Why is my heat pump cooling my DHW?

grnmeira · 2022-12-29T01:07:15Z

I recently moved to a new place with a brand new install of a 5kW gen6 Samsung heat pump using a 200L Joule tank. Even though I'm reasonably happy with the system, something is bugging me. Sometimes when the heat pump kicks in to reheat the hot water, the tank temperature drops sharply. And there's no hot water in use anywhere at home, and no demand for space heating. The temperature is set to 48, when it reaches 43, the hot water demand signals, the pumps start, and the tank temperature drops drastically. In 10 to 15 minutes, it drops to 28 degrees, enough to ruin a bath. After some research and going through lots of Samsung/Joule manuals and some tests, it feels to me that the system is circulating cold water through the coil, but I'm no plumber 😄 If the water outlet temperature (I believe this is what Samsung calls the HP output) is above 40, the drop in temperature is minor, though if the water outlet is cold, then it gets bad. Does this make sense? Shouldn't valves/controller prevent this? Has anyone faced/fixed something similar? I've added a picture of what my installation looks like. Thanks everyone. And congratulations for the forum, really good information around here.

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SUNandAIR

(@sunandair)

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31/12/2022 10:23 am

Just wondered -can you lower the temperature when your hot water reheat starts?
You seem to only have a 5 deg drop. That also means the start of the reheat is at 43 degC which might be a bit high.

I’ve got an 8.5kw HP and have the temp set to 48degC with reheat kicking in at 38degC. That means it’s got a 10 deg drop and so, not switching continually to DHW. And perhaps equally importantly the reheat temp starts when pipe temps roughly match the radiator temps if they’ve been heating the radiators.

I hope this is of interest

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TRBob

(@trbob)

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Joined: 3 years ago

Posts: 49

31/12/2022 10:54 am

Posted by: @sunandair

↑

@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.

My DHW has always been like this, it drops about 7 or 8 degrees if I recall correctly when it starts heating, I attributed it to what SUNandAIR describes. Since summer I have it set to only heat at midday for an hour and do a manual start if more is required, so its not as noticeable because the tank is mostly cold anyway and I don't watch it like I did when it was first installed.

Mitsubishi Ecodan 8.5kWh
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