07706 889614 Contact Us
Radiators Not Heating But Hot Water Works? Here’s Why (Orpington & Bromley) | SKR
Heating mechanics • Orpington & Bromley

Radiators not heating but hot water works?
Here’s why it happens (and what the system is actually doing).

This one confuses people because it feels “half broken”. Tap runs hot, boiler sounds alive… yet the radiators sit there cold like they’re sulking. Nine times out of ten, the cause isn’t mysterious — it’s mechanical, and there are a few patterns I see over and over in Bromley and Orpington homes.

If you want me to sanity-check what you’re seeing, just call Stuart on 07706 889 614. Quick chat, no drama — tell me your boiler type and whether any radiators get warm at all.

Call 07706 889 614 Skip to safe checks
Gas Safe registered • heating fault-finding Checkatrade 9.95/10 • 67 reviews

This page feeds the main hub so Google (and people) can follow the logic properly: see the “hot water but radiators cold” hub explanation here.

What this symptom usually means

On most modern systems (especially combis), hot water and central heating are two different “paths” inside the boiler. You can have a boiler that can still heat water for taps, but fails to send heat around the radiators properly. That’s why this exact symptom is so useful diagnostically — it narrows the field.

And no, it’s not always “the boiler’s dead”. Sometimes the boiler is doing its job, but the circulation is poor (pump / air / sludge / balancing). Other times the boiler can’t switch into heating mode correctly (often a diverter valve story).

If you’re building the bigger picture, the parent guide lives here: the diagnostic hub for hot water working while radiators stay cold. This page is the “why it happens” layer.

A normal call-out (so you can see how this plays out)

Properly normal one: a place not far from Bromley South. Customer tells me, “Hot shower this morning, but the kids’ bedrooms are freezing and the radiators are stone cold.” They’d already topped the pressure up (fair enough) and bled a couple rads (also fair).

Boiler fired. Hot water was perfect. Heating demand came on… and it just didn’t move heat into the circuit. No magical fix, no mystery. We followed the system’s behaviour: is it switching into heating mode? then is the pump actually shifting water? then is the water clean enough to circulate?

That’s the difference between “guess and swap parts” and engineering fault-finding. And it’s also why these pages link into the core heating systems overview — the symptom only makes sense when you understand the circuit.

The real mechanical causes (not the internet list)

Below are the causes that actually show up in Orpington/Bromley properties — combis, system boilers, older rads, newer rads, you name it. I’m keeping this explanation-first. If you want the “what to do next” path, it’s in the hub: start with the diagnostic steps here.

Diverter valve not shifting properly

In a combi, the diverter valve routes heat to either your hot water plate exchanger or your radiators. If it sticks (or only half moves), the boiler can happily make hot water — but heating demand goes nowhere useful. You’ll often hear the boiler fire, but the radiator circuit stays cold or lukewarm.

Pump circulation issues (it’s “on”, but not moving)

Pumps can seize, run weak, or get jammed with debris. When circulation is poor, radiators won’t heat properly even if the boiler is producing heat. Sometimes the boiler will cycle on/off because it can’t push heat away — like a kettle boiling with the lid stuck.

Airlocks & trapped gas

Air (or gas) in the wrong place stops flow. You’ll get cold radiators, gurgling, and weird “some rooms warm, some dead” behaviour. Bleeding can help short-term, but if you’re bleeding constantly, something else is going on inside the system.

Sludge / magnetite restricting flow

Black sludge builds up in older systems and collects where flow is tight: radiator tails, valves, and sometimes the boiler’s internals. The boiler might still heat hot water fine, but the heating circuit becomes lazy and uneven.

Balancing problems (flow rates, not “air”)

If one radiator gets roasting and the next stays cold, it can be basic hydraulic behaviour. Water takes the easiest path. Poor balancing can make it look like “heating isn’t working” when it’s really flow distribution.

Low system pressure can knock heating out first

Lots of boilers will protect themselves and lock the heating circuit out if pressure is too low. Hot water can still appear to work (briefly) because the boiler fires in a different mode. If you suspect leaks, the right place to start is: leak detection in Orpington & Bromley.

Safe checks you can do (without turning this into a DIY experiment)

I’m not going to tell you to strip covers off a boiler. Don’t. But you can gather useful clues in five minutes that make diagnosis quicker (and cheaper).

Do this

  • check your boiler pressure gauge when cold (take a photo).
  • Turn heating on and listen: does the boiler fire continuously or short-cycle?
  • Feel one radiator flow/return: is one pipe warm and the other stone cold?
  • Note whether hot water demand changes anything (weirdly useful).

Please don’t do this

  • Don’t leave the filling loop open “just a bit”.
  • Don’t keep bleeding daily and assume that’s normal.
  • Don’t crank every radiator valve to max and hope.
  • Don’t start swapping parts because a forum said “diverter”.

If you’re stuck right now

If you’re in Orpington and you’ve just got home near the A21 / Orpington High Street and the house is freezing, ring me and tell me what the boiler does when you ask for heating. Call 07706 889 614 — I’ll tell you what’s most likely before you start fiddling.

Call now See what a proper service checks

If this keeps happening every winter, a thorough annual service usually prevents the repeat cycle: full boiler servicing info.

What we actually do on these jobs

I’m not interested in “quick wins” that come back in two weeks. The goal is: heating comes on reliably, radiators heat evenly, and the boiler isn’t being forced to fight the system.

Depending on what the system shows, that might mean: confirming diverter behaviour, checking pump operation, clearing air issues properly, or tackling system water quality (filters, flushing, inhibitor). It’s also why I keep pointing people to the bigger system context page: how heating systems work (and how they fail).

If there’s any hint of a leak or a slow pressure drop, I’d rather prove it than guess: our local leak detection page is here.

What people usually say after it’s sorted

“He actually explained it.”

Not in a lecture-y way — just enough so you understand why it failed and how to stop it happening again.

“No guessing, no upsell.”

The fix followed the evidence. That’s it. If it needed one part, it got one part. If it needed system work, it was discussed properly.

“The house finally heats evenly.”

This is the big one. It’s not just “radiators warm”; it’s consistent heat, no weird cold rooms, and a boiler that isn’t cycling itself to death.

Proof & reviews live here: SKR Plumbing & Heating on Checkatrade.

Who wrote this / who you’ll speak to

About Stuart (SKR Plumbing & Heating)

Stuart - SKR Plumbing & Heating

I’m Stuart. I’m Gas Safe registered and I cover Orpington and Bromley most days. This page isn’t written to “rank” — it’s written because I keep seeing the same misunderstandings on jobs: people treat the symptom (bleed, top up, twist knobs) and the real cause stays there waiting.

Verified reviews: Checkatrade profile (9.95/10 from 67 reviews)

Radiators cold but hot water fine — FAQ

Is this usually a diverter valve?

It can be, especially on combis — but I’d never call it off a single symptom. Diverter faults have a “feel” to them: hot water is strong, boiler fires, heating demand doesn’t translate into heat in the circuit. Still, pump issues and air/sludge can mimic it. Pattern beats guesses.

Why do some radiators get warm and others stay cold?

That’s often flow distribution (balancing) or restrictions (sludge/air). Water takes the easiest route, so the closest radiators can hog heat while upstairs or far rads starve. If it’s happening repeatedly, it’s worth stepping back and looking at the whole circuit rather than “that one radiator”.

I bled the radiators and it helped… then the next day it’s back. Why?

Because the underlying issue didn’t disappear. Air can re-collect if circulation is poor, if the system is producing gas internally, or if there’s ongoing corrosion/water-quality problems. Bleeding is sometimes a useful step — but “bleeding all winter” is a sign, not a solution.

Can low pressure cause radiators not to heat even if hot water still works?

Yes. Many boilers protect the heating circuit if pressure is below the safe threshold. Hot water can still appear to work for short periods because the boiler runs in a different mode. If you suspect a slow pressure drop, don’t ignore it — start with: leak detection across Orpington and Bromley.

What’s the best next page to read if I want the step-by-step checks?

Go here: the main diagnostic hub for this symptom. That’s the “do this first, then this” guide. This page is the mechanical “why”.

Is a boiler service actually relevant to this, or is that just sales?

I get the suspicion — fair. But a proper service checks the stuff that quietly causes these problems: circulation hints, pressure behaviour, filter condition, signs of sludge, and whether the system is being forced to run in a stressed way. If you want the exact details, it’s laid out here: what we include in a boiler service.

Ready when you are

Want it diagnosed properly — not “try this and see”?

If you’re in Orpington, Bromley or BR7 and your hot water works but radiators won’t heat, call me with three bits of info: boiler type, what the pressure is doing, and whether any radiators warm up at all. Call 07706 889 614.

Call 07706 889 614 Send details via contact form

Helpful supporting pages (for the full “system view”): heating systems explained • boiler servicing • leak detection