My Cosy 9 heatpump was installed in the summer and seems to be working well. It recently encountered temperatures around 0 degs C and has been doing some rapid defrosts (4 mins defrost, run for 30 mins, repeat). Over a couple of days I’ve seen about 80 defrost cycles. Is this normal?
You can see my installation and data on HeatpumpMonitor.org and here are a couple of graphs. These defrosts are certainly not helping with system efficiency
This doesn’t seem normal. You’d ordinarily expect to see, in the worst conditions, a defrost cycle around once per hour. Yours is defrosting twice as frequently as that. It might be a particular quirk of the Cosy heat pump hardware and there’s nothing much that can be done about it. But I would ask Octopus, see what they say.
Hi Ian, from your HPM display, it looks like your HP is running flat out 9.5kW between defrosts.
Running at full chat seems to be the most likely way to make a defrost necessary. I also see that your room temps are rising quite rapidly when the HP is delivering 9.5kW
Have you tried settings that reduce the output of the HP? If you could find some settings so it only delivered, say 4kW then you may find the less aggressive heating required fewer defrosts.
I have Daikin not Cosy, but maybe you have settings like Quiet Mode and/or a way to reduce the flow rate. Both of those reduce the heat output of Daikins - and help to keep room temps more constant.
Keep things low & slow 
Good idea I’ve emailed Octopus.
Hi David, I’m on Octopus Go so cheap electricity 00:30 to 05:30, during that period I up the target temp from 21 deg C to 23 deg C to store some cheap heat in the house. During a sub zero night even when running at full power the indoor temperature only rose from 20.1 to 20.9 over 6 hours. It would be interesting, as you say, to reduce the heat output and see what happens. I’m using Octopus weather compensation and I have it set for minimum possible flow temperatures (I can not set flow temp below 30 degs at 12 degs outside temp, or below 50 degs at -10 degs, which gives a minimum flow temp of about 40 degs at 0 degs C). I could go into manual mode and set the flow to 30 degs and see if that makes any difference. But as it behaves well when it’s above 2 or 3 degs outside I might have to wait a while for a cold night to try it. Thanks for your thoughts.
Hi Ian, it’s not so much the weather compensation, flow temps. I’m really focussing on reducing the power that the HP is allowed to deploy.
I take your point about “banking” some heat during off-peak hours, but imo we also need to factor in the “negative heat” consumed by the defrosts.
Last year, if my HP was defrosting frequently eg during nighttime freezing fog atmospherics, I would switch it off for a few hours till the sun came up and cleared the fog.
I reckoned it was a waste of energy running the HP in such unfavourable conditions.
Defrosts are normal, but it looks like your heat pump is running flat out at 100% capacity from 8am-12pm: https://heatpumpmonitor.org/dashboard?id=860&mode=power&start=1763592480&end=1763659560
Running at 100% output will result in frequent defrosts.
It would be more efficient to run the heat pump for longer at a lower flow temperature, this will result in less frequent defrosts.
Since I can see the HP is running at 100%, I’ve added this data to the HPM heat pump database for the Cosy 9. It looks like the Cosy 9 is capable of 7.9kW max at -1C: https://heatpumpmonitor.org/heatpump/view?id=18
Hi Ian, To illustrate the fact that running at full power triggers frosting up, have a look at my system at these times Emoncms - app view
Notice that the only defrost happened when the HP restarted after being off for a couple of hours. It’s an irritating quirk of the Daikin HP that when the HP starts up it always runs at full power before settling down to preferred, programmed quiet settings.
Your HP seems always to be running full power, so IMO you are always prone to getting defrosts in today’s type of weather conditions.
Cheers, David.
Can you move the DHW load to an immersion heater on very cold nights? (Lower cost on cheap rate then heatpump on day rate.)
(A 2nd hand storage heater on cheap rate is also likely cheaper then heatpump on day rate.)
I have had a daikin 9kW for three years and each year it is the same old balancing act: heat v cost.
This year I went down the Ai route to try and solve the problem. I have never had the the power below 1kw, quite often well above this.
I recently had the software update provided by daikin so hoped to achieve the sub 1kW target. I have set the WDC to 43 @ -9 and 25 @21 with a deltaT of 5. My madoka is set to 23C so really does nothing. The system is left at these settings at all times, no setback at night. House sites at just over 20C day and night.
I am now running at 850 - 970w at all times and have seen my COP increase.
Not saying this would work for anyone else but I have managed to get my power usage down to I think the best I can achieve.
How do you find this mode, where do you find it in the menus?
Hi Derek, Here are the relevant settings I use in my 8kW Daikin The thought is to reduce the “accelleration” of the HP - to make it less eager.
Quiet mode is 7.4.3 in the MMI
I don’t know what effect each of these settings has on its own (life is too short), but together they are working well enough for me
| MMIbreadcrumb | description | value @ Jan 2025 | notes |
|---|---|---|---|
| 1.7 | room sensor offset | 1 degree | makes Hall display similar to emonTH |
| 2.5 | WD curve | 36@-2 35@12 | |
| 2.7 | emitter type | Radiator | |
| 2.8.1 | 25 degrees | ||
| 2.8.2 | 65 degrees | ||
| 2.B.1 | target DT for CH | 10 degrees | set so wide that it can never be achieved - to keep the flow rate very low. |
| 2.C.1 | No Modulation | ||
| 2.E | WD curve type | 2 point | |
| 4.5 | Pump mode | Request | |
| 4.7 | Pump limitation | 70% | |
| 7.4.3 | Quiet mode | Most Quiet | puts further limitations on pump agility. |
| 9.2.1 | DHW tank type | 3rd party small coil | selected to reduce power going into tank coil. |
Thanks for information, very useful.
