LWT will only rise if your emitters are giving out less heat than the heat pump’s minimum output. This generally only happens when it is warmer outside. If it is cold outside, your LWT will remain constant at target LWT as determined by your water law settings.
So if your LWT target is say 38C at an outside temperature of 3C, then at that outside temperature your emitters will continue to produce more heat than the minimum ASHP output and your LWT will remain around 38C for hours on end.
In my experience I don’t think under any circumstance LWT should rise above the WL temperature. The WL temperature determines the LWT.
If the emitters cannot emit sufficient heat, the LWT will rise quickly to the WL temperature and the RWT will rise quickly and the compressor will shut off. LWT should not rise higher than the WL temperature.
My test was unsuccessful.
I tried setting 2093 to ‘1’ and there was no change, I also tried changing 2092 to ‘1’ no change again and 2091 was already set to off.
If understand the book correctly
2093 is for the remote controller
2091/2 is for the wired controller (which I’m using using although my old wireless thermostat is wired in as well)
This was just supposed to be a test to see what my systems minimum output is.
This is what i have found even with 9023,9022 set to ‘1’
once LWT reached WL the pump cycled.
Hi Glyn, I have checked this out by setting target WL LWT to 25C (25C at 10C OAT and 25C at -5C OAT). FSV 2093 = 1. As you can see from the graphic below, the LWT continued to rise above 25C and would probably have leveled out around 36C as my emitters are not big enough to produce 3.8kW of heat until LWT approaches 36C. It only cycled off when I ended the experiment.
Interestingly, COP was well maintained over the first 20 minutes, whereas it usually drops to low levels around the 10-20 minute period (< 2 this morning despite OAT of 10C). Perhaps, I will leave this setting until it gets much colder!
It sounds as if you are using an external thermostat, not the wired remote controller. So 2093 setting would not be operative. 2091/2 are used for an external thermostat and both have to be set to 0 (Not use) if you want to use the thermostat in the wired remote controller. If your screen looks like the graphic below, then you are using an external thermostat. Water Law settings will still control LWT.
Hi Mike,
Yes mine does look like that.
If i understand correctly then to do the same sort of test as you did I need to set both 2091/2 to ‘0’ then set 2093 to ‘1’
Then the pump should not turn off and will run at minimum until it reaches equilibrium or I turn it off.
Thanks
Ian
Interesting, I’m using the Samsung controller as wired remote my system which I think is why I get differant operation with LWT always being constrained by WL setting + over shoot
I use the Samsung wired remote controller’s thermostat as well. Your system usually cycles off due to your thermostat settings, rather than the LWT target being exceeded. I think if you set your target LWT really low, say 20C (so your emitters would not be able to match your minimum ASHP output) and your thermostat up to 30C, you would see the same effect as in my experiment.
It may not run at minimum, if by that you mean minimum heat output, especially at colder outside air temperatures. When target LWT has been reached, if your radiator output equals the ASHP’s output at this LWT, then it will run with a constant LWT. eg if your heat loss at 3C outside air temperature is 4kW and your radiators are outputting 4kW, then your heat pump should be producing 4kW - well above the minimum.
However, in the case when target LWT has been reached, but the ASHP output still exceeds the radiator output and ASHP output cannot go any lower, then with FSV 2093 = 1, the LWT will rise until radiator output equals ASHP output (which should be close to minimum output). With FSV 2093 = 2-4, it should cycle off instead.
You would have to experiment to see which setting is more efficient. Your cycle time is long (30 minutes), so is not detrimental and you may find room temperature is better maintained with it cycling rather than running at a higher LWT. As far as I am aware, if you use your external thermostat and set 2091 and 2092 to 1, then this has the same action as setting these to 0 (internal wired remote controller thermostat ) and setting 2093 to 1
I dont think this affects cycling etc, but I’m curious to know why the flow temperature drops by a few degrees after stopping. The red dips below the green by a degree or so… then the two go together as expected.
I do plan experiment with these settings and see how it affects the efficiency and this explanation is very helpful thank you.
Its not as extreme as that I’ve zoomed right in and the drop is approx. 0.4 - 0.5 deg
Could it be a consequence of the flow rate jumping back to max or maybe because my outside unit is 10m away from the house?
Anyone else got theories on this?
I’m intrigued now.
It must be condensed refrigerant evaporating just after stopping (‘cooling’). Old units did this. However, its a tiny amout. I should have thought it through before posting the question. Its good to drill into odd details though.
Hi John, yes you answered own question.
The controller depressurises the refrigerant circuit (by opening the compressor LFR SOV) when the compressor stops, so the liquid refrigerant in the condenser flash cools (to about ambient temperature) and heat flow in the condenser temporarily reverses, cooling the circulating fluid as it re-evaporates the condensed refrigerant.
This doesn’t take long as there’s only about a litre of liquid to deal with, after which heat flow in the condenser ceases and LWT equalises with RWT.
You can check the flash cooling effect by looking at the pressure-enthalpy diagram for your refrigerant.
Hi Ian,
Have you considered a Homely heat pump controller?
I’ve ordered my Samsung R290 (16kw) together with the Homely controller, which seems a no-brainer to me (about £250). But it won’t be installed until next spring due to planning delays…
Yes, I remember now this was quite a thing in the past. Intereting how we dont see it at all on most units now
I’d heard of homely but had not looked into it.
It looks interesting if it can do everything it states, i might give this winter a go myself and see how i get on then maybe look at homely if i can’t or don’t want to tweak myself.
Thanks
