@jamespa 

Heat pump as per my footer, it has an option to use PWM via a cable to the pump. Below is todays graph.It only appears to change the pump speed when starting up and then it settles down to a steady flow. I was operating it at a fixed speed before I changed my pump to one that supported PWM.

Thanks.  To me it looks like pump speed follows compressor modulation (which makes some sense if it is trying to maintain roughly a fixed DT across emitters). 

As  matter of interest why did you change the pump to PWM and did it make a difference?

@jamespa I had a discussion with iancalderbank 

https://renewableheatinghub.co.uk/forums/thermostats-trvs-pumps-valves/pwm-pump-or-not#post-18612

Hard to tell what difference it has made as I was still in the throes of setting things up. 

@jamespa

Hi. Information as requested.

1. Samsung Split Gen 4 with Grundfos UPM3 PWM pump integrated in the internal unit with PHE, running for 10 years.

2. Yes.

3. It MODULATES the flow according to heat load to maintain a steady dT of 5C, which I have given up trying to explain.

4. Yes.

5. No.

I have a UFH system for 200 sqm house with multiple loops, all in parallel. The HP connects directly to the manifold without mixer, and with one flow meter and thermometers for main F/R and all loops. Some secondary loops are kept open to meet minimum flow requirement. The rest, for principal rooms, are controlled by Heatmiser thermostats to deliver the precise amount of heat within the residual modulation range and gives flexibility of programming. As zones turn on and off randomly, the HP modulates its water flow to maintain 5C dT across loops, providing long running periods. The main issue is the Samsung mechanical 7 l/min flow switch which controls the flow alarm and is fitted to all models, even today it seems. This determines the minimum flow @2.5kW which restricts the range of smaller units which have the ability to modulate lower. A 5 l/min or less flow switch does not seem to be available on the market so other tricks have to be made. Samsung could be more intelligent by designing an electronically measured flow meter.

Thanks that's helpful.

Posted by: @heatgeek

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The main issue is the Samsung mechanical 7 l/min flow switch which controls the flow alarm and is fitted to all models, even today it seems. This determines the minimum flow @2.5kW which restricts the range of smaller units which have the ability to modulate lower.

So what do the smaller units do when the demand is <2.5 kW. If the compressor can and does go lower (which you imply it can) but the flow rate remains high, the DT will fall.  Is this what it does? 

Or does it stop the compressor modulating down (even though it could modulate down further), and thus cycle prematurely?

The latter seems bizarre but you seem to imply is the behaviour.

A separate question, but do the latest Samsung models still use hot gas bypass to extend the compressor output range?

Posted by: @jamespa

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So what do the smaller units do when the demand is <2.5 kW. If the compressor can and does go lower (which you imply it can) but the flow rate remains high, the DT will fall.  Is this what it does? 

Or does it stop the compressor modulating down (even though it could modulate down further), and thus cycle prematurely?

The 4kW unit can modulate down to 1.5kW or just below 5 l/min @ 5C dT. However, the flow switch will throw a E911 low flow alarm if you try and go below 7 l/min, which is an aberration. Therefore, one has to maintain a minimum flow >7 l/min (2.5kW @ 5C dT) under all circumstances until cycling, or, in extremis, you have to start to siphon off up to 2 l/min flow after the flow switch e.g. auto bypass valve to top of volumiser on return leg. One can also play some other tricks e.g. increase flow on open loops by dropping dT to 3C and increasing dT on other legs to maintain average 5C, leave an open loop in lounge where there are multiple loops, etc. However these are all nauseous workarounds because of Samsung stupidity. The same flow switch is used for higher power units so the problem is not so acute or non-existent. The Daikin would seem to be better at this as it can modulate down to 5 l/min when running (as confirmed by @toodles) but can have a minimum startup flow of 12 l/min, as per diagram, just to test you. It has other quirks too, according to matt-drummer. Nothing is perfect, but the principle is the same.

The above presupposes also that WC is used. The simple Samsung WC tracks OAT linearly degree for degree for the UFH so there is no issue. For rads, the low end @ 15C OAT would need turning up. It seems to me that nothing much has happened in 10 years except for safer GWP refrigerants, higher DHW temperatures and more colourful controls.

Posted by: @jamespa

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A separate question, but do the latest Samsung models still use hot gas bypass to extend the compressor output range?

I have no idea.

Thanks, thats helpful

Posted by: @heatgeek

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It seems to me that nothing much has happened in 10 years except for safer GWP refrigerants, higher DHW temperatures and more colourful controls.

And painting the outside black...

The UI for many makes has not, IMHO, been designed or developed with the user in mind.  I have no idea why they aren't spending more time on this as its fundamental that you need to present the user with a control interface they can understand, but which 'under the hood' does the right thing.  Some of the European manufacturers have arguably done better than some of the far eastern ones, presumably because they also make boilers.  However I've yet to read an instruction manual (and Ive read a lot) that describes a UI which makes controlling these things simple.  It creates an opening for the likes of Homely, but for bits of kit that sell for 2-4K that should not be necessary!

@jamespa Agree entirely.

1. Samsung AE080BXYDEG with MIME03EN
2. Yes, PWM output 
3. It is meant to reduce the flow rate to maintain a set DT (which is adjustable). It does a pretty poor job if this and the implementation is poor. I have added a "man in the middle" controller to watch what PWM signal the Samsung puts out, modify the signal (to adjust for DHW or Heating modes and apply a suitable offset and to lower the flow rate during valve position changes to minimise water hammer) and then control the Grundfos UPM pump.
4. No
5. Yes

@arundalep thanks, very interesting.