No Hot Water From a Combi Boiler? Here’s the Safe Way to Narrow It Down
This is one of those problems that makes you feel silly fast. The boiler looks “on”. The heating might even work.
Then you run a tap… and it’s freezing. Or it goes warm for ten seconds and dies again.
I’m Abdul (Gas Safe 626557). If you’re in Newham, I’ve had this exact call in places off Green Street and Romford Road:
someone’s trying to get kids washed for school, or they’ve just come back from work and the shower is basically an ice challenge.
This page is the calm version of what I talk you through on the phone — safe checks first, then the point where we stop guessing.
The 90-second checklist (safe, no tools)
We’re not opening the boiler. We’re just confirming what the system is doing.
1) Try hot water from two different taps
Sounds basic, but it matters. Kitchen tap, then bathroom. If one runs hot and the other stays cold, the issue might not be the boiler at all (you’d be surprised what I’ve seen).
If both are cold, it’s much more likely a combi hot water issue or a boiler-side fault.
2) Watch the boiler when the tap turns on
Turn the hot tap on and look at the boiler display. Does it change mode? Any flame icon? Any error? On many combis, that switch into hot-water mode is the clue.
If an error appears, note the code. Don’t guess. Use the error-code guide above and then go to diagnosis.
3) Does it go warm for a moment, then cold?
That “warm for 10–30 seconds then cold” pattern shows up a lot with combi flow/temperature issues, sensor problems, or blockages that reveal themselves under demand.
In London, hard water can make this worse over time (scale builds quietly, then suddenly you notice it).
4) Don’t keep resetting
One reset is fine if it’s locked out. After that, stop. If the boiler is trying to protect itself, repeated resets won’t “convince” it — they usually just waste time.
If the boiler won’t even attempt hot-water mode, the “boiler not firing” guide above is the better route.
What’s normally behind “no hot water” on a combi
Combi boilers do a little dance when you open a hot tap: they detect flow, switch into hot-water mode, fire up, then hold temperature. When hot water disappears, the fault is often somewhere in that chain. The tricky bit is that the symptoms can look similar.
Flow/temperature sensing issues
If the boiler can’t correctly “see” the hot water demand, it won’t switch properly, or it will cut out quickly. That’s why you’ll sometimes get a short burst of warm water then cold.
This is also why I ask people: “Does the display change when you open the hot tap?” It’s not a random question.
Hard water build-up (the London classic)
Scale builds up slowly in components and heat-transfer areas. Most people don’t notice until the boiler starts struggling under hot-water demand. It’s not your fault — it’s London.
You don’t need to diagnose this yourself. The point is: don’t panic and don’t keep resetting.
District proof: Newham “hot tap panic” calls
A really common Newham scenario: heating is fine, then the hot tap goes cold. People assume “boiler’s broken” and start pressing buttons. What usually makes it worse is rushing. One reset, sure. After that, stop.
If you tell me the model, what the display does when you open a hot tap, and whether it ever goes warm for a moment, I can usually tell you which route you’re on: quick fix vs proper diagnostic visit.
Stop DIY here
Please don’t take panels off, don’t “bypass” anything, and don’t keep cycling power. If there’s a gas smell, soot marks, a CO alarm, or leaking near electrics, treat it as urgent and use emergency guidance.
Boiler faults are one of those areas where the safe move is the smart move. I’d rather you ring and it turns out simple.
If you just want a straight answer
Call 07727 154746 and tell me three things: the boiler model, what happens on the display when the hot tap opens, and whether the water ever turns warm briefly. I’ll tell you the likely cause and the right next step.
If it needs a visit, we’ll give you a clear ETA and keep it honest — no drama.
What to do next (the clean routes)
If there are no safety warning signs, the standard route is a proper fault-find and repair booking: boiler repairs & diagnosis. If you have any safety concerns at all, switch route to: boiler emergencies.
And if you want to zoom out and see what we do as a business, you can jump to the ARA Services homepage.
FAQ
My heating works, but the hot water is cold. Is that normal?
It’s common, but it’s not “normal”. It usually points to a combi hot-water-side issue rather than the whole system failing. The boiler can still heat radiators while struggling to deliver stable hot water under demand.
Hot water goes warm for a few seconds then cold — what does that suggest?
That pattern is often linked to flow/temperature sensing or a hot-water-demand chain that starts, then fails. It needs proper diagnosis rather than repeated resets.
Is no hot water an emergency?
Not automatically. It becomes urgent when safety is involved: gas smell, CO alarm, soot marks, or leaking near electrics. If any of those apply, treat it as emergency and follow safety-first actions.
Can London hard water really cause boiler hot water problems?
Over time, yes. Scale can build slowly and then show up as unstable hot water delivery. You don’t need to diagnose it yourself — it just explains why this fault is so common locally.
What should I say on the phone to make this faster?
Boiler model, what the display does when you open a hot tap, and whether you ever get a brief warm burst. That’s enough for me to route you properly.