Rheem Heat Pump Water Heater Schedule

PleaseGIVEmeWATER

We have had our 66-gallon Rheem hybrid heat pump water heater for about two months now. We are trying to figure out the best schedule for minimal energy usage while still having enough hot water for two people. We take showers in the morning. We are in the process of trying different schedules, energy-saver vs. heat pump, and trying to figure out how hot we really need to set it. It is frustrating since the app only shows us the temperature that we have it set at rather than the actual water temperature. If you have gone through this analysis and found a schedule that works well for you, please share! Or even a schedule that you think should work, but they don't, maybe I can learn from that too.

So far we seem to have less consumption while using the Energy-Efficient mode, so we have all four time periods set for that. We tried scheduling "off" for certain periods during the most expensive time frames, but the system would not kick back in when the schedule changed back to on in the energy-efficient mode.

Thanks for your help and input.

EdTheHeaterMan

Meeting at 9:00 AM. Topic: "Things are Heating up"

Coffee at 10:30 with Bradford

11:00 AM flushing

Lunch at noon with Bock 32

1;00PM. AO Smith at the Ritz.

Afternoon free for sauna then a bath

Home by 5:00PM

Family dinner at 6:00 with the girls: Ultra, Platinum, and Classic. Low Boy has a date with a Skinny Model so he will be leaving early.

Larry Weingarten

Hi, What you're asking is much like timing the stock market. Things change all the time so it's just going to be imprecise. Another approach is to put "too much" insulation on anything that can lose heat, so it just doesn't matter when you use hot water. Trying to get the heater to never use the resistance elements will keep the COP high. Keeping recirculation down to the bare minimum will also do a lot to reduce heat loss from the system.

Yours, Larry

ethicalpaul

Set it to "heat pump only" and forget it

If you find yourself running out of hot water then try "energy saver" which is oddly named since it uses more energy than "heat pump only" mode. In fact, their kind of dumb app is always trying to get me to switch from "heat pump only" to "energy saver" supposedly to save energy. It makes no sense.

But in "heat pump only" my wife and I use about 25 cents per day with our Rheem 50 gallon and we never run out of hot water even though she takes long showers and also baths in a soaker tub, so I think you'd be just fine with a 66 gallon.

What temperature do you keep yours at? From their factory they are like 120 degrees. I boosted mine to 130 which of course results in more hot water because of tempering at the faucet.

Summary: don't overthink this, it's not worth it

JakeCK

I run mine at 140f to maximize it as a thermal battery.

While I agree with Paul for the most part, I wonder if there is a point during its cycle while it is getting close to its max temp that COP falls to 1. At this point it would make sense to switch to the elements and just get there quicker since a cop of 1 is the same efficiency as any other heating element. But it would take very close monitoring of all the variables to figure this out.

hot_rod

some useful info and formulas here

https://idronics.caleffi.com/sites/default/files/magazine/file/idronics_33_na.pdf

JakeCK

Here you can see my attempt at both maximizing solar usage, the tank as a thermal battery, and trying to run it at off peak times, to varying degrees of success.

Today was atypical, the wife washed several loads of laundry very late in the evening. The tank is actually drawn down a good bit right now.

And there is a massive difference in performance between late winter and mid summer. I've been debating on pulling the trigger on a drain heat recovery pipe for the main bathroom shower. Run times are nearly doubled in the winter because of the very cold supply.

ethicalpaul

drain heat recovery can’t possibly reach return on investment imo

JakeCK

https://forum.heatinghelp.com/discussion/comment/1803672#Comment_1803672

Yes it can, because it doesnt wear out or break down. Its just a copper pipe. But who said anything about ROI? It can nealy double the amount of time spent in a shower before you run out of water.

But yes it does have an ROI and I can even calculate it using their listed "efficiency" of about 46%. At my current POCo's rate it's roi is about 10 years with how much it'll shave off its run time. Avg $/kwh .12, cost to purchase 735$ off supplyhouse, efficiency 46%, usage per year 1375kwh

1375kwh . %46 = 632kwh recovered.

632kwh . .12kwh = 76$/yr

$735 / $76 = 9.67 yrs.

And this a bit conservative because I did not adjust for inflation over 10 years. Granted the argument could be made that this would only recover from showers, not the dish washer or clothes washer. But there are multiple showers taken a day, we only wash clothes or dishes a couple times a week. In addition to that my kids are young today, but some day they'll be teenagers, and teens take long, long showers.

Now my situation is a bit unique with regard to cost of energy because of the solar panels but a kwh is a kwh. And maybe I want to buy an EV. That 632kwh/yr is about 5 months worth of commuting for me with your average EV.

JakeCK

But wait theres more! Reduced run time means less wear on the compressor, valve, fan, and possibly heating elements. Maybe instead of 10-15 years of life out of this water heater I can push it to 15-20.

How much is that worth? Remember these tanks are ~1500$, and that is buying it from menards and installing it yourself. Think they'll be cheaper in 15 years? Me thinks not, quite the opposite in fact.

ethicalpaul

doubling the time in the shower seems wildly optimistic but ok. Seems like an uninsulated tank in the basement next to the boiler before the water heater would do the same thing

JakeCK

https://forum.heatinghelp.com/discussion/comment/1803686#Comment_1803686

Are you questioning my math or the manufacturers recovery rate? All of the various manufacturers have similar recovery rates. Its a function of length of the hx pipe. 46% doesn't seem like some wildly unrealistic number. And the math is pretty straightforward. If you recover 46% of the heat going down the drain and send it back to the shower/water heater that means you're going to use 46% less energy.

Am I missing something?

And my boiler only runs about 7 months of the year and they're on opposite sides of the house.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1803682#Comment_1803682

Where I am, Washington, DC, new construction can cost upward of $500 per square foot. Just the space savings from a 40 gallon tank instead of an 80 gallon tank would justify spending $735.

Hot_water_fan

Drain heat recovery might have the best ROI of anything proposed on the wall. That said - the cheaper the heat it recovers, the worse the ROI is.

Larry Weingarten

Hi, One more thing about drain water heat recovery is that you can increase overall efficiency by increasing the length of the unit. Here's some info from Power Pipe: https://renewability.com/reference/ It's a counterflow heat exchanger, so is not hard to get efficiencies over 60%. This can have a substantial effect on you hot water making needs.

I'll add that ROI is convenient, but life-cycle cost is probably a better metric if you plan on being in a place for a while. And even if you move after a year, you can show the new owner the low energy bills and perhaps recoup the cost that way.

Yours, Larry

JakeCK

At an interest rate of ~3% I ain't going anywhere anytime soon. Lol I guess thats one advantage of high(er) interest rates. It forces people to stay put and actually invest in longer term improvements to their house.

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1803691#Comment_1803691

no I’m not questioning the idea of water heater recovery time, I’m questioning how much heat you can recover from a drain pipe