I have a 300L hot water tank. It's heated from an air-to-water heat pump. The hot water tank is connected to a heat exchanger. When any hot water faucet is opened, a circulation pump is turned on which pumps hot water from the tank through the heat exchanged, at the same time cold water runs through the heat exchanger, is warmed up and goes to the faucet.

So far everything works as described.

But if no hot water is being used and the circulation pump is turned off, gravitational forces lead to a hot water circulation flow through the heat exchanger. Thus the hot water tank loses it's heat much faster than I liked (sure, some insulation will be added when I think that all is set).

My understanding was, that one needs to add a "thermosiphon" where the hot water would cool, fall to the bottom of the siphon and could not clime up the other side of the siphon. Well, I did not read any technical specs for such a thermosiphon, I simply added a siphon like pipe-loop to the hot water loop from the tank to the heat exchanger. However, I seem to have missed some important sizing or leveling or something else, because my thermosiphon still seems to circulate hot water - at least that's what my impression is - purely based on the speed of temperature drop and the touch on the heat exchanger which is warm also in the morning when no hot water has been used for hours. The U shaped pipe in the picture is supposed to be my thermosiphon. Any advice on better thermosiphoning?

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UPDATE: Emphasis is on BOTH (in- AND out-let of the water tank) that need the "siphon" to prevent heatloss. I added a 4 foot high loop to only one end and while this reduced the heat loss it does not stop it. I will add a siphon to the other end aswell, else I am loosing 1C of water temperature every hour (additionally added insulation to the pipes - but I cannot insulate every piece )

  • What is the mfgr of the heat pump and tank? Where is this installed? What is the power requirement for the heat pump? Nov 24, 2021 at 13:52
  • @JimStewart - Panasonic 5kW heat pump. Installed in Germany. Max. apmerage was 12A and 13A, whereas one is mainly for the heating coil which is disabled in my setup currently. Tank is a no-name, factory insulated steel tank with one 2.1m2 heatexchanger loop where the heat pump is connected to. The hot water is taken from the "real" tank volume.
    – AndyZ
    Nov 24, 2021 at 14:09

4 Answers 4


OK, so you have a 3-loop system. The inner loop is freon, for the heat pump. The intermediate loop is service water (or antifreeze) which you heat/store in a 300l (80 gal) tank. Then you use the hot service water in a heat-exchanger to do on-demand heating of potable hot water, which is delivered to a faucet.

(sure, some insulation will be added when I think that all is set).

There's your problem right there. Cart before the horse.

You think this thing is going to resolve itself magically via ???????, and your logic is, "well, I'm not going to put the insulation back on until ??????? is done, because the insulation would just get in the way of working on it, and how important is insulation anyway?"

The lack of insulation is the cause of the thermosyphon.

The uninsulated apparatus there is working like a "Heat Pipe" on a laptop. Hot water comes to the heat exchanger, cools off quickly because there's no insulation, and falls.

Really, the same could be said for all the poorly insulated service water pipes and hoses in the photo. Perfect world, all that stuff is inside a compartment which is itself insulated, and equalizes out to service water storage temperature. If it is actually in the cold-air handing compartment where the cold air in the freon process travels, that would be the worst-case scenario, since you're just pumping heat in circles at that point lol.

  • @harper-reinstate-monica thanks, now as you describe the heat losses without the insulation it makes perfect sense. Anyway, it probably did not hurt to increase the siphon loop. Change the small loop around 40cm height to a 130cm high loop. Additionally I will now move the priority of insulating the pipes higher in my prio list.
    – AndyZ
    Nov 25, 2021 at 7:53

As mentioned by others, the tubes should be insulated to reduce the thermoflow and losses in general.

If the thermosiphon is not directly installed close to the tank's in- and outlet, the uninsulated tubes/hoses from those tank connection up to the thermosiphon will work like a room heater.

This free forum (in German) has very good technical information about running and optimizing heat pumps, especially the Panasonic Geisha/Heisha/Jeisha etc. series in Middle-European Conditions (many installed and optimized by DIYers). https://www.haustechnikdialog.de/Forum/46/Haustechnikforum?showOverview=true

Links to a comprehensive information site are often included, f.e. for micro-in-tube- circulation: http://www.bosy-online.de/Thermosiphon.htm

  • thanks, yes I know the HT forum, have been reading there a lot. Thats where I got the thermosiphon idea from, but never stumbled across any technical details or requirements like the importance of the distance from the in or out let.
    – AndyZ
    Nov 25, 2021 at 7:57

Easy, increase the depth of that loop, we had a similar issue and need about a 4ft drop to stop the thermosyphon.

  • thanks, will try that as a first step. Another idea I had is that maybe the u-pipe must sit higher than every other pipe in the loop? That would make it a lot harder for me to achive, so will try at first with just a higher U and see what happens.
    – AndyZ
    Nov 24, 2021 at 14:04

You need to prevent thermo-siphoning.

"Heat traps" is a common term for the preventative measures, and there are several approaches - check valves that pump flow will open but which don't open at thermosiphoning pressures, as well as actual "plumbing trap" style u-bends.

Without getting a very good overall sense of your system from the picture, I would guess you might need something on the pump leg as well as the non-pumped leg. Heat traps are generally installed on both the hot and cold lines of a hot water tank.

I would also apply insulation to your pipes.

  • thanks, I will at first try to increase the height of the U-shaped pipe as Solar Mike suggested. If that won't be enought then will add another u-pipe for the other end.
    – AndyZ
    Nov 24, 2021 at 14:02

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