I can't comment on other aspects, but heat pumps do have to work harder at temperatures close to freezing with high relative humidity. This is reflected in Daikin's capacity graphs which show a drop in capacity at ambient temperatures from like 1-5C, then it increases below 0C before dropping off again as it gets really cold.

Defrost cycles could be worse if you have your flow temperature set to higher than it needs to be. Are you running it with weather compensation? 

100% this heat pump is being used like a gas boiler. Turn down the flow temperatures and it'll stop icing up. It needs to be used in the heat pump sweet spot. My flow temperature yesterday was 32° - no defrost

Thanks, yes I am using weather compensation, it may be set a bit high at low temperatures as it never reaches the set value before being turned off by room thermostat controls ( when there isn’t the defrost issue). However it generally cuts out for defrost before it gets much above 50, and it needs to be at least that, so if I set it that low I would expect it to still need to defrost? It seems to be controlling on differential temp rather than leaving temp so I don’t think it would be working any less hard.

Where are you finding the capacity graphs?

@batalto It is being used in accordance with its instructions. Flow temp is a lot lower than it used to be with the gas boiler, but I doubt that 32 deg would get anywhere near heating the house. I have radiators, not UFH. It is set up for those. I will look at setting it lower than it is but I don’t think I have much scope to go lower than it is reaching before needing to defrost.

@bramshot I would just try knocking the weather compensation curve down a little bit. It'll cost less to run, probably have to defrost less, and if you're saying your flow temps never even reach what they're set up to run at, then your house can clearly handle lower temps than the heat pump is aiming for. You should wait 1-2 days after changing weather compensation settings before assessing if the changes have worked. The worst that's going to happen is you're a tiny bit too cold for a few hours, the best is lower bills, less stress on your heating system, and fewer annoying cycles. 

On the capacity graphs, you can get them from this link https://daikintechnicaldatahub.eu/en-GB   otherwise they'll be in the installer's manual, but I wouldn't worry about it too much. 

@bramshot I've got rads. Honestly use some weather comp. You can go below 37° you'll be fine.

@oswiu I have reduced the temperatures, but I have already seen a defrost cycle as the ambient falls tonight. Even though the house can cope with the lower temperatures, surely it is at those lower temperatures that the frosting is occurring? It never gets to deliver the higher temperatures that were set. I don’t think reducing the setpoint reduces the demand on the heat pump, it is the actual temp and delta T achieved that is determining how hard it is working. I will however persevere and keep reducing the temp as I think it is the way to go, just not sure if it ( the heating) will be effective when low enough to prevent frosting.

Thanks for the chart location data, though like most Daikin info it is not easy to understand especially when the axis labels are messed up. This info is NOT in the installation guides etc that I have seen. Mine is an EPRA18DAV3, which is not specifically mentioned in those listed. 

@batalto my rads are not oversized, as it is a High temperaure heat pump and supposed to work with ‘ normal’ sized rads. Are yours the same? Also an oldish house so not as well insulated as a modern one. Although the rads were surprisingly effective at lower temperature when the weather was warmer I really doubt that anything below 50 to 52 will do as ambient falls to zero.

Posted by: @bramshot

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@oswiu I have reduced the temperatures, but I have already seen a defrost cycle as the ambient falls tonight. Even though the house can cope with the lower temperatures, surely it is at those lower temperatures that the frosting is occurring? It never gets to deliver the higher temperatures that were set. I don’t think reducing the setpoint reduces the demand on the heat pump, it is the actual temp and delta T achieved that is determining how hard it is working. I will however persevere and keep reducing the temp as I think it is the way to go, just not sure if it ( the heating) will be effective when low enough to prevent frosting.

Thanks for the chart location data, though like most Daikin info it is not easy to understand especially when the axis labels are messed up. This info is NOT in the installation guides etc that I have seen. Mine is an EPRA18DAV3, which is not specifically mentioned in those listed. 

You will probably still get frosting occurring on the Evaporator, but at less frequency as the LWT is lowered. This will happen in particular when there is low ambient temperatures, but above zero, and high humidity.

Remember that the refrigerant gas could be at -40C inside the Evaporator, so any moisture touching the outer metal is likely to freeze.

Are you controlling your system using weather compensation?

@bramshot my rads are all normal double type rads. Honestly I'd give lower flow temps a go. The worst case is you turn it back up. Low and long is the aim

@derek-m yes weather compensated. Will try lower temperatures tomorrow.