Posted by: @ecobaker

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The required volume of water in the system is more to do with the defrost cycle, and has little effect on mild weather cycling. Please read the post I sent previously.

@derek-m Oh doesn't the volume of water circulating into the rads determine how much energy is being out putted from the heat pump? A system with more volume of water requires more energy and thus the heatpump will meet is minimum power requirements. Or have I got that wrong?

The loading on a heat pump is determined by how much thermal energy is being emitted by the heat emitters, which of course will be greater if the radiators are physically larger and have greater heating capacity. But since the size and type of radiators is fixed, then to output more thermal energy it is necessary to increase the Average Water Temperature. Increasing the water flowrate may slightly increase AVT, but the quantity of thermal energy will be determined by the temperature difference between AVT and IAT. 

@iancalderbank Thanks thats really helpful.

I've shifted control to the Hive stat in my living/dining room by setting 2091 to 1, I will keep an eye on it from there. It 12c outside and house has been holding at 21c nicely with the pump off over an hour. I guess I make small adjustments to the weather compensation and see how that effects things over the new few weeks. 

I have the 8k Gen6, so was thinking the minimum output would be 40% of that hence 3.2kw, you think it could be lower. 

Posted by: @ecobaker

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I have the 8k Gen6, so was thinking the minimum output would be 40% of that hence 3.2kw, you think it could be lower. 

there is one G6 8kw on HPM.org, here   , low point seems to be about 2.5kw. The owner of that one might be on this board as well, I can't remember.

I should add, it depends on the system, as well as the HP. I.e. the radiators, the circulation design (LLH/Buffer or single loop), flow resistance. The better designed and implemented this all is, the more stable at low modulation.

Posted by: @ecobaker

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I've shifted control to the Hive stat in my living/dining room .....

I don't know what the Hive product is, precisely, but I suspect that it does dot operate as a simple thermostat.  I have a Drayton 'Smart' thermostat and timer on my gas boiler at present that short cycles six times per hour when the boiler can't modulate to a lower output. I've been trying to determine the control parameters of my boiler as its only sensor is a thermistor (other than overheat thermostats).  There must be some derivative control that I can't easily identify.

@ianmk13 Yeah I wondering about that, I think its just calling on or off for the heat, then the weather compensation on the unit is adjusting the output of the HP. I would like to just use the samsung wired controller, but it's installed on the box in the airing cupboard, so not great for getting accurate room temps. I think I need to move it.

Things have got worse 🤔 

I have removed the Hive thermostat, unwired it from the control unit, and changed FSV 2091 and 2091 to 0, as it says in the manual if you want to use the wired controller to control heating. 

I have moved and rewired in control panel, to a good location in my upstairs landing, rather than in the airing cupboard. I set the controller to use Indoor temp this produced error code e121. Set room temp to 21c and turned on heating.

Despite WC being set up the outlet temp shot up to 48 degrees. However there was no hot water circulating in my rads. What I think was happening was, I have a LLH fitted and the HP was heating and pumping water round and round through the LLH and back to the HP. The pump on the heating flow pipe was not pumping, although it was working fine with the Hive wired in. Not sure why it was putting out water at 48c, maybe as there was such small volume running through it, just the water in the pipes from HP to LLH and back. 

Error code 121 in the user manual.

"Short- or open-circuit error of the room temperature sensor of the Zone 1 indoor unit (detected only when the room thermostat is used)"

But I'm not using a room thermostat, I have it turned off in FSV, so maybe there is some other wiring issue that is causing this. 

Any ideas as to what is wrong? 

@ecobaker the description of what the system did makes total sense if there was nothing running round the rads. as you have an LLH you need the secondary side pump to be running as well. It may have been wired so that it was only activated by the Hive. you need to take a look at the wiring to the secondary-side pump.

look at this document

your plumbing layout should ( hopefully) be basically the same as page 1.

study the wiring diagrams on page 6 and 7 for secondary pump wiring. 

I don 't have a secondary pump so I can't speak from first hand how these two options operate.

Posted by: @ecobaker

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I had my system working quite well, a cop of around 3.8, and I had a plan about how to improve it with a few new radiators. I was controlling the system with a Hive, that my installer wired in for me.

• I set the Hive to about 28 degrees, it would fire up the compressor I left it like this 24/7
• I would adjust the outlet temp to get the room temps I wanted, with no weather compensation.
• Rather manual I know, but I wanted to get a feel for what flow temps worked well given the outside temp.
• Its a simple set up, two bed terrace house, one open loop heating zone, all radiator valves full open. 

However I wanted to try running the system without the Hive, using the standard Samsung wired controller, and setting up weather comp, so I disconnected the Hive.

I'd set the system to run based on the water outlet temp, which I had set at 30, which would have been about right for today.

But without the hive the compressor wont stay on. What happens is the compressor turns on for maybe 3 mintues, water outlet shoots up to 38 to 40ish, the compressor turns off. The water outlet temp very slowly drops until below 33, then fires up again for a few minutes. 

Any ideas on what is wrong?

-- Attachment is not available --

@iancalderbank Thansk Ian, that document is useful. 

I figured out my system, after a good 40 mins in the loaf working out what everything does. I have automatic pumps, that just pump at a fixed flow rate so not controlled by the board. I also don't have a three way valve, but I do have three mechanical isolation valves, one of the flow pipe to the LLH, one on the DHW flow and one after the LLH on the flow pipe. That second valve on the flow out of the LLH was wired into B19 and B22, and the hive stat joined the connection between B20 and B22, since that wasn't there any more I just wired in a loop to connect them and everything works now. 

I'm still getting the same error message E121, so not sure what that is about. 

@ecobaker you might want to check that the right valves are open and not open when wanted. I suggest drawing a plumbing diagram and a wiring diagram at this point.

I've seen that error message when changing between external stat and controller-as-stat - there was nothing actually wrong, it just got confused by the change. I switched the controller off and on and it went away. not saying yours is the same issue but try it.

@ecobaker Samsung have reverted back to me and said the manual is available for installers only on their installer portal, but have shared the following which I hope will be useful: 

Vmin = (tmin × Qmin) / (C × dT)

Minimum volume is found as a function of the time of operating a minimum output capacity, related to the specific heat capacity of the medium and the temperature rise by the heating cycle. The catch here is that the capacity of the heat pump increases towards the point where we no longer need heating. This adds roughly 25% of capacity to the minimum output. How much is the minimum output of a compressor? On average 25% of nominal. But a 12kW and 16kW could share the same compressor, 25% of 16kW = 4kW. But then for the 12kW unit, it becomes 33%. So please be aware. Rule of thumb says 10l~15l/kW makes it work smoothly also during (extreme) part load operation. Part load is when you cannot operate full speed. Extreme part load is when the system produces more minimum output than required. This triggers On/Off cycling. Not more than 4 per hour is advised.

@editor Thanks for that. 

I managed to get the reference document at this link provided by @iancalderbank quite a few useful docs there.