Best practise for heat <-> dhw cutover

[gvzdus]

Hi, with my LT cascade installation I am heating up once per day the ~500l DHW buffer (operationMode 3 ; forceDHW 1).
I notice that each change on the Three-Way-Valve seems to trigger a compressor stop by the built in control SW of Panasonic. Probably they do so because of worries that during change of the Three-Way-Valve there might be some seconds of complete blockage. However, my Orkli 40713200 valve seems to be designed like "always non-blocking".
So I am thinking of

• managing the valve directly by one of the Heishamon Large relays
• do the DHW heating in heating mode (max. 55° possible due to SW limit)
  Goal 1: Avoid 2 compressor starts per day during heating time.
  Goal 2: Optimize the target temperature during heating: Initially: Start with current DHW buffer temp + 1K, then increase to target temp +2/3 K. (Idea: Do never cool down the upper hot layer in the buffer).

Any thoughts / experiences? Or even confirmation of my observation "switch over always causes a compressor stop"?

P.S. Here a graph about a DHW cycle, started at 12:02:

• WP1 (heatpump 1) turns off for a few minutes
• WP2 is set by my smarthome system to 55°
• WP2 goes into defreeze
• 12:25: DHW heating done, WP1 turns off again, while I gradually decrease the target temp for WP2 to keep it from shutting down.

From the logfile:
2025.01.07 12:25:13 1: Cooling down: wp2out=52.50 wp2target=55 wptarget=39 newtarget=51
2025.01.07 12:26:13 1: Cooling down: wp2out=46.25 wp2target=51 wptarget=39 newtarget=45
2025.01.07 12:27:13 1: Cooling down: wp2out=41 wp2target=45 wptarget=39 newtarget=40
2025.01.07 12:28:13 1: Cooling down: wp2out=40.50 wp2target=40 wptarget=39 newtarget=39

[geduxas]

No, stop is to prevent overheating.
Also did you tried dhw smart mode? I haven't time to proper test it as how it works in total..

[gvzdus]

I don't get how it should be helpful:
"During SMART DHW start time 20:00 (Default Setting) to SMART DHW stop time 05:00 (Default setting) Heat pump re-heat the tank water only when tank temperature drop below 20°C (Default setting)
Time between 05:00 to 20:00
Heat pump reheat the tank water when tank temperature as below condition
Reheat when tank temperature below tank set temperature + R/C (Tank re-heat Temperature) - 3°C"

I might try to decrease the max. temp. of my DHW buffer a bit, but afraid to risk my marriage.
It never goes to < 20°. Also in "Haustechnik Dialog", they speculate this is only supported by Panasonic "all in one" devices.

[geduxas]

I think i misunderstood that smart function.. thout it is smarter :D

So yes last effort to go without stop is somehow adjust temperatures.. i think there was some discussion about it. You could search for it.

[stumbaumr]

The compressor turns off because after switching from DHW to heating the target temperature drops so much that the difference between target and outlet is bigger than 2K.
With my rules I modify the heating curve and re-adjust it as soon as the outlet temperature fits again the previous target temperature...

[MiG-41]

Is it also checking , if i.e. external thermostat is on ? ( if both are off it will turn off anyway , no need to do this magic. with changing curves)

[stumbaumr]

I only use a heating curve with watertemperature. The separate room thermostats (https://www.mi.com/de/product/mi-temperature-and-humidity-monitor-2 ) display the temperature in by the missus required rooms for reaffirmation that it is not to cold.
Goal is to have her complain that it is to warm and then tell her to bring fresh air in by opening the garden door and the roof windows for a short period of time.
I have a screenshot of a timeperiod without defrosting where you can see that the compressor never halts even after DHW-production. My average compressor runtime - including the two starts when defrosting - is 2 1/2 hours...

[gvzdus]

Your experience is certainly longer than my one.
For "cooling down" after DHW mode my rule is running once per minute and decreases the target temp to max(int(outlet_pipe_temp)-1; target_heat_temp).
This is what I do on my second (non-managing) HP, which stays in OperationMode 0 (heating).
I think about moving the 3way-valve from control by HP1 to "direct control by Heishamon relay" and also let HP1 run always in OperationMode 0.

[stumbaumr]

Since there is no official picture in the Pana documentation for cascaded setups...
...are you running them in parallel or sequential?
A picture of your setup would help - something like this:

[gvzdus]

Parallel!
Searched 3 minutes for the mail with the setup plan - giving up now.

[MiG-41]

I know simpler way to change from DHW to Heat without stopping ( but it have to be a twin-fan outher unit). When DHW is already heated to set temperature it waits 2(or)3 min and then changes to Heat. Just before that ,first you do Force Defrost , and after 30 s you switch to Heat. Some little defrost ( if any ) will perform , but it will be in Heat mode.

As for single fan units each defrost couses at least one stop of compressor , it will not make this trick.

[geduxas]

Wow interesting, so when dhw reaches setpoint temperature, it will wait for 2-3mins? Or when it reaches supply temperature to it's setpoint?

[MiG-41]

Yes , just to be sure ,that for sure DHW have this temperature ( becouse it can start fluctuate for some time ,before reach setpoint for sure. That is why this time.)
When it reaches the target of water temperature ( in heating loop)on outlet ,it is the same like during heating , it starts modulate , and when it will be above 2 degree of target temperature in time 3 min it will stop .