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I have an electric boiler equipped in my house, and also a timer that make me schedule the time in which I want to turn on the boiler and heat water.

I am looking for the most efficient way to run the boiler (i.e. reduce the boiler's electricity use). Let's say that, on a daily basis, I normally use only 5-10% of the total boiler capacity.

These are the alternatives that comes to my mind:

  • Turn on the boiler every day for a small amount of time (e.g. 30 minutes? this is what I am currently doing and I always have hot water available)
  • Turn on the boiler every 2 days for a longer period of time (e.g. 1 hour).

Let's say both ways produce enough hot water for my demands. Which is more energy-efficient? Is that any other, more energy-efficient way to handle the boiler?

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    Energy use is directly related to "on" time. It's that simple. After that it's a matter of whether you're meeting your usage needs.
    – isherwood
    May 1, 2023 at 21:45
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    If you manage to equate "electric water heater" with "electric boiler" you may find this answer illuminating: diy.stackexchange.com/a/244644/18078 Functionally, if you are actually using hot water at all, you might as well just leave the thing on, unless you can get a favorable off-hours rate. Do be sure you have everything (pipes included) well insulated, but it must be fairly well insulated based on your statements in the question.
    – Ecnerwal
    May 2, 2023 at 0:25
  • @isherwood that's more true for electric heating than gas with its temperature-dependent efficiency, but still doesn't capture the whole story: in a boiler-based system (as opposed to an immersion heater) there's a loop that heats up and cools down. Each time it does so it wastes heat into the house (this can be good in a cold climate, but is bad if you don't want to warm the house). Doing that less often wastes less heat. It feeds into the runtime of the boiler
    – Chris H
    May 2, 2023 at 10:28
  • How can heat be wasted into the house? Unless it's overheating, that's nonsensical to me.
    – isherwood
    May 2, 2023 at 12:49
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    @isherwood: Energy use only has a direct relationship with “on” time if the power draw is constant. As for waste: Resistive electric heating is usually undesirable (heat pumps would be much more efficient) and for several months per year most people would rather keep their homes cool than warm.
    – Michael
    May 2, 2023 at 13:36

2 Answers 2

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None of the above.

If the boiler is actually well insulated enough that you get sufficient hot water even with it is only on for a short time each day then it is not losing a lot of heat to the surrounding air. Which is a good thing. But if that's the case then you really aren't saving much because the boiler will, very likely, run non-stop during the times it is on. In other words, you might have something like 30 minutes per day instead of it running a few times each day for a few minutes after you use some hot water.

What I would do is adjust the temperature to the lowest usable temperature. If it is set to 125 F and you always mix it with some cold water, try 120 F or 115 F, etc. until you get the lowest temperature that works for you. Then just leave it on all the time, unless you are going away for several days. The boiler will run automatically when it needs to, as you use water. But then it will stop sooner than it would in a "only on some of the time but set to a higher temperature" because it doesn't have to run to the same high temperature.

Note however, that both my advice and your current practice do not work if you have a concern about growing bad stuff in the tank. If that's the case, you need to set to a higher temperature (I think typically 140 F) and keep it set high.

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  • Yep here is some advice on avoiding Legionella and it recommends storing the hot water hot (60C/140F+).
    – psmears
    May 2, 2023 at 9:20
  • The problem is really bacteria. If it were not for bacteria: Energy transfer/loss is directly related to the temperature difference and time. It’s more efficient to heat up for a short time when you need it and then let it cool back down rather than keeping it at temperature.
    – Michael
    May 2, 2023 at 13:38
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    Regarding the set temperature, some systems involve a thermostatic mixing valve to control the actual output temperature. If the OP has such a system, then they can set that temperature as low as they want while keeping the tank temp high to prevent bacterial growth, and it will still save them money (because they will be pulling less hot water from the tank to achieve the same output temperature). May 2, 2023 at 17:16
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Check out the bottom for the best solution.

Your use of "boiler" is confusing because this can describe either a tank water heater, or a hydronic house heating system which often includes water heat also. But either way...

Time-of-use or variable energy tariff from your utility

Turn on the boiler every day for a small amount of time (e.g. 30 minutes? this is what I am currently doing and I always have hot water available)

And you may notice your phone does the same thing - you get to use it anytime you want, but you only have to charge it occasionally.

"Well of course it does that, Harper, it has a battery!" Well, a tanked water heater (often called "boiler) is a thermal storage load. It has a huge and very well-insulated water tank, and acts like a huge battery - just a hot, wet one.

Now, a great many utilities offer variable-rate tariffs based on time of day, market conditions, etc. So you can set your water heater to do its heating during those times, if it is financially advantageous to do so.

If you are doing home heating with your boiler, you could have a very large and very well-insulated cistern where you keep a large mass of water that you heat when tariffs are favorable. In fact British people have storage radiators, right in rooms, which heat up bricks at night on cheap power, and then radiate that into the room during the day.

Other than that, there's not a huge savings.

And that's because water heater insulation is already pretty good. You're welcome to supplement it with more - there's no fire hazard to adding more insulation to an electric water heater, and they make insulation kits for that purpose.

When we've looked at this question in the past, we just didn't find a better answer than that. The idea of shutting it down while not in use and letting it cool down, then having to reheat it, saves a little... but it doesn't save much. Also it lowers the temperature into the "breeding bacteria" range, which is a health risk.

The best for last: Heat pump boiler. Hands down. Not even a question.

Now if we're talking "water heater" there are complications, because if it's interchanging heat in the utility room it's filling the surrounding space with cold air, which means ???. (party because of free A/C, or run the furnace more, or both depending on season).

However, in the greater space of "boilers" there certainly are outside-air-sourced heat pump boilers, either for water heat or for home heating (though more easily found for the latter, which is typically a hydronic system providing both home heat and water heat). These move the heat exchange outside, eliminating the cold utility room. And yes, better ones can work down to very cold temperatures.

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    Your first section isn't likely to be much help - we don't know that the OP has the option to make major changes to their home (it could be rented) let alone the money for an expensive upgrade (and suitable outside space etc.). The 2nd section could well be a good answer
    – Chris H
    May 2, 2023 at 10:30
  • @ChrisH are you British? If so you may be making assumptions about what "boiler" means. (your assumptions might be in sync with OP, but Stackexchange is a site that serves the whole world.) These questions endure, and are seen by hundreds of others. Now, are you saying I should delete the best option, hands down on the off-chance OP can't use it? That makes no sense. I for one trust OP to know when an option won't fit them. Also, part 1 included unitary heat pump water heaters, which are drop-ins - discussed in the videos I linked and cued up. May 2, 2023 at 19:05
  • yes, because the use of "boiler" for a heating device seems heavily skewed to British English. The question is about "how should I use...", not "what should I install..." leading with an answer to the latter makes little sense.
    – Chris H
    May 2, 2023 at 19:18
  • @ChrisH It would be negligent of me to delete the silver-bullet solution. However, I am willing to move it to the bottom. May 2, 2023 at 20:31
  • outside-air-source water heaters do exist, but depending on the net installed cost (after any government or incentives) may not pay off during the equipment's lifetime.
    – stannius
    May 2, 2023 at 21:36

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