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: @Anonymous

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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?

This output rating is very much of interest since a digitally altered HP may possess all the limitations of the higher output model such as minimum output operation. I believe this may be the case for the 6kw Mitsubishi which has similar lower output limit of the 8.5kw model.

another reason for knowing the true design output is relevant since it would appear that most of not all HPs appear to be operating at their best COP when operating at 40% level of output, with poorer COPs above and below that level. So how does anybody target their best output/efficiency level?

So the question becomes - 40% of what?

(info courtesy of @jamespa post of the John Cantor cop chart) 

Posted by: @sunandair

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best COP when operating at 40% level of output

There are various ways to look at things related to CoP. Best CoP doesn't always mean the lowest running cost. Things like PV generation, time of use tariff all play a part.

I encourage mine to run at 33 to 35 degrees (WC setting starts at 27 degs), when I have decent battery charge and producing from PV, via a double set point volt free contact. So effectively get a cop of 4 to 5 while running. During this period it's also playing catch-up trying to heat a big slab of concrete, so runs at full load, does not stop running for an hour or two or more. In shoulder months I can nearly heat the house for free, but daily CoP is about 3, because I leave the circulation pump on 24/7 to balance the heat around the house. But best part of zero cost, so not really caring.

WC when set up the heat pump isn't playing catch up, so by default will firstly try to establish return temperature while maintaining dT, then will modulate to maintain a fixed or variable dT, depending on heat pump. Trying to achieve best efficiency point.

I wouldn't try to overthink it, are you on a time of use tariff, run hotter during these periods if you can or charge battery if you have one or both. Not on time of use run 24/7 on WC.

I am currently cooling,.but run that on almost straight WC with a 1 Deg boost setting when there is excess solar, otherwise I run could into condensation issues.

@johnmo 

I think we’ve been trudging through the same black hole of heat pumps for roughly the same time.

My post above wasn’t really about user tactics for settings, as you have kindly offered, although your combination of tariffs etc looks like it works well.

My comments above are really aimed at the manufacturers role in providing useful product performance data so any or all homeowners and users can make informed decisions about how to set up and get the best out of their HP. 

It is continually surprising to me that so many pitfalls from design, pipework, buffers or cycling seem to still exist around installing and operating a heat pump, when it should be seen as a trouble free transition. This must put a slur on the image of heat pumps at a time when the image should be bright and optimistic.

So in the above case the fact that HPs have a peak efficiency at a certain modulation which may be higher than the minimum output would be useful to know when anyone wants to set up a WCcurve.

Yet it’s not conveniently catalogued by most manufacturers in a format for homeowners literature so they can make use of it.

here is the chart from John cantors video on cycling which shows varying efficiency at different percentage outputs which I think is helpful.

On a personal level I think we’ve got ours set up as good as we can without making material changes since it’s an old house with an all radiator setup. We are also using thermal solar for water at this time of year but we don’t have PV as yet.