Central heating: reading the symptoms before you call

The symptom reading table

Falling pressure: the three suspects

A sealed system does not consume water. If you have to top up every week, water is going somewhere or the installation no longer handles expansion.

1. A leak

Sometimes it is obvious — a stain under a fitting, a mark on the ceiling — and sometimes discreet: a joint buried in screed, a radiator weld that weeps and dries instantly on hot metal. A useful clue: if pressure falls even with the boiler off, the leak is a genuine one.

2. The expansion vessel

This is the most common cause and the least investigated. The vessel holds a diaphragm and a cushion of compressed air that absorbs expansion. When the cushion is lost, heated water has nowhere to expand: pressure climbs sharply as the boiler fires, the safety valve opens and discharges, then pressure falls below normal on cooling. The giveaway symptom is pressure yo-yoing between hot and cold.

3. The safety relief valve

A scaled or tired valve no longer seats fully and lets a thin stream escape continuously. The discharge pipe should never run in normal operation; if it does, it is either the valve or the vessel upstream of it.

Radiators: cold at the top, at the bottom, or all over

Where the cold sits on the radiator tells you exactly what is happening inside. It is the simplest diagnosis in the trade, and the one most often read backwards.

• Cold at the top: air, being lighter, collects at the highest point. Bleed at the small valve on the top of the radiator until a steady trickle of water comes out. Then restore system pressure, since bleeding lowers it.
• Cold at the bottom: that is sludge. It comes from internal corrosion — black magnetic iron oxide — and settles by gravity in the base of the emitters. Bleeding does nothing here: the system needs flushing, sometimes with the affected radiators removed, then protecting.
• Completely cold: no flow is arriving. A closed isolating valve, a fully closed lockshield, or a thermostatic head whose pin stayed pushed in after a summer without heating. That last case is often solved by unscrewing the head and freeing the pin by hand.

Sludge and limescale: two different enemies

They are often confused, but they have neither the same origin nor the same treatment.

Sludge is born inside the sealed circuit, from corrosion of the steel in radiators and the heat exchanger. It reduces flow, insulates the exchange surfaces and eventually jams the pump. The treatment is a power flush, followed by a corrosion inhibitor and, ideally, a magnetic filter fitted on the return.

Limescale comes from the mains water and settles wherever water is constantly renewed: the domestic hot water circuit, the exchanger of a combi boiler, the cylinder. On hard water like that of many Algiers districts, this insulating layer cuts hot water flow and pushes consumption up. The treatment is descaling, and the prevention is a softener or a scale-reduction filter.

When the boiler locks out

A lockout is not a breakdown: it is a protection that worked. Resetting without understanding is like silencing a fire alarm. There are only a few families of causes.

• Low water or low flow: pressure too low, pump seized after the summer, clogged filter. The boiler senses that it is heating without the heat going anywhere.
• Overheat: same circulation origin, or a closed valve on the main circuit.
• Ignition fault: fouled electrode, insufficient gas pressure, or simply air in the line after a supply cut.
• Obstructed flue: blocked duct, cluttered terminal, bird's nest. This is the fault never to bypass: the pressure switch that cuts the boiler is doing exactly its job.

If the boiler locks out again within the hour after a reset, stop and get it looked at. The third reset in a row has never fixed anything.

Noises, and what they are telling you

• Gurgling and bubbling in radiators: air in the system. Bleed, then find out why it keeps coming back.
• Continuous whistling in one radiator: the lockshield is closed too far and water is squeezing through too small an opening.
• Cracking at start-up and shutdown: pipe expansion restrained by an over-tight clip or a wall penetration with no clearance. Harmless, but avoidable in new work.
• A humming boiler when firing: often exchanger scaling, with water flashing locally against an over-hot surface.

The annual service: what it actually contains

A service that amounts to a wipe with a cloth and a relight is not a service. Here is what must be done, and what you are entitled to ask for.

1. Inspection and cleaning of the heat exchanger and the burner.
2. Check of ignition, flame appearance and absence of soot.
3. Check of the flue and its slope, of the terminal and of the ventilation grilles.
4. Check of expansion vessel pre-charge and of the safety relief valve.
5. Check of system pressure, bleeding of high points, pump test.
6. Check of safety devices and control settings.
7. A written report of what was measured and what should be watched.

Pre-winter checklist for Algiers

• Fire the heating up once in late October, not on the evening of the first cold snap: a fault is better discovered calmly.
• Check cold pressure and bleed the radiators on the upper floors.
• Free the pump if it has not turned all summer.
• Clear the ventilation grilles and check none was blocked during building work.
• Inspect the flue terminal on the façade or roof after the autumn winds.
• Test the carbon monoxide alarm and change its battery.
• Clear curtains and furniture away from the radiators.

Frequently asked questions

What pressure should my system run at?

Cold, most domestic systems sit around 1 to 1.5 bar, with the exact figure depending on building height. What matters as much as the number is the gap between cold and hot: a large gap points to an expansion vessel that needs checking.

I bleed my radiators every month — is that normal?

No. Bleeding once a year is normal; bleeding monthly means air is getting in somewhere: a high point with no automatic vent, a micro-leak that draws in air when the system stops, or a failed vessel. It is a symptom to investigate, not a routine to accept.

Is power flushing really necessary?

It is when radiators are cold at the bottom, when the water coming out at bleeding is black, or when the pump is struggling. On a healthy system protected by an inhibitor there is no reason to do it preventively every couple of years.

Can I top the boiler up myself?

Yes, filling is a simple operation on most boilers. But if you have to do it every week, topping up is no longer maintenance: it is masking a leak or a failed vessel, and fresh water brings in oxygen each time, which accelerates corrosion.

How long does a repair take in Algiers?

Most call-outs are settled in one visit. Send us a photo of the pressure gauge, the fault code on the display and a description of the symptom: we arrive with the most likely part instead of ordering it afterwards.

About the author

This guide is general information. It does not replace an on-site diagnosis by a professional, and no work on a gas installation should ever be attempted by a non-professional.