@ggw I think you've nailed the issue. Very odd. 

I think you may have a faulty outdoor temperature sensor probe, if the unit is under warranty this will be covered by Grant. You've enough evidence based on your own monitoring to ask them to come out and replace it. Fault when running only may be vibration caused, so it might be OK when the unit is not running, but goes doo-lally when it is. Just because it's not open circuit doesn't mean it's not faulty, a poor connection onto the PCB or in the thermistor unit itself could mean vibration causes incorrect readings.

I have the exact same 10kW Aerona and the outdoor air sensor is pretty accurate. It can be slightly affected by heat from the coil when it defrosts during which time the fan stops for a while, other than that it's been within 1degC or so of where it should be. As the outdoor temperature goes above 17degC the unit starts to cycle on and off for long periods, as the heat load for a typical 21degC set point falls below the unit minimum unit turn down of about 25%. Above 20degC outdoor air temperature heating is disabled regardless of the indoor thermostat setting or demand.

@ggw

In addition to the suggestions by AllyFish, another reason could be electrical pickup from a nearby power cable, that is only live when the heat pump is running. Whenever possible, all cables should be kept separated from each other, and in particular that would apply to power and signal cables.

Do you know what flow in l/m your unit is showing? @allyfish

How do i check this? is it a parameter on controller? i've not seen it in manual. Tho, with the bewildering amount of parameters, that wouldn't be a surprise.

@derek-m @allyfish

Both Good shouts - ill make sure to tell Grant that if they ever commit to coming to look/fix. Noise affecting resistance would be a new one for myself. never considered vibration either.

BT is sometimes right, It's good to talk...

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   @ggw there should be a manual flow setting and indicator gauge on the system pipework. Grant fit one looking like this. It indicates flow in litres/minute

Posted by: @allyfish

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   @ggw there should be a manual flow setting and indicator gauge on the system pipework. Grant fit one looking like this. It indicates flow in litres/minute

Nope, mine does not have this - not on the system anywhere.

Posted by: @ggw

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@derek-m @allyfish

 

Both Good shouts - ill make sure to tell Grant that if they ever commit to coming to look/fix. Noise affecting resistance would be a new one for myself. never considered vibration either.

 

BT is sometimes right, It's good to talk...

 

It is possibly electrical interference on the cable rather than affecting the thermistor. The controller will be measuring the OAT by passing an electrical current down one core of the cable, through the thermistor, and back to the controller via a second core. An electrical interference entering the controller could affect its reading.

I presume that you have measured the air temperature, near the sensor, with the heat pump operating, to be certain it is not a true reading?

@ggw This is a requirement of the Grant pump - Section 9.13 of the Installation Manual: "A flow regulator is supplied with the heat pump. This must be fitted in the return pipe to the heat pump."

If it's not present then speak to the installer/builder.

There's no parameter on the Grant controller which shows the flow, so looking at that is the only way to find out (outside of adding your own flow meter).

@mikefl it ain't there - the plumber clearly missed it.

SITREP #2 

Learning experience - that's what his first winter has been... The future, maybe - the present... If willing

Temperature Sensor issue is still unresolved - Grant have talked a good game on phone - still no one is yet to turn up to fix (1 month) - they offered to post a replacement sensor - I declined, worried that if I install and something fails, bad job for the warranty. So they grudging said they would organise someone.. still waiting

Fixed temperature has worked well above freezing. 33Deg LWT is lovely and warm, even when 4Deg OT.

Our recent 0Deg weather has stressed the 10kW unit, cannot get to 33Deg, best I've seen it is 29Deg - then defrost - then 29deg - rinse and repeat. Double the electrical cost for less heat - oh the joys. Found reducing the zones down (heat only living room and bedrooms) has been effective in both getting heat into the property and slowing the defrost rate to closer to 60mins. That is my decision, and other parts of the house are *much* cooler.

Another issue - DHW

I have 3rd party controls on my S-Plan PDHW setup. For CH, works great. For DHW, the timings to get to 50Deg vary depending on the outside temp. Normal so far...

If I set the DHW time to 90min run, the unit will heat up to 50Deg - stop - but is keeping the pump running. So taking heat away from the Tank. I've had several days with no HW. I understand how this is happening, but how to correct? I need the pump to stop when it reaches set point. ? If using the 3 way valve, it will changeover to CH but as I am 3rd party setup, the unit doesn't talk to valves.

Need to get the engineering hat on - might try and Jerry rig the connections for the 3 way valve on the unit to provide an interlock

Thoughts welcome..