This is strange @martyj76, if the unit was installed by a Grant G1 installer, and has been annually serviced by a Grant G1 service engineer, it will benefit from a 7 year Grant warranty. But it sounds like you've got issues on the system side, not with the ASHP unit. They won't cover that.

The Grant Aerona 10kW is a reliable unit, I've got one, it never misses a beat. On a clean water circuit there's seldom any issues, and you know your ASHP is working fine in DHW mode. So, something is up with your heating circuit?

From cold, you would expect the ASHP to work hard, drawing high power (up to 3kW) to raise the water temperature to target flow rate, either weather compensated, or fixed flow temperature, depending on how it is set up. (Should be weather compensated control.) It will raise the water temperature slowly - the rate of rise of the leaving water temperature is linked to the rate of rise of the return water temperature. The unit will raise the water temperature by 5 or 6degK with a normal flow rate when working flat out, and will drop to as low as 2 or 3degK when the unit is modulating under inverter control and ticking away, consuming much less power. You can read off the unit flow and return water temperatures (to the nearest 1degC) from the Grant controller - user parameters 9 and 0 respectively. 

Your rads are barely getting warm, and the ASHP is cycling off on what appears to be low/no load. That's when there's so little heat energy lost between leaving and return water temperatures that the ASHP 'has a rest' for 5-10 minutes and stops, with only the circulating pump running. The heating demand is way lower than the minimum turn down of the Grant unit, which on the 10kW is down to as low as 2.5kW. That suggests a lack of flow through the heating circuit, caused by, for example, a secondary pump fault/failure, an airlock, or a blockage. Dumb question time - is a manual service valve closed? Check for the obvious!

There's several installation schemes in Grant's R32 Aerona Installation Guide, all have a secondary pump fitted for the central heating circuit, (Very old school, but that's Grant) Sometimes just the built in pump in the ASHP is used for hot water. Some schemes use a 3 port diverter valve, some use 2 x 2 port valves. That's what makes me suspect your secondary circulating pump on the heating side, (assuming it's fitted per Grant schematics). Sounds like you're getting no flow on the secondary side of the circuit around your radiators. Find the pump, is it running nice and quiet? It should be If it sounds like it's full or air, or noisy, it could be faulty. If it's not running but CH is called for, maybe it's failed?

If it appears OK, open everything up, all TRVs to max, set the thermostat to continuously call for heating. Is there is significant temperature difference between the leaving & return water temperature? The Grant controller can display user level data, press and hold button 4, scroll up or down between 0&9 (buttons 8), press enter, (button 7), press and hold button 4 to get back to the home screen. There should be 4 or 6degK difference between flow and return from cold. the delta T is almost nothing, then that suggests water isn't circulating, & no heat is being transferred to your radiators to warm the space.

@allyfish Dear Allyfish, that is so very kind, to respond to the extent you have and with the content information contained.

I will take it from here with the checks suggested and will endeavour to read parameters. I'm now disappointed not to have had a flow regulator installed to read off of, think that will be a must when it can be afforded.

Am fortunate enough to have a day off tomorrow, so going to work through yr suggestions one by one, and am pretty sure I might be able to deduce or pinpoint an area Ive missed. One thing is for sure, the posts I read from experienced contributors on this site merit much respect, that people are kind enough to give their time and thought for others.

I am concerned it is worth all the time one can give to getting to know their heat pump, and to get it running as it should do best, for every obvious reason. It irks me a little that these Aerona Pumps are not installed with the appropriate setup to allow them to work as is best for everyone (and the environment).

I will endeavour to offer up results on my checks tomorrow, I hope this post too amongst many others, might help someone or offer a start point that might assist others with an issue or a query, in the future.

@allyfish Wanted to update with results, and thank @allyfish for your input. I spent the last two days in various configurations testing everything suggested:

The pump is indeed working, i checked this by removing the silver cover and using a flat head screwdriver to see it was turning AOK. Then fired system up and felt the pump for vibration and listened carefully, it was def working. I couldnt get as far as checking flow and return on the Grant Controller, it wouldnt run for long enough to get through the list of parameter settings (glad for the diagram and instructions you attached, this is going to come in extremely useful indeed!).

Yesterday by chance, for no fathomable reason than as I'll explain in due course, the system spent the best part of the day (and evening) in and out of Frost Protection Mode? I have no idea why!? And it hasnt done it today. This (fortunatly) however happened to keep the fan going in between periods of me demanding heat and the system cutting out. The temperature outdoors shown on the Grant controller was thereabouts 12degrees until the eve came. I thought I understood that Frost Protection shouldnt kick in until around 4 or 5 degrees on Grant Aerona units? I then came to all sorts of conclusions such as the outdoor temperature probe was giving the unit itself false readings, among many more bewildering theories.

Anyway the system came up to a sort of lukewarm heat around all the 8 rads (now set to max temp), was able to discern that heat was passing the motorised valve (2 port). But not confident the valve was operating as it might, I set about taking the head unit off and checking settings lever which following Auto operation doesnt seems to jump out of full Auto position to somewhere in the middle of its line of travel. I tried to cable tie it off to prevent it from moving, but this offered no difference in results> The lever wont stay in manual position at all either, it jumps out of what i assume is its locking position, and no amount of cable tying would keep it there, meaning that the micro switch doesnt stay depressed.

I'd like to think I solved things today as a consequence of this, but not 100% sure: I took the head off the valve unit, leaving the valve in what I presumed was the open position in the (very minimal) turn available to it. Thereafter I used a clothes peg to keep the microswitch in the unit depressed. Lo and behold, the system fired up (after I returned power to it....I made sure to isolate power before removing the head of course). It remained in operation mode and for 1/2 an hour I got all radiators up to temp!!

I discontinued this experiment thereafter for fear I might damage things with the aforementioned parts out of situ as they were. I am now 95% confident the problem must be a faulty 2 port valve head? Any thoughts, anyone? Ive found a new valve head and valve on ebay. Next step will be to order up and swap the old head unit out. Incidentally, the valve is manufactured by Neomitis (model MTV222a). From what Ive read, these units rarely fail on new installs (mine is 2 years old), but it isnt unheard of? Can anyone shed any light?

I cannot think what else could be happening. If , with the head unit in place, I shift the head unit lever to manual and let it settle back to the position it returns to, I can hear a distinct click on and then off in the power supply box.

Re. the aforementioned Frost Protection activity yesterday....I found a loose earth cable of about 5 inches in length coming off a waygo triple connector inside the pump unit. the other two earths were firmly in position on the 30 port bus? connectors on the PCB, cant remember which no's they were. I see the controller is wired in (black and white cables) into ports 1 and 2 of this bus. Ports 3 to 15 as I remember are empty. I think port 3 is labelled up GND, which I assume is Ground? Somewhat naively (some might say stupidly), i took it to mean this loose earth of the Waygo, belonged in port 3 and took to resituating it. Can anyone shed any light if this is (probs unlikely) right?

I am hoping, with the advent of a new 2 Port valve head, my woes might be over, and my house warm again. i will have learned much in this process, and importantly that I will have to get an engineer out to put a missing flow regulator in line. Not sure where this should go, but I shant be trying to freeze pipes myself for sure!!

So glad this site exists! Any thoughts on motorised valves anyone?

Great to know you've made good progress @martyj76, and ruled the pump out. The two port valve could well be faulty, it might be a fault with the valve itself rather than the motorised actuator. Honeywell changed their motorised valve OEM a few years ago and their reliability for a couple of years was woeful - the valves kept burning motors out. So even the big supposedly trusted brand names cock it up trying to save a few pennies. If the valve is sticking that can easily present as what appears to be a faulty actuator. For what it costs, I would suggest you purchase a complete valve, then at the worst you have a spare valve body if it is just the actuator head that is faulty.

When you manually operate a motorised valve, the operation level stays partly slack unless latched. Its just how they are, but it feels a little odd.

Section 8.4 of the Grant installation manual covers frost protection, and what can initiate frost protection mode by the unit. It's not just the obvious - outdoor air temperature, but also flow temperature and room air temperature measured by the Grant controller. If the controller is in an unheated space <14degC (i.e.: not an airing cupboard) frost protection mode starts. Yes, 14degC by default!! Parameters 4301 to 4322 control frost protection. You can change the parameters to stop frost protection starting unnecessarily based on a room temperature <14degC, but I would not advise lowering the 4degC outdoor air temperature or circulating fluid temperature set points.

The electrical installation can be shoddy, even though there are few cables required on the Grant system between wiring centre on the cylinder, Grant controller and the outdoor unit. I had a half-finished wiring of the 3kW immersion heater in the low loss header, with cables just dangling loose and unterminated, resulting in the heater control relay never operating as it had no neutral connection. An earth in the multicore has been used for a live 230V control signal to the relay as well. In my day, potential on earth cores was an absolute no-no, and we were told to terminate all cores of a multicore, land them on unused terminals if not required. Seems sparkies these days don't care much for any of that.