You turn the hot tap on — it’s warm. The boiler fires. Everything looks fine.
Then you check the radiators and they’re barely heating.
This is one of those faults that seems simple on the surface, but keeps catching engineers out, especially in spring. Across trade discussions and posts on X, a recurring pattern shows up every year: hot water works, heating doesn’t, often after the system has been running hard all winter.
The symptoms aren’t always clear-cut. In reality, it’s rarely a complete failure. More often, hot water is only lukewarm, radiators get slightly warm but never properly heat up, or the whole system feels inconsistent rather than fully broken. That’s what makes it easy to misdiagnose.
The most common causes, in order of likelihood:
First, diverter valve issues. On combi boilers, the diverter valve controls whether heat goes to hot water or central heating. Most commonly, it sticks, only moves partially, or fails to switch fully between modes. The result is that the boiler continues prioritising hot water and the heating side never receives full flow. This is particularly common after winter when the valve has been switching constantly under load.
Second, circulation problems. If the boiler is producing heat but it’s not moving effectively, the issue shifts from direction to flow. Common causes include a weakening or sticking pump, partially closed valves, sludge restricting flow, or air trapped in the system. Typical signs are some radiators warming while others stay cold, or the system never quite reaching full temperature.
Less obvious but still important causes include faulty temperature sensors giving incorrect readings, automatic air vents sticking or drawing air in, and zone valves or wiring faults on more complex systems. These faults can mimic circulation or diverter issues and are often overlooked.
Rare but worth ruling out are heat exchanger issues affecting heat transfer, severe sludge or internal restrictions within the boiler, and hydraulic imbalance in older or modified systems. These are less common but do show up, particularly on ageing systems.
Why this keeps happening is fairly predictable. After winter, components have been under constant load, debris and sludge have built up, and moving parts like diverter valves begin to stick once demand drops. At the same time, engineers are often under time pressure, so it’s easy to reset, bleed, top up, and move on. That works temporarily, but it doesn’t solve the underlying issue.
Hot water but no heating isn’t usually a failure — it’s a direction or flow problem that hasn’t been properly identified.
A typical example would be an 8 to 12-year-old combi in a three-bed semi. The customer reports hot water working but radiators only getting slightly warm. On the first visit, the system is repressurised, radiators are bled, and the boiler is reset. Heating improves briefly. A few weeks later, the same issue returns. On closer inspection, the diverter valve is only partially switching, and the pump is beginning to weaken. Once both are addressed, the system returns to normal operation.
On site, these jobs are rarely major failures, but they can easily turn into repeat visits. Each return trip eats into your day, disrupts your schedule, and reduces customer confidence. From the customer’s perspective, they don’t see an intermittent fault — they see a problem that hasn’t been properly fixed.
A more effective approach is to think through the system logically. First, confirm heat generation by checking hot water performance. If that’s working, the issue is not with heat production. Next, check whether the heat is being directed correctly, which points to the diverter valve. Finally, assess circulation by looking at the pump, valves, sludge, and air within the system. If those all check out, move on to secondary causes like sensors, controls, or internal restrictions.
KEY TAKEAWAYS
Low heating with working hot water is usually a direction or circulation issue, not a complete failure.
Diverter valves are the most common cause on combi boilers.
Circulation problems are the next most frequent issue.
Less obvious faults like sensors and air vents are often missed.
Seasonal changes after winter are a major trigger for these problems.
Fixing the root cause properly prevents repeat callouts and builds trust.
Fixing these properly the first time isn’t about doing more work — it’s about doing the right work in the right order.
If you found this useful, Heatlab designs innovative, problem-solving tools for plumbers and heating engineers — built from real on-site experience.
