On a Viessmann Vitocal heat pump, fault F.74 means the hydraulic system pressure is too low, and the heat pump switches itself off. Viessmann’s service instructions say to top up with water and vent the system. F.75 is the related fault for no flow in the secondary circuit, or a faulty flow sensor. If F.74 returns after a top-up, there is a leak, an expansion vessel problem or a sensor fault, and you need an engineer.
If it keeps coming back, book an engineer rather than topping up or restarting repeatedly. You can book a heat pump repair online or call 0333 041 5130, and our full Viessmann heat pump error codes guide covers every other code.
Please read first: this article is a guide only. We make every effort to keep it accurate, but manufacturers update manuals, and codes can differ between models and software versions, so always check the official Viessmann Vitocal service instructions for your unit. Do not attempt anything dangerous or anything that could void your warranty.
What do F.74 and F.75 mean on a Viessmann Vitocal heat pump?
F.74: hydraulic system pressure too low. The pressure sensor in the heating circuit has read below the set minimum. The heat pump is switched off until the cause is fixed. Viessmann’s measures are to top up with water and vent the system. If the fault recurs, the service instructions tell the engineer to check the system pressure sensor against an external pressure gauge, check the pre-charge pressure of the expansion vessel, and check the system pressure and range settings.
F.75: no flow in the secondary circuit or flow sensor faulty. The heat pump is switched off. The measures are to check the secondary pump or heating circuit pump, check the heating and cooling circuit, replace the pump if required, and check the flow sensor.
F.982: heating circuit pump running dry. The heat pump does not start. The listed measures are to check the shut-off valve, check the heating circuit pump connection, and fill the system. This is another way low water shows up.
These entries come from the Vitocal 200-S and 222-S fault tables in Viessmann’s service instructions for contractors, which also cover the 150-A, 250-A and related models. Fault codes and wording can vary between models and control software, so use the table for your unit. The visible text does not give a bar figure for the correct system pressure. It refers to a set system pressure and a range that are configured on the system, so ask your installer what yours is set to rather than guessing.
F.87 is the opposite problem: system pressure too high, where the safety valve in the indoor unit has opened. If you see F.87 after repeatedly topping up, you have over-filled.
Why does a Vitocal heat pump lose water pressure?
- A leak at a joint, valve, pump connection, radiator or underfloor manifold.
- An expansion vessel with the wrong pre-charge, or a failed diaphragm.
- Air in the system that is vented by the automatic air vent, taking water with it.
- The safety valve discharging after a pressure spike and not closing fully.
- Recent bleeding or radiator work, where water was released and not replaced.
- A faulty pressure sensor, which reads low when the real pressure is fine.
- A closed shut-off valve or blocked filter, leading to F.75 or F.982 faults on top of low pressure.
Underfloor heating systems add a lot of pipework, so a small leak under a screed can lose pressure slowly and quietly. If you top up more than once, treat it seriously.
It is worth understanding why Viessmann switches the heat pump off at all. With too little water in the circuit, the pump can run dry and the heat pump cannot move heat away from the refrigerant circuit. Stopping early protects the compressor and the pump. That is also why F.982, the dry-running fault, is listed alongside F.74 as another sign that the system needs filling.
If you have had a top-up that cleared F.74 but the fault comes back weeks later, the system is losing water somewhere or the expansion vessel is not doing its job. The service instructions specifically list checking the expansion vessel pre-charge as a follow-up measure for a recurring fault, so it is worth asking your engineer to confirm it was done. A pressure sensor that disagrees with an external gauge is the other classic cause.
What can you safely check yourself?
- Look at the pressure gauge on the system and write down the reading. Check whether it is a cold or warm system.
- Look for visible drips, damp walls or ceilings, and wet floors near manifolds and pipework.
- Check all radiator valves, zone valves and shut-off valves are open.
- Note the full fault code and anything else showing on the controller.
- Top up through the filling loop only if the manufacturer’s manual allows it and you are competent. Fill slowly, close the loop and disconnect it afterwards.
- See our guide to a heat pump losing pressure for what to expect and when to worry.
What not to do:
- Do not touch refrigerant circuits. Refrigerant work is for F-Gas certified engineers only.
- Do not remove panels or covers from the indoor or outdoor unit.
- Do not keep over-pressurising the system. That can lift the safety valve and trigger F.87.
- Do not ignore repeated top-ups.
- Do not change the system pressure or range settings in the controller unless you are an engineer.
When should you call an engineer?
- F.74 returns after a top-up.
- You are topping up more than once a month, or more than once at all in a short period.
- F.75, F.982 or F.87 appears.
- You can see water where it should not be.
- The gauge looks normal but the fault remains, suggesting a sensor problem.
- You have no heating or hot water.
Book early. Pressure faults often start as a nuisance and end as a shutdown at the coldest time of year.
How can you stop it happening again?
- Have an annual service. Our annual service checklist covers pressure, the expansion vessel, filters and pumps.
- Have drips investigated early. Our guide to a heat pump leaking water helps you decide what to look for.
- Make sure the system was filled and vented properly at installation. Our commissioning checklist sets out what should have been done.
- Look at your heating before winter. Our heat pump winter tips cover what to check before the cold weather arrives.
- Check the gauge monthly and keep a note of readings.
Other Viessmann codes you might see with low pressure
These are the pressure and flow faults we could verify from Viessmann’s service instructions for the Vitocal 200-S/222-S.
| Code | Meaning | First step |
|---|---|---|
| F.74 | Hydraulic system pressure too low; heat pump off | Check the gauge and top up if competent, vent, then call an engineer if it returns |
| F.75 | No flow in the secondary circuit or flow sensor faulty | Check valves are open and book an engineer to check the pump and sensor |
| F.982 | Heating circuit pump running dry; heat pump will not start | Check shut-off valves and pressure, then book an engineer |
| F.87 | Hydraulic system pressure too high; safety valve has opened | Do not top up. Book an engineer to reduce the pressure safely |
| F.797 / F.799 | Secondary pump or heating circuit pump 1 off | Isolate power, restart once, and call an engineer if it persists |
Need help with low pressure on a Viessmann Vitocal heat pump?
UK Heat Pump Servicing engineers find the leak, check the expansion vessel and air vent, test the pressure sensor against a reference gauge and carry out heat pump repairs with a 12-month guarantee. We also offer an annual heat pump service and cover plans. Contact us or call 0333 041 5130.
Frequently asked questions
Will the heat pump restart after I top up the pressure?
It should, once the pressure is back in range, though some units need the fault acknowledged on the controller. If it does not restart, leave it and call an engineer rather than trying repeated resets.
Can the pressure sensor be wrong?
Yes. Viessmann’s own measures include checking the sensor against an external gauge. A reading that does not match the gauge points to a sensor fault, not a leak.
Does cold weather lower the pressure?
Cold water contracts, so a cold system can read lower than a warm one. A small seasonal change is normal, but a steady fall over weeks is not.
Why did the safety valve open after I topped up?
The system was probably over-filled, or the expansion vessel cannot absorb the expansion when the water heats. Do not add more water, and have the vessel checked.
Is a Vitocal fault covered by my Viessmann warranty?
That depends on the cause and the terms. Keep your service records, because skipped servicing can affect cover.
