Central heating systems in Devon homes fail in recognisable ways. Most faults follow a pattern that points clearly to the cause — if you know where to look. This guide walks through the most common central heating problems, what causes them, and what to do at each step.
Note: the checks below are safe for any homeowner. Anything involving the gas or oil supply, the boiler's internal components, or the flue must be handled by a qualified engineer — Gas Safe registered for gas systems, OFTEC-registered for oil.
No Heat at All — Boiler Running, But No Warmth
If the boiler appears to fire normally but radiators are cold across the whole house:
1. Check the room thermostat. The most common cause. A thermostat set below the current room temperature, a flat battery, or a wireless thermostat that has lost connection to its receiver will shut the system down entirely. Check the thermostat setting — it should be at least 2°C above the current room temperature — and replace batteries if relevant.
2. Check the programmer. If the heating is on a timer and the programmer has lost its schedule (after a power cut, for example), it may be in an off setting. Reset or override the programme manually to force the heating on.
3. Check the system pressure. Most combi boilers require a working pressure of 1.0–1.5 bar when cold. Check the pressure gauge on the boiler — usually a small round dial on the front panel. If it reads below 0.8 bar, repressurise through the filling loop (refer to your boiler manual). If pressure drops repeatedly, there is a system leak that needs investigating.
4. Check for a boiler fault code. Modern boilers display fault codes when they lock out. The code narrows the possible causes significantly. Common Devon fault codes: low pressure (water loss), ignition failure (common in oil boilers during winter cold), and thermistor faults (sensor failures).
Some Radiators Cold, Others Warm
If some radiators are warm but others — particularly at the far end of the circuit, or upstairs — are cold, there are two likely causes:
Trapped air. Air in the radiator collects at the top. The bottom is warm (water is circulating) but the top is cold. Fix: bleed the radiator. Use a bleed key on the bleed valve at the top corner. Open slowly until water — not air — escapes, then close. See our full guide: how to bleed radiators — Devon step-by-step.
Sludge deposits. Radiators cold in patches — cold at the bottom but warm at the top — indicate magnetite sludge settled at the base, reducing flow. Bleeding will not fix this. A power flush is required if the problem is systemic. Individual heavily sludged radiators can sometimes be removed and flushed off-circuit. See our power flushing cost guide.
Balancing issue. If radiators close to the boiler are always hot first and far radiators barely warm up, the system needs balancing — adjusting the lockshield valve on each radiator to restrict flow to near radiators. This is a DIY task but requires patience and a clip-on thermometer.
Cold Radiators After a Power Flush
If your system has recently been power-flushed and radiators are still cold, the most likely explanation is a stuck thermostatic radiator valve (TRV). TRVs can seize in the closed position, particularly on radiators that have been turned down for years. The TRV pin — visible under the valve head — can be freed by gently removing the TRV head and moving the pin manually. If the pin does not move freely, the TRV body needs replacing.
Boiler Fires But No Hot Water (and Heating Works)
On a combi boiler, separate faults can affect heating and hot water circuits independently. If central heating works but there is no hot water:
1. Check the diverter valve. The diverter valve in a combi boiler directs flow to heating or hot water on demand. A failed or sticking diverter valve can deprive the hot water circuit while leaving heating unaffected. This is a repair job for a Gas Safe engineer.
2. Check the flow rate. Combi boilers require a minimum flow rate through the hot water circuit to activate heating. If the hot water tap is partially blocked with scale — possible in Devon's moderately hard water areas — the flow rate may drop below the trigger threshold.
Boiler Short-Cycling (Firing Every Few Minutes)
A boiler that fires, runs for a few minutes, then shuts off and fires again is short-cycling. Common causes:
- System pressure too high (above 2.0 bar when cold)
- Overheat thermostat triggering due to restricted flow
- Pump running at incorrect speed
- Expansion vessel fault (pressure relief valve opening, losing water, triggering refill and pressure fault)
Short-cycling causes premature wear on the heat exchanger and ignition components. A heating engineer can diagnose the specific cause within one visit.
Pressure Drops Repeatedly
If the boiler pressure falls below the operational minimum repeatedly, there is a water leak in the system. This may be:
- A weeping radiator valve or TRV
- A pinhole leak in a radiator
- A failing pressure relief valve (PRV) — check for damp around the condensate or overflow pipes
- Microbore pipework corrosion (common in Devon properties built in the 1970s–1980s)
A leak detection survey will locate the source, including hidden leaks under floorboards or within walls.
When to Call a Heating Engineer
- Call immediately for:
- Any smell of gas or oil fumes
- A yellow or orange flame visible through the boiler inspection window (should always be blue for gas)
- Carbon monoxide alarm activating
- Persistent fault codes that cannot be cleared
- Call — not necessarily as an emergency — for:
- Repeated pressure loss despite no visible leak
- Short-cycling that persists after basic checks
- Motorised valve faults (zone valves failing to open or close)
- Pump failure (no flow through the system, pump cold to the touch)
- System-wide sludge (cold radiator bottoms throughout the house)
Frequently Asked Questions
Why does my central heating work but my hot water is cold? On a combi boiler, the most likely cause is a sticking diverter valve. This requires a Gas Safe engineer — it cannot be fixed by the homeowner.
Why are my radiators cold at the bottom? Magnetite sludge settling at the base of the radiator. Bleeding will not fix this. A power flush, or individual radiator removal and flush, is required.
Why does my boiler keep losing pressure? A water leak somewhere in the system. Check all radiator valves for weeping, and have a leak detection survey carried out if no visible source is found.
Why is my central heating making a banging noise? Kettling — a banging or gurgling noise from the boiler or radiators — is most commonly caused by scale on the heat exchanger or sludge restricting flow. A magnetic filter, inhibitor dosing, and possibly a power flush is the usual remedy.
Our central heating team covers Tavistock, Okehampton, Launceston, Yelverton, and the wider Devon service area. For boiler faults requiring immediate attention, our boiler repair service is available across the area. If sludge is confirmed, power flushing can restore full performance to the system.