Vaillant Boiler Pressure: Too Low, Too High, and Topping Up

How to read the pressure on a Vaillant

Vaillant boilers show pressure in one of two ways. Some have a dial gauge, usually on the underside or behind the front flap, with a needle and a green band. Most ecoTEC models show it as a number on the digital display, such as 1.2, next to a small bar symbol.

On older ecoTEC plus boilers with a two-digit display, the screen normally shows the flow temperature. Press the minus button and it switches to the pressure reading for a few seconds before going back to the temperature. On current ecoTEC plus models with a text display, the pressure is under the Information menu as Water pressure. If a separate dial gauge on the pipework disagrees with the display, trust neither until an engineer has checked the sensor, because a failed pressure sensor is a known Vaillant fault.

• Cold, 1 to 1.5 bar: where Vaillant recommends it
• Cold, below 1 bar: top up now, before it trips
• Hot, up to about 2 bar: normal expansion
• Hot, 2.5 bar or higher: act today, it is close to the 3 bar relief point

Topping up a Vaillant that has dropped

When the pressure gets too low the boiler stops and shows F22 on a Vaillant. On most ecoTEC plus models the pressure figure flashes below 0.8 bar and the boiler stops below 0.5 bar (0.3 bar on the newest models), one of the commonest reasons an ecoTEC will not turn on. On some models a low reading also brings up F75, because the pressure sensor saw no rise when the pump started. Both usually clear once the system has water in it again.

Many Vaillant installations have a silver braided filling loop between the cold mains and the heating circuit, with a valve at each end. Many ecoTEC combis have a built-in filling valve under the boiler instead, a small knob near the front pipes that you turn to let water in and turn back to close. The steps below are for the braided loop; with a built-in valve, open it slowly and close it at the same reading. The job takes five minutes.

• Switch the boiler off and let it cool, so the reading is a cold one
• Check both ends of the filling loop are tightly connected
• Open the first valve: its slot or lever runs along the pipe when open
• Open the second valve slowly; you will hear water hissing in
• Watch the display or gauge and close the second valve at 1.2 to 1.5 bar
• Close the first valve, switch the boiler on and press reset if the code is still showing

If the pressure falls again as soon as you close the valves, stop topping up the boiler. A big leak or a stuck-open relief valve will just keep dumping the water you add, and fresh water brings in oxygen that corrodes the system.

Why is my Vaillant boiler pressure too high?

High pressure on a Vaillant almost always comes from one of four places, and the pattern of the rise tells you which.

A filling loop left open, or one whose valves no longer seal, lets mains water creep in. The rise is steady and happens whether the heating is on or off. Closing both valves firmly and draining some water off usually settles it; if the pressure then climbs again with the valves shut, the loop is passing.

An expansion vessel that has lost its air charge gives a different pattern. The pressure is normal cold, jumps towards 3 bar within minutes of the heating starting, and falls back when it cools. On some ecoTEC combis the vessel sits at the back of the casing, so engineers often fit an extra vessel on the pipework under the boiler instead of taking the boiler off the wall.

A cracked plate heat exchanger on a combi, one of the parts behind a Vaillant with heating but no hot water, lets mains water into the heating side whenever the hot tap runs. If the gauge rises after a shower or a bath, this is the prime suspect. The fourth cause is simple overfilling after a top-up done while the system was warm, which one careful drain-off fixes.

How to bring the pressure down

Turn the heating off and give it an hour. Then bleed the radiator nearest the boiler, a quarter turn at a time, with a cloth under the key, and watch the reading fall. Close the bleed screw at 1.2 to 1.5 bar. Our guide on how to reduce boiler pressure covers the drain-off valve method for bigger drops.

Do not twist the red or black cap on the pressure relief valve to let water out. Once a relief valve has been opened by hand, or has discharged at 3 bar, it can fail to reseat and drips through the copper pipe outside. Then the pressure falls day after day and you have swapped high pressure for a boiler that keeps dropping pressure.

When to call an engineer, and what they check

Call if you top up more than a couple of times a year, if the pressure rises with the filling loop shut, if the pipe outside is dripping, or if the display and a dial gauge disagree. None of those will fix itself.

The diagnostic order on a Vaillant is usually the same. The engineer compares the boiler reading with a test gauge to rule out the pressure sensor, isolates the filling loop to see if it passes, then drains the boiler side to zero and checks the expansion vessel charge through its Schrader valve. On a combi they run a hot tap with the filling loop shut to test the plate heat exchanger. Everything past the front panel is engineer work, and gas work is carried out by Gas Safe registered engineers. This is what a Vaillant boiler repair visit covers.

Typical UK market ranges, parts and labour, are roughly £100 to £200 for a pressure sensor, £90 to £200 for a relief valve, £150 to £350 for an expansion vessel and £300 to £600 for a plate heat exchanger, depending on model and access. Our visit starts with the £60 call-out, and the repair price is agreed with you on site before any work starts.

If you can smell gas at any point, open the windows, shut the meter valve if it is safe to, leave switches alone, then get out and call the National Gas Emergency Service on 0800 111 999.