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A cold tap has a short list of usual suspects, and most of them stand outside the boiler casing where you're allowed to check them. Work the list — unless you smell gas, in which case skip straight to the red rule.
No hot water is a triage tree, not a mystery. Check the boiler's pressure gauge (around 1 to 1.5 bar cold), the timer and thermostat, and the consumer unit for a tripped switch — those three catch a surprising share of "dead" systems. Smell gas at any point? Leave the property and call the National Gas Emergency Service on 0800 111 999 from outside. Otherwise run the checks below, then ring 020 4577 2888 any hour to be connected with a local plumber.
Pressure, controls, power — three gauges, five minutes. Most cold-tap mornings end at one of them.
Start at the boiler's front. Most sealed systems idle around 1 to 1.5 bar cold; below about 1 bar plenty of boilers simply refuse to heat water, and topping up through the filling loop — manual open beside you, loop closed properly afterwards — is a routine householder job. Then the controls: a timer knocked sideways by a power cut, a thermostat nudged down, a hot-water schedule that thinks it's still the weekend, a smart control quietly offline. Mundane causes solve more "no hot water" calls than anyone likes to admit.
Last, the consumer unit. A tripped switch kills a boiler as thoroughly as any breakdown — reset it once. Trips again? Stop: something is tripping it for a reason, and that reason wants a professional. And in a hard frost, remember the frozen condensate pipe — the small plastic pipe outside can ice up and lock the whole boiler out, hot water included. The frozen pipes guide covers the gentle thaw.
On a combi, that split is the classic sign of a diverter valve fault. The diagnosis is yours to spot; the repair isn't yours to make.
A combi boiler uses a diverter valve to switch between feeding the radiators and heating water on demand. When that valve wears or sticks, you get exactly this lopsided picture: radiators piping hot, hot tap running cold — or occasionally the reverse. It's one of the most common combi faults there is, and knowing the symptom's name is genuinely useful currency on the phone.
Which is also where your involvement ends. The valve lives inside the boiler casing, and the casing comes off for nobody but a suitably registered gas engineer — that's a legal and safety line, not a judgement on your competence. Note the symptom, note the boiler's make and model, and ring.
Your suspects are the immersion heater, its thermostat, and the switch on the wall. Check all three before assuming the worst.
Homes with a tank in the hot press heat their water via the boiler, an electric immersion heater, or both — which means you may have a backup you've forgotten about. If the boiler side is down, find the immersion's wall switch (often labelled, often switched off since last winter) and check the consumer unit for a tripped fuse. An immersion that trips repeatedly, or a cylinder that runs scalding then suddenly cold, points at its thermostat — that's a book-a-visit fault, not a live emergency.
One more cylinder-country quirk: the airlock. A hot tap that coughs, spits and dies after a tank refill or recent plumbing work has most likely swallowed air. Sometimes it clears itself within a day; if not, purging an airlock is quick work for a plumber.
Gas smell — instantly. Leaks and tripping electrics — same day. A cold tap alone can usually wait for a civilised hour.
The red rule outranks everything on this page: if you smell gas, leave the property, touch no switches, and call the National Gas Emergency Service on 0800 111 999 from outside. Nothing else happens before that.
Short of that, triage honestly. Water pooling under the boiler, pressure that collapses daily, an immersion that keeps tripping the electrics — those are same-day problems, because each one is quietly getting worse. A house that's merely cold-tapped, with the kettle picking up the slack? Ringing in the morning rather than at midnight often costs less — the costs guide explains why — and either way, say what you've already checked. That one sentence of triage shapes what comes out in the van.
Usually yes — repressurising through the filling loop is a routine householder job with the manual open beside you, and the sweet spot is around 1 to 1.5 bar cold. What you can't fix from outside the casing is a pressure that keeps falling: that pattern means the system is losing water somewhere, and it deserves a professional look.
On a combi boiler that lopsided pattern usually points to the diverter valve — the part that switches the boiler between radiators and taps. It's a common fault with a known fix, but the valve sits inside the casing, so diagnosis is as far as a householder goes. Quote the symptom when you ring; it genuinely speeds things up.
Air in the pipework, most likely — an airlock, common after a cylinder refill or recent plumbing work. Sometimes it works itself out; if a tap stays sputtering or dies entirely, a plumber can purge the run quickly. Mention whether any work was done on the system recently — it's the first question they'd ask anyway.
No. Anything beyond the external controls, the reset button and the filling loop on a gas boiler is work for a suitably registered gas engineer — that's the law and the safety line in one. And if you ever smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999 before anything else.
The main page — the triage board and the areas this line covers.
Go to home →Pressure drops, no heat, error codes — and the gas rule.
Read the guide →The sixty-second triage for escaping water.
Read the guide →Prevention that costs pennies, thawing that doesn't end in a flood.
Read the guide →The signs, the stopcock test, and when a damp patch turns urgent.
Read the guide →What to try, what never to pour, and when it's NI Water's sewer.
Read the guide →No invented prices — how charging works and what to ask first.
Read the guide →Ring any hour with what you found — pressure, symptom, model — and be connected with a local plumber covering Derry and the villages around it.
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