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What happens when there is no heat demand? By heat demand I mean there is no demand for space heating or domestic hot water. Is the water inside the pipes still flowing? Is this why bypass valves exist?

I am trying to characterise the output of a condensing boiler(s) (connected in series) which are generating the heat demand required by a sum of apartment blocks. I have assumed that each apartment flat has a Heat Interface Unit. I know how to calculate the required flow rate to each HIU when there is heat demand. However, I don't know what happens when there is no demand to an apartment. Is there water still flowing to the HIU but bypassing inside the unit? What happens to the return temperature of the bypassing unit?

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    Hello, and welcome to Stack Exchange. What kind of heating system do you have? Forced hot water? And, is the domestic hot water tank heated by your boiler? – Daniel Griscom Jun 8 '16 at 1:08
  • @DanielGriscom Let's assume that I have a condensing boiler connected to a heat interface unit and then to the customer. – Farhad Azer Jun 8 '16 at 9:20
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    ... still not getting it. What's a "customer"? Is this a hypothetical question? If so, then you need to be a lot clearer on what you're asking before you're likely to get an answer. – Daniel Griscom Jun 8 '16 at 11:17
  • @DanielGriscom You are right, sorry for summarising so stupidly. So I am trying to characterise the output of a condensing boiler(s) (connected in series) which are generating the heat demand required by a sum of apartment blocks. I have assumed that each apartment flat has a Heat Interface Unit. I know how to calculate the required flow rate to each HIU when there is heat demand. However, I don't know what happens when there is no demand to an apartment. Is there water still flowing to the HIU but bypassing inside the unit? What happens to the return temperature of the bypassing unit? – Farhad Azer Jun 8 '16 at 19:16
  • In the detail department, it's best to edit this information back into the question, so that potential answerers have a better chance of seeing it in context. However, this site is focused on what a non-professional individual might do to his home; designing a heating system for a block of flats really isn't what we can address. – Daniel Griscom Jun 8 '16 at 19:38
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In a normal hot water system heat demand triggers a main circulating pump and a zone valve or zone pump.

Without demand, heat can slowly migrate through the pipes by the laws of thermodynamics.

This migration can be stopped by the zone valve or a zone pump with a built in check valve.

Hope this helps your thought process.

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