Self-regulating water heater


Most everyone is looking to live a little greener these days, with motivating factors typically being the preservation of the environment or financial considerations. [Fabien] fit into the latter category after realizing that about 25% of his monthly gas bill went to heating the water he and his family use each day. After a few calculations, he found that they only required hot water 68 of the 168 hours per week that the water heater was typically running. He figured the best way to save a few dollars was to rig the water heater to turn itself down when it wasn’t being used.

He connected a servo to the temperature control knob on his water heater, allowing it to be adjusted by a microcontroller. Having a rough idea as to the schedule his family keeps during an average week, he wrote an application for his Netduino that would actuate the servo when needed. A DS1307 real-time clock was wired to the Netduino for accurate timekeeping, so as to ensure [Fabien's] wife never had to endure a cold shower.

It’s a shame that most water heaters don’t ship with some sort of programmable thermostat like you see with newer HVAC systems, but this hack is definitely a step in the right direction.

Continue reading to see his power-saving water heater in action.

74 thoughts on “Self-regulating water heater

  1. This will not save as much as you think.

    The cost to heat the water and cool and heat and cool and heat is more than what it will take to maintain the temperature with a properly insulated water heater. (Same concept applies to driving for MPG’s, a constant 45mph is better than 60mph accelleration, stop, 60mph accelleration, stop. etc.)

    I have a geothermal heat pump with an auxiliary output to assist the hot water tank. A programmable thermostat is contraindicated to well insulated low output heaters.

  2. For those of you in Europe who are appalled at the US usage of tanks, the jury is still out on what is more efficient. It all depends on usage patterns. Different isn’t always worse (or better).

  3. Gas prices in the US must be notably cheaper than in the UK then, because I know personally two people who have saved over $500 a year in switching to a tankless system. As for them not heating at the correct rates etc, you’re wrong unless you’re working on 80s technology. You buy a boiler for the max draw and it self-adjusts gas downwards to maintain efficiency for the given heat requirement – using a small tap, tiny flame, using two large showers – full flame. For a medium sized house here you can buy one for £800, 1200 fitted, and they require sod all maintenance.

    Believe what you like, the physics of storing large tanks for occasional use makes no sense at all unless you have amazing insulation or ambient temperature not far off the water temp (then you might as well go solar heated).

  4. “The cost to heat the water and cool and heat and cool and heat is more than what it will take to maintain the temperature with a properly insulated water heater.”

    Fundamental misunderstanding of physics. Steady state temperature loses more energy to environment than allowing it to drop between uses because heat loss is governed by delta T. Unless your heater system is horrifically inefficient when working at full chat and vastly efficient when trickling along (and in reality they’re not that much different) the steady state loss will be much larger than the variable loss.

  5. @Anonymouse @CutThroughStuffGuy etc

    I used 10 Btu/h loss as a simple mathematical example to help you understand the physics that I was describing because people seem to understand numerical example better than theory (proven by the completely wrong understanding of what I described in my first comment). Show me the fallacy in my scientific argument and I will glady admit I made a mistake. In the meantime stop taking small pieces of what I said out of context then using hand waving and oft wrong intuition based arguments to say I am wrong or downplay the truth of the scientific principals.

  6. ““It takes 12,000 BTU to equal 1 ton…”

    In case you were wondering, that’s 9,338,031.15 foot-pounds.”


  7. Maybe there is better technology now for tankless, but when I checked it out (8 years ago) when I replaced my water heater all the stated issues were there.

    How do I save $500/yr when

    Gas Water Heater installed : $400
    Tankless Gas Water Heater Installed: $1,300

    My cost to operate Gas Water heater $240/yr

    Lets ASSUME the tankless is twice as efficient as conventional AND assume that both will have the same maintenance costs (which is not true). Even with these assumptions I can save $120/yr by converting which will take me 7.5 years to break even.

    Now back to a more realistic comparison according to some research I just quickly did, Tankless is about 24% – 34% more efficient IF you use less than 41 gallons a day, so lets split that and use 29% efficiency gain. I currently pay $240/yr and that would goto about $170/yr a savings of $70/yr. Now is going to take 12.8 years to recover the additional cost and that is without factoring in the additional maintenance required for tankless systems.

    Now if we add in recommended maintenance to flush deposits out of the boiler of a tankless system, an additional $150 – $200/yr that is not present in a conventional system and run the numbers again

    For tankless $170/yr for Gas plus $150/yr Maintenance or a total operating cost of $330/yr

    For conventional $240/yr. So if I go tankless how much money do I save?

    To further support my position the below is taken from Consumer reports website.

    Heating water accounts for up to 30 percent of the average home’s energy budget. Some makers of gas-fired tankless water heaters claim their products can cut your energy costs up to half over regular storage heaters. So is it time to switch?

    Probably not. Gas tankless water heaters, which use high-powered burners to quickly heat water as it runs through a heat exchanger, were 22 percent more energy efficient on average than the gas-fired storage-tank models in our tests. That translates into a savings of around $70 to $80 per year, based on 2008 national energy costs. But because they cost much more than storage water heaters, it can take up to 22 years to break even—longer than the 20-year life of many models. Moreover, our online poll of 1,200 readers revealed wide variations in installation costs, energy savings, and satisfaction.

    So I am really doubting someone who claims a $500/yr savings by switching, at least in a residential setting.

  8. @swighton

    not necessarily. your setback period needs to be long enough that the energy used to recover is less than the energy to maintain.

    because tank hot water heaters loose so little heat, it takes very little to maintain temp. lets pick an easy number and say 1 degree an hour.

    over 8 hours of having the thermostat lower, you’ll loose 8 degrees, which will force the boiler to fire long enough to bring the water back up 8 degrees. likewise if the thermo was constant, it might have fired for 1/8 the total time 8 times over those 8 hours. same energy input.

    next to no efficiency gained or lost, energy lost tinkering with t-stat.

    if, as in my example, you only need warm water most of the time, say 100F, for hand washing, but will need 140F (and a safe temp mixing valve for sinks) to run the dishwasher one day a week, then you’ve got a period of longer than the 40 hrs necessary for the temp to drop to 100, and every hour over that 40 you’ll be saving energy.

    in an un-insulated tank, or very cold utility room, you might start to see savings over half a day, but until the boiler is starting to fire at the lower temp to maintain that point, it’s not more efficient.

    science FTW.

    (i do realize that as temp differences between tank and ambient decrease, thermal loss decreases, do the 1 # isn’t constant, but it made the math easy to explain, and the difference in this case wouldn’t matter much)

    now, on to tankless on demand systems. they shine for efficiency where you have a need for continuous hot water (because you’re not keeping a battery of water stable) and/or where you only need hot water less often than the thermal loss time for a tank. there’s a time and a place for most tools, recognizing which one is the most apt is important.

  9. @metis

    “i do realize that as temp differences between tank and ambient decrease, thermal loss decreases, do the 1 # isn’t constant, but it made the math easy to explain, and the difference in this case wouldn’t matter much”

    That is a very telling statement: the whole point of this debate centers around the fact that your thermal losses (energy leaving that must be replaced) are less as the temperature decreases, and thus the water heater uses less energy when you turn the heater off then warm the water back up. Ignoring this to make your math (which was also wrong by the way) easier made you completely miss the entire point.

    I have conflicting feelings about rebutting your comment because your reasoning shows a very flawed understanding of science (temperature is not a unit of energy!!!!) which will probably result in very little change in your understanding, however I must.

    First things first – the first law of thermodynamics – it informally states that in any system energy is conserved. This means that any energy put into the system stays in the system until you take it out somehow (via heat transfer or work which there wont be any of). This means all that energy used to heat the water stays in the water until it is transferred out to the surroundings.

    In the case of the water heater, since the temperature will be at or above ambient in normal operating conditions the second law of thermodynamics (once again informally) tells us that energy will propagate from regions of higher temperature to lower temperature, thus the hot heater will transfer energy into the water in the heater, and the warm water heater will transfer energy to the surroundings because it is warmer. Those are the two ways that the internal energy of the heater can change. Holding the heater at a constant temperature will have an associated heat transfer out (loss of energy) that must be replaced by the heater in order to hold a constant temperature. As I explained previously Turning off the heater and allowing the temperature to drop results in less energy leaving in a given time. Yes the energy that left will have to be replaced to bring the water back up to temperature, however the total amount of energy used will be less than holding it at a constant temperature because less energy will have left. If you can’t understand this I can do nothing for you except to say again that temperature is not a unit of energy.

    I do agree with you on one point though – Science FTW, assuming you’re using it right.

  10. “I have conflicting feelings about rebutting your comment…however I must.”

    And thank you, @swighton, for doing so. @metis’ misconception on this topic seems to be a common one, and while I’ve thought that said misconception can’t be true, people seem to believe it so wholeheartedly that I’ve wondered what the right answer is. Thank you again for putting a solid, reasoned, and understandable answer to it!

  11. @everyone who thinks turning a system off uses more energy

    Because it can be confusing, I try to avoid bringing mathematics into a discussion and rely on theory, but I feel that they might be necessary in this situation.

    I’ve used some nominal values to calculate the energy used in water heater that is left on and a water heater that is turned off then turned back on to show mathematically that there is an energy savings. Note that these are nominal values and you CANT draw conclusions about the actual savings in a real system (e.g. don’t say 3% savings is nothing!). I’m just illustrating the energy savings. You can check out the math at the link below.

  12. @swighton

    MathCAD approved :p

    Correct me if I’m wrong… But roughly 100kJ in saving per day, for one year is ~0.34187 Therms per year saved.

    At the current rate around $0.90 per therm for natural gas, that translates into an economic savings of $0.31 per year.

    Using the 3100000J to maintain temperature for 10 hours…

    310000J per hour
    7440000J per day
    2715600000J per year
    226300000J per month (year/12)

    That’s a savings of About 25 therms per year – At the same ~$0.90 per therm cost – that’s $22.50 per year.

    If that amount of energy came from electric sources… Savings comes out to ~$75/year (@10 cents per kWh) – a fair bit closer to economically viable.

  13. eek! I botched my language from getting distracted in the middle of writing (face palm)

    >That’s a savings of About 25 therms per year – At the same ~$0.90 per therm cost – that’s $22.50 per year.<

    Should have been…. It cost $22.50 per year to maintain the water tank temperature

    Similarly, it cost $75/year to maintain water temperature…

  14. @mass_producer

    You can’t really use my calculations to prove the savings – I had to lowball the convection coefficient to get the biot number (which is used to determine if treating a body as a lumped thermal capacitance is appropriate) small enough to justify the mathematical model I used.

    Real world values for the convection coefficient would probably be larger which would increase the losses and thus the savings – but would require a much more complicated mathematical model to prove/get good numbers.

    I’m not all that concerned with what the savings are – just the fact that they exist.

  15. Problem being that you’re calculating this assuming 2 things: 1.) a non-insulated tank and 2.) that the heater for the tank runs continuously.

    When you consider point 1.), the surface temperature of my water heater at the time of writing this felt to be about 80 deg. Fahreheit (in an approx. 70 degree room). That makes the delta T for your convection equation much lower, therefore making the SS energy consumption lower as well.

    Consider this for point 2.): motor vehicle fuel consumption; specifically, let’s look at the fact that cars have two gas mileage estimates on them – one for city driving where there is a large amount of fluctuation in speed and one for highway mileage where we maintain a steady-state value. Which consumption rate is smaller (i.e. 1/mileage)? The highway mileage. It supports the notion in controls engineering that maintenance of the steady state of a system is much more efficient when you allow for small transients rather than large ones.

    One could make the argument that this hack follows the same idea as a programmable HVAC thermostat. While true, there are much larger thermal losses over a whole-house system vs. the water heater at hand, thus making the water heater model much more efficient. Also of note in this case, air has a much lower thermal coefficient than does water, making it easier (read: cheaper) to heat and cool per unit mass than water.

    A shower timer reducing the amount of water consumed as a whole would have been a much more energy-saving option.

  16. Alston,

    It doesn’t matter if its insulated – insulation affects both cases equally. In the case of keeping water hot, it loses less energy to the environment. In the case of coast and burn – it loses less energy and requires less energy to restore operating temperature.

    Using surface temperature doesn’t tell us much. The surface temperature is dependent on the water temperature and temperature of the environment. You’d need to know one of those and the R-value of the tank to then come to the same conclusions as swighton. That said – you *can* use surface temperature to add a radiation component to heat loss, but that only adds to the case where savings exist.

    No – it’s only measuring the amount of heat necessary to maintain temperature. How fast it does this is another matter. If you need 100 watts per hour (for example), you can do that by adding 1.666 watts constantly 1000 watts for 6 minutes.

    Hrmmm – I think we can all agree that we can’t drive a water heater to work mostly because cars are not water heaters. Cars have a lot more sources for energy IO.

  17. @Alston

    mass_producer explained it perfectly so I won’t go into too much detail except to say that you can’t use the outside temperature of the water heater for the delta T because the insulation will make the surface temperature much lower than the fluid temperature. In fact I just realized something…. you just game me some real world data for the surface temperature which makes for a very simple calculation…. YES! Thank you!

    Let me preface by saying that I way lowballed the convection coefficient to use the lumped capacitance as a simple model – lowballing the convection coefficient must have helped to account for insulative effects. Real world convection coefficients are typically over an order of magnitude larger that what I used – more like 5-15W/m^2K so your steady state energy loss will be (using the typical low end 5W/m^2K and your real world data)

    h*A*(deltaT)= 5 [W/m^2-K] * 2.87 [m^2] * (80 [F] – 70[F]) = 79.7W

    That’s remarkably consistent with my lumped capacitance model (I got 86 W using that one).

    Now I need to point out that you have made a very bad comparison of a water heater to a car. I’ll take it from the top: power is the rate of using energy. In SI energy is measured in Joules (which is force over distance) and power is measured in watts (one watt is one Joule per second). Cars use more energy in the city (less MPG) because you are constantly having to accelerate them from a stop which requires an input of energy (because you are accelerating a mass which requires a force given by F=MA, and force*distance is the energy usage) that wouldn’t normally be required driving at a constant rate and is due to the fact that you forcibly pull energy out of the car with the brakes. Stopping a car then starting it again is roughly equivalent to adding ice to the water in the tank then heating it up to the hot temperature. It is totally different than what is going in this situation and demonstrates that you have an incomplete understanding of the system.

    If I was modeling the energy consumption of a car I would use what I just described to model it and prove that a car uses more energy by stopping and starting which is the same thing that I did for the water heater. Lets make something clear: Car != water heater, you are making a big logical fallacy when you say:

    “It supports the notion in controls engineering that maintenance of the steady state of a system is much more efficient when you allow for small transients rather than large ones”

    My mathematical model models the TRANSIENTS that you speak of to show the energy loss and to prove that a heater that is turned off and back on uses less energy over time. I don’t like to be harsh or aggressive, but clearly you don’t know what you are talking about so I’m going to stop this discussion now.

  18. good grief.

    one of my favorite physics course explanations began with “assume a horse is a sphere…” we know a horse is not a sphere, but we do know enough about it to make some educated guesses for the purposes of an example. temp is not energy, however the delta temp corresponds to the delta energy, and is easier to be illustrative with. considering the actual losses over a period of 10 hours, there’s less skew.

    to make this work, you need to actually not add energy long enough to for system to have lost enough energy that the input energy required to bring it back to operation is higher than the maintenance cost.

    you entirely missed the point of my explanation to prove possible $20 cost savings over 10 hours of off time over the course of a year.

    insulation slows thermal loss. this is a well reported reasonably well documented idea. out of curiosity, i turned my water heater off last night. (typical home unit, in 65ish degree basement) this morning, i turned it back on. it did not fire. i hopped in the shower, and as usual, about half way through it fired. (i can hear the boiler fire as the shower is ~10′ away) this means that the maintenance energy for it overnight was functionally zero. for all practical purposes, the same as had i left it on. anecdotal evidence yes, but it proves my point that there’s not buckets of heat leaving.

    assuming 8 hour work days, and 8 hours of sleeping, and wanting to have hot water when you wake, and an anal retentive schedule, it’s reasonable to have the heater off for (2) 7.75 hour cycles per day.

    if i was going to be away for the weekend, or out of town for a week, it makes perfect sense to turn down the tstat on it, however on the average day, significantly less so, probably none at all in my basement.

    yes, it *might* save me $20 a year, however take into consideration the parts cost and time needed to fabricate program and debug the thing. lets be conservative and call it an hour of build, hour of programming, an hour of install and $10 in parts. figure a billable rate lowball of $100/hr for someone trained enough to do soldering and simple programming, and the actual user cost of the thing is ~$300, or about 15 years before ROI, and longer than the life of the heater. your time off isn’t free, it still has value. (not to denigrate the fun of doing something like this, but remember it still has a cost, just different costs from a game of billiards at the pub)

    engineers who do this professionally have not deigned it necessary to put water heaters on timers to gain efficiency. odds are they know how to do their job better than we do, and have decided it didn’t produce any real savings that are highly sought after for marketing purposes.

    given the stupid levels of home automation out there for nearly every system, it seems strange that no one has jumped on this were there any real energy savings. maybe because there aren’t any for an occupied home.

    now, that said, an uninsulated heater, going to higher water temps, an uninsulated mechanical room, stupid high gas costs, long periods of no demand, are all reasons why this can be a great energy saver. for most folks, it’s moot.

  19. @metis

    I can agree with your reasoning – it is clearly well thought out and logical and most importantly doesn’t violate any scientific laws or try to shoehorn anecdotal evidence to disprove one. You’ve made a good case for the low cost savings.

    I personally can’t make any remarks on savings from a system – my concern is simply to refute the misconception that more or the same amount of energy is used when the heater is turned off and then back on.

  20. Yay! Fallacies in my logic induced by not thinking before typing…

    Dumping ice into the water heater would be the equivalent of taking a shower and the hot water getting replaced by incoming cold water. So, I didn’t really explain that point fully… It’s futile at this point; I’ve destroyed my credibility in this argument.

    If I go back and re-examine my logic, then yes you’re correct, it could lower the energy usage by a small amount. If you use equal amounts of lower and higher temperatures, you’re effectively splitting the difference between the temperature settings. But then everything metis said kicks in.

  21. @Alston

    All calculations (and arguments) by me are for the energy saved when the water heater isn’t being used (no cold water in/hot water out) with the intent of showing that it doesn’t take more energy to allow the water to cool then heat it back up (centered around misunderstanding of the conservation of energy and the rate of energy loss from a heated system being tied to temperature).

    You seem to have a good mind – apologies if I was harsh – for some reason when someone uses a logical fallacy (accidentally in this case) to disprove me it really gets me going.

    The energy usage of the water heater while being used is well beyond the scope of what I was arguing and to be honest to try to model it is a lot more thermo-fluids than I would care to do. Would you believe it if I told you that I hate (like with passion) thermodynamics? Anyway I think were pretty much in agreement. :)

  22. One thing everyone seems to be missing is that if you live in the colder regions of the world (like Norway), the water heater won’t waste any electricity no matter what (except for when we’re lucky enough to have a few warm weeks of summer).

    The heat “lost” from the water heater goes into the total heating of the house, which lessens the demand on, say, thermostat controlled electrical heaters.

    So, unless you’ve got it so hot outside you actually need active cooling inside your home, the trusty old hot water heater isn’t really robbing you at all. If you are lucky enough to live in Hawaii or Thailand… what on earth do you need a huge reservoir of hot water for anyways?

  23. My water heater has a draft blower, so it requires electricity to heat water. So, I used a cheap Ikea timer to turn the water heater off in the evening, and back on again shortly before rising in the morning.

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