Topping up your boiler: step-by-step plan, correct pressure and tips per brand

Short answer: if the water pressure of your boiler is below 1 bar when cold, top it up to 1.5–2 bar. You do this with the filling loop on the filling valve, with the boiler switched off and cooled down, and with the valve opened only slightly. Then bleed the radiators and check the pressure again. If you need to top up more than once or twice per heating season, something else is going on.

How many bar should a boiler have, and when should you top it up?

A central heating system is a sealed system. The water expands as it heats up and contracts as it cools; the expansion vessel absorbs this. The pressure you read therefore depends on the temperature. So always measure with a cold system: in the morning before the heating comes on, or a few hours after switching the boiler off.

When cold, the pressure should be between 1.5 and 2 bar. Below 1 bar most boilers switch off for safety and show a fault code (see below by brand). When hot, the pressure may rise to about 2.5 bar. Above 3 bar the pressure relief valve opens and water runs out through the drain under the boiler.

Pressure (cold boiler)	What it means	Action
Below 1 bar	Too low; boiler shows a warning or shuts down	Top up to 1.5–2 bar
1 to 1.5 bar	On the low side, usually no fault yet	Top up at the first opportunity
1.5 to 2 bar	Good	No action needed
2 to 2.5 bar (cold)	Too full; when hot the pressure rises too high	Drain some water via a radiator bleed valve
Above 3 bar	Pressure relief valve discharges	Boiler off, drain some water; if it happens again, have the expansion vessel checked

What you need

• A filling loop with a union nut and rubber washer at both ends. Most boilers come with one; otherwise it is available separately, also as a filling kit with valve.
• A filling valve on the system and a small tap with a hose connector on the water supply, usually under or next to the boiler. The filling valve is almost never inside the boiler itself, but in the pipework below it.
• A bucket and a cloth for the drips when connecting and disconnecting, and a radiator bleed key for the radiators.

Not sure which tap is the filling valve? Follow the hose connector: it is the tap the filling loop fits onto and that is connected to the water supply. The installation manual for your appliance includes the filling and draining diagram.

Step by step: topping up your boiler

1. Switch the boiler off and let it cool down. Turn the thermostat down or put the boiler on standby and wait until the system is cold.
2. Connect the filling loop to the water supply and purge the air from the loop. Hold the loose end over a bucket, open the tap briefly until water comes out without air bubbles and close it straight away. That way you do not pump air into the system.
3. Connect the loop hand-tight to the filling valve and check that the washer is seated in the union nut.
4. Open the filling valve first, then the mains water tap very slowly. A quarter turn is enough. Keep an eye on the pressure gauge or display; the pressure should rise slowly.
5. Stop at 1.5–2 bar. Close the mains water tap first, then the filling valve. Overfilled? Let some water out via a radiator bleed valve.
6. Disconnect the hose with the bucket underneath and hang it up to drain. Do not leave it connected.
7. Bleed the radiators, from the lowest to the highest, until only water comes out. See bleeding your boiler and radiators.
8. Check the pressure again and top up again if necessary. Bleeding makes the pressure drop a little again; repeat steps 2 to 6 until the cold pressure is between 1.5 and 2 bar.
9. Switch the boiler back on, reset any fault message as described in the manual and check after an hour that the pressure stays below 3 bar when warm.

By brand: reading the pressure and finding the filling valve

The filling valve is part of the installation, not of the appliance; the installer decides where it goes. How you read the pressure does differ by brand and series. If in doubt, consult the user manual for your model; you can find it via the rating plate.

Remeha

Remeha boilers with a pressure sensor (including the Tzerra, Calenta and Ace versions) show the pressure in bar on the display and warn or lock out if the pressure is too low. Older Avanta models without a sensor have a pressure gauge underneath the boiler. See Remeha fault codes en Remeha spare parts.

Intergas

On the Kombi Kompakt HRE, HReco and Xtreme you read the pressure on the display or, on older versions, on the pressure gauge underneath the appliance; the manual for your model tells you which button shows the pressure. The filling valve is in the connection kit underneath the appliance. See Intergas fault codes en Intergas spare parts.

Nefit / Bosch

Nefit and Bosch boilers with a display (ProLine, TopLine, TrendLine, Bosch Condens) show the message H07 when the water pressure is too low and stay off until the pressure is sufficient again. The pressure is shown on the display or on the pressure gauge in the bottom panel. After topping up, H07 usually disappears by itself. See Nefit and Bosch parts.

Vaillant

The Vaillant ecoTEC shows the water pressure digitally on the display; if there is a lack of water, code F.22appears. Top up to the value given in the manual, normally between 1 and 2 bar. See Vaillant spare parts.

ATAG

ATAG boilers from the A, i and Q series show code 118 when the water pressure is abnormal and display the pressure on the screen or via the ATAG One thermostat. See ATAG spare parts.

Topping up safely

• Never with a hot boiler. Cold water in a hot heat exchanger puts stress on the material, and you will read an incorrect pressure.
• Do not overfill. Above 2 bar cold, the pressure relief valve will open as the system heats up, and a valve that has discharged once sometimes keeps dripping.
• Above 3 bar: drain off. Switch the boiler off and let water out via a radiator bleed valve or the drain cock until it is below 2 bar.
• Do not leave the filling loop connected. A permanent connection between drinking water and heating water is not permitted without the correct backflow prevention, and it masks a leak.
• Do not keep topping up. Every litre of fresh tap water brings oxygen and limescale into the system, and that attacks the boiler and radiators.

When topping up does not help

If you need to top up more than once or twice per heating season, water is escaping somewhere or the expansion vessel no longer works. The most common causes:

• Expansion vessel without pre-charge pressure or with a ruptured diaphragm. Recognisable by high pressure when warm and low pressure when cold. View expansion vessels by brand and capacity.
• Pressure relief valve that keeps leaking. Water drips from the discharge pipe under the boiler, even at normal pressure. View pressure relief valves and other valves.
• Filling valve that does not close, or a leaking coupling. Look for damp patches, limescale rings or rust under the boiler and at the underfloor heating manifold. A new filling valve is a small part.
• Pressure sensor giving an incorrect reading. Is the boiler complaining while the pressure gauge shows a good pressure? Read replacing the water pressure sensor or view pressure sensors.

The full explanation, including the expansion vessel self-test, is in boiler water pressure keeps dropping: expansion vessel, leak or valve?. A code you do not recognise can be looked up in the overview of boiler faults and codes. Instalpunt supplies the parts; you check whether a part fits your appliance using the model number and manual. Instalpunt does not install; if you would rather not replace an expansion vessel or valve yourself, have a qualified local installer do it during a service visit.

Frequently asked questions

How many bar should a boiler have?

Cold between 1.5 and 2 bar; when warm the pressure may rise to about 2.5 bar. Below 1 bar most boilers show a fault, above 3 bar the pressure relief valve opens.

Hoe vaak moet ik mijn cv-ketel bijvullen?

Normally no more than once or twice per heating season, for example after bleeding. If you need to do it every few weeks, there is a leak or the expansion vessel needs replacing.

Should the boiler be off while topping up?

Yes. Switch the boiler off or to standby and let the system cool down. That way you read the correct pressure and avoid damage from cold water in a hot heat exchanger.

Top up first or bleed first?

Top up first to 1.5–2 bar, then bleed, then check the pressure and top up again if necessary. Bleeding with too low a pressure draws air into the system.

What if the pressure rises above 3 bar after topping up?

Then there is too much water in the system or the expansion vessel is not working. Switch the boiler off, drain off until below 2 bar and see whether it comes back the next time it heats up. If so, have the expansion vessel checked.