@heatgeek thanks! I contacted them too.

Posted by: @heatgeek

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From the data tables attached, it is clear that the Vaillant has a 14.474 l/min flow or 0 flow (5kW unit). The Daikin has current average flow of 6.54 l/min with peak flow of 23.835 (8kW unit). Both units are outputting around 2 kW.

Unfortunately for your argument the best performing Daikin on heatpumpmonitor is the one owned by matt-drummer and from how he explained his control setup it is very much not following a 5C dT! Because the Dakin controller tries to maintain a fixed dT and oscillates badly as a result, he uses a fixed flow temp that he controls from the phone app (no weather compensation at all!) and has set the dT to 10C, which with the flow temp typically being 29-31C it can't achieve and so the flow rate is floored at its minimum (you'll notice that it is always about 7 lpm). So there might be examples of well performing heat pumps that actively maintain a 5C dT but this isn't one.

For another point of reference, here's a fixed flow rate Mitsubishi running at 2kW output and dT of around 2C just fine without cycling.

I received the new upgraded quote from SmartHeating (HeatGeek). The Maxflow is now at 45C instead of 47.

What's interesting is that I now have a proportionally higher efficiency (360 vs 330%) though the estimated saving is negligible (15 vs 13£ - though relevant when cumulated over time). Also, the cost is now 7,838 vs 8,000£ (I was expecting more than less...) I will share the pdf version as soon as I get it. Still waiting for the updated VBaillant-based quote from Sustain...

Posted by: @eliuccio

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Also, the cost is now 7,838 vs 8,000£ (I was expecting more than less...) I

How odd.  Or maybe they have now realised that they are in a competition!

Posted by: @jamespa

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Water Pump modulation

Sorry but again I think you are mistaking modulation of water flow rate with modulation of heat output and then making linkages which have no apparent basis and statements based on what others have said without any supporting argument or evidence.  You have provided a lot of data, but I apologise that I still cant follow your argument that maintaining a particular deltaT specifically of 5C is advantageous.  Perhaps you could summarise why this would be the case in a sentence or two in order than can better understand what you are trying to say.  

Apologies for the delay in response due to other heated issues (pardon the pun).😊  Oh boy, why is this so hard? I am not mistaking modulation of water flow rate with modulation of heat output. Modulating flow rate does modulate heat output! To reiterate, heat delivery in kW is governed by the formula

Water Specific Heat (4.2) x Flow Rate (l/sec) x delta-T (C)

I have shown by this calculation that a Daikin HP modulates its heat flow by maintaining a constant 5C dT (as confirmed by @toodles) and modulating the water flow rate. The Vaillant (and others) has a fixed high water flow rate matching its peak power output and seems to modulate the heat output through changing the dT (lowering it). They can both deliver the same amount of heat using two different techniques and, some will argue, with the same efficiency, regardless of dT. Brendon Uys will probably disagree. The question is, however, what is happening at the radiator? The Daikin maintains a 5C temperature differential across the radiator, assuming also that flow temperature varies with WC. What happens if the differential drops to 2C? NOTHING? Also, turning down the pump speed means much less energy consumed than running at top speed all the time and is significant. The problem becomes more acute with the Vaillant if you over-size it. The fixed flow rate is then much higher therefore the modulation range is more restricted by dT variation only.

Posted by: @jamespa

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You do not provide any evidence for the assertions or conclusions nor do you know how this particular machine was controlled or what the circumstances were (for example the sensor may be in a room subject to lots of solar gain!).

In terms of control strategy, the title says it all. Uncontrolled or controlled, which is better in this instance, regardless of circumstances? A system varying room temperatures from 25C to 20C means heat is being delivered for an average room temperature of 22.5C (given the wide timeline shown) as opposed to an average of 20.5C for thermostatic control with 1C hysteresis. I am not going to argue as to which consumes more energy and which is more comfortable with varying conditions, e.g. solar gain, cooking, entertaining, etc.

As regards weather compensation, I do not need educating on what WC is and have not disregarded it. WC is taken for granted in terms of flow temperature but has NO effect on the type of modulation. WC is straightforward if you have a UFH system as temperature variation can be linear but it is a bit more complicated if you mix UFH and radiators. Maybe, playing with multiple WC curves may have more to do with varying rad dt’s….or not??

As regards heat pump sizing, I was being naughty by being facetious a la Brendon. Apologies.

Posted by: @heatgeek

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Modulating flow rate does modulate heat output!

Thats simply not true (to any material extent), which is where you are going wrong!.  Essentially you are mixing up cause and effect.

The heat output of the heat pump determined by the compressor.  This (through the primary control loop) responds to the heat lost by the radiators, or more accurately it responds with the objective of keeping the flow temperature constant and equal to the target flow temperature, which (due to conservation of energy) requires an amount of heat equal to the sum of the heat lost by the radiators and any 'lost' heat as the water passes through the pipework etc.

The heat lost by the radiators is dependent on the difference between radiator temperature (to first order flow temperature) and room temperature.  So in reality the heat output is determined by the radiators and the flow temperature.  The amount of heat the radiators lose (determined to first order by the flow temperature and room temperature), together with the flow rate, result in a deltaT which must satisfy the equation you quoted.  

Put another way, the heat pump cant force the radiators to deliver more heat by somehow increasing deltaT, in fact the opposite happens, if deltaT increases the radiators deliver slightly less heat.  Likewise the heat pump cant force the radiators to deliver less heat by somehow reducing deltaT, in fact the opposite happens, if deltaT decreases the radiators deliver slightly more less heat.  

Posted by: @heatgeek

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they can both deliver the same amount of heat using two different techniques and, some will argue, with the same efficiency, regardless of dT. Brendon Uys will probably disagree

True  As set out above its driven by the radiator not the heat pump, the heat pump merely responds to the heat lost by the radiator, it cant force the radiator to deliver more heat by throwing heat at it!

Posted by: @heatgeek

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the question is, however, what is happening at the radiator? The Daikin maintains a 5C temperature differential across the radiator, assuming also that flow temperature varies with WC. What happens if the differential drops to 2C? NOTHING?

Not sure what you mean here by nothing.  The radiator emission (for the same flow temperature) will be slightly more if the differential drops to 2C, because it is proportional to (average radiator temperature)^1.3.  This is why, with a lower deltaT, you can operate at a (slightly) lower flow temperature for the same output, and thus higher thermodynamic efficiency.

Posted by: @heatgeek

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Also, turning down the pump speed means much less energy consumed than running at top speed all the time and is significant.

The first bit is true, the statement that it 'is significant' may or may not be.  In a very low loss house the water pump energy is certainly significant.  In a house with a higher loss it becomes insignificant - so for example much of the time my 7kW house runs at 4kW.  With a COP of 4 that means an energy consumption of 1kW.  The water pump is 50W tops, so a slight increase in the power consumed by the water pump is quickly dominated by a relatively small improvement in COP due to lower DT.  

There is actually an analysis of this over on openenergymonitor   Their presumption, rightly or wrongly, is that the equation is a straight trade off between the improvement in COP resulting from a lower DT, and the increase in energy consumed by the pump because it has to run faster to achieve a lower DT.  The implication is that they haven't heard of or don't believe the argument made by Brendon Uys which, given that he hasn't substantiated it with an explanation or evidence, is a fair position to take).

Posted by: @heatgeek

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The problem becomes more acute with the Vaillant if you over-size it. The fixed flow rate is then much higher therefore the modulation range is more restricted by dT variation only.

Modulation range is determined by the compressor not by flow rate variation.  You are confusing two things.  Oversizing any heat pump will reduce the ability to deal with lower demand scenarios.  Many heatpumps (currently) have a modulation ratio of about 3 so for these the behaviour will be quite similar. 

There are two principal classes of exception to the 'about 3' number:

• Some heat pumps have software limited outputs, for example the Daikin 8kW model is also sold (under a different model code) with a lower maximum output, but this is just a software limit.  This class of heat pumps will have a lower modulation ratio (many manufacturers do this trick, not just Daikin) because the minimum output is the same for all of the software limited model variants.
• At least one heat pump, the Mitsubishi R290 8kW pump, actually has two compressors (in this case a ~6kW variable output compressor and a 2kW fixed output compressor).  This has a larger modulation ratio of about 4. 

Posted by: @heatgeek

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In terms of control strategy, the title says it all. Uncontrolled or controlled, which is better in this instance, regardless of circumstances? A system varying room temperatures from 25C to 20C means heat is being delivered for an average room temperature of 22.5C (given the wide timeline shown) as opposed to an average of 20.5C for thermostatic control with 1C hysteresis

Nobody is suggesting running any heating without control, other than perhaps a log burner.

However I am not sure what two cases you are trying to compare here.  By 'not controlled' do you mean controlled by weather compensation and are you asserting that this results in large temperature swings?   If so how do you come to this assertion please (it is not in general true)?  If this is not what you are saying, please clarify the two cases you are comparing.

@eliuccio

I was wondering how things are going with the quotes.

Following the rather esoteric discussion about water pump modulation I thought it might be worth repeating something said very early on, which may have got lost.

System design matters more than choice of unit.  In particular you want it to be designed for low flow temperature and a direct connection from the ASHP to the emitters (ie no buffer, low loss header or plate heat exchanger).  These two factors must take priority over choice of unit.

If you havent already I would ask about buffer/LLH/PHE.  A 2 port volumiser is acceptable, a 3 or 4 port buffer, PHE or LLH not.