SP welcome to the Grant Aerona 17kw club. 
Ours was not set up correctly on handover and WC was not selected just 50c full beans. 

Roll forward a couple of years and a visit from an Elite Heat Geek and WC is on. 
You need a massive amount of patience that’s for sure. 

60kw for 240m2 doesn’t sound too bad. We are mid 70’s for 300m2 in these lower temperatures. 

Unfortunately I’m still not convinced. It’s far from a switch on and forget about the gas cost boiler previously. 

The need for lower electricity prices is what we need for folk with Ashp. 
but that ain’t gonna happen. 

Hello - to cover my understanding of the Grant (mine's 10kW but the same controls I assume):

1. For delta T, the Grant instructions themselves are not consistent - sometimes mentioning a target of 5C and other times 8C. I don't know anyone who gets near to 8C, and even 5C is good. Mine is similar to yours; generally around 2-3C when it's running in a "steady state" long-running mode. Obviously the closer your room gets to its target temperature the less energy it will draw, so a lower delta T will be observed. Moreover with a lower water temperature, the chance of a larger drop reduces (i.e. 38C water loses heat much slower than 70C water).

However, as you state the flow regulator exists to temper the flow rate to enable the heat emitters to have the chance to draw energy from the water. Assuming your installation follows Grant's recommendations, you'll have a low loss header (LLH) and probably know the trick is to balance the flows either side of the LLH, but to start with you need to know the flow rate you're after. Do you know this? Without the flow regulator you're fixed to one of the 3 offerings the dip-switches on the HP unit can give you, and whatever the head of the system is. The system shouldn't have been handed over by the installer without the flow regulator, so I'm surprised how a Grant installer failed to include this.

2. Using the Chofu controller I think isn't possible, if the HP has been installed as per Grant's installation guide. The guide isolates the controller functions by sending the 'on' signal out of the EP002 box directly to terminal 20 (?) of the HP unit, that being controlled by whatever 3rd party room heating controller you have (wired into the EP001 unit). This is how Grant bypasses the Chofu controller. This can be reversed out to put the Chofu controller back in charge (and some users have done this) but I think you'd need wiring changes at the HP PCB end, and amendments to parameters in the '51' group, but not something I've looked into (although, as I say, others on this forum have).

The room heating controls are usually fairly standard in terms of connecting back plate so I'd think replacing those with ones that provide more time slots would be easy to do, although you'd still not reap the full benefits of having the Chofu controller (which seems much smarter than what Grant supply). Another option would be Grant's new 'smart' controller although that doesn't seem to be offered to existing users.

Hi @steam-powered, @mikefl makes good points above, especially balancing the low loss header if you have one. If you don't have any flow setting or monitoring, you can do this by buying dome cheap digital thermostat probes, and measuring the flow temperatures in and out on the top (leaving water temperature) and bottom (return water temperature) sides. Get them to match. If you have a LLH you'll have a least one additional circulating pump after it, on the secondary side. Check the speed setting, it's highly unlikely to be a PWM speed controlled pump, and will be manually set to one of three speeds. You can adjust the secondary and primary pump speeds to balance the flow, check the Grant Aerona manual for the primary pump - there's a simple dip switch adjustment at the control PCB in the ASHP unit, which is factory defaulted to max speed 3. Chances are your installer didn't think or know how to adjust it down.

The low deltaT between supply and return isn't anything to be concerned about, it tend to indicate your flow rate is high, and/or your heat loss low. My 10kW Grant Aerona maxes out on deltaT at about 5.5degC going flat out, but usually sits around 2.5-4degC. My flow rate is mid-range for the 10kW model, but the torturous wet radiator circuit I inherited with the house means the secondary pumps has to be on speed 3 to match the primary pump on speed 2 and get sufficient flow to balance 20 radiators.

70kWh isn't a large consumption in 24h for a 17kW unit. Most of my all-electric house electricity consumption is the 10kW ASHP, it's averaging at around 2.6kWh pretty much constantly right now = 62kWh a day. I'm on Octopus Cosy and leverage the advantage with Solar PV + battery storage, charging the batteries in the low tariff periods, to see me through as much of the normal and high rate periods as my 13kW of storage will allow.

There isn't a simple way to offset the standard Grant + Chofu Aerona3 R32 controller to offset the weather curve down a notch during the higher rate periods, and boost it up during the lower rate periods - that would be a way of minimising your electricity consumption in the more expensive tariff periods, by using more in the lower tariff periods. The new Grant Smart controller does include this feature however. You could do it with some of the in-built functionality in the Chofu controller using low tariff mode function, but you will need an additional timer and wiring onto auxiliary terminals in the ASHP PCB to initiate the low tariff modes. I had a look at this option, but decided it was not something I want to consider further for now.

My 10kW is a little under-powered, should have been a t 13kW but DNO issues, so I use the 3kW LLH immersion heater as a supplementary heater sometimes to boost start-up from cold, kicks in if circulating water below 30degC, drops out when reaches 37.5degC. Also energised during defrost cycles, to stop LWT dropping to low. Consumes a bit more power of course, but mitigates against a target LWT shortfall and very sluggish warm-up or defrost recovery times. In the current cold snap we're on 24/7, and I don't need this enabled as the house will stay around 20-21degC.

@RoamingBull, you are yet another Grant owner who didn't have WC enabled when Grant commissioned it. That's a common story. Same with me, 50degC LWT with no WC enabled. I'm now running at 45degC max LWT.

Thank you all for sharing your experiences and great advice which I have been applying over the last day or so and taking advantage of the current cold snap to run some tests.

The low loss header was in perfect balance on flow and return as the system was set. In the absence of a main flow regulator I switched the Aerona pump down from L3 (handover condition) to L2. 

The main circulation pump is actually variable speed (Primatec HE-V 710 02 123) and was set at fixed L3 from handover, running at constant 45W. I switched this to the "Auto" mode and it soon modulated down to c 24W against the Aerona L2 pump setting. The HP soon modulated down to longer cycles. It had been heating for 5-6 mins then shutting down for 2-3 mins and then repeating, pulling c 1.8 to 2.5 kW, sometimes with short peaks c 3.4 kW. (The Chofu book says there is a minimum 3 min dwell between compressor stop/starts, but mine seems to re start a bit sooner sometimes). With the L2 rate it was running for c 7-8 minutes with a 2 -3 min dwell time, pulling 1.5 to 1.8 kW, with the occasional peak to 2.2 kW. 

I then dropped the Aerona to L1 pump and it was heating for 11-12 minutes with 2-3 min dwells pulling 1.2 to 1.5kW, peaking at 1.8kW. The Primatec pump modulated down to 18W at this lower flow rate. By this time most zones were reaching their upper room stat limits and with the lower demand the HP moved back to shorter 4 min heating cycles, so trial now paused, just as I reached the afternoon cheap rate tariff....

I have been really encouraged by this step change today, it shows (I think?) that the flow regulator on the return from the LLH to the HP is a must to get it into continuous running. It was great to see the controller really starting to modulate the compressor downwards and with stable running at c 1.2kW a possibility it seems 24hr running may be more attractive than switching around the Octopus Cosy timeslots. Thank you all for helping me get to this point so quickly!

As an aside, after the above changes to the pump rates the LLH balance was -1 degree on the return from the heating system v the return to the HP. The flow side remained in perfect balance. I doubt -1 degree is that critical ???

Currently waiting for my builder to respond on getting the flow regulator fitted. I also need him to move the Mag One filter as it was fitted on the return between the LLH and the HP. My understanding is that it should be on the heating system return to the LLH, I cant see much debris accumulating in the short loop from HP to LLH. Any comments on that please???

I switched my DHW from HP to the immersion heater in December as the HP was pulling 4.8kW in the DHW cycle. Does the COP factor actually make that high consumption more efficient that my 3kW immersion heater? I was doubtful given my current inefficient short cycling and my gut feel was that the immersion would be a better bet, at least until the HP flow rates are under control. I have read that others on this forum have also made that change.