Vaillant [F.22] fault codewhat it means and how to fix it

F.22 is the heat pump switching itself off because the heating water pressure has fallen too far, and the thresholds are published precisely. Below 0.7 bar you get a maintenance message, M32, which goes out on its own once pressure rises above 0.7 bar again. Below 0.5 bar for longer than one minute, the fault message F.22 and the current filling pressure appear alternately on the display, and that goes out when pressure rises back above 0.5 bar. Vaillant give 1 to 1.5 bar as the correct cold filling pressure, higher on a house of several storeys, and 0.7 bar as the minimum operating pressure.

What to do about it is where Vaillant’s own documents part company, and you should know all three. The operating instructions for the uniTOWER plus and the hydraulic station — the indoor units on aroTHERM plus and aroTHERM split systems — say that if the filling pressure falls below the minimum the heat pump is switched off automatically, and then: inform your competent person so that he can top up the heating water. The operating instructions for the earlier aroTHERM monobloc say the opposite in three words, "Top up with water", under checks carried out by the operator. And Vaillant UK’s owner-facing fault code page splits it by pressure: if the gauge shows less than 1 bar, this can be solved without an engineer if you are competent to do so, by locating the filling loop and slowly raising the pressure to between 1 and 1.5 bar, at your own risk; if it shows more than 1 bar and the fault is still there, it is a case for a Vaillant engineer, because the causes then are a faulty pressure sensor or its wiring, or the expansion vessel.

Two things are worth adding that Vaillant do not spell out on that page. Their own manuals strongly recommend antifreeze in parts of the installation at risk of frost, and some aroTHERM plus systems have a primary circuit filled with a propylene glycol mixture — topping those up with plain water dilutes the protection, which is a good reason to know what your system holds before you open a filling loop. And a heat pump circuit has a great deal more pipework than a boiler’s, so pressure that keeps needing attention means a leak to find rather than a level to correct. One last trap: the troubleshooting section of the same indoor unit manuals heads this fault F.723, "Building circuit: Pressure too low", while the display and the appendix call it F.22 and F.022. They are the same fault.

1. 1

   Read the pressure. On the indoor unit it is under Menu, Live Monitor, Water pressure; some systems also have a gauge. Below 0.5 bar is what produces this fault; below 0.7 bar produces the M32 maintenance message instead; 1 to 1.5 bar cold is where Vaillant want it.

2. 2

   Look for the leak. Check around radiator valves, under the indoor unit, and at any visible pipework — a small weep that dries between visits leaves a stain or a bloom of limescale.

3. 3

   Check whether an outside discharge pipe has been dripping. That points at the pressure relief valve rather than a leak in the house.

4. 4

   Decide whether to top up yourself. Vaillant UK’s page says you may, if you are competent, by locating the filling loop and slowly raising the pressure to between 1 and 1.5 bar. The indoor unit manual says to inform your competent person instead. If you know where the filling loop is, know the system is plain water, and this is the first time, the top-up is the same job as on a boiler. If any of those is not true, ring your installer.

   Vaillant recommend antifreeze in parts of a system at risk of frost, and some aroTHERM plus installations have a glycol-filled primary circuit. Adding plain water to those dilutes the protection. If you do not know what your system was filled with, that is the reason to make the call rather than open the valve.
5. 5

   If the pressure is above 1 bar and F.22 is still showing, do not keep adding water. Vaillant UK make that a Vaillant engineer case — the pressure sensor, its wiring or the expansion vessel — and topping up further only overpressurises a system that is not actually low.

6. 6

   Tell whoever attends how often it has happened. Pressure that needs topping up more than about once a year means a leak to find rather than a level to correct, and Vaillant’s own instruction is to identify and eliminate the reason for the loss.