Heating is on, house is cold — here’s the real reason

You notice it most when the weather turns: the radiators are hot, the thermostat says “on”, yet the house still feels stubbornly cold. In many homes the culprit isn’t the boiler at all - it’s the circulation pump failing to move hot water around the system properly. If heat can’t travel, it can’t reach the rooms that need it, and you end up paying to warm the pipework instead of your living space.

The giveaway is frustratingly subtle. One or two rooms might toast nicely while others stay tepid, or the heat arrives late and never quite “fills” the house. The real reason is usually flow - and flow has a few common choke points.

The real reason: you’ve got heat, but not enough movement

A heating system is basically a loop. The boiler makes hot water, the circulation pump pushes it through radiators, then the cooled water returns to be reheated. When that push is weak (or blocked), the hottest parts of the loop hog the heat and the rest of the house gets leftovers.

This is why you can have radiators that feel hot at the top but the room still feels chilly. Surface warmth isn’t the same as steady circulation across the whole system, especially if only part of the radiator is actually heating properly.

Heat is only useful when it reaches the far rooms. Most “cold house” problems are really “slow flow” problems.

Quick signs it’s a flow problem (not a boiler problem)

Look for a pattern rather than one odd radiator.

• Upstairs stays cold while downstairs heats (or vice versa).
• Some radiators get hot fast, others never really catch up.
• Radiators are hot near the pipes but cool across the panel.
• The boiler fires, stops, then fires again in short bursts (cycling).
• You hear gurgling, rushing, or “kettling” type noises when the heating runs.

If your hot taps are fine, that often points away from “no heat” and towards “heat isn’t being distributed”.

The circulation pump: what it does and how it quietly fails

The circulation pump sits in the heating circuit and keeps water moving at the rate your system needs. When it’s struggling, you can still get warmth near the boiler, but the far end of the circuit starves.

Common pump issues in UK homes include:

• Stuck pump after summer: the impeller can seize after months of inactivity.
• Wrong speed setting: too low, and the flow won’t overcome resistance in larger systems.
• Air in the pump housing: reduces pumping effectiveness and adds noise.
• Wear and tear: older pumps lose power gradually, so the change feels like “the house is just colder this year”.

A simple check you can do safely

With the heating on, carefully feel the flow and return pipes near the pump (or near the boiler if access is limited). One pipe should be hotter (flow) and the other slightly cooler (return). If the difference is extreme and distant radiators stay cold, it suggests heat isn’t travelling well.

If you’re not confident identifying components, don’t remove covers - use the symptoms below instead.

Radiators themselves: the three problems that mimic “bad heating”

Even with a healthy pump, radiators can block heat delivery.

1) Air trapped at the top

Classic sign: radiator is hot at the bottom and cool at the top, sometimes with trickling noises. Bleeding helps, but persistent air can indicate low system pressure or a leak.

2) Sludge at the bottom

Classic sign: radiator is hot at the top and cold at the bottom. That’s usually magnetite sludge settling and restricting flow through the panel, which also makes the pump work harder.

3) Poor balancing

Classic sign: the nearest radiators to the boiler get roasting hot, the far ones lag. The system is taking the easiest routes first. Balancing adjusts lockshield valves so each radiator gets its share.

If the first two radiators are scalding and the last one is barely warm, that’s rarely a “more powerful boiler” problem.

The 15‑minute triage: what to try before you call anyone

These steps are low-risk and often reveal the real fault quickly.

1. Check boiler pressure (combi systems)
   If it’s low, radiators often heat poorly and air enters more easily. Top up only to the manufacturer’s range (commonly around 1–1.5 bar when cold).

2. Bleed the coldest radiator first
   Work from the lowest floor upwards. Keep a cloth and container ready. Recheck pressure afterwards.

3. Turn every radiator fully on for a test run
   Open TRVs and lockshields temporarily to see if heat can reach everywhere at all. This isolates “valve set too low” from genuine circulation issues.

4. Listen near the pump/pipework
   A healthy system has a steady hum and gentle whoosh. Loud buzzing, repeated surging, or silence when heat is demanded can point to a struggling pump.

5. Feel for “fast hot, fast cold” cycling
   If the boiler gets hot quickly then shuts down, it may be hitting temperature because water isn’t moving away fast enough.

When the fix is balancing (and why it changes everything)

Balancing is unglamorous, but it’s often the difference between “boiling hot hallway” and “even heat everywhere”. You slightly restrict the radiators closest to the boiler so water is forced to travel to the farthest ones.

A quick clue you need balancing: if you can make a cold room warm by turning down other radiators, the system is competing for flow rather than lacking heat.

A basic balancing approach (without getting obsessive)

• Start with all TRVs fully open.
• Fully open all lockshields, then close them.
• Open the lockshield on the closest radiator a little (small fractions of a turn).
• Open further radiators progressively more as you move away from the boiler.

You can get very technical with thermometer readings, but even rough balancing often fixes the “cold back bedroom” problem.

If you suspect sludge: the “hot top, cold bottom” red flag

Sludge doesn’t just make a radiator cold - it can make the whole system inefficient. It narrows passages, reduces heat transfer, and strains the circulation pump.

What helps depends on severity:

• One cold radiator: remove and flush it (often a plumber job, but straightforward).
• Several affected: powerflush or targeted chemical clean, plus a magnetic filter if you don’t have one.
• After cleaning: inhibitor added to protect the system.

If your system has a magnetic filter, cleaning it can sometimes restore flow faster than you’d expect.

The overlooked culprit: a stuck valve (especially after summer)

TRVs and lockshield valves can stick closed, so a radiator stays cold even though everything else works. If a radiator never heats and the pipes into it stay cool, it may not be getting flow at all.

A light tap and gentle pin movement can free a stuck TRV pin - but forcing it can cause leaks. If you’re unsure, get help.

When it’s time to call an engineer

Call sooner (and save money) if you see any of these:

• Boiler keeps shutting down with fault codes.
• Pressure repeatedly drops after topping up.
• Pump is very noisy, very hot, or clearly not running when heat is demanded.
• Multiple radiators show sludge symptoms, or the system water is black when bled.
• You’ve bled radiators, pressure is correct, and the far rooms are still cold.

A good engineer will look at pump performance, system differential pressure, balancing, air ingress points, and whether the boiler is short-cycling due to poor circulation.

A simple “cold house” checklist you can keep

• Confirm boiler pressure is in range.
• Bleed radiators and recheck pressure.
• Check whether the pattern is “near hot, far cold” (balancing/flow) or “tops cold” (air) or “bottoms cold” (sludge).
• Fully open valves for a test run.
• If symptoms persist, suspect circulation pump performance or system restriction.

FAQ:

• Why are my radiators hot but the rooms still cold? Often the system is heating the panels unevenly (air or sludge) or hot water isn’t circulating well enough to sustain heat output, especially in distant rooms.
• Could it just be the thermostat in the wrong place? Yes. If the thermostat is in a warm hallway or near a hot radiator, it can switch the heating off before colder rooms catch up.
• Does turning the boiler temperature up fix poor circulation? It can mask the problem briefly, but it usually increases cycling and cost. If flow is weak, you’re better addressing air, balancing, sludge, or the circulation pump.
• Is a noisy pump always a bad pump? Not always - noise can come from air in the system or restrictions. But persistent buzzing, grinding, or surging is a strong hint the pump needs attention.