I recently had an indirect-water tank added to my multizone gas-powered hydronic heating system. The supply line from the boiler to the water tank heats up considerably every time one of my heating zones is enabled, even when the circulator for the water heater is off. This seems wasteful. I also worry it will cause irregular (and perhaps dangerous) domestic hot-water temperatures, since heat is reaching the water tank in an uncontrolled manner.


  1. Is there a simple piping fix (e.g. heat trap?) that will prevent boiler water from reaching the water tank unless its circulator pump is enabled?

  2. Is this evidence of an installation/design error, or is this a common drawback of indirect water tanks?

Edit: this diagram, with zone circulators but without zone valves, matches my install: enter image description here

  • Had that problem once, a bit backwards from your design. A solenoid shut off valve wired into the pump should fix it.
    – crip659
    Sep 2, 2023 at 17:28

1 Answer 1


When we design a heating system we give domestic hot water (DHW) priority, we allow a heatinhg circuit via gravity to flow through the DHW tank, it's logical that DHW should be heated to set temperature as long as heat is created, by gas or any means, the heat allowed in the tank is controled via a cylinder thermostat and DHW zone valve.

Your questions:

  1. The transfere is on purpose, the idea being supply DHW flow on any demand, but will stop when tank reaches set temerature.

  2. Answered in answer 1

  • Good to know this is by design. Is the zone valve built into the tank? Is it powered? I ask because I don't see it, or how it is powered. In my system, I only see a circulator pump, controlled by a thermostat, that actively pumps water through the tank. The pump, however, doesn't do anything to stop passive flow from gravity. So I don't fully see how the temperature is regulated unless the valve you mention is somehow integrated into the tank.
    – user169686
    Sep 3, 2023 at 21:57
  • The zone valve will be on the pipework before it enters the tanks heating flow circuit, if you edit your post and show a picture of your tank I can label the equipment. Sorry did not see your edit, it's the system zone valves Sep 4, 2023 at 1:49
  • Thanks for the reply. Unfortunately, I do not have zone valves, only circulator pumps. Hence, my concern that boiler water is heating the water tank in an uncontrolled manner.
    – user169686
    Sep 4, 2023 at 16:58

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