@allyfish Ditto for my Grant 10kW - it gets closer to the target LWT after each iteration following the defrost cycle, but never seems to quite make it, then gradually loses the battle as the evening progresses. It's a fatal strategy - the more the HP is running, the more likely defrost cycles are required. It's not quite as annoying as the HW cycle cooling the water in the water tank, but it all adds up.

The optimal figures will be different for each house, but those figures don't look wildly out.

21 01 is the fixed LWT (leaving water temperature); so with weather comp on, it's ignored, although making it the same as 21 02 isn't a bad idea (in case you decide to switch WC off at some point).

21 02 is the maximum LWT you'll ever produce with WC on. It'll produce* that when the outdoor temp hits '21 04' or lower i.e. at -4C or lower you'll be targeting a LWT of 52C

20 03 is the minimum LWT the HP will produce, and it'll do this when outdoor temp is '21 05' i.e. 30C. If the outdoor temp is above '21 05' the HP won't produce any output.

Between those two extremes there's a straight line drawn, so at say 15C it'll produce an LWT between the 30 and 52C.

You can probably reduce '21 05' to 20C, as if it's 20C outside, there's usually no need for heating indoors. This will slightly increase the gradient of the line, but in the current climate you're so far towards the '21 04' end it'll make little difference. 

*"try to produce" ... as in colder weather the defrost cycle is constantly knocking back attempts to reach that LWT. Low outdoor temps are a double-wammy of badness

@mikefl I have a theory the closeness of the outdoor air sensor on the Grant Aerona's may be causing some adverse response. During defrost the outdoor air sensor is subject to a little radiated and convected heat from the evaporator when warm, it registers this as a rise in OAT. That then drops demand on the ASHP, and there's a sluggish response after defrost. It's just a theory, which I plan to test by moving the outdoor air sensor further away from the coil face. It may be complete Bo$$ox, but I'm going to have a play with the sensor location 😉

@mikefl thanks Mike. I see. 

so if I increase 2102 and 2103 slightly I might get a small increase in output on the understanding it will use more electricity?

also do you know how I can see the current LWT as it actually is as opposed to the LWT setting?

really appreciate the guidance from you on this, it’s making sense.

@allyfish Hi, I have been monitoring the parameters from our aerona 6kW over Modbus. The OAT sensor does not get affected by the defrost cycle.

@marvinator80 You can see the LWT (and Return WT) on the Controller. Slightly easier than getting to the other parameters, you press the 'return' key (looks the same as 'undo' in this editor I'm currently typing in, after the + in a circle) for a few seconds, change that blinking number to 9 using up/down, them press the tick key. 09 shows the LWT; 00 shows the RWT.

I have a couple of sensor thermometers held against the pipes as this is easier to do than all the key presses. Grant say you should aim for a difference of 8C between LWT and RWT, but that's if the HP isn't modulating down, and all heat emitters are fully open, so not a typical scenario. 

@ghayne @allyfish I think I agree; the OAT seems fairly resilient to what the HP is actually doing (I get a bigger influence from sunshine with HP on south-facing wall). The impact of defrosting on LWT (and of heating on the defrost temperature) is what's most noticeable. During 'non-heating' periods (10:00 to 14:00 here) the defrost sensor temp tracks OAT quite well.

(there's more going on than this graph reveals maybe, as I'm only grabbing data every 5 mins; thus missing some defrost triggers, and only seeing the impact on LWT)

@mikefl thanks very much.

37 going out, 31 coming back. I’ve not long changed up to minimum up from 35 to 40 from so will be interesting to see what effect this has. Currently 0.5 outside.

Hi, sorry i don't want to hijack the thread, but i was wanting to ask the experienced Grant aerona 3 owners about the defrost cycle on the grant, and it seems to have sort of popped up here anyway.

Specifically,  does anyone understand what exactly triggers a defrost cycle? 

I've noticed that at low ambient temps, when it receives a demand for heat, it initially starts to heat then does a defrost before going back to heat. Fine.

Once it's going it sometimes seems to defrost at a reasonable point (i.e. before the hx is completely iced up), but at others, the frost is clearly blocking the airflow, the 'defrost temp'( parameter 05 on the user menu has dropped to -20  (as low as it goes according to the manual)), the Dt has fallen off and the power consumption is cranking up, but it still just keeps on going!

I can't find any parameters in the manual to tweak (grant or chofu) such as min run time between defrosts for eg, or max OAT - defrost T perhaps.

And tips would be appreciated. 

Marcus

@mab interesting question.

@mab As far as I can see, (at least for the Grant) there's no 'user control' of the defrost cycles (maybe wisely). The 'defrost temperature' is poorly named as it's actually the temperature reading from the defrost sensor, which is somewhere near to the fans, and it's this which seems to trigger the cycles (probably). If you look at the graph in my post https://renewableheatinghub.co.uk/forums/postid/23922 you'll see that whenever that is below -10-ish a defrost cycle is triggered. As I say, only taking readings every 5 mins you can miss the exact moment, and I have got minute-by-minute data to analyse at some point to better understand this.

Having said that, my HP seems to defrost every 42 minutes - maybe that's just how long it takes for a zero-ish OAT to plunge the fan below the trigger temperature (OAT has been fairly even on recent cold nights/days) - I know it's 42 minutes as the cloud of water sets off my security camera.

Obviously an operating/heating HP creates more defrost cycles than a dormant one; in fact I'm keen to see if an OAT of maybe -10C will trigger a defrost cycle when the HP isn't actually heating. Although at -10, I'd like my HP to be working.

The coming days should provide ample opportunity for more evidence.

Thanks, 

I've made a note of your 42min interval as i have a suspicion there's a minimum defrost interval but haven't worked out what it is. The other day when i was watching it i saw the defrost temp drop quite quickly from about -4 down to -20 (OAT >0) and it didn't defrost. In the end i turned the room stat down to turn off the heating, then back up again as I'd worked out that it would do a defrost when it restarted - but clearly i don't want to have to micromanage the central heating to keep it working efficiently.