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Heat pump delta T too low - 2c

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(@derek-m)
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Posted by: @william1066

@scrchngwsl That is a good question, one I am trying to work out for myself.  Ideally you need to know your target flow rate to start.  This simple number is not so easy to get as it is likely to be variable, and ideally set by the heat pump controller and depend on a number of factors.

Assuming you have a flow rate range that will be required for the majority of the time.  I guess you can then look ats the spec sheet of the zone valve vs the full bore diverting valve and do some maths on cost, some experiments on noise etc, look at your pump head spec etc. 

A non trivial exercise which I need to undertake, and which could take some time.  As a diy'er I tend to save time by doing rough calculations and over spec, not ideal, but one way to approach it.  That is what I did with my pump and actually it turned to be only just big enough.  I calculated 6.5 mh2o, it seems the pump needs close to 10mh2o pressure to get to 30 litres per min.  And this is a bit too noisy for me right now, so I am looking at options (including moving the pump outside the house)

Why do you require a flow rate of 30 litres per minute? Having a DeltaT of only 2C would indicate that the flow rate is too high for the present heat demand. The quantity of heat energy being transported from your heat pump into your home is a product of the DeltaT and the flow rate, to transfer the same quantity of heat energy with the normal DeltaT of 5C would be achieved with a much lower flow rate.

If you have a flow rate of 30 litres per minute with a DeltaT of only 2C, but you cannot achieve the desired indoor temperature then the problem is a lack of heat energy from your heat pump, or incorrect settings and/or losses within your system.

 


   
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(@william1066)
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@derek-m  mostly because that is what the documentation that came with the heat pump says, and I quote

"... and 30l/min on the 12 and 16kW unit."

Although .... the Samsung manual says the E911 fault will trigger if flow is less than 12 l/min for 15sec (30sec if starting). 

At the end of the day I am looking to make this as efficient as possible, ignoring the fact that to meet the heat loss of the house at -2 degC I would need delta T of 7 and flow of 30 l/min (15kW).  lps * 4.18 * dT (0.5*4.18*7 = 15ish). 

My understanding is to maxi mise efficiency I need to minimise flow temperature, which means maximising flow and minimising delta T.

Right now with a dT of 3 degC our dining/kitchen is pretty warm, despite being -4 outside.  But yes, I need to understand this a bit better, taking a holistic view on this rather than focusing on just 2 parameters.  Thanks for your insights and challenges, much appreciated, I need to invest a bit more time in understanding this.

 

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(@william1066)
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Decided to check the specs of one of my Zone valves vs ESBE @ 45 lpm - so about 4x

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At more modest flow rates there does not seem to be much difference

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(@derek-m)
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Posted by: @william1066

@derek-m  mostly because that is what the documentation that came with the heat pump says, and I quote

"... and 30l/min on the 12 and 16kW unit."

Although .... the Samsung manual says the E911 fault will trigger if flow is less than 12 l/min for 15sec (30sec if starting). 

At the end of the day I am looking to make this as efficient as possible, ignoring the fact that to meet the heat loss of the house at -2 degC I would need delta T of 7 and flow of 30 l/min (15kW).  lps * 4.18 * dT (0.5*4.18*7 = 15ish). 

My understanding is to maxi mise efficiency I need to minimise flow temperature, which means maximising flow and minimising delta T.

Right now with a dT of 3 degC our dining/kitchen is pretty warm, despite being -4 outside.  But yes, I need to understand this a bit better, taking a holistic view on this rather than focusing on just 2 parameters.  Thanks for your insights and challenges, much appreciated, I need to invest a bit more time in understanding this.

 

image

 

The heat loss from your home is dependent upon the temperature difference between inside and outside. This can vary quite considerably throughout the day.

Maximum efficiency is normally achieved when the heat pump is supplying just the correct quantity of heat energy, at the lowest LWT, to meet the heat loss. In most systems the LWT is set as a fixed value or by means of weather compensation, with a few offering indoor temperature modulated control.

Let's consider what happens in weather compensation mode if the WC curve is a little too high. The vast majority of heat pumps are designed to operate with a DeltaT of 5C between LWT and RWT.

If the required LWT is say 35C, with a corresponding RWT of 30C, at a water flow rate of say 25 litres per minute, this will meet the present heat loss and keep the indoor temperature at approximately 20C. But if the actual LWT is 38C, with a RWT of 33C, and water flow rate still at 25 litres per minute, the heat emitters will produce more heating than required, and the indoor temperature will start to increase. As the indoor temperature increases, the difference between the heat emitters temperature and the indoor air temperature will decrease, so the quantity of heat energy actually transferred from the heat emitters will start to decrease. There is too much heat energy going to the heat emitters, but the heat emitters cannot dissipate all of this heat energy into the rooms, so the RWT will start to increase.

There are two ways in which the system could be re-balanced, one being to lower the LWT, but this is fixed by the WC curve, the other method is to lower the water flow rate, so that there is less heat energy being transferred each minute.

I'm afraid the statement below is not fully correct. 

My understanding is to maxi mise efficiency I need to minimise flow temperature, which means maximising flow and minimising delta T.

Lowering the LWT should indeed improve efficiency, but this should be at the minimum permissible water flow rate, at the designed DeltaT of 5C.

I appreciate that this is a difficult concept to understand, and equally difficult to explain, since there are quite a number of different parameters which are continually balancing and re-balancing.

The important thing is to adjust the WC curve to match the heat loss of your home as closely as possible. The heat pump controller will then be in a better position to control the LWT, and also vary the speed of the water pump to maintain a DeltaT of 5C. If the heat pump controller cannot actually control the water pump, then the DeltaT could vary outside the designed limits.

 


   
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(@benseb)
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Ah this may make sense why my Ecodan struggles to maintain DT5 when at milder temps. The circulation pumps are either on or off so no modulation of flow rate. 

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


   
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(@ajdunlop)
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Posted by: @derek-m

The heat pump controller will then be in a better position to control the LWT, and also vary the speed of the water pump to maintain a DeltaT of 5C. If the heat pump controller cannot actually control the water pump, then the DeltaT could vary outside the designed limits. 

How many Heat Pumps can actually vary the pump speed.

I know the Ecodan can't. It seems odd that they all can't control the pump speed to maximise efficiency and have something else they can vary to match the output to what is needed.

 


   
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(@derek-m)
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Posted by: @ajdunlop

Posted by: @derek-m

The heat pump controller will then be in a better position to control the LWT, and also vary the speed of the water pump to maintain a DeltaT of 5C. If the heat pump controller cannot actually control the water pump, then the DeltaT could vary outside the designed limits. 

How many Heat Pumps can actually vary the pump speed.

I know the Ecodan can't. It seems odd that they all can't control the pump speed to maximise efficiency and have something else they can vary to match the output to what is needed.

 

Someone can correct me if I am wrong, but I was under the impression that the Ecodan has an internal water pump, the speed of which is controlled by the FTC controller.

If your system also has external water pumps, it may be possible for these also to be controlled using Pulse Width Modulation (PWM), but only if the water pump has this capability.

Please provide more details of your system.

 


   
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(@william1066)
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@derek-m I am back on this topic.  So I looked at Grahams post on the carnot cycle, and some other stuff.  And flow rate does not come into the efficiency formulas that I can find.  Using Grahams post on theoretical efficiency I get the following.  It seems to me the focus for efficiency is ultimately a focus on the LWT [direct system, with no mixing, and low temp emitters etc].  Of course as you increase your flow rate, you can reduce [to a point] your LWT to meet your heat load.  (assuming constant dT and suitable emitters).  Hence my obsession with flow rate.  I think that the delta T is really dictated by your emitters as it does not seem to be a parameter wrt efficiency calculations, at least not directly.

image

   
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(@sunandair)
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Posted by: @ajdunlop

How many Heat Pumps can actually vary the pump speed.

I know the Ecodan can't. It seems odd that they all can't control the pump speed to maximise efficiency and have something else they can vary to match the output to what is needed.

Hi your statement has spiked my first hand knowledge pot. 
When was your HP fitted?

how do you know pump speed cannot be controlled?

have you located your pump speed setting  on the “service screen?

Is it located in the “service screen” menu?Or is it located in the “auxiliary settings screen”? 

 

 


   
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 robl
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@william1066 

I think that you're right - dropping the LWT always improves the COP of the compressor.  The DeltaT is pretty much irrelevent to the compressor - inside the heatexchanger there is a gas->liquid phase change at a precise temperature, and that's close to the LWT, irrespective of DeltaT.  This is different by the way to a gas boiler - there it's the return water temp that counts, as it's not extracting heat primarily from a phase change.   

I think the reason why 5C is bandied about is a rule of thumb.  If your DT is less than 5, it's likely that the pump driving the waterloop is taking a lot of power, and the overall compressor+fans+pumps COP is lower than it could have been.  A heatpump with inbuilt pump, or PWM control of a pump, may well very the speed of the pump to hit DT=5C for this reason.


   
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(@derek-m)
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Posted by: @william1066

@derek-m I am back on this topic.  So I looked at Grahams post on the carnot cycle, and some other stuff.  And flow rate does not come into the efficiency formulas that I can find.  Using Grahams post on theoretical efficiency I get the following.  It seems to me the focus for efficiency is ultimately a focus on the LWT [direct system, with no mixing, and low temp emitters etc].  Of course as you increase your flow rate, you can reduce [to a point] your LWT to meet your heat load.  (assuming constant dT and suitable emitters).  Hence my obsession with flow rate.  I think that the delta T is really dictated by your emitters as it does not seem to be a parameter wrt efficiency calculations, at least not directly.

image

The quantity of heat energy produced by a heat pump is given by the formula:-

Q = mcDT

Where Q = Heat Energy, m = Mass, c = Specific Heat Capacity and DT = Change in Temperature.

Since the volume of 1kg of water is approximately 1 litre around the temperature of a central heating system, it is accurate enough to assume that 1kg of water occupies a volume of 1 litre.

The above formula could therefore be re-written:-

Q = lcDT     where l = litres.

The Specific Heat Capacity of water in the temperature range for central heating is approximately 4184 Joules, which equates to approximately 1.16 Watts of heat energy required to raise the temperature of 1kg of water by 1C.

If a heat pump is receiving water at a RWT of 30C, and is heating it by 5C to a LWT of 35C, it will require 1.16 x 5 = ~ 5.8 Watts per litre.

The same would be true if the RWT is 40C and the LWT is 45C.

So using the above formula, the determining factor is the volume of water being heated.

If the flow rate is 10 litres per minute, then the volume of water each hour will be 600 litres.

So to heat that quantity of water will require 600 x 5.8 Watts of energy, which is approximately 3.487kWh.

If the flow rate is now increased to 20 litres per minute, the quantity of energy required is now 1200 x 5.8, which is approximately 6.973kWh.

So from the point of view of heat energy produced, the water flow rate is a prime determining factor.

Heat pump efficiency is quite a simple calculation, since it is 'energy out' divided by 'energy in', but in the real World it is a little more complex.

Whilst 'energy in' can be obtained with reasonable accuracy using electricity power meters, 'energy out' is more problematic, since it requires reasonably accurate measurement of LWT, RWT and water flow rate.

Although, as stated above, it is the DT which determines the quantity of heat energy produced, rather than the actual LWT, the LWT determines how much 'energy in' is required to produce that quantity of 'energy out'. Because a heat pump has to work harder to provide higher LWT, it must therefore increase the 'energy in'. This increase in 'energy in' is not directly proportional to 'energy out', but actually increases as the LWT is increased.

Increased LWT being supplied to a home, will increase the quantity of heat energy being transmitted by the heat emitters, which in turn will require the heat pump to produce more heat energy, which is normally achieved by increasing the water flow rate.

If the heat loss, and hence heat demand, of a home is reasonable well known, it may be possible to obtain an approximation of heat energy produced by a heat pump, utilising reasonably accurate measurement of indoor temperature, outside temperature and LWT, though I doubt that this would be as accurate as measuring the actual 'energy out'.

 


   
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(@william1066)
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@sunandair

> how do you know pump speed cannot be controlled?

Yes my HP can control the pump, but I did not know that at the time as I was following non Samsung instructions.  I found out later, from someone on this forum PWM is supported (Samsung Gen6).  Am going to rectify that this coming weekend.  (had to buy a PWM to analog converter as my [expensive magna3] pump expects a 0-10v signal).


   
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