January 22nd, 2023

Setting up a Rheem Heat Pump Water Heater, Part 2

(Updated February 1, 2023 after a long series of experiments revealed surprising results)

How to Save a New Schedule

After opening the app (EcoNet version 6.x, not the slight variant called Rheem EcoNet nor the other slight variant called Ruud EcoNet), click the central area of the first screen to go to the screen which shows the circular temperature graphic.  It ranges from 110° F to 140° F.  Open the Schedule pane by clicking the Schedule icon in the lower left.  It always opens with today's schedule selected and shown.  If you want to edit a different day, choose that day so you can edit it instead, or you can easily copy the schedule for any day of the week to any other day(s) of the week.

Then, for each of up to 4 events for that day, you can change the time (in 15-minute increments), the operational mode (Energy Saver, Heat Pump, etc), and the target temperature (110 to 140, or even higher with a special override on account of the extreme scalding danger).  Click the right arrow for a given event, change the settings, then hit Save to save that event only.

Then, if you want to apply that same schedule to any or all other days of the week, click Copy then select any of the other six days and click Save.

Finally, save the entire schedule that you’ve just edited, remembering that this is actually 7 schedules for 7 days, by clicking Save Schedule!

To be sure you’ve saved them and remembered to set everything as intended, re-open the Schedule pane and look again to double-check one or more days that you changed.

Note: The “Available Hot Water” report shown on the second screen (with the circular temp graphic) attempts to explain what approximate fraction of a tank is ready for use, meaning approximately fully heated up.  This readout has to deal with two kinds of conditions: the entire tank having cooled down from being off for many hours or otherwise in between heating cycles and some of the tank having been replaced (a portion at the bottom) with cold water as a result of hot water use.  It is quite approximate as a result of this.  I've mostly seen it saying "Full" but sometimes it shows the tank graphic looking less than completely full, and at times I've seen it also labelled as ⅔ or even ⅓, though at no point should our tank have actually been that low.

Miscellaneous Tips:

For any given target temperature, the water heater will keep the water leaving the top of the tank within a range of plus or minus probably about a degree and a half of the target, so I would guess that setting 112° F will result in outgoing water temps of about 110.5 to 113.5° F.  As with any thermostatically controlled system, there has to be a fluctuation which corresponds to the On/Off cycle of the heating system(s).  There are no partially on modes of either of these two heating systems (heat pump or resistive elements).

Contrary to the implication of Rheem’s frequent urging, the most energy efficient operational mode of the water heater by a large margin should be the Heat Pump mode, meaning heat pump only [but amazingly enough, as borne out in the experiments I did over the next eight days or so, at least under the conditions tested, the Energy Saver mode actually consistently used 15 to 20% less electricity than the Heat Pump mode!  See the Part 3 article for the details.].  The rate at which the heat pump alone can heat the tank is fairly low compared to conventional electric or natural gas water heaters and it varies according to the ambient temperature, so recovery (meaning heating up cold water that enters the bottom of the tank because we use hot water from the top of the tank) is slower in Heat Pump mode than when the energy hog, resistive heating elements are also used. [It looks as though artful use of one of the two resistive elements can actually help with energy efficiency in certain ways, as well as replacing heat pump operation when inlet air temps get too low.]  As a general rule, this slow recovery is of no consequence because we have a tank that starts off with a LOT of hot water, which is generally all ready to go, at whatever temperature we’ve got the system set for.  But the intricacies of heat pumps apparently offer opportunities for the resistive elements to help out in ways that don't simply have to do with cutting recovery times.

Our “40 gallon” model is rated by the EPA as 36 gallons.  We never use more than 12 gallons in a given hour, and never do that more frequently than once every few days, so we have tons of reserve for two people and would be just fine with four people too.  If you have crummy shower heads that waste a lot of water (and heat), unlike ours which use 1 gallon per minute (I measured the most-used one, a Grohe model), your showers might well use more than twice as much water.  Even so, most households wouldn’t need to resort to using one of the least efficient modes or buying a larger capacity tank so as to increase short-term capacity.  Have you noticed how many motels waste immense amounts of hot water with their ridiculous shower heads?  If you want to be a profligate water user, eating beef or having a slow leak in a toilet are likely the two top ways to do that.  As of forty-something years ago, the total water consumption of the average Californian was a staggering 5,000 gallons a day!  Of that less than 100 was for domestic use (we use less than 20 each), and 3,500 gallons was for making — beef.  Feeding alfalfa to cows is ultra water-intensive.  Toilet tanks have two valves in them.  One going in and one going out.  If either is leaking, the water is pouring down the drain and it adds up ferociously.  We need toilets that never leak, but until then, keeping a watchful eye (and ear) out is a good idea.  Does the tank always fill after a flush?  Does the inlet valve shut off 100% when the tank has filled?

When the hot water tank has cooled down by, say 6 to 10° F because you’ve turned it off during many hours of peak power pricing plus late evening or overnight, and you use Heat Pump mode to warm it back to your target temperature, it looks like the tank will warm at about 4° per hour.  If you use Energy Saver mode, I’m guessing it will heat the tank at something like 8° F per hour with our model and at around 12° F per hour with most of the Rheem models.  Our model is a 15 Amp 240 Volt model, where most are 30 Amp 240 Volt.  But when you’ve used a fraction of your tank by actually using hot water, the heating is going to be concentrated in that incoming, cold water fraction of the tank, so it will effectively heat that smaller volume much faster.  The EPA’s first hour rating for our model is 46 gallons, but they don’t specify which operational mode that’s for: "Heat Pump" or "Energy Saver".

So please just be sure to use the most energy efficient mode all the time.  I ran all of the hot water out of our tank once when leaving on a trip (after turning it to Off), so as to avoid 14.7 psi of extra, negative pressure on our pressure tank’s air bladder caused by shrinkage of the water in our closed system once the whole-house valve had been closed.  As I emptied the hot water, cold water flowing in to replace it, once the temperature of the water started to drop, it was pleasantly gradual, precluding the old problem of running out of hot water quickly while being covered with shampoo.

I may acquire a whole-house, automatic shutoff valve that operates 24/7 to reduce flooding risks, as further protection.  If you have a hot water heater, you need to understand what a closed system is, the multiple ways it can be created*, and what it will mean for pressure increases as water is being heated up.  Without a way for water to get back out to the street, a 40-gallon tank heating up by 50° F (e.g. from 65° to 115° F) will increase in volume by about 50 ounces — a quart and a half.  This could increase your water pressure from about 50 psi to about 250 psi!  A typical water heater TPR valve will release pressure (if it’s working right) at 150 psi.  The tanks are designed to hold 300 psi, but don't imagine that your entire system can!  A 2-gallon (or larger, depending mostly on water heater tank size), air bladder pressure tank can solve this serious problem, and is now required by code, so long as the bladder hasn’t ruptured, which they eventually do, and much sooner if they're not inflated to the correct pressure during installation as is often the case!  They ship with only 20 psi in them, sometimes up to 40 psi, depending on the brand, but no higher for shipping safety reasons.  They need to be inflated to match your system pressure, when there's no pressure above ambient air pressure pressing on the water side of the bladder in order to achieve their expected life span.  Various other pressure relief valves are available which can work at much lower pressures, e.g. 75 or 80 psi, to provide more protection against a burst pipe or a burst washing machine hose or some other component.  I like the Watts line of valves and plumbing components.  They have a nice video explaining how to inflate the air bladder of one of their pressure tanks.

    *1) A whole-house shutoff valve being closed, 2) A pressure reducing valve being between the water main and your building, 3) A backflow preventer having been installed by the water company at the water meter, and 4) Maybe some other such devices!  All can turn your whole house into a water-heater driven pressure cooker, capable of extreme pressure fluctuations that you can't see or feel.  I installed an inexpensive (~$25), hose thread, water pressure gauge at the unused hot water hose connection of our laundry sink.  It has a red "telltale" needle that gets moved by the main indicator needle to show you the max pressure since you last positioned it.  These telltale needles go by many names.  Our system, being set to 50 psi, has not exceeded 60 psi and sometimes briefly drops to as low as 35 during water use.  If the bladder ever fails, I'll be able to see that it has when I next look at the pressure gauge.  Pressure tanks are not expected to last as long as water heaters.  Many say water heaters only last 12 years, but our last one was good for 32 years with almost no maintenance.  That is surely exceptional, but some water heaters are better than others, some water chemistry is easier on water heaters than others, and recommended maintenance and lower temperature operation are... recommended.  Twelve years isn't nearly long enough!

I will soon begin a series of experiments where I use the water heater for whole days in both Energy Saver mode and Heat Pump mode, in an effort to measure the relative energy consumption of the two modes.  Barring an astonishing surprise (that Energy Saver really does save energy better than or even as well as Heat Pump mode) I will not have to change what I’ve written, based on common sense and understanding physics [Note, Feb. 1: I did have to change what I'd written!  Energy Saver was better at saving energy, despite that seeming to be an impossibility, given how the two modes are described].   The third mode, which uses both heating systems to the max should be called Max Recovery.  And the fourth mode, which foregoes the heat pump altogether, should be called Conventional Energy Waster mode, but it's needed for a maintenance procedure, it might also function in the event of a failure of the heat pump, though that seems quite unlikely, and if the heat pump can’t operate or is far less efficient than normal because the incoming air is too cold, e.g. below 35° F, then you can rely on the energy-hog resistive heating mode to still have water heating capability.  So I agree that it’s a good thing to have in this system — so long as people don’t use the inefficient modes for no good reason.

Note that the Sanden heat pump only water heater system uses CO2 as the refrigerant, which is not only ultra-safe for the climate if it were to leak out but which allows the system to operate down to 20° F below zero.  I.e. about 55 degrees colder than the ubiquitous American heat pump models, which currently use R134A refrigerant and which put the small heat pump on top of the water tank, unlike the Sanden, which puts it outdoors on a concrete pad, separate from the tank.

Soon, the R-134A will be replaced by a superior refrigerant, from the HFO family, which will have the great benefit of being essentially harmless to the climate in the event of a leak.  I am told by a Rheem telephone tech support person that leaks have occurred but they are rare.  Both refrigerants are about equally energy efficient in use.  Neither is harmful to stratospheric ozone in the event that they leak into the air.  But the R134A, a preeminent member of the HFC class of chemicals, is an extremely powerful greenhouse gas.  One pound of it leaked into the sky warms the planet as much as several thousand pounds of CO2, for so long as both are still actually in the sky.  It’s official atmospheric lifetime is 14 years, but I would guess, based on the IPCC's atmospheric degradation curve for methane, that this means it will only be 90% degraded after about 30 years.  The most likely new refrigerant, R-1234yf, is also a very powerful greenhouse gas but it's roughly 400X less capable of warming the surface of the planet because of an extremely short atmospheric lifetime of just 10 days.  So, part of what we need is to be sure that these refrigerants don’t leak, mainly as a result of improper end-of-life disposal!  Please help to make sure that the refrigerant in your unit is recovered and not vented!

The people who know best about the ozone and climate threats are thoroughly in support of the use of these and other heat pump devices, despite this weakness of the current refrigerants, because of their huge power savings and an understanding of the complete picture.  The new refrigerants have been in the works for many years and are close to being widely deployed into new devices in the U.S. and around the world as of now (January, 2023), thanks to the Kigali Amendment to the Montreal Protocol, which is phasing out most of the entire class of HFC chemicals, which are primarily used as refrigerants, over a period of years which necessarily differs somewhat by country.  Heat pumps are destined to become a universal ingredient of good living.

Things to Consider When Devising a Schedule

1) The wisdom of setting the lowest temperature that you require for your hot water.  The hotter the water that you store in your tank, the greater your standby losses, but also the greater your system heat capacity.  Hopefully your tank volume is more than sufficient for your actual needs or likely future needs, so that you can operate the water heater at low target temperatures.  I find that 111° or 112° F are ideal as a target in our house, perfect for showers, while sometimes needing to add just a little cold water to the mix.  Lower temps should also make the water heater last longer, and the hotter you store your water, assuming you’re not using a mixing valve at the tank outlet, the greater the scalding danger becomes.  Imagine accidentally pouring 125°F water on a baby!

2) There have been many mentions of a threat of a pathogenic bacterium, Legionella, growing in hot water systems if they're not set to 125° F or higher.  According to the WHO this water-borne bacterium and pathogen, which carries the threat of severe lung disease for some people, can "live and grow" at between 68° and 122°F (below which it would not thrive and above which it would perish).  In the U.S., the rate of infections is about one in 80,000 people per year.  Inhalation of small particles of contaminated water is usually involved and timely administration of antibiotics is always required to reduce the infection's lethality, when an infection in the lungs does take hold.  Either periodic or continuous maintenance of high tank temperatures could be important in some situations for minimizing the chance of this very serious infection from occuring.  A main question I have is the extent to which post-treatment chlorination at the water treatment plant is successful at doing what it's designed to do, to maintain a pathogen free water distribution system all the way through to the entrace to the sewer system.  This good article on the subject from our water agency provides a smart overview, and explains that the residual chlorine carefully added to water leaving their treatment plants protects against such contamination, but under circumstances of disuse, water can become stale and can lose its chlorine protection.  I do not feel any need to raise the temperature of our water heater as a means of killing Legionella bacteria.  Your circumstance might differ, for various reasons.  If your water treatment is less trustworthy or if someone in your home is at greatly increased risk because of an acute immunodeficiency it might be wise to set your water heater to 125°F or even somewhat higher, and then perhaps incorporate a mixing valve to deliver water with less of a scaling danger, though that wouldn't heat-sterilize a shower head.  I don't see any advice regarding the periodicity that might be required if intermittent tank temperature increases are to be considered.  Remember that larger swings in the temperature of a tank of water can induce very large spikes in water pressure if the building's system is a closed one.

3) Obviously keeping your hot water needs low is important, including using high quality shower heads, avoiding pointless use of hot water for clothes washing, and avoiding water leaks of any kind, which can immensely increase total water use.  Insulating your hot water pipes well is also helpful.  And being aware that anytime you turn on a hot water tap, fully hot water is leaving the tank and will be losing all of its heat, regardless of how much heat you’re yet feeling coming out of the tap.  I enjoy treating that resource gently, but of course hot water is very nice to have, if not a necessity.  It’s also a precious resource, both the heat and the water.  Each is naturally energy intensive, and remember: all of the water we use has been stolen from somewhere (a river, a groundwater aquifer...)!

4) Know when your electricity provider charges more for power, if you are on a time of day plan.  The price can vary a LOT.  The tank of a modern tank-type water heater offers the ability to easily avoid using the expensive power, e.g. by using Rheem’s “EcoNet” smart phone application to set up a schedule, though for unknown reasons, this system still uses power at an average rate of 0.6 kWh/day when it's supposed to be off.

5) If you have a rooftop solar PV system, it will usually be quite advantageous to do your water heating while your system is generating more power than you’re otherwise using, which will tend to be mid-day.

6) Power grids are most rich with power overnight, after most of us have gone to sleep, with the hours of midnight to 3 AM or perhaps 4 AM being those which can absorb the most new demand.  Though this runs counter to the time of solar PV (photo voltaic) power production, and is a time of relatively cool air temps from which the heat pump must extract heat, the grid benefits are high.

7) The heat pump is most efficient when the air from which it’s taking the heat is warm and is also benefitted some by higher relative humidity as I recall.  So this also argues in favor of mid-day or afternoon operation and against nighttime operation, especially in the hours approaching dawn, which are the coldest outside.  Aren’t we lucky that Earth’s day length is 24 hours?  If it were 48 hours, this would be a miserable place.  Super hot in the daytime and super cold at night.

8) When do you need water that’s fully warmed up?  Only for showers?  Is it perfectly OK if your water has cooled down 5 or 10° F from your target temp when you know you’re very unlikely to want a shower?  This can really help to clear space in the schedule for keeping the water heater’s power consumption at or close to zero during long periods when costs are higher and/or solar PV production from your roof is low and/or the grid is relatively overloaded with demand.  Usually peaking power generation is the most polluting and expensive, by far, so helping out the grid will soon be widely understood to be important and routine.  We can have a 100% clean grid if we want it.  Lots of progress toward one is underway.  We’re going to need more long-distance, high-voltage power transmission.

9) Bigger changes in water temperatures in the tank will drive bigger pressure fluctuations in any building that's a closed system, as many are these days, including ours, by virtue of our pressure reducing valve, which takes ~125 psi street pressure down to ~50 psi.

That may be about it.  But it’s enough that figuring out an ideal schedule will be pretty hard.  On the other hand, figuring out a good schedule is pretty easy.

Here’s the schedule that I’ve settled on for our house as of a couple of days after initially writing and publishing this article, and after making the observations noted at the end.  Our house has a PV rooftop system sufficient to make about 87% of our current, average annual use and which generates more power after high noon that before high noon:

02:00 AM Energy Saver mode 110° F (minimum water temp is likely to be at least 108° after cooling down from about 115° at 3:45 PM)

09:00 AM Energy Saver mode 112° F (a little bump with solar power)

02:00 PM Energy Saver mode 115° F (a larger solar-powered bump to increase the solar part of the total energy used and to give nicely tempered water for possible showers in the evening)

03:45 PM Off 110° F * (to end power consumption for the 4 to 9 PM peak pricing period, for the late evening medium-heavy grid demand period, and when we have no solar power)

* The actual lower limit of the Off setting is the ambient temp.  The actual lower limit of the Vacation mode setting is 65° F.

If we didn't have a PV system, I'd lean more toward adding heat to the tank in the middle of the night.  Grid use in general for the year, on average, in the service area of our utility, Pacific Gas & Electric (PG&E), which is most of Northern California, is high from about noon to about midnight and according to PG&E it’s highest from 4 to 9 PM.  The peak load shifts toward afternoon during the warmer months and toward the evening in the cooler, darker months.  So if I can load up on stored water heat mainly in the afternoon between 2 and 3 o’clock, that’s economically good for us and for what the utilities claim about the grid these days.  It also aligns well with our solar PV power generation.  I want to be sure we can shower in the morning or the evening, but I don’t care about perfectly warmed up water between perhaps 10 PM and perhaps 7 AM, so by goosing the temp briefly, starting at 2:00, I can get a bump up of a few degrees completed well before the 4 PM price increase.

About the Usage Report Screen

Power usage in kilowatt hours (1 kWh = 1,000 watts used for one hour) is shown for each of 24 periods per day (in the Daily graph).  And please: a kilowatt is a rate of flow, not an amount of energy!  Energy usage is measured in kWh, not kW!

Any usage greater than zero is only shown in the time period during which the usage ended, regardless of how many time periods that given usage occurred over, except that no energy usage is ever reported in any hour period immediately following an hour period in which a usage was reported.  In other words, if the compressor runs from 12:30 to 2:15, power was used in the 13th, 14th and 15th hours of the day.  All of that power use will be reported as having occurred in the 15th hour.  And if there is a usage in the next hour, it will not be reported until the hour after that.  So there are two main ways in which power usage is reported late.  Energy usage is never reported during two hours in a row!  I see no good reason for this limitation other than cutting internet traffic from Rheem and Ruud water heaters to Rheem's servers.

If a usage ends during a given period, the amount is reported immediately, and can then be shown in the EcoNet application (if and only if the screen is reloaded!  There are no push notifications), or within several minutes, rather than waiting for the end of that period to be plotted, and the usage is apparently never then later modified to add in a second usage which likewise ends in the same period.  It may or may not be the case that such a usage never occurs.  In one of the hybrid modes, with faster heating, that would be more likely to occur.

For unknown reasons, the water heater always uses either 0.10 or 0.11 kWh of power once each 4-hour period of the day, even when the water heater is in Vacation mode or set to Off.  This is quite odd, to say the least and it is not supposed to be doing this, according to Rheem telephone support and to common sense too.  In both Vacation mode and when set to Off, the water heater should user zero power for heating the water except when the water has cooled all the way down to 65° F and the water heater is in Vacation mode, at which point it will provide a little heat to the water, sufficient to protect against freezing, though 65° F seems like overkill for that purpose.  One exception to this rule is that at times the water heater instead uses 0.15 kWh each 6 hours.  In any case, while the water heater is set to Off or Vacation mode, it uses ~0.61 kWh per day!  Why?  I am almost 100% certain that the usage is not by the compressor (the heat pump) and that it is by the upper resistive heating element, where it is apt to be turned on for about 5 minutes.  Why?  I have not managed to measure power going to the water heater myself using a clamp-on ammeter, but I've been tempted to try.  There is never any hint of warmth at the outer surface of the water heater and plainly this reported power usage, if real, has nothing to do with the computer or wireless features of the water heater.  If real, it has to be one or both of the heating elements, but a Rheem phone support person told me that Energy Saver mode uses the upper heating element only, as a supplement to the heat pump. 

Today’s experiments have cleared up three more details of how the Usage Report screen works:

1) If a heating task is too great to be completed in the scheduled time, a programmed Off period will end the heating operation at the scheduled time.  The confusion created by the wrong reporting times for energy usage had made it unclear whether this was the case, but actually watching the water heater revealed this.  Also, I have yet to see a single instance where the EcoNet description of whether the compressor of the heat pump is operating or not was incorrect.  EcoNet reports that the compressor is on when it's on but says nothing about it when it's off.  I've also never seen it say anything about resistive heating being in process.

2) The timing reported in the graph for usage amounts doesn’t simply follow the aforementioned rule that all of the usage is reported in the last whole hour period during which that usage occurred, even if the usage occurs in two or more usage periods, rather it also will only occur in an a period which does not follow a period in which a different usage was reported.  The water heater gets into a rut and reports all usages in either odd or even hours only, but occasionally flips from odd to even or vice versa.  I've seen it flip four to six times in the data going back five and a half months.  Thus if usage occurs in the 15th and 16th hour periods, it won’t be reported until the 17th period if it's currently into odd-hour reporting!

3) And when will a usage appear if it's being reported in the hour after a usage ended? Right at the beginning of the hour as you would expect?  No!  I saw it take something like 20 minutes!  Needless to say, this means that looking at the energy usage data is very likely to leave you grossly misled regarding exactly when the water heater is actually using power.  But if you instead look at the water heater itself, in person, or if you read the notices in EcoNet regarding whether the compressor is said to be operating, then you can observe the changes of operation which give away the real times of at least the great majority of the energy use.

These are good examples of less than stellar programming.  Apart from these faux pas in the energy use reporting, EcoNet is pretty great software.  A lot of other things were done right.  Maybe they’ll fix it up when they read this, but at least now I know how to set the schedule for our water heater!  I still don't know why it consumes a tenth of a kWh every four hours when it's set to either Vacation mode or Off, however.  I'll try again to get an answer from Rheem about that over the phone, but I'm not expecting to hear a real answer.


Update Jan 24, 2023:

1) According to Rheem telephone technical support, the water heater is definitely not supposed to be using any power in either Off mode or Vacation mode, either when using a schedule or when setting those modes directly on the water heater, yet the EcoNet app is showing a very regular ~0.61 kWh of use per day when in either of these modes while using a schedule and it also did so when we were actually away for a week and I had set the heater heater to Off on the water heater itself.  The usage pattern reported is a series of 0.10 to 0.11 kWh usages which occur once each four hours, or sometimes usages of 0.15 kWh which occur each six hours. According to EcoNet telephone technical support, the app simply reports what the Rheem server tells it.  I.e. the water heater talks to your local WiFi network, and through it to a Rheem server, then the EcoNet application talks to the server to get the data reported out by the water heater.  The usage data are not stored on your device(s), and yes, data really is a plural word despite common usage.

So now I need to verify that the water heater is actually using power periodically when it’s set to Off in the schedule by standing in front of the water heater periodically to see if the heat pump is running at a time that corresponds (in the usual, tricky way) with the time of usage reported by the app.  [I did but the compressor never ran.]  By the way, the heat pump is not silent!  It can be heard easily outside our uninsulated garage or in our kitchen with the door to the garage closed if the kitchen is quiet.  The sound level of the compressor isn’t bad, but it is louder than our refrigerator, for example.  Assuming it is cycling on as reported by the app, I will have to again let Rheem know that the water heater is apparently malfunctioning, as I did this morning after a half-hour wait on the phone to talk to a Rheem rep.

2) It turns out there is only one EcoNet app for controlling water heaters, presented in versions for three different brandings: none, Rheem and Ruud. There was an older version, 5.xx and now an improved version 6.xx.  The old version had a dark UI and the new version a mostly white UI.  So there were six versions on the App Store only for a short while.  Mystery solved.  It may or may not be the case that all three can work with any given model of Rheem or Ruud heat pump water heater.  Ruud is essentially a division of Rheem.  The only data stored by the app itself is your settings, including your saved schedule, if any.

3) When asking for a second time what’s up with Rheem’s insistence that we should use “Energy Saver” mode instead of Heat Pump (only) mode for “best efficiency” or words to that effect, I got an answer that was again rather garbled but nevertheless somewhat illuminating.  I was told that in Energy Saver mode the top heating element only is used (so far so good), together with the heat pump, in an effort to extend the capacity of the tank, by not more than half a tank at a time.  Something like that.  If Rheem’s engineering team designed this mode so that it never turns on in order to simply maintain tank temp (offsetting standby losses), and if they only turn on the upper (of two) resistive heating elements when they figure that the system is fairly certain to be on course for running short of hot water, e.g. after a large fraction of the hot water has already been used, causing a corresponding rapid influx of cold water to the bottom of the tank, then maybe, depending on exactly how conservative they were in deciding how long to wait to turn on that energy hogging, upper heating element, then Energy Saver mode could be a good thing.  Because it would actually only turn on when you really need it to avoid running out of hot water because, for example, four or more people where showering in rapid succession.  But it’s impossible to know future demand so any guesses they may have made will be just that.  Additional heat can be stored either by running the water heater hotter or by buying a larger tank, of course, but I didn’t like either of those options.  The larger tank is both… larger, and it’s heavier and more of a seismic problem, harder to successfully brace and strap to the wall(s) securely enough.

I will have to run some experiments (single-days using Energy Saver all day, whenever Heat Pump mode had otherwise been scheduled) to see how much power is used, both on days where negligible hot water is used and on days when about 12 gallons of hot water is used. It would be nice if Rheem would tell us how the system actually works.  Part of what they have motivation to do is to please the EPA with respect to obtaining Energy Star ratings.  One requirement for that has been to achieve 50 gallons of first hour hot water, and my unit is rated at 46.  It would be quite ironic if the best, high-efficiency, single home water heater model in the country were not to be given an Energy Star rating.

I shall report further as I learn more.  Today I looked back through our usage patterns, clear back to the day it was installed in August.  I was pretty shocked by the variability of what I saw, given that the water heater was operated in the same, simple mode for almost the entire time.  For one thing, I saw it flip back and forth a few times between only reporting power usage for odd-numbered hours of the day to only even-numbered hours of the day! Lately it’s been all odd numbered hours (1st hour, 5th hour, 7th hour, 13th hour, like that).

The behaviors I saw in the record today are making me think this system may be almost undiagnosable, because there was a radical level of usage variation that made no sense, way beyond what I’ve been seeing lately.  Maybe it’s got something to do with improvements to the software since then.  I still don't know whether it's possible for the water heater's firmware to be updated, or whether its operation can otherwise be updated in ways that users cannot.  I also took another dive into the universe of water heaters in general, looking at unfamiliar EPA and DOE materials on the topic.  This DOE document:


states: "The energy usage per day in the above equations is based on the DOE test procedure for hot water heaters, which assumes an incoming water temperature of 58°F, hot water temperature of 135°F, and total hot water production of 64.3 gallons per day, which is the average usage for a household size of three people."

Good God, people!  This is appallingly wasteful of heating energy and water too.  Don’t forget that it takes a whole lot of energy to get purified, cold water to your house.  I've guesstimated that we use at an average of 6, maybe 7 gallons of hot water a day between two people in our house — seven times less per person!  And it’s not been heated to 135° F, rather only to about 112° F.  This hopefully pessimistic DOE estimate helps to explain why the estimated annual power consumption of an ordinary electric hot water heater works out to nine times as much as we’re using — about thirteen kWh per day.  OMG, that's as much power as our whole, 2700 square foot house!  So efficiency of the machine is supremely important, but so is simply not wasting hot water.  Showering every day is stunningly wasteful.  Profligate!  Washing clothes in hot water is a total waste.  And so is washing them before they need it!  Stop wasting hot water, America!  Our resource use damages the world, so just try to know the consequences of your actions in detail so you have a clue about it.