WElcome to the forum.  My first post was eaten by some gremlin, so I am trying to reproduce it here.

Firstly, we need some information about the system itself, does it fo hot water as well as the heating?  What sort of controller do you have for it, maybe a brand name and type or a photograph of it?  Did the builder leave you any proper instructions about how to operate it, or even a manual? 

Are all the thermostats set to the same temperature? 

@guthrie 

It does do hot water as well but we’ve currently switched it off as we felt it wasn’t coping doing water and heating. So the immersion is doing the hot water for now. 

The controller is just the standard Grants controller. We don’t have a hub or anything.

We’ve been left with no manual. But i did have it at one point and it was useless. I think it was more for the installer than us.

 The builder is clueless too! Which is why I’m becoming desperate.

Downstairs the thermostats are set to 20 degrees. Upstairs are set between 16.5 and 17.5 because I’ve got to the point where I know it’s not going to reach higher temperatures and I’m trying not to let it get any colder!

I really need help. 

Please post some photo's of the equipment within your system, so that the forum can get some idea of how your system has been installed.

Hello and welcome. The Grant Aerona units are known to spend a lot of time in defrost in winter conditions, usually every 45-60 minutes. That does seem more than other brands - weather such as the damp and relatively mild winter weather we've recently experienced. This reduces the unit heat output and it can struggle to reach target heating flow temperature. That in turn means insufficient heating into the UFH system. It also means the electricity consumption rockets too, as the unit is working more or less flat out between defrosts - it never gets chance to modulate down to a low speed control.

It might seem perverse, but it could be the 6kW unit is undersized for a 3bed new build. The heat loss calculation done by the developer or builder might be less than 6kW, but in the weather we've recently had, the Grant Aerona unit might not be outputting 6kW, probably somewhere between 4-5kW based on several members here with varying capacity Grant units who have all seen flow temperatures drop several degC below target set point, me included. Grant has a product called a 'volumiser/low less header' which has a 3kW immersion heater that can be used as supplementary or emergency heating. It's an expensive power consuming option however. It does rather hint that they realise there are times their Aerona branded ASHP products don't provide enough heat output

The Grant unit controller is complicated, most Grant installs use a simpler external digital programmable thermostat and water time clock + cylinder thermostat. The Grant controller is able to do all that, and much more besides, but it has to be said programming it and adjusting parameters on it isn't easy or intuitive. Fear not, plenty of people here can help.

Grant has a history of commissioning systems with weather compensation disabled, (WC is an automatic modulating control where the heating flow temperature increases as the outdoor air temperature decreases and vice versa), and their installs have too much reliance on on/off room thermostat controls instead. That's not good for ASHP efficiency.

The grant unit can only supply water at one temperature, so having downstairs and upstairs zones with different thermostat settings may complicate things. Your UFH might have some thermostatic blending or flow control for each zone, but this often isn't the most efficient heating method with ASHP. As @Derek-m and @Guthrie have mentioned, put some details up on here and you'll certainly get some help and advice.

Quite a few Grant Aerona owners have found their units struggling this last week or so, (me included), but the weather has been somewhat exceptionally cold. 

Hopefully things have improved now the cold snap has reached its end ... ?

There are many (too many) parameters to consider to improve performance, but it might also be worth checking some obvious things first. For example, check the airflow around the outside unit (I found at the weekend a gutter was dripping water onto it; the heat pump air caused that to freeze so there was a frozen waterfall blocking part of the fan flow) to ensure nothing's in the way; try to identify if you have an auto bypass valve near to the low loss header, and if so, make sure it's practically closed; if you have a secondary pump try a day on a different speed to see if it's better or worse.

Sorry that you are having difficulties. Installers ought to know more about that they're doing, and do a proper handover.

We may have similar and could share experiences:

I have a Grant Aerona 10 KW ASHP installed in 2022.

My problem: With the Honeywell Home thermostat  temperature is set at 23 degrees, the room only reaches 18 C.

I have tested the system with the thermostat batteries temporarily removed. Although in these circumstances the heat pump should continue working unregulated, with the room temperature continuing to rise. This didn't happen, it still only reached 18 C. Other tests confirmed the thermostat was regulating at different target temperatures up to 17.5 C, so I have no reason to think the thermostat is faulty.

The flow and return temperatures show about 5 or 6 degrees difference early in a program cycle and then fall to about 3 degrees as the room temperature reaches a maximum.

My attention was drawn to the possibility of the modulator switching off the electrical load too early. I don't know if the modulator can be tested, are settings adjustable or if faulty, will it need replacing?

The best evidence of insufficient load is the daily overnight Octopus History Chart. Once the batteries are charged as early as 2am and the room is at about 18 C the KW load drops off typically to between 1.5 and 2Kw and remains at this level until the end of the low cost energy slot at 4.30am.  This I believe is sufficient to maintain room temperature but not increase it. At other times, when raising the temperature up to 18C the load is typically between 3 and 4KW.

Fault codes have not been displayed on the Grant control screen and I don't know if there are any fault codes that relate to the modulator. Perhaps there are other aspects that can reduce the KW load but I have no idea what they could be. The unit had an annual service last November and in every other respect the installation, including batteries and solar panels appears to be working perfectly.

You don't say what flow temperatures you are running at, are you using weather compensation or a fixed flow temperature?

What are the flow and return temperatures?  You say there is a difference of 5C so everything seems to be working correctly.

If the room isn't getting beyond 18C then you are either not running the flow temperatures high enough when its cold outside, or your radiators are too small to transfer enough heat, or you aren't running it 24 hours a day.

The heat pump likely doesn't know the temp inside the house so its just supplying a set flow temp and once the flow and return get close enough it will ramp down power to maintain that offset.

@gary Thanks the reply a little more details. The radiators are more than adequate and everything was fine last winter. As an experiment last night, I fully turned on more radiators just to observe the difference. The load increased from typically 1.5Kw to 2.75Kw as would be expected with more radiator open but the lounge, that has the highest ratio of radiators to space and is the best insulated room, still reached 18 degrees. So the system will deliver more KW for more radiators as it will to raise temperature up to 18C . In the best insulated room, once that lounge temperature is 18C the load cuts back. It is not the room thermostat that is regulating and doesn't appear to be related to time but it does appear that perhaps another thermostat somewhere in the control mechanism is turning off the load. I suspect it must be measuring the circulating water but I have no idea what it could be other than the modulator, which was a suggestion from others, which I understand has its own thermostat.  Even of the few warmer days we had recently with temperatures reaching the low teens the lounge temperature still only got to 18 degrees. I have three  friends who are not heating engineers but retired Instrument Engineers and a Physicist. All, independently, have the suspicion a temperature measuring device embedded in the instrumentation is reducing off the KW load. Thanks again.

@hillswick if you engage more radiators then it will take longer for the flow and return temps to stabilise so the heat pump will run harder.

what flow temp you are running at?

 If the rads are getting warm but you aren’t getting to temperature then it’s just not high enough flow temp or your running it for a minimal time it needs to run 24/7 output enough heat 

If it was working okay last winter, and not this, then implies something in the set-up has changed, or something is broken (but if that was the case I'd expect an error code if something was cutting out on overheating).

The value of parameter 41 00 should be '1' (control by water temp) and 42 00 should be '0' (always on). Check these to ensure nothing important has been updated.

And then as a test I'd switch off weather compensation, 21 00 → 0, and have fixed LWT of 55C (21 01 → 55) and your house should keep getting warmer and warmer (obviously set any room stats to 30C target or similar).

I used this set-up when balancing radiators, and the house went over 25C.

Sorry, I should have covered these points.  Whilst there is a good deal of truth in running 24 hours there are two aspects to the calculation. Heating efficiency and financial cost. In my case, facing south any sunshine in colder months slows down the heat loss and even on overcast days the heat loss is slow because of the exceptionally good insulation. Last winter, even on days when I was away and the heating turned off, the room temperature reached the target temperature in an hour or so which was little different to leaving the pump ticking over and turning the thermostat up.The proof ultimately is in the annual kWh. For the year ending 28th Feb I purchased 3486.3 kWh plus 1119.6 kWh of Solar. On a day by day basis I felt I consumed more kWh when the pump was running continuously. A normal day time comfort temperature is 16.5 degrees a little higher on an evening. The reason I want to raise the temperature is overnight to capture the Octopus cheap rate. The house superheated, then takes longer to cool meaning I don’t need to use batteries until later and this in turn reduces the day time kWh I need to import in the evening due to the battery capacity lasting longer. Day rate is about three times the cost of the night rate. This difference more than compensates for a slight fall in efficiency. I stress, last year I could meet these objectives easily. Nothing has changed regarding lifestyle. This year,  once the room reaches 18C the load drops from between 3 and 4 KW to 1.5 and stays at this level. To reach higher temperatures, surely the load would need to continue at the higher load  before reducing.
The only reason I ran with more radiators was to make sure it was able to deliver more KW when the lounge had reached 18 degrees. It did an extra 1KW+, the extra energy for the additional radiators but the lounge stayed at 18 degrees.

The lounge is above average size.

Once the room reaches temperature the Flow is 50c and the return 46. Which I was told was fine but obviously if the load was higher, the flow temperature would be higher, the radiators would be hotter and the room temperature would rise but I’m new to Heat pumps and have poor understanding so could be wrong.

Regards