- Heat pumps
- Air-to-water heat pump
F788
Vaillant F.788 on an aroTHERM plus heat pump: building circuit pump fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On the aroTHERM plus, F.788 means the heating circuit water pump has locked out after an internal fault. Check the pressure on the display and that the system valves are open; leave the rest to a competent person.
Safety first
Stop and check this first
- Vaillant states that users must not carry out maintenance or repairs on the product and that a competent person should top up the heating water. Do not open the heat pump or the indoor unit to bleed the pump.
Safe checks you can perform
- Water pressure on the display
- System valves
- Pump, filter and connector
Leave these tasks to a qualified professional
- Do not attempt this professional task: Purge, filter and flow before replacing the pump
- Do not attempt this professional task: Find the loss if the pressure dropped
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Purge, filter and flow before replacing the pump
What they doThe technician purges, cleans the dirt filter, checks the pump connector on the board and measures the circuit flow.
They must show youthe circuit pressure after refilling, the clean filter and the flow reading against nominal.
In writingwhat they found, what they did and, if the pump was replaced, its part number and the reason.
Find the loss if the pressure dropped
What they doIf water had to be added, the technician locates where it is lost before closing the job.
They must show youstable pressure after a few days or the leak point found.
In writingthe starting and final pressure and the leak repair.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Water pressure on the display
- Who:
- you.
- How:
- on the interface go to Menu, Live Monitor, Water pressure, and note the value with the system cold.
- Result that matters:
- Vaillant gives a filling pressure of 1.0 to 1.5 bar when cold; below 0.5 bar the unit shows a pressure fault.
- If the pressure is low:
- dry running from lack of water is likely; tell the competent person so they refill and look for the loss.
- If the pressure is correct:
- lack of water is ruled out; air, filter, connector or the pump remain.
- Evidence to keep:
- a photo of the pressure reading with the date.
Evidence: Vaillant VWZ AI heat pump appliance interface, operating and installation instructions 0020291573_01
System valves
- Who:
- you, without dismantling anything.
- How:
- check that the visible stopcocks are open and that not all thermostatic valves on radiators or underfloor loops have been closed.
- Result that matters:
- that the water has somewhere to flow. Vaillant links low flow to closed stopcocks and closed consumers.
- If a valve was closed:
- open it and note whether F.788 returns after the next start.
- If everything was open:
- a closed system is ruled out; leave the rest to the technician.
- Evidence to keep:
- a note of which valve was closed, if any.
Evidence: Vaillant VWZ AI heat pump appliance interface, operating and installation instructions 0020291573_01
Pump, filter and connector
- Who:
- a competent person.
- How:
- they leave the heat pump without power for at least 30 seconds, check the pump plug contact on the board, purge the circuit, clean the dirt filter and test the pump.
- Result that matters:
- whether the pump starts and the flow reaches at least 35% of nominal, the figure Vaillant gives for status S.275.
- If the pump works after purging and cleaning the filter:
- the cause was air or dirt, not the pump.
- If the pump stays locked with a full circuit and a firm connector:
- the pump or its supply is the suspect.
- Evidence to keep:
- a job sheet with the measured flow and the filter condition.
Opening the unit requires isolating all poles and waiting at least 3 minutes for the capacitors to discharge. Competent person only.
Evidence: Vaillant VWZ AI heat pump appliance interface, operating and installation instructions 0020291573_01
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Heat pumps
- Brand
- Vaillant
- Product type
- Air-to-water heat pump
Known code variants
- aroTHERM F.788
- aroTHERM plus F788
- bomba circulación F.788
- circulating pump F.788
- error F.788 aerotermia Saunier Duval
- F788
- Genia Air F.788
- Genia Air Max F788
- GeniaSet F.788
- Saunier Duval F788
- Saunier Duval heat pump F.788
- Vaillant F788
What it means
Vaillant defines F.788 as a building circuit pump fault: the high efficiency pump electronics detect dry running, blockage, overvoltage or undervoltage, switch off and lock. Before that, status S.277 stops the heat pump for 10 minutes and retries; after three failed restarts F.788 appears. The code does not say whether water is missing, air is trapped, the dirt filter is blocked, the plug on the board is loose or the pump itself failed.
Warnings and stop conditions
- Mediumsource verified
Vaillant states that users must not carry out maintenance or repairs on the product and that a competent person should top up the heating water. Do not open the heat pump or the indoor unit to bleed the pump.
Evidence: Vaillant VWZ AI heat pump appliance interface, operating and installation instructions 0020291573_01 · Vaillant aroTHERM plus VWL 35/6 to 75/6 A S2, operating, installation and maintenance instructions 0020330791_03
Symptoms and causes
Probable causes · in order
- Air or too little water in the circuitCommon
Dry running appears when the circuit holds air or too little water. It usually comes with low pressure on the display or radiators that sound full of air.
Evidence: Vaillant VWZ AI heat pump appliance interface, operating and installation instructions 0020291573_01
- Blocked dirt filter or seized pumpPossible
Vaillant asks to check that the dirt filter lets water through. A full filter or a seized pump causes a lockout even with correct pressure.
Evidence: Vaillant VWZ AI heat pump appliance interface, operating and installation instructions 0020291573_01
- Board connector or supply voltagePossible
Overvoltage or undervoltage and a poor contact of the pump plug on the board give the same code. Only measurement with the casing open separates them.
Evidence: Vaillant VWZ AI heat pump appliance interface, operating and installation instructions 0020291573_01
Virtual technician
Does F788 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Was this entry useful?