Posted by: @mike-h

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Posted by: @jamespa

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So in general terms using a thermostat (or Hive or similar) as the primary way of controlling a heat pump is to be avoided.  Unfortunately there are still a lot of people (and it seems many installers) that don't understand or don't accept this.

I feel that I need to defend the use of thermostats with heat pumps, particularly in the case of heat pumps that are oversized and don't modulate down sufficiently and end up cycling if LWT goes below 33C. One of the top performing Samsungs on heatpumpmonitor never goes below a LWT of 33 because of a tendency to cycle and run times are controlled by the wired controller's thermostat. My Samsung is controlled in a similar manner, although not as efficiently. What it loses in efficiency it gains in economy. Low and slow would require a minimum of 18kWh of electricity usage even in milder weather (24 x 0.75kW), whereas I rarely use that much. Average daily usage in January was 18.1 kWh and less than 12 kWh for the other months.

Fair enough.  There is an exception to every rule of course (I said 'in general terms'), which is why heat pump systems (and central heating in general) need intelligent design.

Also, in the case you quote, is the external control being used as a timer (effectively to enable batch heating) rather than a thermal controller.  My comment (which was made in the context of a discussion about external thermostats that were switching the heat pump on and off several times per hour) was intended to relate to the thermal control (hence the word 'primary'), not time control. 

I can easily accept that an element of on-off timing (on the timescale of hours, ie batch heating) when a heat pump is oversized (and in the low season even when it isn't) may well make sense.  I can also accept that, if the house has a long time constant, a thermostat with an appropriate hysteresis is a way to automate 'batch heating' without having to expressly put in time periods.  I don't doubt that there are other 'edge' cases where external controls can prove useful.  

However in general terms the thermal control of a heat pump (in the sense of matching the heat output to the heat demand from the house) is, in most cases, best achieved by adjusting the WC curve correctly (or as correctly as possible) and then letting the heat pump sort it out, not by modulating the heat pump on and off using an external thermostat.  This is very different to the way we use boilers in the UK, where we have almost totally shunned weather compensation and typically operate at a flow temperature well above that needed to deliver the required amount of energy to the house, using thermostats/TRVs to match demand to supply using on/off modulation.  It is because of this that the difference needs emphasising from time to time. 

I hope we can agree on that, as well as the inevitability of exceptions to the general rule. 

Posted by: @jamespa

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I hope we can agree on that, as well as the inevitability of exceptions to the general rule. 

@jamespa Yes I totally agree with your comments and you have explained the difference between timed and thermal control very well. I haven't yet worked out whether the wired controller's thermostat is just an on off thermostat or whether it is cleverer than a 3rd party thermostat as the Midsummer guide suggests. In my experience it doesn't alter the compressor's consumption as set temperature is approached, but it does seem to have reduced cycling.

@mike-h 

Hi Mike,

Following my recent discovery that my Samsung  "Short Cycling " was caused by the "Cycles per hour " settings of my Honeywell "Home" thermostat Could I ask the following Question:

Continuous or Thermostat Heat Pump Control?

My Heat Pump Power is continuously variable between the Minimum power and the maximum power and can  , presumably,  be controlled by a continuous Sensor for example a thermistor .

When asked to control the power less than the minimum power my Samsung Heat Pump will  cycle.

Why is my Samsung controlled by a Thermostat and not a continuously variable Thermistor or other linear sensor?

I note that my Samsung has three options for  Standard Temperature settings, all continuously variable :

1: Output Water Temperature , presumably a continuously variable Temperature Sensor attached to the output Heat Pump water pipe.

2: Room Temperature, a continuously variable  Thermal sensor attached to the rear of the LCD Panel.

or

3: An external Continuously variable Thermal sensor available as a kit .

Again, Why then is my Heat Pump Room Temperature set by a Binary Thermostat ?

Many thanks, in advance for clear , unpretentious answers to my Heat Pump Philosophic and  practical questions , as per @technoogeek?

@iantelescope Unfortunately the electrical interface on the Samsung is only binary for external thermostats. These are terminals B22 (WL1) and B24 (WL2). The third option is the internal Samsung controller which is I think a continuous signal but I believe it has its own bespoke protocol because other config commands are sent along the same wires.

To have an external thermostat such as the Honeywell, to provide a continuous signal input, the Samsung controller would need to support the universal OpenTherm protocol, which it does not sadly. So unfortunately you are limited to the binary interface (Honeywell does support OpenTherm).

From your various descriptions, I think your issue maybe that the Honeywell is cycling and your heat pump is further sub-cycling due to it being on setting 4 (3mins on, 7 mins off).

You need to slave the heat pump to the Honeywell so that the heat pump only switches on and off in synch with the thermostat. As the thermostat demands heat for longer periods per cycle the running time of the heat pump increases accordingly. To acheive this you need to change the config to use option 3 for #2091 / #2092. You can look at #2041 to verify which water law (WL1 or WL2) is being used for automatic heating weather compensation

If this does not resolve the issue you can always return the config back to the original settings. If it is successful then further system improvements maybe made by reviewing the hydraulic configuration of your system to see if buffers etc are required or not 🙂

Regards

@technogeek 

Again, many thanks for your direct and unpretentious reply.

My Multiple causes of "Short Cycling".

I am painfully aware of the "short Cycling" caused by the Samsung 2091  "Water Pump 3" option .

I have repeatedly, and unsuccessfully,  tried to limit "Short Cycling" by altering each of the "Water Pump Options" only to recently discover that the 10 minute Cycle period was caused by the Honeywell Thermostat Cycle Rate setting.

Following your advice, I have, again,  returned my 2091 Water Pump Option to Water Pump Option no 2. Again, many thanks for your help and advice ......

Standard Temperature Option ..............External Sensor ?

Why not  control the heat pump with a External Room Thermostat , a continuous device , selected by the Standard Temperature External option ?

Cycling superseded by the "Inverter"

A linear and continuous device, a thermistor  controlling an inverter,   is preferable to a Thermostat controlling a refrigerator .

I appreciate that Heat Pumps may  still require some  cycling control because of the nature of refrigeration cycling.

Some cycling is required .....

ian 

@iantelescope with the set up you have and like mine, the thermostat will cycle up to 3 times an hour. I do not believe this is an issue and potentially save money as the pump will not be running continuously 24 / 7. If you have changed the option of #2091 /#2092 and cycling is now limited to the thermostat cycling rate then the thermostat "on" time can be further increased by changing your weather curve to slow the rate of heating which will make the thermostat demand heat for longer per cycle (requires a little experimentation).

Posted by: @iantelescope

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I have repeatedly, and unsuccessfully,  tried to limit "Short Cycling" by altering each of the "Water Pump Options" only to recently discover that the 10 minute Cycle period was caused by the Honeywell Thermostat Cycle Rate setting.

I do not believe cycling 3 times an hour is short cycling. If this was the case then my oil boiler for example would have serious issues. I would class short cycling as "on" for 4 mins, "off" for 3 mins, "on" for 3 mins etc.

When the temperature drops my thermostat is demanding heat for 98% of the time and backs off as the external temperature increases, however this is the best you can acheive with the equipment you have. You could always set the thermostat to an unachievable set temperature such as 30C and that will give you a permanent thermostat "on" signal and providing the heat pump is on option 2 or 3 (#2091 /#2092) it will run continuously and be controlled purely on the weather compensation parameters.

The only other options are rewire the built in thermostat to your Honeywell location or something like Homely which continuously talks to the heat pump via the ModBus interface.

With my similar set up as yours I am getting in the region of 4.3 COP with combined DHW & CH and consuming on average 16Kwh a day power at 7C OAT, so a small amount of cycling is not really an issue I do not think. If my pump ran continously, my electric bill would be more like 30Kwh+ and I would be switching the heat pump off permanently in favour of the oil boiler! 😀

Posted by: @technogeek

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and your heat pump is further sub-cycling due to it being on setting 4 (3mins on, 7 mins off).

@technogeek FSV 2091/2092 Setting 4 causes the water pump to run 3mins on and 7 mins off, not the heat pump. However, I have no idea why Samsung provides three different options for how long the water pump runs when the water law discharged water temperature turns the compressor off.

• setting 2 water pump turns off after 1 minute
• setting 3 water pump stays on
• setting 4 water pump runs for 3mins and then off for 7 mins and repeat

If you are listening solely to the water pump, then setting 4 may make it seem that the heat pump is cycling, but unless the compressor restarts, this is not cycling as I understand it. Observing the smart meter makes it clear when the compressor stops and starts.

Posted by: @mike-h

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Posted by: @technogeek

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and your heat pump is further sub-cycling due to it being on setting 4 (3mins on, 7 mins off).

@technogeek FSV 2091/2092 Setting 4 causes the water pump to run 3mins on and 7 mins off, not the heat pump. However, I have no idea why Samsung provides three different options for how long the water pump runs when the water law discharged water temperature turns the compressor off.

• setting 2 water pump turns off after 1 minute
• setting 3 water pump stays on
• setting 4 water pump runs for 3mins and then off for 7 mins and repeat

If you are listening solely to the water pump, then setting 4 may make it seem that the heat pump is cycling, but unless the compressor restarts, this is not cycling as I understand it. Observing the smart meter makes it clear when the compressor stops and starts.

Setting 2 is possibly used to get the remaining thermal energy from the heat exchanger within the heat pump after the compressor has stopped.

Setting 3 may be to extend the period between cycling during milder weather conditions, since the thermistors within the heat pump which initiate the stopping and starting of the compressor will cool at a slower rate if the warmed water within the heating system is being recirculated. Obviously this may reduce efficiency since the water will be cooled as it passes through the outside unit and the water pump will use more energy.

Setting 4 may be to prevent the water from freezing if an overnight setback is employed and the compressor is off for a lengthy period of time.

@technogeek 

Again, many thanks for your excellent advice , I appreciate it!

Cycling :

Since Cutting the Cycling Rate from 6 per hour, i.e. a 10 minute period with a 4 minute Run time to 3 per hour my Cycle Time having increased to ~20 minutes with a Run time of ~6 minutes.

 Run Times over the last 150 days showing the sudden improvement since reducing the Thermostat Cycling Rate to 3 per hour.

I will take some time to measure the effects of the Thermostat Cycle Rate change.

However, the Average COP over the last 150 days is 2.97.

My Heat Pump is followed by Two motors , a Heat exchanger, a 50 litre Buffer tank and a forest of pipes with losses of circa 25%..29 %

The Actual Average COP as measured at the start of the Radiator circuit is  1.98 over the last 150 days.

The COP and the COP at the start of the radiator circuit with the associated losses over the last 150 days are :

 Where the blue line gives the COP output from the Heat Pump itself and the Red line gives the COP at the input to my Radiator circuit.

Measurements were here made with a Sharky Water Power meter fitted at the start of the radiator circuit. Hot Water was not used.

I am hopeful that the Honeywell Thermostat Cycling Rate reduction will improve matters.........hopeful but realistic given my Heat Pump  "installation" was "started" exactly two years ago.

ian

@mike-h I cannot speak for other Samsung models but I have a Samsung Gen 6 controller connected to a 12Kw HT Quiet. Standing in my plant room observing the behaviour of option 4 (#2091/#2092) the entire system starts up, runs for 3 minutes and completely shuts down for 7 mins, ie Water pump, compressor and the controller goes into standby, then the entire system fires up again for 3 minutes.

It was not playing nicely with my thermostat so slaved it with option 3 and from a control point of view is much better 🙂

Regards

@iantelescope I briefly switched my system on yesterday to give you some comparison figures. In this warm weather my system ran for 2 cycles of 5 mins each for the one hour it was on. The only reason it ran for 5 mins is because the thermostat minimum run time is set to 5 mins otherwise it probably would have run an even shorter time per cycle.

The problem is, trying to set up a system at this time of year is near impossible requiring educated guess work etc. Once we have a sensible control configuration improving the hydraulic performance would be the next step in my opinion. I also think starting a new separate thread to discuss this would at this point be a good idea as we have seriously gone off topic 🙁

Hope that helps

Regards

Posted by: @technogeek

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@mike-h I cannot speak for other Samsung models but I have a Samsung Gen 6 controller connected to a 12Kw HT Quiet. Standing in my plant room observing the behaviour of option 4 (#2091/#2092) the entire system starts up, runs for 3 minutes and completely shuts down for 7 mins, ie Water pump, compressor and the controller goes into standby, then the entire system fires up again for 3 minutes.

My apologies! That behaviour is quite different to my Gen 6 12kW Samsung and to what my manual suggests. Option 0 - just using the thermostat in the wired remote controller seems to work well for me.