Filling the Heating System – Final Stage

A full refill is needed when the system has been empty or part of it has been drained. The usual situations are:

• First start-up of the heating in a new build or after a full renovation
• Replacing or moving a radiator, thermostatic valve, pump or boiler
• Flushing the system to remove sludge and rust
• Repairing a leak where the water had to be drained

Topping up is far more common. Pressure in a sealed system can drop slowly, for example after bleeding radiators, because a little water escapes along with the air. If you find yourself topping up often, say every few weeks, water is leaking somewhere or the expansion vessel is not working. Topping up alone will not fix that.

Many older homes still have an open-vented system with a small feed and expansion tank in the loft. These systems refill themselves through a float valve, so the steps below apply mainly to sealed systems with a pressure gauge on or near the boiler.

In a block of flats with communal heating, the system is normally not filled from inside your flat. Draining and refilling the risers is decided by the building management or owners' association, so let them know in advance before any radiator is changed.

A heating engineer usually works in this order:

1. Check that all joints are tight and drain valves are closed. On a new installation, carry out a pressure test for leaks.
2. Flush older pipework or a freshly installed system so no swarf, jointing compound or sludge is left inside.
3. Switch off the boiler and fill only a cold system.
4. Open the radiator valves so water reaches every circuit.
5. Fill slowly through the filling valve or filling loop while watching the pressure gauge.
6. Bleed the radiators from the lowest floor up to the highest, then bleed the boiler and pump as well.
7. Top up to the target pressure, start the heating and bleed again once the system is hot.
8. On a new or altered system, balance the radiators so every room heats evenly.

Air comes out of the water gradually. So it is normal to bleed the radiators again and add a little pressure for a few days after filling.

In a house, the pressure in a cold sealed system is usually somewhere around 1 to 1.5 bar. The exact figure depends on the height of the building and the setting of the expansion vessel, so follow the boiler manual or your heating engineer's advice. As the system warms up, the pressure rises a little because hot water takes up more space. The pressure relief valve on the boiler typically opens at around 3 bar. If the pressure creeps towards that figure while the heating is on, the system is overfilled or the expansion vessel is not doing its job.

Why does the expansion vessel matter so much? It absorbs the extra water as it expands with heat. If it loses the air or gas charge inside, the pressure shoots up when the system heats and falls when it cools. The vessel charge is checked with the system drained or depressurised, ideally during the annual boiler service.

Mains water is normally used for filling. In hard water areas, and especially on larger systems, treated or softened water helps stop limescale building up in the boiler. Some boiler manufacturers specify water quality directly. Many installers also add a corrosion inhibitor after filling or flushing, following the boiler manufacturer's guidance. Only use antifreeze if the system design and the boiler and pump manufacturers allow it.

A small, simple system in a flat or a single-storey house, with a filling loop at the boiler and working bleed valves, is quick to fill. It takes longer with:

• A two-storey house with several circuits, or radiators combined with underfloor heating
• Old pipework that needs flushing first
• Seized or leaking valves that have to be replaced
• Radiators and bleed points that are hard to reach
• Water treatment or radiator balancing on top of the filling itself

You can top up a sealed system in your own home if the filling loop and pressure gauge are easy to reach. Leave the gas side of the boiler, the pressure relief valve and the expansion vessel alone. Any work on the gas parts of a boiler must be done by a qualified, registered gas engineer.

1. Switch off the boiler and wait for the system to cool.
2. Check the boiler manual for the recommended pressure.
3. If the boiler has no built-in filling key or tap, fit the filling loop between the cold mains connection and the heating return, making sure both ends are secure.
4. Open the valves slowly and watch the pressure gauge.
5. Close the valves once the target pressure is reached, then disconnect the loop if it is an external one.
6. Bleed the radiators with a radiator key until water runs out without bubbles.
7. Check the pressure again and top up if needed.
8. Switch the boiler back on and, once it is hot, check that all radiators are warm.

• Filling too fast, which traps air in the system
• Filling while the boiler is running or the system is still hot
• Overfilling above the recommended pressure, so the relief valve drips when the heating warms up
• Skipping the bleeding, so radiators are warm only at the bottom and the pipes gurgle
• Missing a small leak that only shows up in the middle of winter
• Topping up again and again without finding out why the pressure drops

In practice it often turns out that people have been topping up for years without knowing the expansion vessel has lost almost all its charge. Fresh water also brings in oxygen and minerals, which speed up corrosion and deposits inside the system.

When the job is handed over, check the following:

• The cold system pressure matches the boiler manufacturer's recommendation
• Once hot, the pressure rises only slightly and the relief valve does not drip
• All radiators heat over their whole surface, including on the top floor
• There is no gurgling or rushing sound in the pipes
• Joints, valves and the area around the boiler are dry
• The boiler shows no fault and does not fire up unusually often
• The engineer has told you what pressure to keep and how to top up
• After a few days, the pressure is still where it should be