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I've got a combi boiler which heats the hot water tank using the indirect method. I use Hive to control it.

I'm trying to work out: if the hot water is set to on for an hour, will the boiler constantly heat the hot water tank for the entire hour, or will it detect when the water is hot and then turn off? and in like manner, if it is set to come on, and the tank is sufficiently hot from a previous slot in the schedule, will it still try and heat the tank?

Just trying to work out the best schedule and can't find an answer to this anywhere...

From a comment by the OP:

I'm trying to set up an efficient Hive hot water schedule, to give me hot water when I need it. In Hive hot water settings you can set to on/off only, believe temp control is managed by the tank... (don't have the models of tank / boiler to hand). So I wanted to know if I set the Hive schedule to On several times a day, whether it would burn gas still even if the water is already hot. I was guessing there's a temp cutoff, but wanted confirmation as I couldn't find any useful info about it elsewhere.

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    Most hot water tanks use a thermostat to control temperature, you do not want water much above 130 or 140F degrees burning your skin. Controlling water temperature by time is bad idea, it will either be too cold or more likely close to boiling and give you bad burns. – crip659 Feb 14 at 22:00
  • Do an internet search of "boiler explosions" and I think the answer will be self-evident... – Jimmy Fix-it Feb 15 at 0:03
  • Where on this planet are you? We'll need more info to provide better answers. What make/model combi boiler do you have? For many decades, water heaters are controlled by a thermostat. Some are powered via a timer to only use power in places that vary power prices by time of day. But that shouldn't interfere with the WH thermostat operation, unless I'm really missing something here. – George Anderson Feb 15 at 0:44
  • if the hot water is set to on for an hour” what does that mean? Hot water heaters are not set to on, they heat up water when the thermostat tells it that the water temperature inside has dropped below the setpoint. Then it only stays on long enough to bring the water up to the setpoint. Those are generalizations and we will need specific information about your set up and you were understanding of it in order to answer your question. – Alaska Man Feb 15 at 3:24
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    This sounds like an XY Problem. Why don't you tell us what problem you're trying to solve, instead of asking about a specific solution. You're likely to get a variety of answer that you'd never considered - there's a lot of knowledge floating around here. – FreeMan Feb 15 at 18:35
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If the water is still hot enough, the control system would not start reheating it. There is a hysteresis involved.

Explosions should never be a problem, if safety standards are followed. The redundant devices are:

  1. Safety thermostat which interrupts the energy input (switch for electric systems or oil burners, valves for gas) at a level below 100 degree Celsius.
  2. The control thermostat which is used by the control system - it must be completely independent of No. 1
  3. A safety pressure relief valve that opens the boiler water line if a high pressure is detected, f.e. 300 kPascal.
  4. Also a safety issue: The Legionella problem should be considered, i.e. once a while the complete hot water system should be heated up to at least 60-70 degree Celsius and all hot water faucets should be run for a minute.
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  • This is an excellent answer for a question about boiler explosions. Unfortunately, this question isn't about that. (At least, not yet...) – FreeMan Feb 15 at 18:36
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    @FreeMan Doesn't the very first sentence answer the question? And since boiler explosions were mentioned in comments, some infos about safety might be useful as well. – xeeka Feb 15 at 18:47

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