My heat pump is a sold as a 6kW. But it does almost the same kW at any flow temp and any outside temp varies between 5.88 (-15) and 7kW (+30).

So strongly suspect it's really an 8kW with a software gag on output. The 4kW is pretty similar almost flat output of 4kW, but the 8kW output varies by quite a margin, with -15 performance pretty similar to the 6kW unit, at just over 6kW.

Posted by: @editor

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Until then, it is likely that homeowners will continue to be misled by kW labels that are, at best, incomplete, and at worst, materially misleading. Why does Brand X’s 8kW unit perform like Brand Y’s 5kW? The lack of standardisation feels intentional, and it’s no wonder so many systems end up undersized, oversized or wasteful. This confusion must end.

In fairness to heat pump manufacturers the output inevitably (unless artificially restricted in software) varies with oat and FT. Furthermore the variation is different between manufacturers and models, depending on compressor characteristics and how they have chosen to tune performance.

There is no way that a single figure can describe this complexity and thus it is unrealistic to expect all models branded as having similar 'headline' outputs to have the same output under all relevant conditions.  So we either have to dispense altogether with headline figures or accept that they tell only a small part of the story.

To add to the challenge, design outdoor temperatures, even within one country eg UK, vary quite significantly so a 7kW unit in Glasgow may be good for 8kW in Plymouth and probably 5kW in Prague.

For me the current approach of specifying 'headline' output at a given set of conditions is a practical one, and in reality probably the only practical one.  One might reasonably argue that the specified condition should be at a lower oat than 7C.  I don't know where that figure came from, presumably it's an EU standard, but if so it's a bit odd because there can't be many parts of the EU where this would be a sensible design temperature. 

What I do think is missing, however, is the detailed design tables specifying max and min output as a function of ft and oat (including the effect of defrost)  Some manufacturers are very good at this (eg Mitsubishi which publishes very comprehensive data), some mediocre (eg Vaillant which does publish detailed tables but only in Czech on its Czech website, and says nothing about defrost) and some dreadful (eg Daikin which hides most of the data unless you are an installer, and Samsung which seems to publish very little also).  My view us that the open publication of this detailed data should be mandatory and if it were there would be no excuse for designers to get it wrong.  

Posted by: @jamespa

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What I do think is missing, however, is the detailed design tables specifying max and min output as a function of ft and oat (including the effect of defrost) 

Think you can find the data, if willing to search, but it should be freely available to anyone, it's not exactly that much data. 1 table says it all, or 2 tables to include cooling.

A lot of heat pumps are just name badge engineering, the issue is the company willing to slap a badge on someone else product, is just playing, not really that involves or really that bothered. 

Our heat pump is a Maxa, they are the actual manufacturer, was and still is badge engineered by many big companies including Viessmann.

Posted by: @johnmo

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Posted by: @jamespa

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What I do think is missing, however, is the detailed design tables specifying max and min output as a function of ft and oat (including the effect of defrost) 

 

 

Think you can find the data, if willing to search, but it should be freely available to anyone, it's not exactly that much data. 1 table says it all, or 2 tables to include cooling.

That's my point really, it should be both freely and readily available.  As you say it's just a table (or set of graphs) of cop and output vs ft and oat at max and min modulation frequency, and maybe a couple of intermediate frequencies for good measure.  Mitsubishi can manage that, why not everyone?