@technogeek 

Again, Many thanks for your input.

Given two years of frustration with my Samsung Heat _Pump I am, now reduced , to  constantly attacking myself over my Heat-Pump Fiasco.

The Discovery that my apparent "Short Cycling", was,  in fact , principally the result of the Thermostat settings has left me viscerally  attacking myself .

Not a good place to be, cursing oneself over a Heat Pump!

What Now ?

Your comment to Derek_m is especially appreciated :

"I appreciate what you are saying and in an ideal world it would be best to remove everything and start again however I get the impression that is not an option and all we can do is try and improve on what we have.

Given the £14,000 spent on an  incompetently Installed Heat Pump, I cannot , ultimately,  blame anyone other than myself for this fiasco.

I have learnt to trust no-one, especially myself !.

I cannot , as a 76 year old , therefore , confidently, remove and or replace  the Heat Exchanger , Buffer-Tank or Hot-Water Tank.

With the "Short Cycling issue " now resolved and with the improvements in COP resulting from Water flow reduction I am a wee bit more positive.

Water Pump Control ? :

Question 1:

For a Samsung Heat Pump should I control the pipe water flow using the INV Pulse -Width -Modulation inputs to the Grundfoss Motors? Case 1 in the Samsung Installation instructions. The PWM Board Output is faulty .

Question 2:

For a Samsung Heat Pump should I control the pipe water flow using the ON-OFF Relays controlling the Grundfoss Motors? Case 2 in the Samsung Installation Instructions.

A visiting French-Samsung Engineer advised the use of the ON_OFF Relays , case 2 in the installation instructions BUT with the installation of a more substantial Relay in Series with the Control Board Motor relay as shown in the Installation Instructions.

Excessive ON_OFF SWITCHING would, he said, result in the destruction of the on board Relays.

The French-Samsung Engineer also said that "He did not understand the INV PWM Option, case 1, saying that "Only the Germans really understood the PWM System!".

My System is fitted with Both INV PWM and ON_OFF Relay Water Control.

PWM Water Pump Control  using an Arduino:

I have successfully increased the Delta_T across  the start of the Radiator Water Circuit by reducing , and controlling , the Water Flow rate using an Arduino PWM output port.

Samsung Control Board replacement ?

Given the faulty PWM output from the Samsung Control Board should I replace the control Board?

@technogeek

Again, many thanks for your understanding and help.

Kendra and Parallel Buffer Tanks:

Buffer tanks and "Octopus "Cosy" Tariffs:

Because of the costs involved in the replacement of the Heat-Pump, Hot Water-Tank, Heat-Exchanger and buffer tank  I will have to use the system in it's current configuration.

I will use the Buffer tank , in it's current Parallel configuration to Store heat when the Electricity supply is cheapest releasing the Heat when Electricity is most expensive, using the Octopus "Cosy" Tariff  .

Buffer tanks and Back-up winter heating:

Given excessive Electricity consumption  of my Samsung when the outside Temperature falls below +3 C ,  I will use a back-up Immerser Heater inserted into the Buffer Tank.

Water-Pump Control:

I am increasing the Delta_T Temperatures across both the Primary and Secondary Ports of my heat Pump by reducing ( and controlling) the water flow rate using an "Arduino" PWM port.

The PWM signal to the Primary Heat-Exchanger port is calculated from the  Delta_t across the Primary Heat-Exchanger port.

The PWM signal to the Secondary , Radiator Heat-Exchanger port is calculated from the  Delta_t across the Secondary, Radiator Heat-Exchanger port.

Any suggestions for the PWM Signal would be appreciated.

Control-Board replacement:

Given a  faulty PWM output signal , and  the damage inflicted on the Control Board Motor Relays by Two years of hard ON_OFF Switching I will have to replace the Control Board.

@iantelescope

Posted by: @iantelescope

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Given the faulty PWM output from the Samsung Control Board should I replace the control Board?

Given the performance of your past installers, what are the reasons for concluding the PWM section of the Samsung board is faulty?

I am assuming when I ask these questions that you have Samsung installation manuals etc, but has the wiring been checked to see if the primary pump is correctly connected to the board and the INV 100% Pump option has been selected in configuration (#4051 = 1)? There is a note in my manual that says "If wrong wiring between PWM and reference, INV Water Pump may not work or wrong operation".

As far as I understand (but could be incorrect), PWM water pump control is to adapt the flow rate to cater for changes in your hydraulic circuit such as the operation of mixing valves, underfloor heating zones switching in and out etc, that would drastically change the Delta T value your heat pump sees. If like myself, you have a set of balanced radiators that do not have any TRV's operating on them, then you are going to have a fairly constant flow rate. With the correct flow rate set and constant, you will only need to change the water temperature thus maintaining a fairly efficient COP throughout the outside temperature operating range. So I guess its up to you, if you just have balanced radiators then set appropriate flow rates in the primary and secondary circuit pumps and leave them as is.

From my understanding of your plumbing diagram, you have a buffer tank in parallel with your heat exchanger. I would as a minimum recommend the buffer being moved to be in series in the return pipework (I do not think it is needed as I cannot see what it does to improve things being in the return pipe, however others on the forum disagree so I would put it in as it will not make things worse and you have covered yourself). Moving the buffer should allow maximum amount of heat to be transferred to your secondary circuit before entering the buffer.

I will be out of the country for the next 10+ days so will not be able to respond to further messages

Cheers

@iantelescope

I suspect that for the Kendra buffer tank arrangement to function correctly, there must also be a water pump located within the shown heat pump unit, otherwise the return water from the heat emitters would flow through both the buffer tank and heat pump PHE, causing mixing and reduction in water temperature and efficiency. There would therefore need to be two water pumps in the primary circuit of your system, rather than just the one that you have shown.

Because your schematic shows a water pump prior to the buffer tank, but does not show one after the buffer tank, prior to the heat exchanger, then the water in the primary circuit coming from the heat pump will flow through both the buffer tank and the heat exchanger.

It may be useful to put in some figures to try to explain what may be happening.

If the system starts from cold, with a desired LWT of 40C, a desired heat pump DT of 5C and a water flow rate of 10 lpm, then the heat pump should be developing approximately 3.5kW of thermal energy.

If, for simplicity, it is assumed that 5 lpm flows through the buffer tank and 5 lpm flows through the PHE, then 1.75kW of thermal energy will be going into the buffer tank and 1.75kW of thermal energy will go into the PHE. If it is assumed that the water pump in the secondary circuit is running and the heat emitters are fully functioning, then the 1.75kW of thermal energy will be transferred by the PHE from the primary circuit to the secondary circuit. The remaining 1.75kW of thermal energy will start to heat up the 50 litres of water within the buffer tank. If the water flowing into the top of the buffer tank is at a rate of 5 lpm then it will take approximately 10 minutes to push the cooler water from the buffer tank and replace it with the warmer water coming from the heat pump. Obviously in the real world the water coming from the heat pump will not instantly jump in temperature to 40C, but I assess that within 15 to 30 minutes the water going into the buffer tank and also coming out of the buffer tank could be at a temperature of 40C. The buffer tank is therefore no longer presenting a load to the heat pump.

If the PHE is still presenting a load of 1.75kW to the heat pump, then the heat pump must reduce the compressor speed to try to match the output to this reduced demand. Once the compressor reaches minimum speed then the only remaining option is to switch off and on, i.e. cycle.

If you have isolation valves on the inlet and outlet pipes to the buffer tank I would suggest that you close one of these valves to stop the water flow through the buffer tank. This should therefore force the full primary flow through the PHE, thereby transferring the maximum amount of thermal energy to the heat emitters and also providing the maximum loading on the heat pump.

@derek-m

Many thanks for your thoughtful , and thorough ! explanation.

Using my Electrical background , could I paraphrase your explanation by saying that the Buffer tank is shorting the Heat Exchanger?

I may have misread, or misunderstood  Kensa,  in his statement that " only under very light load will most of the flow enter the buffer- so the buffer only comes into play when actually necessary".

The Actual Quote from Kendra:

A better arrangement (and the Kensa recommended way), is to connect a buffer tank using two
connections only. This will prevent most of the flow passing through the buffer under most
operating conditions, so minimising mixing, only under very light heating load will most of the
flow enter the buffer – so the buffer only comes into play when actually necessary.

Kendra's Jpg & PDF on Buffer tanks:

Buffer tank Positioning:

Kendra's quote No 2:

The unwary could easily think that fitting a thermal store was a good substitute for both a buffer
tank and a hot water cylinder, but when sourced from a heat pump this is not a good idea. Although
the thermal store will usually function OK as a buffer, they don’t make good partners for heat
pumps as all the space heating load then has to be served at DHW production temperatures – this
will typically lead to very poor efficiency and result in high electricity bills.

Again , I am probably mistaken , but Kensa is recommending that :

they don’t make good partners for heat
pumps as all the space heating load then has to be served at DHW production temperatures – this
will typically lead to very poor efficiency and result in high electricity bills.

My Buffer was installed at the input to the Heat Exchanger to avoid using Water at the Temperature of the Hot Water , ~52 C.

Buffer Valve:

My System is fitted with a single valve to use the Buffer , or Not.

I Do not need to remove the Buffer tank, but would do so if I could afford it.

Cost:

I would like to remove the Heat Exchanger , the Buffer Tank , and the Hot Water Tank.

I would like to replace the Heat Pump with the 6.5 kW Heat Pump as Specified in the original design.

I would like to replace the 200 L Hot Water-Tank with an On-Demand Under table Electric Water Heater.

However, whilst everybody agrees that this is the desirable outcome I would be charged a further £16000 for the privilege.

I will get none of the above!

Confidence:

I would like a Arithmetically designed Heat Pump system,  providing confidence.

I would like all heat pump put on-line with automatic and regular On-Line system performance analysis .  

I will get none of the above!

@iantelescope

kensa's example is similar to Kirchhoff's 1st Law, which states that the sum of the current flowing into a junction will equal the sum of the current flowing out.

The Kensa example assumes that under normal operation the water flow rate (current) going into the top junction is equal to the water flow rate (current) flowing from this junction to the PHE. Applying Kirchhoff's Law would indicate that the water flow rate (current) through the buffer tank would therefore be zero. For this to be true would require there to be a water pump pushing flow from the heat pump towards the junction and a water pump sucking water from the junction and pushing it to the PHE. The pressure (voltage) across the buffer tank being zero when there is no flow through the buffer tank.

As described in the Kensa text, if the water pump sucking water from the junction is slowed or the flow is restricted, then the system is unbalanced and water will start to flow through the buffer tank.

In your system there is no second water pump, so the flow coming from the heat pump is being shared between the PHE and the buffer tank. It is like having two resistors in parallel connected across a single supply, with the current being split between the two. If these resistors are instead connected in series, then the same current will flow through both.

The cheapest solution is to close the buffer tank valve and stop the water flow through the buffer tank. Alternatively a decent plumber could re-pipe the system to put the buffer tank 'in series' in the return pipework, thereby providing a store of thermal energy to help with the defrost cycle.

To get the best from your system I would suggest that you at least isolate the buffer tank.

You have reduced the cycling effect caused by the room thermostat, though if you still have problems you could consider a replacement  thermostat with no cycling capability.

Any TRV's need to be opened fully so that the maximum capacity of the heat emitters is available, though you may need to balance the system to get the desired room temperatures.

The system should be operated in Weather Compensation (WC) mode which should be adjusted so that the system operates at the lowest acceptable LWT.

The primary and secondary water pumps should be adjusted to achieve the most efficient transfer of thermal energy through the PHE.

The PHE is similar to having a transformer in an A.C. circuit, increasing the temperature (voltage) on the primary side will increase the temperature (voltage) on the secondary side. This in turn will increase the thermal energy (power) dissipated in the secondary circuit, which will also cause an increase in thermal energy (power) being drawn from the primary supply. There will of course be a reduction in efficiency due to losses within the PHE and increased losses within the power source (heat pump).

@derek-m 

An excellent analysis derek.

Please bear with me and read the points I have repeatedly made over the last two years.

Series or Parallel Buffer tank ?

The Serial tank:

The MCS provided a statement of "How to Calculate the Volume of a IN-Series Buffer tank.

The MCS calculated the Size of the Buffer tank "on the Assumption that a Temperature Drop of 5 C could be afforded."

With the Heat Pump switching between 35 C and 41 C and a  Temperature drop Across the Heat Exchanger of 6.5 C the Radiators were operating with water between ~28 C and 34 C.

The Radiator Water Temperatures of 28 C were regularly measured at the Radiator furthest from the Heat Exchanger.

Further reducing the pipe water Temperature  by  5 C was not an option.

I may have been mistaken, but I considered a 5 C Temperature drop unacceptable.

The Serial Tank was NOT an option !

The Serial Tank was however the most efficient in that the 5 C Temperature Drop was much lower than the 35 C Temperature drop of the Parallel , Kendra , tank. *

The Parallel tank:

With a In-Series Tank ruled out I searched the internet for a Parallel Buffer Tank, eventually finding  Kendra .

I was sceptical about "Shorting the input to the Heat Exchanger with a Buffer tank".

I insisted that the Tank be fitted with a valve to reduce the greatly increased  losses caused by the operating Water Temperature of 35 C.**

As the MCS say, A buffer tank trades Stability against Energy loss.

I could not position the Parallel Buffer Tank directly across the Heat Pump since the Maximum Water Temperatures would have increased to 52 C ( Hot Water ) and 58 C * ( legionella ) resulting in unacceptable Energy  losses.

The Buffer tank installation:

Two "Engineers" appeared on 26th Oct 2022 to fit the Buffer tank recently "discovered" under the desk of the now fired "Technical director ".

They admitted that they did not know where , or How,  to install the Buffer Tank, eventually accepting the Kendra Parallel Buffer tank solution.

Buffer Tank or No Buffer tank:

The Buffer Tank Rap produced by @Mars makes a humorous, but telling  point .......................

*    Energy Stored in a  5 L Series Tank is 4.18 X 50 X 5 = 1045 kJ

**   Energy Stored in a 5 L Parallel Tank at 35 C is 4.18 X 50 X 35 = 7300 kJ = 7.3 MJ

  Energy Stored in a 5 L Parallel Tank at 52  C is 4.18 X 50 X 52 =  10800 kJ = 10.8 MJ

* Energy Stored in a 5 L Parallel Tank at 58  C is 4.18 X 50 X 58 =  12100 kJ = 12.1 MJ

@iantelescope I have managed to find a little wifi where I am.

If you have the ability to isolate the buffer tank why not try this first? As @derek-m has said tune your water pump speeds to get the most efficient heat transfer across your heat exchanger. Its better to have all that heat going into your house than trying to heat buffer tanks etc in addition to your house. For the sake of just switching off a valve its worth a try and you may find the buffer is not needed if the water pumps are set correctly 😀

@iantelescope

Your system differs from most in that you have a PHE between the primary circuit and secondary circuit and also a buffer tank. Your system was further compromised by having a room thermostat that would appear to have been set to cause cycling at a rate of 6 times per hour. As has been stated 'cycling thermostats' should not be used in heat pump systems.

It may be useful to consider a typical example of a system like yours, for which you can change the values to more appropriate ones.

A somewhat typical home with a calculated heat loss of 12kW at an IAT of 21C and an OAT of -3C. For each 1C difference between IAT and OAT the actual heat loss will vary by 0.5kW, so for an IAT of 21C and an OAT of 9C, the heat loss should be approximately 6kW.

If the total heating capacity of the heat emitters, at the standard DT50, is 24kW, then it should be possible to approximate the average heat emitter temperature required to supply the 6kW heat loss. The attached spreadsheet shows that the temperature of the water going to the heat emitters would need to be 40.7C, with the return water being 35.7C. This of course is at the secondary side of the PHE.

To get thermal energy to transfer from the primary side of a PHE to the secondary, there must be a temperature gradient, which according to PHE manufacturers is often in the region of 5C.

In the above example the primary flow temperature would therefore need to be 45.7C with a primary return of 40.7C.

To get the 6kW of thermal energy to be transferred from the heat pump to the heat emitters, via the PHE, obviously requires the water carrying this thermal energy to flow around the two circuits. It takes approximately 1.16W of energy to heat 1kg (assume 1 litre) of water by 1C, so it can be calculated how many litres of water that can be increased in temperature by 5C using 6kW of energy.

6000/5/1.16 = 1034.5 litres.

But since this is the quantity of water that would need to flow around the secondary circuit in 1 hour, then the required flow rate would be 1034.5/60 = 17.24 lpm.

Because the primary circuit contains anti-freeze the Specific Heating Capacity (SHC) will be lower, so if it is assumed that it takes 1W to heat 1 litre, then the primary flow rate would need to be 6000/5/1/60 = 20 lpm.

The actual loading on a heat pump is not set by the heat pump controller, but by the thermal energy demand, which itself is determined by the actual heat loss of the home.

So in the above example what happens when the heat loss reduces due to an increase in OAT. The IAT may start to increase so the quantity of thermal energy being absorbed and dissipated by the heat emitters will start to reduce. This in turn absorbs less thermal energy from the primary side of the PHE, which causes the Return Water Temperature (RWT) to the heat pump to start to increase. An increase in RWT will start to cause an increase in LWT, which the heat pump controller will measure and reduce the compressor speed to compensate.

There will come a point when the compressor speed cannot be reduced any further, so the increase in RWT and LWT cannot be prevented by the heat pump controller. I am not certain if it is the rising RWT or LWT which causes the compressor to be stopped.

A further factor that needs to be ascertained is whether the primary and secondary water pumps continue to operate. Why could this be important?

Once the compressor has stopped the RWT and LWT will start to reduce. The rate at which these temperatures fall will be dependent upon the location of the thermistors, the volume of water around them, and if the water pumps are continuing to circulate water and hence tend to keep the thermistors warmer. Often the RWT and LWT thernistors are located within the heat pump unit, which is subject to the present OAT, which could be 10C to 15C. If the primary water pump has also been stopped, then the small volume if water around the thermistors will cool more rapidly from its initial 30C to 40C temperature. Once the temperature reaches the lower limit the compressor and water pump will restart ( fast cycling).

If the primary water pump was kept running, a greater volume of water would be being passed by the thermistors, so it is likely that they would cool at a slower rate. (slower cycling). Increasing the volume of the primary circuit using a volumiser should also slow down the cycling rate.

If the secondary water pump was also kept running, then it is likely that thermal energy would be absorbed by the primary circuit from the secondary, further reducing the rate at which the RWT and LWT fall and further slowing the cycling rate.

I would suggest that you monitor the operation of the pumps/valves etc. within your system along with the rate at which the temperatures increase and decrease. Also check the settings within the controller that initiate the starting and stopping of the compressor and water pumps.

Once you have achieved the slowest rate of cycling it should then be possible to concentrate upon system balancing and improving efficiency.

@derek-m

Sorry for the delay in answering your excellent and thoughtful response !.

Thermostats and Buffer :

My  Thermostat in controlling  the Cycle Time AND the Value of the Power demanded from the Heat pump is a Pulse Width Power Modulation Thermostat.

With the Thermostat controlling the Power and Cycle Time,  "Short Cycling" will be inhibited when the Heat Pump reaches it's minimum INV  continuous Power as given in the MCS Documentation.

The Honeywell Thermostat Advantages and Disadvantages!

Advantages:

With Power controlled by a continuous PWM Signal the Honeywell Thermostat, is , ironically, preventing , or limiting "Short Cycling".

Disadvantages:

The Thermostat minimum cycle time of twenty minutes, with a PWM Run time of between 3 and 8 minutes, the Grundfoss Motors and the PCB Relays are being damaged.

A visiting Samsung Engineer recommended protecting the on-board relays with an additional relay , as shown in case 2 of the installation instructions.

Water Quality BS7593:2019

The pipe water quality is deteriorating with excessive ON/OFF SWITCHING totally  failing BS7395:2019.

The Thermostat Fiasco:

My Heat Pump was not , initially provided with a Thermostat, with my "installer" saying that he did not appreciate that a Thermostat was required.

Without no Thermostat NO Heat was forwarded to my Radiators!

The Honeywell Thermostat was provided three weeks after the start of the "Installation".

Recommendations for a Thermostat?

Given the problems , and the PWM control of my Thermostat can anybody  recommend a replacement Thermostat?

@derek-m 

HI , I must congratulate you on the excellent clarity of your system description, first class "Technical authorship"!

System Temperature Measurements.

I am in the process of fitting DS18B20 Temperature Sensors across the Primary , Heat Pump and secondary , Radiator Heat Exchanger Ports.

Following your advice I will attempt to measure, and store ,  the Temperatures from each of 10 X  DS18B20's connected externally to the pipework.

A Sharky Power meter with Internal Water Temperature Sensors will also be used to measure the internal pipe Radiator Water Temperatures at the start of the Radiator water pipe circuit.

All of the DS18B20 's will be connected to a PC via a pair of "Arduino's" .

Posted by: @iantelescope

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Disadvantages:

The Thermostat minimum cycle time of twenty minutes, with a PWM Run time of between 3 and 8 minutes, the Grundfoss Motors and the PCB Relays are being damaged.

As I have mentioned in a previous post, if you are in mode 3 field option #4091 if I remember correcly, you can increase the run time of the compressor and water pump per cycle by lowering your flow temperature to the point it maintains room temp but maximises thermostat "on" cycle time

Regards