It sounds to me like the dhw switchover is co-inciding with a defrost cycle and the HP is drawing heat for the defrost from the coil in the cylinder.

That’s all I can think of.

We occasionally get this and I’ve put it down to our buffer tank that’s part of our hot water cylinder (which appears to be what you have).

These types of stratified water tanks work by using a series of layers, or "strata," of water at different temperatures within the tank.

At the bottom of the tank is a layer of cold water, which is constantly being replenished as the hot water is used. Above this layer is a layer of hot water, which is heated by an external source, such as a boiler, heat pump or solar panels (via an iBoost, for example). Below this layer is the buffer. 

Now I don’t know what these tanks look like inside or how they filter and control water exactly, but I’ve definitely seen our HW drop significantly on occasions when the valve moves from CH to HW. For some reason, under some circumstances, it must allow the cold water to mix with the hot.

I’ve found it more pronounced on good solar days where the HW has hit 60C, then the valve has changed to CH, and the HW drops to 50C. It’s not super common, but does happen fortnightly, and you’re right @grnmeira, you don’t need to be using HW for this to happen.

Maybe @heacol can demystify this for us?

Posted by: @hughf

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It sounds to me like the dhw switchover is co-inciding with a defrost cycle and the HP is drawing heat for the defrost from the coil in the cylinder.

That’s all I can think of.

I checked the "status menu", and it shows the defrosting cycle is not active.

Posted by: @editor

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We occasionally get this and I’ve put it down to our buffer tank that’s part of our hot water cylinder (which appears to be what you have)

@editor in this particular case I'm using a high gain Joule Kodiak cylinder ( https://erpfichetool.jouleiot.com/#/specification/Joule%20IE/HUKH-G6200-L3C). It shouldn't have a buffer tank.

If the temperature drop happens because of cold water circulating (while the heat pump output is not warm enough). Given the system works with 3 2-port valves (2 zones and 1 DHW), the only alternatives I see to get this temperature drop not to happen are:

1. Leave all valves closed (bypass) until the heat pump reaches a reasonable temperature before opening the DHW valve.
2. Leave the heating zone valves open (circulating water through the radiators only) before opening the DHW valve.
3. A mechanism to quick start the heating process in the HP (like an electric heating element inside the HP).
4. Reduce the flow, maybe? While the heat pump is still heating up? My pumps don't seem to be controlled by the Samsung controller though.

Does that make sense? I don't see anything in the manuals that can help me with anything of those alternatives. Could some of the motorized valves be wrongly wired?

Check that the motorised valves are actually fully closed when they are supposed to be. If you have three 2-way valves, one for DHW and two for CH, then when the DHW valve is open both CH valves should be closed and vice versa.

@derek-m what should the valves do while the heat pump is not "hot enough"? Should they bypass the zones and the coil? Because otherwise that'll cool down the water anyways, correct?

Posted by: @grnmeira

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@derek-m what should the valves do while the heat pump is not "hot enough"? Should they bypass the zones and the coil? Because otherwise that'll cool down the water anyways, correct?

For best overall efficiency, the temperature of the water going to your central heating should be as low as possible that meets the heat demand. The water supplied for central heating is therefore often cooler than that required to produce hot water in your cylinder. If the DHW valve does not fully close when the system is in CH mode, then the now cooler water from the heat pump will continue to flow through the heating coil within the hot water cylinder, and hence have a cooling effect. Check the operation of the actuated valves.

Also check the pipework around the hot water cylinder for thermal siphoning.

Do you have a secondary circulation pump to ensure hot water is readily available at the taps? If so, this could be draining the heat energy from your tank.

Posted by: @derek-m

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The water supplied for central heating is therefore often cooler than that required to produce hot water in your cylinder. If the DHW valve does not fully close when the system is in CH mode, then the now cooler water from the heat pump will continue to flow through the heating coil within the hot water cylinder, and hence have a cooling effect.

One thing to note is there's no demand for space heating while the problem happens. And if space heating is working, there's no drop (except for the expected heat loss) in DHW temperature.

Actually, if space heating is running and DHW demand kicks in, it actually seems to help to attenuate the temperature drop.

Posted by: @derek-m

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Do you have a secondary circulation pump to ensure hot water is readily available at the taps? If so, this could be draining the heat energy from your tank.

We do have a pump for tap/shower hot water, though the sharp drop happens when there's no usage of hot water at all.

The cylinder seems to hold the temperature quite well. The problem happens when the system is totally idle, no demand for space heating, no use of hot water. When the temperature drops to 43 (due to normal loss) and the HP starts up to reheat the DHW back to 48, that's when it does the sharp drop. 

I would suggest that you monitor the temperature of the pipework around your hot water cylinder to identify where the heat energy is being transported when the reheating cycle occurs. Then post some pictures.

Unfortunately I don't have the tools to do that properly. I probably can show some pictures of some of the values from some sensors and check if a couple of pipes are warm to the touch.

I do notice that when the sharp is drop, the input pipe (coming from the heat pump) is quite cold and same for the return pipe.

If you don't have a suitable thermometer then use your hand. You could monitor the sensor values during the cooling process which may throw some light on the problem.

@grnmeira

“The cylinder seems to hold the temperature quite well. The problem happens when the system is totally idle, no demand for space heating, no use of hot water. When the temperature drops to 43 (due to normal loss) and the HP starts up to reheat the DHW back to 48, that's when it does the sharp drop.”

I think I’ve encountered something similar. Your paragraph above seems to describe my thoughts. 
If HP was idle then the primary pipes are probably cold. HP is slow to build temp up so while water is circulating it’s re absorbing heat from your DHW cyl. This could be confusing for the system (for a while) since the return will be warmer than the flow. 

perhaps try to coincide DHW reheat while the space heating has been running then the primaries will already have some heat in them. Just a thought from a newbie.