Bosch fault code AA
Fault code AA on a Bosch boiler (including ZSN and ZWN) means: On boilers ZWN during hot water draw-off. Plate heat exchanger is scaled up. Reset the appliance once. If the code comes back, a part is usually faulty. You can check the water pressure and settings yourself; leave work on gas-carrying parts to a certified gas engineer.
After a safety alert (gas, flue gas, combustion), reset no more than once and call in a certified gas engineer. If you smell gas, call your national gas emergency number or 112 immediately.
Cause
Fault code AA on a Bosch boiler indicates: On boilers ZWN during hot water draw-off. Plate heat exchanger is scaled up.
Fixing fault code AA: step by step
- Read the water pressure on the display or gauge. With the system cold it should be between 1.5 and 2 bar.
- Bleed the radiators with a radiator key, then check the pressure again and top up if needed.
- Check that the radiator valves and underfloor heating circuits are open, so the water can circulate.
- Reset the appliance once. If the code comes straight back, do not keep resetting it.
- If the fault keeps returning, or the boiler gurgles or knocks, the heat exchanger may be fouled. Flushing or replacing it is work for a qualified heating engineer.
Applies to
Bosch ZSN, ZWN, 18-6 KE, 24-6 KE.
Fault code AA on other Bosch series
The same code does not mean the same thing on every series. Check the rating plate to see which series you have:
- AA on VRC, 24 Open: Only during hot water draw-off. Plate heat exchanger is scaled up.
Matching parts
Bosch parts are available from Instalpunt; shipping to EU countries from €12.95 (€16.95 for more distant countries), usually 3–5 working days.
For fault code AA, the part involved is usually in the category: heat exchanger.
This message does not point directly to one faulty part. If the code keeps coming back, view the Bosch parts:
Fix it yourself or call an engineer?
Always consult the manual for your appliance and work safely. If in doubt or if the fault comes back, call in a qualified professional. A question about a part? Write to us at info@instalpunt.nl.
Frequently asked questions
What does Bosch boiler fault code AA mean?
On boilers ZWN during hot water draw-off. Plate heat exchanger is scaled up.
How do I fix fault code AA on a Bosch boiler?
First check the cause described above. Then restart the appliance using the reset button. If the message comes back, a part usually needs replacing or the fault needs to be repaired by a qualified professional.
Which parts do I need for Bosch fault code AA?
That depends on the exact cause. In the Bosch parts overview at Instalpunt you will find the current price and stock for each part.
Can I fix fault code AA myself?
You can reset the appliance and check things like the water pressure or settings yourself. Work on gas-carrying parts must be left to a certified gas engineer.
All Bosch boiler faults and codes at a glance →