(OK, not helpful maybe but it *is* on topic... :-) )

Overall household kWh (the ASHP is not separately metered) goes off the scale (because of the ASHP) in freezing weather but it is strongly influenced by humidity (I keep a daily log of humidity and max/min temperatures as well as kWh). It is generally very damp (humidity >95%) in this part of West Oxfordshire but on a good winter day can be as low as 75%. This means that on high humidity days the ASHP can go into defrost at as low as 5C outside temperature. But on those rare good days of low humidity it can avoid defrost even when the air temperature is below 0C.

This last few days it has mostly been very damp as well as sub-zero so the ASHP spends much time defrosting. Daily electricity goes up to 75kWh to 80kWh (from around 50kWh on less cold days). In our all electric home the ASHP runs continuously. I've experimented with turning off the UFH at night to save electricity. The internal temperature drops about 1C. But in the current cold weather (about -3C here) the ASHP struggles to recover this during the day and just uses more kWH to make up (especially if the humidity stays high). The net result is no saving in kWh.

This is a bad start to the year. It is a worse start than in 2025 which itself was worse than 2024. But experience shows that over a full year the overall average daily kWh tends to largely even out. In 2025 the total kWh was about 500kWh more than in 2024 (ie less than 1.5kWh per day), about £125 on our bill or £10 per month. That's 2 pints in our local. I'm not indifferent to this but it is small change compared to a single call out charge by a local heating engineer to try and seek improvements to the system.

Mike

You system isn't too different from mine. I don't have UFH and my house is a 2 bed mid-terrace so maybe a bit smaller. Heat loss 3.3 @ -3C though overestimated.

Is your emitter type UFH or radiator? I wouldn't expect fan coil with UFH. Try modulation at 4/5. Which may slow temp gain but with longer cycles isn't such an issue.

Your LWT of 39C could probably be brought down nearer to 36C at the cold end of the WD curve. There's plenty of flow rate capacity in the 4KW Daikin unless you have particularly restrictive pipe work.

Most of the time my LWT is ~33C with very low flow (7 l/m). WDC = 36C @ -5C & 26C @ 12C (I don't need heating above 12C OAT).

Thanks for your response. 

I have UFH downstairs and radiators upstairs; and as I am in pure weather compensation there is no modulation setting. I was using madoka thermostat in milder weather and didnt find it comfortable. Indoor temp is now more stable, the only issue is the frequent defrosts when it gets colder. 

LWT of 39c is for outdoor temp of 3c, I'll try to come a bit lower (eg set 44-45c for -3) to see if it makes a difference. I dont have restrictive pipework and there are no zones in the system, everything is open. 

It is a steep learning curve for me if you imagine we had 3-4 frost days which are the first ones for the system....

You system isn't too different from mine. I don't have UFH and my house is a 2 bed mid-terrace so maybe a bit smaller. Heat loss 3.3 @ -3C though overestimated.

Is your emitter type UFH or radiator? I wouldn't expect fan coil with UFH. Try modulation at 4/5. Which may slow temp gain but with longer cycles isn't such an issue.

Your LWT of 39C could probably be brought down nearer to 36C at the cold end of the WD curve. There's plenty of flow rate capacity in the 4KW Daikin unless you have particularly restrictive pipe work.

Most of the time my LWT is ~33C with very low flow (7 l/m). WDC = 36C @ -5C & 26C @ 12C (I don't need heating above 12C OAT).

I'm already up to 10 today, it's played havoc with keeping the house up to temperatures and hammering the electric consumption.

I suspect, but have no evidence, that defrost is very variable according to location and time.  There are places in the UK that are very humid and places that are very dry.  This is going to make a huge difference as is any front passing through.  Probably best not to worry unless its causing a problem that occurs frequently!  Obviously if it does cause a frequent problem that's a different matter!

I'm already up to 10 today, it's played havoc with keeping the house up to temperatures and hammering the electric consumption.

Regards, Toodles.

I wish I had only 9 defrosts in that timeframe.  I had 16 in the early morning of just the 26 Nov.

Considering I live within a mile of the River Thames (as the Fly Crows), this seems reasonable to me. A cycle takes ~8 mins. from start to finish; the radiators go cool for about ~ 3-4 minutes but otherwise, I would be unaware of the procedure happening at all.

BTW, I had an email today from Homely to say the beta testing of the new Homely programme starts today but I don’t know if my smart controller is in the trial or is one of the ‘control’ units they have decided on.

Regards, Toodles.

It defrost more when working hard in cold/humid weather. The only way to get it to defrost less is to make it work less hard and produce less heat.  If it defrosts like that, keeps you warm and is at your design temp, then I would not worry about it, its not often at design temp. Its when ASHP are slightly undersized and they work that hard in more normal days that it becomes an issue. There has been some problems with units defrosting x 2 or 3 and then not outputting what they are meant to which can spiral temps in the house down, the machine works harder to raise the temp, its cant, defrost more..... That's a defrost spiral.  I''ve been in a few on them and have to manage my unit carefully and manually in cold weather 

Do you live somewhere damp even for the UK?

Cambridge, not the most humid place in the UK but there has been a lot of rain last few days so gardens are still soaked....