Hi, are you able to see the delta between GT8 vs GT9 temps? If this is more than 10 degrees then it would suggest that there is a flow issue again. When you changed the hoses did you flush out the system, especially the heat exchanger?  I had the same errors as well as high HTF before I changed out the hoses, does the electrical cassette trip out?

The temperature diff remains fairly low during operation.

I didn't do the hose change myself, but I know that the radiator circuit was flushed out.

And yes, the "electrical cassette" error, as well as the "high pressure switch" error happen and cause the heat pump to cut out. Acknowledging the error resets things and allows the heat pump to run again (until the next error). These errors are happening several times a day.

This only happens when running the hot water. The radiators are not problematical (not that they are on now). I seem to recall that the same happened last year - these problems started once the radiator heating was no longer needed.

I know you will probably have tried this, but if the GT8/GT9 temp differential is low, it may not solve the issue? However,  Pressure Switch High (HP) error indicates there is not enough flow over the heat pump. Check the heat carrier pump is working (it's the central heating type pump just to the left of the compressor), and try increasing the speed if it is. There is a switch on the pump, which has 3 settings, turn anti-clock to speed 3.

Also check that there is no air in the hot water tank. There is a thumb screw valve at the top of the tank.

As you have said that it's in hot water mode then there is no other valves or filters, unless your technician added these when changing the flexi-hoses?

Poor flow over the cassette would normally trip the overheat protector (not just tripping out the heatpump), this is a small button on the control box below the cassette. It is not obvious that this has tripped and even less positive to reset, which you do by pushing it back in. 

Other than that it could be poor or corroded connections, or a problem with the PCB or sensors, sorry not very helpful. 

How long ago did you change the hoses?

Here are the reported most common causes for both.

Common Causes High Pressure Switch error:
Blocked or dirty condenser coil (insufficient airflow)
Faulty fan (not expelling heat from the condenser)
Overcharged refrigerant
Non-condensable gases in the system
Faulty high-pressure switch
Closed or restricted water circuit (e.g., blocked heat exchanger)

Troubleshooting Steps:
Check outdoor fan operation – Is it running when the unit is on?
Inspect and clean condenser coils – Remove debris and dust.
Verify flow in water circuit – Check for pump operation, airlocks, or valve closures.
Measure refrigerant pressure – Requires a licensed technician.
Reset the system – Some models allow manual reset after the pressure drops.

Common Causes: Electrical Cassette
Faulty PCB
Loose or corroded electrical connections
Power fluctuations or unstable voltage
Failed sensor input or internal relay

Troubleshooting Steps:
Power cycle the unit – Turn off power for a few minutes, then restart.
Check cable connections – Look for loose terminals or moisture in connections.
Inspect PCB for burnt spots or corrosion.
View error code specifics – IVT systems often display a numeric code; let me know the code for detailed steps.
Check fuse and power supply – Ensure stable voltage and no blown fuses.

@abbote thanks for that advice.
There was a bit of air in the cylinder, but removing it hasn't helped.
The trip on the electrical cassette seems OK (in that it can't be pressed in).
The hoses were changed last summer.

I looked at increasing the pump flow. Presumably this is the pump you refer to?

There seem to be 3 settings, controlled by the green button. The symbols suggest the are something related to the heat curves. It's not obvious which is the current setting

OK, so when the HP comes on it becomes obvious.
It's on the setting on the right (one one where the curves are curves, not straight lines).

Your system seems to be more modern than mine, mine was installed in 2011. My advice would be to speak to Zak @Alto Energy. zak.crozier@altoenergy.co.uk, www.altoenergy.co.uk. www.altoenergy.co.uk Tel: 01993 220699. They were extremely helpful if you can't get hold of Zak try Sam Keats who is their design manager. They also sell spare parts. I got my hoses direct from them and saved over £500 from what a 'so called' heat pump expert maintenance company tried to charge me.