Search with Wattson

Notifications

Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

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.

Page 2 / 2 Prev

Air Source Heat Pumps (ASHPs)

Last Post by TRBob 4 years ago

14 Posts

6 Users

2 Reactions

10.3 K Views

RSS

SUNandAIR

(@sunandair)

Honorable Member Member

Joined: 4 years ago

Posts: 570

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

ReplyQuote

TRBob

(@trbob)

Trusted Member Member

Joined: 4 years ago

Posts: 61

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 8.5kW
4.4kW Solar PV
5.2 kWh Battery Storage
1983 build, 300mm loft insulation, cavity wall insulation (beads)

ReplyQuote

Page 2 / 2 Prev