To add to the conversation, we've been using about 70-75kWh of electricity per 24 hours running on weather comp for our 18kW ASHP. House is lovely and warm, 21C in most areas. Compared to a winter a few years ago when we ran at 45C set point, we were doing 110kWh per day. Glad to see temperatures rising to double digits in a few days time, which will see our consumption drop well below 50kWh for 24 hours.

@iancalderbank - your second chart, zooming in on a single defrost cycle, is interesting, in that, apart from the RWT, and a slightly different but not by much time frame, the temps follow exactly the same pattern as seen in my zoom in on a single defrost chart. Interesting, in that two different installations with different brands of heat pumps do the same thing.

I wonder, does your heat pump sit in it's own microclimate? You have the same rise in OAT during the defrost as I do, which according to @derek-m's theory means that at other times the normally running heat pump is lowering the OAT. My own view, as you will know, is that this is normal behaviour that applies to many if not most heat pumps. 

The failure of my system to reach the desired/design IAT in cold weather is I think largely another problem: the heat pump is simply under-specced for the loss/demand. Of course other factors affecting efficiency don't help, but the root cause is 11kW supplied will never meet a 12kW loss. 

@hughf Thanks 🙂 Im glad my self-install worked as I had hoped even though I used plastic piping for the new circuits.

@bontwoody My self install seems to be working nicely too, despite me ignoring the manufacturers instructions and running it direct, open loop... I strapped the buffer sensor to the CH flow pipe after the 3 port 😀

@hughf Makes you wonder doesnt it, if total amateurs like us can get it right how do so many installers manage to get it wrong 🤣 Dont mention the dreaded B word!

@bontwoody I think the answer is that we are "total amateurs" who really really gave a damn. read every document. watched loads of training and education video's.  studied best practice. paid care and attention to absolutely everything. I wish there was a way to make use of this without actually becoming an installer!

@iancalderbank we can argue with installers on Linkedin about how they're mostly incompetent 😀

Posted by: @cathoderay

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@iancalderbank - your second chart, zooming in on a single defrost cycle, is interesting, in that, apart from the RWT, and a slightly different but not by much time frame, the temps follow exactly the same pattern as seen in my zoom in on a single defrost chart. Interesting, in that two different installations with different brands of heat pumps do the same thing.

I wonder, does your heat pump sit in it's own microclimate? You have the same rise in OAT during the defrost as I do, which according to @derek-m's theory means that at other times the normally running heat pump is lowering the OAT. My own view, as you will know, is that this is normal behaviour that applies to many if not most heat pumps. 

The failure of my system to reach the desired/design IAT in cold weather is I think largely another problem: the heat pump is simply under-specced for the loss/demand. Of course other factors affecting efficiency don't help, but the root cause is 11kW supplied will never meet a 12kW loss. 

I always see a dip in the OAT as measured by the heat pumps own sensor whilst its running. I don't think there's a microclimate. There's simply a massive flow of air, that puts a bit of wind chill on the sensor ,  which is physically on a short wire on the upper back of the unit. If the wire was longer, I could site it well away from the unit . In that case I'd expect it not to vary with run state.  But given this is what the manufacture gives us, and its the same, on 10000's of deployed units, I assume they must allow for this behaviour in their controls.   In defrost mode, there's a large cloud of warm water vapour thats pushed into that same physical area. It briefly clouds up the window of the nearby utility room, so its no surprise it moves the OAT sensor higher for a few mins. 

ref your heat loss - If you were able to run at a lower LWT, then IIRC the theory says they defrosts would be less frequent and thus effective output slightly better.  Its also the case that often the capacity varies by LWT , its never the "nominal plated number" . So (speculating) you might perhaps get an extra 0.5kw out of it at 45C vs 55C. Suggest checking the data book to see if going through the emitter-side work to get yourself a lower LWT gains capacity.

@iancalderbank

A sensor should not suffer from 'wind chill', since it is not a heat source.

@derek-m maybe thats not the right technical term. I'm just telling you what it does. If you stand by the heat pump its a LOT colder when its running.  I had to do some minor property maintenance the other day in the full blast.  7 layers on and still numb in 5 mins, had to give up. Ok so the sensor isn't directly in the blast - its on the upper back . but it doesn't surprise me it drops a bit at all. Not sure how many people actually graph this level of detail from a sensor that is physically on the unit.

Posted by: @hughf

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@iancalderbank we can argue with installers on Linkedin about how they're mostly incompetent 😀

thinking about the installers (and related roles) that I follow , who discuss this subject area on Linkedin, they are largely the good ones I think. Its the ones not discussing how best to make these systems work,  just "banging them in" that are the issue.

@iancalderbank

Yep mine is near 2 degrees colder when it's running according to reading on my unit. Noticed it few times.