I don't know anything about your heat pump or the water tank but it's following a pattern so it sounds like a programming input issue.

Is the hot water immersion coming on for the weekly legionnaire cycle that should only run for an hour? Or are you intending to heat your water by immersion heater?

Can it not be changed to one hour weekly at midnight until 0100? (or whatever you want) Or is it over-riding manual input? 

There's some real experts in this forum and I'm not one of them but hopefully my reply will bump your post up for a response. 

Thanks for your response Lucia. My knowledge of heating systems is minimal but I will give some more information on the setup installed. Outside is an air source heat pump (ASHP) and close by internally - 180 litre hot water tank (HWT), 2 motorised valves, one for hot water in the tank, the other for central heating radiators. Attached to the
HWT is a manual cylinder thermostat. Three controls - One each for the central heating, hot water, and immersion switch for the Legionella cycle. In the house is a programmable RF Thermostat & receiver.

Grant Aerona3 Air Source Heat Pump HPID10R32 - 10kW
Grant 180 litre Tank QRSC180SL
Manual cylinder thermostat, Code 542902
System Wiring Centre Model EP001
Heat Pump Remote Controller CMR-2613
Domestic Hot Water DHW Programmer HPIDTM4 (Heatmiser 4 zone touchscreen)
Greenbrook T205-C Timer Spur Switch
Programmable RF Thermostat & Receiver CP4 EPH Controls

The system is very good. The ASHP is quiet and heats the water for the HWT quickly. The central heating works well also. The problem we have is not being able to control confidently the timing of the domestic hot water.

The installers heating engineer manager visited yesterday and reset the DHW Programmer. Off all 4 zones and all 4 times. Set it to come on at 1000hrs till 1400hrs every Saturday, no other days or times. We will see what happens tomorrow. I have set my alarm for 2355hrs tonight (Friday) to see if it behaves itself at midnight. I will also watch the ASHP controller at 1400hrs tomorrow (Saturday) to see what happens. It usually displays "Off until 2400hrs".

The engineer found a problem when checking the system. Turns out the manual cylinder thermostat is faulty. It is set for 45C degrees but checking the water in the HWT with a probe found it to be 52C. That means that the ASHP would keep heating but not turn off at the set temp of 45C. The engineer will return to change the thermostat and see if it is now behaving itself. The engineer is determined to find out and fix the problem for which we are very grateful. I did phone Heatmiser and asked if the Domestic Hot Water Programmer has a "Factory Reset" function but was told "no" for this model.

Posted by: @mellow

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Turns out the manual cylinder thermostat is faulty. It is set for 45C degrees but checking the water in the HWT with a probe found it to be 52C. That means that the ASHP would keep heating but not turn off at the set temp of 45C.

Hi @Mellow, I hope I can help, as I have the same Grant 10kW ASHP, Grant branded Heatmiser 4 channel DHW thermostat and a 210L Grant pre-plumbed cylinder.

Why is your DHW coming on at midnight?: This screen showing in Zone 1 (the only zone wired in as DHW only uses a single zone) indicates your programmer WILL come on at midnight. It's stating it's off until that time. So there is something in the programmer that is switching it on at that time. That's why it is coming on. You need to clear that. The screen should state 'Off until Sat 10:00' if that's what you want programmed.

Strange thermostat readings: the contents of the DHW cylinder are not even temperature, water is intentionally held stratified, with high temperature at the top, where water is drawn off, and almost cold incoming mains temperature at the bottom. That prevents the leaving water temperature cooling every time make-up cold water is admitted, which would otherwise happen. The height at which the measure the temperature is measured will result in different readings. Cylinder stats usually measure between mid-height and 2/3 height. They are notoriously inaccurate as a result. That does not mean they are faulty however. If the Grant cylinder thermostat green LED is flashing then it is calling for heat [cylinder sensor temperature < set point]. If the LED is solid green then demand is satisfied [cylinder sensor temperature = or > set point]. When scheduled DHW is set on a timer, say, 90 minutes daily, the LED will usually be flashing. It will only have demand satisfied for an hour or two after recharging until standing losses cool the contents by 1 degree. It will call for heating thereafter until the next DHW timer on period.

Second, why are you only charging DHW for 4hrs on a Saturday? Have you another source of DHW? The tank will not keep temperature for a week! Sorry, I must be missing something here. I charge daily for 75 minutes at 1pm, or until demand is satisfied, whichever occurs sooner. With the ASHP set to 60degC max flow temperature in hot water mode that gives me a tank of DHW at 55degC each day by around 2pm. Thermal losses are a degree of indicated temperature on the digital thermostat every 3-4 hours, so it's still at 50+degC next morning when we shower. Plenty warm enough for ablutions and washing up.

Thanks AllyFish for your help. It will take me a while to read through and understand. The engineer has cleared everything in the programmer except for on at 1000hrs and off at 1400hrs Saturday - tomorrow. I will look at the programmer to see what happens and report back. 

Have read your helpful comments AllyFish and now understand what you are saying. My knowledge on this subject has now increased a little.

I only have the hot water timer on for just one time a week because of the problem with it coming on, without being programmed to, at night. Once the problem is solved I will follow your practice of having it on around midday, every day, for just over the hour and take advantage of the solar panels.

Our cylinder thermostat must be different to yours, no green light, just a manual dial temperature range 25degC to 65degC.

We use the ASHP for heating the DHW not the electric immersion heater, and the Greenbrook T205-C Timer Spur Switch is set to off for the present.

So briefly the programmer is set for Saturday 1000hrs till 1400hrs no other days or times. Certainly not programmed to come on at 2400hrs. (That time of 2400hrs is possibly of interest if you read the latest observations.)

Friday night ASHP comes on at midnight, display is "ON UNTIL 1400"

Saturday at 1400hrs display is "OFF UNTIL 2400" (no day specified)

Sunday, Monday, Tuesday, Wednesday, Thursday, nothing happens. Then midnight Friday ASHP comes on and the cycle repeats itself.

Will post latest observations later. 

Stayed up yesterday evening (Friday) to see what happens at midnight. As expected the time displayed went from 2359 to 0000 and the demand for hot water did not come on. Expected because the timer had been reprogrammed by the installer on Thursday. What is interesting to me though is it did not display 2400 but 0000 (hrs). This is the first time I have seen this.

Woke up at 0234 to visit the loo and could hear the system in the background. Went to the setup and saw the motorised valve for hot water was on (orange light). Looked at the programmer and the ASHP was running. The timer display was "ON UNTIL 1400". 

Thermostat was set to 45degC, turned the dial down to 30degC. ASHP shut down. 2 minutes later went to bed.

1119 hrs. Have reset thermostat dial to 45degC.

I will watch the timer at 1400hrs when the timed period will end and no doubt it will then display "OFF UNTIL 2400" (no day specified) and no doubt the whole cycle will repeat again, again and again! Unless the installer, I which is very unlikely or any of you kind people can come up with the solution. Thanks.

@mellow

Is it possible to set this timer to be 'off until 10:00' and then 'on until 14:00'?

Just checked and yes today (Saturday) after 1400 hrs the display is "OFF UNTIL 2400"

Good point @derek-m, I will look into that.

Off topic I know but it looks like a should address other members with the @ symbol so that

they will be informed by email when mentioned in a post. I'll start doing that.

Hi again @mellow, had a play with my DHW timer to see if I can replicate your issue. Right, there are four zones and each can have up to 4 time periods on a Mon-Fri & Sat-Sun, total 32  possible programmed time settings. Assuming your DHW uses zone 1, you’ve got up to 8. You need two identical ones Mon-Fri & Sat-Sun for daily hot water.

You need to check them all to see if the screen shows this, which is a blank time period:

if it shows this, it’s not blank:

You need to use the down button until the time shows 4 dashes and is empty. That completely clears any time in the setting.

You need to do this for the Mon-Fri settings & Sat-Sun. Suggest delete everything and double check all zones & day settings empty. Then start over programming just the periods you want. Use ‘mode’ button to set all the other zones except zone 1 to ‘off’ (options are off/constant/auto) see how you get on. Fingers crossed.

Grant has tried to simplify the wiring and controls for the installer to make like a bog standard Y or S plan gas boiler. The original Chofu (company that actually makes the Grant heat pump) controller has all these functions built into it, but the installer just tucks it the way, never to be seen again. If was installed as Chofu intended you would have one combined controller, thermostat and timer.

Posted by: @allyfish

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Hi again @mellow, had a play with my DHW timer to see if I can replicate your issue. Right, there are four zones and each can have up to 4 time periods on a Mon-Fri & Sat-Sun, total 32  possible programmed time settings. Assuming your DHW uses zone 1, you’ve got up to 8. You need two identical ones Mon-Fri & Sat-Sun for daily hot water.

You need to check them all to see if the screen shows this, which is a blank time period:

-- Attachment is not available --

if it shows this, it’s not blank:

-- Attachment is not available --

You need to use the down button until the time shows 4 dashes and is empty. That completely clears any time in the setting.

You need to do this for the Mon-Fri settings & Sat-Sun. Suggest delete everything and double check all zones & day settings empty. Then start over programming just the periods you want. Use ‘mode’ button to set all the other zones except zone 1 to ‘off’ (options are off/constant/auto) see how you get on. Fingers crossed.

From reading the manual some time ago, you need to ensure that DHW and central heating are not called fir at the same time h but check your manual. Believe there is a risk of CH getting 60 Deg water