Random googled data:
Water heaters must be less than 140 F per IPC, can be higher if approved mixing valve is installed on output.
120 F is considered the limit for scald protection.
110 F is ADA/ child safer.
The other issue is that setting the temperature lower also promotes the growth of legionnaires. Most recommend a minimum of 120F to prevent the growth of legionnaires.
This limits your ability to use your hot water heater as a method of energy storage safely, unless you do add a thermostatic mixing valve which allows you to set your water heater temperature higher.
The rheem system uses their proprietary Econet platform. It's all cloud controlled. Some people have figured out certain endpoints. I found one that lists the power usage and published my findings on the 'smarthings' website.
I haven't tried pushing any settings yet, just reading the values.
I just installed a Rheem 50 gal Heat Pump Water. The short-review: It's quiet (can barely hear it on the other side of a wall), and if the consumption data being provided by the Econet app is to be believed, it's very efficient in heat-pump mode only using about 1.4 kWh / day on average for the past 3 days with typical hot water use. This is in San Diego, so relatively warm inlet water temps (after the first fill, tank measured ~70F) and relatively warm ambient temps (garage ranging from 70-75F).
This replaced a 12 yo gas water heater rated to use 250 therms/year, but I estimate only used 84 therms/year or 7 therms/month based on my billing data from non-heating months (hard to completely separate furnace gas usage from water heater gas usage).
If the Econet energy usage data is correct, it's on track to only use 40-50 kWh / this month. This is the equivalent of about 1.4-1.7 therms of gas / month in raw energy. If the heat-pump has an UEF of 3.5 and the old gas heater an UEF of 0.6, does this add up? 50 kWh of heat pump energy over a month turns into 175 kWh of energy into the tank. A therm of gas has 29.3 kWh equivalent, of which only 60% makes it out of the tank - 7 therms * 29.3 kwh * 60% = 123 kWh of energy. Hmm, so maybe not as efficient as it claims? Either way, I'm not even going to able to notice 50 kWh / month on my energy bill.
As far as using Econet to assist with energy storage - one real drawback is that you have to use the app to program the water heater for different temperatures based on time of day - you can't do it from the control panel. The app is also fairly buggy, at least on Android - it crashes regularly and had some weird issues setting up the time periods in the schedule. It gives you the ability to set 4 different time periods with different operating modes, allowing you to set the time, temperature and operating mode (heat pump, energy saver, high demand, electric). But given the safety issues with varying your temperatures from 120F, that significantly limits the amount of load shifting you can get away with.
A thermostatic mixing valve would let you safely set much higher water temps, but this does come at the expense of a reduction in efficiency - the heat pump will be less efficient in heating water up to higher temperatures than lower temperatures.
For now, I am trying out setting the target temperature to 115F during utility peak rates and 125F the last hour of super-off-peak rates (and 120F the rest of the time), but I think that eventually I will end up narrowing this range down given the low overall usage of the heater.
As an aside - why did I choose this over another gas furnace? At current electricity / gas rates here in San Diego, there is no financial benefit to doing so - in fact, even with the vastly higher efficiency, running the old burner is slightly cheaper - gas is about $1.40/therm, electricity varies from $0.10-0.50/kWh (average around $0.25/kWh). But with the heat pump I get a chance to eliminate a large chunk of my natural gas usage, I get to cool the garage in the summer instead of heating it and I have the ability to add more solar (would only take a single solar panel) to offset the energy usage of that appliance.
