faults, triaged properly
Boiler fault codes
What your fault code actually means
Pricing
How our quote-only pricing works
Areas we cover
All 33 London boroughs
Boiler repair guide
The complete guide to boiler repair in London
Emergency guide
What counts as urgent, and what to do first
Repressurise safely
The homeowner-safe steps, and when to stop
Power flush guide
The signs that mean sludge, not a simple bleed
London-wide
No Heating / No Hot Water across all 33 boroughs
Same diagnosis-first approach from Barking to Richmond. Pick your borough for local housing detail.
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Tell us the symptoms and your postcode. We'll diagnose the fault and confirm the price before any work starts — never a guess billed by the hour.
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