Topping up heating water: pressure and steps

In short: if the pressure gauge needle has dropped below the green zone, or the boiler shows a pressure fault, the system is short of water. To top it up: with the heating off and cold, connect cold water to the filling valve, open it slowly and close it as soon as the target pressure is reached, usually between 1 and 1.5 bar cold in a single-family home. In Switzerland, the fill water must be demineralised.

Why does the pressure drop?

A heating system is a closed circuit: in principle, it does not lose water. Still, a small drop from time to time is normal:

after bleeding the radiators, which lets out a little water along with the air;
when dissolved air in the water escapes through the air vents, especially in the weeks after filling;
when the expansion vessel has lost part of its pre-charge pressure.

If you have to top up often, for example every few weeks, that is no longer normal: see “If the pressure keeps dropping” below.

What pressure should you aim for?

The pressure is read on the boiler or heat pump gauge with the system cold, because it rises as the water heats up.

In a single-family home, the cold pressure is usually between 1 and 1.5 bar, slightly higher in a house with several storeys.
It must be at least 0.3 bar above the pre-charge pressure of the expansion vessel. See our guide How to choose an expansion vessel.
The most reliable reference is the green zone on the gauge or the reference needle set by the installer.
Don't overfill: the pressure rises when the system is hot. If it gets close to 3 bar, the safety valve may open.

Which water should you use?

In Switzerland, the SICC BT 102-01 guideline requires fill and top-up water to be demineralised: total hardness below 0.1 mmol/l (about 1 °fH) and conductivity below 100 µS/cm. Tap water is often hard and leaves limescale in the heat exchanger of the boiler or heat pump: efficiency drops, and the manufacturer's warranty may be lost.

The simplest solution: a demineralisation cartridge placed between the water tap and the filling hose. The water flows through it and enters the system demineralised, with no chemicals and no power supply.

Which Purotap solution should you choose?

For an occasional top-up: the Purotap disposable cartridge, in the 500 or 1000 version. It connects between the water tap and the filling hose with a 3/4" thread. The threaded adapters reinforce its connections.
For filling and topping up over several years: the Purotap micro DS filling station, supplied with a wall bracket, a 3 m filling hose and adapters. It treats about 300 litres of water at 10 °dH, and its cartridge changes colour when it is exhausted.
For a permanent connection: the Purotap micro Multiflex, with a built-in type BA backflow preventer, stays installed next to the boiler or heat pump.

Where is the filling valve?

On many wall-hung boilers, it is built in under the appliance: a tap or a knob, sometimes with a supplied hose. Check the manual: some models can be filled without a hose.
Otherwise, the system has a fill and drain valve with a hose connector, usually on the return near the boiler or heat pump. Example: DN 15 fill and drain valve.
Some systems have a permanent filling unit with a backflow preventer, sometimes with a built-in demineralisation cartridge, such as the Purotap micro Multiflex or the Caleffi automatic filling kit.

The connection must stop heating water from flowing back into the drinking-water supply: that is the job of the backflow preventer. If you fill with a temporary hose, always remove it after filling.

Topping up, step by step

Switch off the heating and let the system cool down, so you can read the pressure accurately.
Prepare the hose: connect it to the cold water tap, with the demineralisation cartridge if you are using one, then let a little water run through to push the air out of the hose. Otherwise you send air into the heating system.
Connect the hose to the system's filling valve, without opening it yet. Keep a bucket and a cloth to hand.
Turn on the water, then slowly open the filling valve. Keep your eyes on the gauge: the pressure rises quickly.
Close the filling valve as soon as the target pressure is reached, then turn off the water.
Disconnect the hose over the bucket and put the cap back on the valve.
Switch the heating back on. Fresh water contains air: bleed the radiators after a few hours of heating, then check the cold pressure again. See our guide Bleeding a radiator: when and how to do it.

Overshot the target pressure? Let a little water out through a drain valve or a radiator bleed screw until you are back at the right value.

If the pressure keeps dropping

Topping up a little from time to time, for example after bleeding, is normal. If the pressure drops within a few days or weeks, look for the cause:

a dripping safety valve, often because the pressure rises too much when hot: see our guide Heating safety valve dripping;
an expansion vessel that has lost its pre-charge or has a damaged membrane;
a small leak at a fitting, a radiator valve or in the boiler: look for signs of damp or limescale;
an automatic air vent that lets water through.

Topping up without fixing the problem brings in fresh water and oxygen each time, which encourages corrosion. Have the system checked by a heating engineer.

Frequently asked questions

Can you top up while the heating is running?

It is better to switch it off and wait for the system to cool down. When hot, the gauge shows a higher pressure: you risk not filling enough.

What pressure for a single-family home?

Usually 1 to 1.5 bar cold, and at least 0.3 bar above the pre-charge pressure of the expansion vessel. Go by the green zone on the gauge first.

Can you use tap water?

The Swiss SICC BT 102-01 guideline requires demineralised water, including for top-ups. A demineralisation cartridge lets you fill straight from the tap with compliant water.

My boiler shows a pressure fault: what should I do?

Many boilers go into safety lockout when the pressure is too low. Top up to the right value, then reset the boiler as described in its manual. If the fault comes back quickly, there is a leak or a problem with the expansion vessel.

What about after draining the system completely?

Refilling a system completely, after work or draining, means flushing it, using demineralised water, bleeding it thoroughly and recording the water values, as the guideline requires. That is a job for a heating engineer.