Cool radiators despite the boiler operating, a humming boiler, and a falling pressure gauge reading are the most common signs that the central heating system lacks water. As a manufacturer of pumps and equipment for heating systems under the IBO and IPRO brands, we regularly answer installers’ and homeowners’ questions about how to identify this problem and resolve it safely before it leads to a more serious failure. You can find the full range of equipment supporting the proper operation of heating systems at dambat.pl. Below, we explain how a lack of water differs from air in the system, what pressure drop is still within the normal range, and how to top up the water in a central heating system step by step.
Symptoms of a lack of water in a central heating system
Too little water in a central heating system most often manifests itself in a way that can easily be mistaken for another fault—which is why, rather than relying on intuition alone, it is worth basing the diagnosis on regularly checking the water level and pressure in the system. The most characteristic signs are:
- Cold zones on radiators—the upper part of the radiator is warm while the lower part is cold, or the entire unit remains cool despite the boiler operating.
- Unevenly heated radiators—some rooms heat properly, while others, usually the ones farthest from the boiler, remain cold.
- Cold heating surfaces in underfloor heating systems—a drop in floor temperature without changing the thermostat settings.
- Frequent starting of the circulation pump—the pump switches on and off at short intervals, attempting to maintain the required pressure.
- A falling pressure gauge reading—the clearest symptom, described in more detail later in the article.
When diagnosing these symptoms, installers are increasingly turning to a thermal imaging camera—it allows them to precisely locate cold zones on radiators and distinguish a local problem (e.g. a single air-bound radiator) from a general lack of water throughout the system. It is also worth remembering that the frequent activation of the circulation pump is a signal that directly affects the operating equipment—prolonged operation in this mode shortens its service life.
Not enough water or air in the system? How to tell the difference
A lack of water and air in the system produce very similar symptoms—cool radiators and a noisy pump—which is why users often confuse these two causes. The key difference lies in the pressure gauge reading: with air in the system, the pressure usually remains within the normal range, and the problem disappears after bleeding individual radiators or the pump. In the case of a genuine lack of water, the pressure gauge reading remains below the value recommended for the system and does not return to normal after bleeding alone.
Many installations also use an additional air separator—a device that automatically captures and removes small air bubbles from the heating water circuit, reducing the risk of airlocks and the operating noises associated with them. If you suspect that the problem is air in the system rather than a lack of water, see our complete guide to bleeding central heating and hydrophore pumps. It is also worth ruling out the circulation pump itself as the source of the noise—we described this in the article about improper circulation pump operation.
Permissible pressure drop — when is it still normal?
A small, gradual pressure drop in a closed central heating system is natural—it may result, for example, from bleeding radiators or slight evaporation during servicing. A problem arises when the drop is rapid, recurs every few days, or is accompanied by visible symptoms from the previous section.
One common cause of a permanent pressure drop is a damaged or incorrectly selected expansion vessel—it then fails to properly accommodate fluctuations in the water volume in a closed system. If you suspect this component of the installation, see our guide how to select a diaphragm vessel for central heating and domestic hot water systems. Expansion and diaphragm vessels for heating systems can be found in our range in the central heating, domestic hot water, and solar vessels category.
What pressure should there be in a central heating system?
The correct operating pressure depends on the height of the installation, its configuration, and the boiler manufacturer's recommendations, so it is always worth verifying the value in the documentation for the specific heating appliance. As a guideline, for a typical installation in a detached house, values of around 1.0–1.5 bar are assumed “when cold,” while the operating pressure (when hot) is slightly higher.
The content has been prepared based on general data. Contact us for details.
For ongoing monitoring of pressure levels, a functioning pressure gauge is essential—we explain how to read and select this instrument correctly in the article Dambat pressure gauges — measuring water pressure.
Water loss in a central heating system — when is it normal, and when is it an alarm?
A one-time, small water top-up after a seasonal inspection is typical and should not cause concern. However, having to add water regularly, for example weekly, is an alarm signal — it usually indicates a leak in the system, a damaged connection, or a faulty valve. Ongoing monitoring of the water level and noting how often it drops significantly helps the installer make an accurate diagnosis.
In this situation, it is worth asking an installer to carry out a pressure test, which will precisely locate the leak. Equipment for this type of testing, namely pressure testing pumps, is an installer’s working tool — it is used to check system tightness, not to routinely top up water in an active heating system.
Ignoring recurring water loss for an extended period also poses a risk to the circulation pump itself — a significant drop in the water level may cause it to run dry. We describe the consequences and ways to protect against this phenomenon in the article pump dry running — consequences, causes, and the best ways to prevent it.
How to safely top up water in a central heating system
The water in the central heating system should be topped up when the system is cool and in the following order:
- Cool the system — do not add water to a hot system.
- Check the current pressure gauge reading and the water level in the expansion vessel, and compare them with the value recommended for the system.
- Open the filling valve and slowly introduce water while monitoring the pressure reading.
- Close the valve when the pressure reaches the recommended “cold” value.
- Bleed the radiators if signs of air in the system appear after topping it up.
Water added directly from the mains introduces minor contaminants into the heating water, which gradually settle on the system components and circulation pump. Therefore, it is worth ensuring filtration every time you top up the system — proper protection of heating systems with magnetic filters can extend the service life and improve the efficiency of the system. Filling valves and other installation accessories for circulation pumps can be found in the accessories category.
Excessive pressure after topping up — how to correct it
Refilling water too intensively leads to the opposite problem — excessive pressure in the system. The first sign is usually a dripping safety valve, which deliberately releases water after the limit has been exceeded, protecting the system from damage. In systems with extensive automation, the differential valve also performs a regulating function by equalizing the pressure difference between the supply and return lines. If the overpressure problem keeps recurring, it is worth considering installing a pressure reducer — we described this in the article how to protect a pump from excessive pressure. You can find safety valves and other protective fittings in our fittings category.
Most common mistakes when refilling water
- Refilling water while the system is hot and operating instead of waiting for it to cool down.
- Failing to monitor the pressure gauge while filling — risk of exceeding the pressure limit and activating the safety valve.
- Skipping regular maintenance of the heating system — inspecting the valves, checking for leaks, and checking the condition of the expansion vessel.
- Refilling water without filtration, which accelerates the buildup of deposits in the circulation pump.
- No check valve at the connection to the water supply network — risk of water flowing back from the heating system.
- Failing to regularly check the pressure gauge after the heating season — a slight drop in pressure is easy to overlook if it is not checked periodically.
You can read more about the role of this last component in the article check valves — types, applications, and the IBO Dambat range. Smooth operation of the entire system is ensured by a circulation pump selected for the installation — you can find our energy-efficient models in the energy-efficient circulation pumps category, and the full range in the circulation pumps category.
Dambat – a trusted manufacturer of pumps and hydrotechnical equipment with quality certificates.
Frequently asked questions (FAQ) — too little water in the central heating system
How can you tell that there is not enough water in the central heating system?
The most characteristic symptoms are cold radiators despite the boiler running, a falling pressure-gauge reading, and the circulation pump starting up frequently for short periods. A thermal imaging camera can be helpful for diagnosis, as it precisely shows cold areas on the radiators.
What is the difference between a lack of water and air in the system?
When there is air in the system, the pressure shown on the pressure gauge usually remains within the normal range, while noise and cold radiators disappear after bleeding. When there is a lack of water, the pressure reading remains consistently below the recommended level and does not return to normal without adding water. We describe the complete procedure in the guide to bleeding central heating and pressure booster pumps.
How often can water be added to a central heating system?
A one-time refill after a seasonal inspection is normal. If you have to add water regularly, for example every week, this indicates a leak in the system that requires diagnosis by an installer rather than continuous refilling.
Which expansion vessel should I choose if the system pressure keeps dropping?
As a manufacturer, we recommend first checking whether the existing diaphragm expansion vessel is correctly sized for the system’s capacity and whether it has retained the correct pre-charge pressure. We describe the detailed selection process in the article how to select a diaphragm expansion vessel for central heating and domestic hot water systems Expansion vessels from IBO are also available in the central heating, domestic hot water, and solar expansion vessels category.
Is dry running of a circulation pump due to a lack of water dangerous?
Yes — prolonged operation of a circulation pump without an adequate amount of water in the system can cause it to overheat and suffer permanent damage. We describe the details and protection methods in the article pump dry running — effects, causes, and the best ways to protect against it.
How can I look after a circulation pump to reduce the risk of failures caused by a lack of water?
Regular pressure checks, filtration when replenishing the system with mains water, and responding to the first symptoms, such as the pump switching on frequently, are essential. Our technical department can help you select a circulation pump and equipment for your specific system — contact us if you need individual advice.
Where can I buy equipment for replenishing and monitoring pressure in IBO and IPRO central heating systems?
As a manufacturer, we do not sell directly to customers. Filling valves, pressure gauges, expansion vessels, and IBO and IPRO circulation pumps are available from our trade partners and installation wholesalers throughout Poland. You can also find our full range at dambat.pl.
