Start with the rule that governs everything else on this page. Only a Gas Safe registered engineer may work on a gas appliance. That is the law in the UK, not a professional courtesy and not an insurance preference. Do not open a boiler, do not remove its casing, do not disturb its gas supply, and do not follow any advice — online or otherwise — that suggests you should. If you can smell gas, do not investigate: leave the property, do not touch switches, and call the national gas emergency service.
With that established, there is still a useful amount you can safely work out yourself before anyone attends, and doing so tends to get the problem fixed on the first visit rather than the second. Most of it involves looking at a display, listening to what happens, and knowing what kind of system you have.
Which problem do you actually have?
"No hot water" is three different faults wearing the same coat. Establish which one you have before anything else.
No hot water, heating works. The boiler is firing. Something is stopping that heat reaching your taps. On a combi, the usual suspect is the diverter valve — the component that decides whether heat goes to the radiators or the hot water. On a system with a cylinder, it points at the cylinder, its coil, its motorised valve, or the controls that call for hot water.
Heating works, hot water lukewarm. Often a partly failed diverter, a scaled heat exchanger, or a cylinder thermostat set too low or failed. London water is hard and scale is a real factor in this stock.
Neither works — no heat at all. The boiler is not firing. Now you are looking at power, gas supply, pressure, a lockout, or a failed component. This is the one with the most things you can usefully check yourself.
Hot water runs hot then goes cold. On a combi, that is usually a flow or scale problem, sometimes a diverter sticking. On a cylinder, that is simply the cylinder emptying — you have run out of stored hot water, which is a sizing or reheat issue rather than a fault.
Hot water is fine but weak. That is not a boiler fault. That is flow, and our guide to low water pressure in a London flat is the right place to start.
Write down which of these you have. It is the single most useful thing you can give the engineer.
What kind of system is it?
Most people do not know, and it changes the whole diagnosis.
Combi. One box on the wall, no cylinder, no tanks. Heats water on demand from the incoming main. Very common in refitted Central London flats because it frees a cupboard.
System boiler with an unvented cylinder. A boiler plus a large insulated pressurised cylinder, usually in an airing cupboard. Good pressure at hot taps. Note that unvented cylinders have their own safety requirements and their own competence regime — they are not something to poke at.
Conventional or gravity system. Boiler, a vented copper cylinder, and tanks above — a cold storage tank and a small feed-and-expansion tank, typically in a roof space or a high cupboard. Still very common in mansion block stock across Maida Vale, St John's Wood and Marylebone.
Communal or district heating. No boiler in the flat at all — a heat interface unit fed from a central plant room. Increasingly common in newer blocks around Paddington. Crucially, if you are on communal heat, your fault may not be yours: it may be the plant, and the route is the managing agent, not a plumber.
Go and look before you call anyone. If there is no boiler, that answer alone reframes everything.
Checks you can safely make yourself
None of these involve opening anything.
Is the boiler powered? A blank display means no power. Check the fused spur or switch next to it — these get knocked off, particularly during other work. Check the consumer unit for a tripped breaker or RCD. If a breaker keeps tripping when the boiler is on, stop and get that looked at; that is an electrical fault as much as a boiler one.
Is there a fault code? Most boilers show a code on the display. Photograph it. Boiler manufacturers publish what their codes mean, and a code turns a mystery into a diagnosis. This alone frequently determines what part the engineer brings.
What is the system pressure? On a sealed system there is a gauge, usually reading somewhere around one bar when cold; the manufacturer's guidance for your specific appliance is what counts. If it reads zero, the system has lost water, and that means there is a leak somewhere — in a radiator, a valve, or under a floor. Repressurising it and walking away treats the symptom. If it keeps dropping, find the leak.
Is the gas actually on? Try another gas appliance — a hob, if you have one. If nothing gas-fired works, the issue may be your gas supply or a prepayment meter rather than the boiler.
Are the controls doing what you think? Programmers get changed by cleaners, guests and clock changes. Smart thermostats lose their pairing and their batteries. Cylinder thermostats and motorised valves get left in the wrong position after other work. Check the timer, check the mode, check whether the hot water schedule is on at all. This is not a joke — it is a meaningful share of no-hot-water calls.
Is it frozen? A condensate pipe running to an outside wall can freeze in a cold snap, and the boiler locks out because it cannot drain. Symptom: it stopped overnight in a hard frost, often with a gurgling noise and a specific fault code. This is one of the few things a homeowner can often resolve safely — the manufacturer's instructions for your appliance will say whether and how to thaw it. Do not improvise with anything hot or naked-flame.
Have you reset it once? One reset is reasonable. If it locks out again, stop. Repeatedly resetting a boiler that keeps failing is masking a fault, and the fault is the reason it is doing it.
Anything beyond this list — the casing, the gas, the burner, the diverter, the heat exchanger, the expansion vessel — is Gas Safe territory. Not because it is difficult. Because it is illegal to do otherwise, and because the failure mode is carbon monoxide.
What is usually actually wrong
Diverter valve. The classic combi fault: heating fine, hot water cold or lukewarm. The valve sticks. It is a known wear item.
Scale. Hard water plus a plate heat exchanger equals gradual decline in hot water performance over years. Central London is firmly in hard water territory and this is one of the most common underlying causes here.
A failed motorised valve or cylinder thermostat. On cylinder systems, cheap components with a real failure rate. Symptom is often heating fine, hot water absent.
A cylinder coil that has failed. Less common, more expensive, and usually means a cylinder.
Pressure loss from a leak. Sealed systems do not lose water for fun. A pressure gauge that keeps falling means water is going somewhere, and in a flat, water going somewhere often ends up as somebody else's ceiling — see our guide to a leak under the bath for how these things travel.
An old boiler at the end of its life. Sometimes parts are obsolete, and repair stops being sensible. That is a conversation about replacement, and it should be an honest one rather than an upsell — a Gas Safe engineer should be able to tell you whether the part exists and whether the appliance justifies it.
What it costs
Two published figures are relevant here, and both exclude VAT.
A plumbing call-out is from £80. That is the visit: someone attends, looks at what is actually happening, and tells you what it is. For the water-side faults on this page — a motorised valve, a cylinder thermostat, a control that has been left in the wrong position, pressure dropping because something is leaking — that is where it starts, and any work that follows is fixed-quoted before it is done rather than accruing while you watch.
A boiler installation is from £700. That is the installation, and it is worth understanding what does and does not sit inside it. It does not include the appliance. It does not include the answer to any of the Central London complications below — a flue that has to move, a cupboard that has to be rebuilt around a new unit, a cylinder that has to come out of a fourth-floor flat, a conversion from a stored-hot-water system to a combi. Those are alterations rather than swaps, and they are priced by quotation because no two of them are the same job.
The reason to know both numbers is that they frame the decision most people are actually making at eleven at night with no hot water: repair or replace. That decision should be made on whether the part exists, how old the appliance is, and what the flue and the lease allow — not on a number invented on the doorstep. A fixed itemised quote is confirmed before any work begins, so the choice is yours to make with the figures in front of you.
The Central London complications
Three things make this harder here than in a house.
Flues. A boiler needs a flue, and the flue has to terminate somewhere acceptable. In a listed building, or in a conservation area, putting a new flue through an elevation is not a free choice — works affecting a listed building's special character can require listed building consent, and conservation areas add planning controls. Boilers in this stock are frequently located where they are because it was the only place the flue could go, not because it was convenient. That constrains replacement, and it is a question to resolve before choosing an appliance. Confirm the position with your council and, if relevant, your freeholder.
Leases and consent. Most flat leases require freeholder or managing-agent consent for alterations, and moving a boiler, changing a flue position, or converting from a cylinder system to a combi is usually an alteration rather than a repair. Often that means a licence to alter. Requirements vary between leases and between freeholds — check yours rather than assume.
Access and cupboards. The airing cupboard in a mansion block flat is frequently the tightest working space in the building, and the boiler was often fitted before the cupboard was rebuilt around it. Getting an old cylinder out of a flat is sometimes a joinery job before it is a plumbing one.
If you are a landlord
Two things are not optional. Landlords must have an annual gas safety check carried out on gas appliances and flues by a Gas Safe registered engineer, under the Gas Safety (Installation and Use) Regulations 1998, with a record provided to the tenant. And in the private rented sector in England, electrical installations must be inspected and tested at least every five years under the Electrical Safety Standards in the Private Rented Sector (England) Regulations 2020.
A tenant with no hot water is also a repair obligation, and one that tends to escalate quickly in winter. Our maintenance service covers the planned side of this so that the annual check and the fault call are not the same phone call.
What to have ready when you book
The make and model of the boiler — usually on a label behind or under the casing door, no tools needed. A photograph of the fault code. What kind of system it is. Which of the symptom patterns above matches. Whether the pressure gauge has moved. Whether anything changed recently — work in the flat, a power cut, a cold snap, a new thermostat. That is a five-minute list and it regularly turns two visits into one.
How we handle it
Gas work is done by Gas Safe registered engineers, and we will not pretend otherwise to save a visit. Where the fault is on the water side rather than the gas appliance — a valve, a cylinder control, pipework, pressure loss from a leak — that is ordinary plumbing, from £80 excluding VAT, with a fixed price agreed before we start. Where the appliance itself has reached the end and replacement is the honest answer, boiler installation is from £700 excluding VAT, carried out by a Gas Safe registered engineer. Where the honest answer is that you are on communal heat and the fault is in the plant room, we will tell you that and point you at your managing agent rather than charge you to discover it twice.
We work across W1, W2, NW8 and W9. Send the model and the fault code through the quote form, or reach a plumber in St John's Wood or Marylebone the same way. If this is urgent and something is leaking as well as failing, our guide to an emergency plumber in Central London covers what to do first.