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Tankless water heaters are terrible....

phigment

Member
Aug 31, 2015
656
1,063
Waterloo, Ontario
Last night I finally got my preliminary monitors setup for my Rheem ASHP. I logged some data points from midnight to about 11am (after we had 3 showers). Here's the graph if anybody is interested:
Screenshot from 2019-10-09 11-12-19.png
 

phigment

Member
Aug 31, 2015
656
1,063
Waterloo, Ontario
Note, the rapid drop in the lower tank temp starting right before 7:00 was because there were 2 showers happening at the same time. The third show started at 7:45ish.

I have my temperature set point drop to 110F at 07:00 am when the TOU rate increases.
 

ReddykwRun

Member
Jun 5, 2019
187
150
LA -Lower Alabama
This cropped up on my Facebook feed... not sure if it was intentional but it looks like they're fishing for prospective model 3 owners...

What really irritates me is how they take advantage of general ignorance around water heaters. 99% efficient sounds good to the lay person but that's a COP of <1 which is TERRIBLE from the perspective of a modern water heater. Heat Pump Water Heaters START at ~2 and go up from there.

Besides low efficiency... tankless heaters are BRUTLE on the grid. No one is going to schedule their shower around off-peak hours. You can use a heat pump water heater to store energy and schedule it to heat water during off-peak hours. Some tankless heaters can pull up to 18kW when heating water. It doesn't take many simultaneous showers to really start adding up.
The power co-op that provides power to my location agrees, they hate them, and you do not realize any savings when they get through with the service requirements and billing. I real deterient to electric tankless, gas is o.k. from what I know, (scaling ???) but I personally love my Heat Pump Water Heater and it helps cool my garage.
 
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ReddykwRun

Member
Jun 5, 2019
187
150
LA -Lower Alabama
I have had a Takagi gas tankless water heater for over 5 years and it has worked well and reduced my NG use. I can't provide any hard numbers.

In the future I plan to replace it with either a heat pump or an electric tankless but haven't done much research yet. This thread is interesting.
Please have a talk with your power provider first when considering using electric tankless, they are a problem.
 
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phigment

Member
Aug 31, 2015
656
1,063
Waterloo, Ontario
OK, that is awesome. How did you get that data?
I have a simple python script that connects to the econet service and parses out the temperature data. I could also query their power usage, but I have a separate power monitor connected to the heater to give me accurate readings.

One thing I did notice is that the temperature data provided through econet is not updated frequently. It ran all day yesterday for 8 hours while nobody was home and the temperature values didn't change. I think it needs to change by a certain threshold before it sends an update over the cloud.

I'm happy to provide the script to anybody who is interested. Just PM me. If there is enough interest I can just post it here.
 
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bxr140

Active Member
Nov 18, 2014
2,993
4,759
Bay Area
Please have a talk with your power provider first when considering using electric tankless, they are a problem.

It depends on the application. In almost all cases (and especially whole home) there are obviously better choices, but as with most things in life there is no one size fits all solution.
 

mspohr

Well-Known Member
Jul 27, 2014
10,067
12,654
California
Please have a talk with your power provider first when considering using electric tankless, they are a problem.
My house is one of seven 200 amp services on a 25 kW transformer. I keep thinking that the power company should upgrade it but I guess they're fine until it blows up.
 

SSonnentag

Let’s go Brandon!
Apr 11, 2017
1,751
2,360
Arizona
Last night I finally got my preliminary monitors setup for my Rheem ASHP. I logged some data points from midnight to about 11am (after we had 3 showers). Here's the graph if anybody is interested:
View attachment 464238

I'm very interested in replacing our 12-year-old gas water heater with one of these HP Rheem units. Is it possible to force the resistive heat elements to stay off? Am I reading your graph correctly in that the unit draws less than 500W of power when heating? What is the amount of power draw should both resistive heating elements kick in? What is the breaker size required? The Rheem website claims only 1 back-to-back shower, but you've reported doing at least two. Looking for feedback on your experience with this.

Thanks!
 

brkaus

Well-Known Member
Jul 8, 2014
8,097
6,650
Austin, TX
I'm very interested in replacing our 12-year-old gas water heater with one of these HP Rheem units. Is it possible to force the resistive heat elements to stay off? Am I reading your graph correctly in that the unit draws less than 500W of power when heating? What is the amount of power draw should both resistive heating elements kick in? What is the breaker size required? The Rheem website claims only 1 back-to-back shower, but you've reported doing at least two. Looking for feedback on your experience with this.

Thanks!
It is possible to put them in eco mode via a menu. The one I looked at required 240v/30a circuit. I don't recall the wattage of the elements or the draw of the heat pump. You still need to size the circuit for the full power draw listed on the appliance.

I'm thinking about putting one before our gas tankless. We have propane and it's quite expensive. The water heater would be in the attic - in the summer it would practically be free hot water. The tankless saves a HUGE amount over the two 40gal gas tanks we had before, but electric is so much cheaper where we live. Anyway, that raises all sorts of issues w/ the allowed inlet temp and the temp control of the tankless. And would likely need a mixing valve. And the drywall work to put in a new circuit...

I guess I could also put it after the tankless and only turn on the tankless when we have company. Still the same issues with work to get electric to the unit.
 
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phigment

Member
Aug 31, 2015
656
1,063
Waterloo, Ontario
I'm very interested in replacing our 12-year-old gas water heater with one of these HP Rheem units. Is it possible to force the resistive heat elements to stay off? Am I reading your graph correctly in that the unit draws less than 500W of power when heating? What is the amount of power draw should both resistive heating elements kick in? What is the breaker size required? The Rheem website claims only 1 back-to-back shower, but you've reported doing at least two. Looking for feedback on your experience with this.

Thanks!
For this unit there are multiple modes of operation. The app lists ("Off", "Energy Saver", "Heat Pump", "High Demand", "Electric").
Heat pump forces it to only use heat pump.
Energy saver uses heat pump most of the time and kicks in electric to boost recovery time.
High Demand uses both heat pump and electric.
Electric only uses the coils and is recommended only for service mode.

I think it's about 3.5kW for electric only... but don't quote me on that.

My family is Me, my wife, my 12 year old son and my 7 year old son (who showers with me sometimes to speed things up in the morning).

Yeah, I have 3 back to back and have done 4 showers without problem. But I did just install some low (1.5 and 1.25 gpm) flow shower heads (high sierra and Niagara Earth Massage 1.25GPM Low flow). They work fantastic!
 

maverick3n1

Member
Apr 30, 2019
44
27
Oceanside
Every heat pump water heater on the market is a 'hybrid' design. They have resistance elements for backup. I agree there are niche applications where tankless heaters are more appropriate but in general any application where a traditional water heater is in use a heat pump can offer a replacement with no change in overall convenience. What bothers me is when tankless heaters are marketed as 'more efficient'.

If unlimited hot water is the goal then ideally you could put a tankless heater downstream of a heat pump heater... best of both worlds :D

It really depends on your application. A homeowner such as myself for example, that rarely uses hot water (take a shower, then I'm gone all day), would benefit greatly from a tankless (gas tankless for me). The gas turns on ONLY when I demand hot water, then turns off as soon as I'm done with the hot water.

As it stands now, with my tank water heater, the 100k+ BTU burner turns off and on all day long while I'm not home, to keep a tank of water hot until I need it for that one time shower, or dishwasher/clothes washer run.

If you are using hot water all day long, then I agree, it's not very efficient, but if you are only using it once or twice per day, heating up what you need when you need it seems much more efficient, then holding 40-50 gallons of water hot indefinitely.

As for commercial use, I own a restaurant, and we use a hybrid with a 130 gallon tank. This increases longevity of the water heater 3-4 fold. The average water heater in a restaurant lasts 3 years before it needs to be replaced, as you have a 200k BTU burner on the bottom of it, running all day long trying to keep hot water. In addition, as you use hot water, cold water is being dumped into the tank, cooling the water still in the tank, causing even more inefficiencies.

The hybrid, runs all water through the tankless to heat, and enters the water storage tank hot. If the tank drops below a specific threshold, a circulation pump will run to re-heat the water. This gives us a very high demand water heater capable of supplying more than 200 gallons of hot water per hour, while having a water heater that lasts 9-12 years in such a demanding environment.
 

ajdelange

Active Member
Dec 10, 2018
1,077
638
Virginia/Quebec
Instant-on hot water heaters are great! I run them from propane and feed with water that has been preheated by the some of the superheat from my heat pump. The propane only adds the last couple of degrees.
 

nwdiver

Well-Known Member
Feb 17, 2013
8,358
11,426
United States
It really depends on your application. A homeowner such as myself for example, that rarely uses hot water (take a shower, then I'm gone all day), would benefit greatly from a tankless (gas tankless for me).

Even if your only use is a daily shower you're still going to use less energy overall with a heat pump water heater than with a gas tankless. The energy rating of a tanked water heater includes the idle period.

As mentioned previously the reality is that we need to transition away from fools fuel... electrification of everything is one of the steps we need to take and electric tankless is worse than horrible...
 

Dave EV

Active Member
Jun 23, 2009
1,886
1,543
San Diego
I'm happy to provide the script to anybody who is interested. Just PM me. If there is enough interest I can just post it here.
Yes, please PM me, or post it, or perhaps even put it up on github?

I'm very interested in replacing our 12-year-old gas water heater with one of these HP Rheem units. Is it possible to force the resistive heat elements to stay off? Am I reading your graph correctly in that the unit draws less than 500W of power when heating? What is the amount of power draw should both resistive heating elements kick in? What is the breaker size required? The Rheem website claims only 1 back-to-back shower, but you've reported doing at least two. Looking for feedback on your experience with this.
Most of your questions have been answered already, but yes, in Heat Pump mode, the electric elements generally stay off. They did kick on when I first fired up the heater, despite putting it in Heat Pump mode.

The most common one appears to be the Rheem Platinum Series, which are only available with a 30A breaker requirement. But the Rheem Prestige Series appear to have both 30A and 15A versions - I'm guessing they forgo one of the resistive heating elements.

We are running ours in Heat Pump mode exclusively right now. It does start running out of hot water if you take back-to-back showers AND you have the temperature set lower. For example, I turn down the temp to 110F from 120F during on-peak hours - the issue is that once the outlet temps start cooling down, you need a higher volume of hot water to cold water in the shower to maintain the same temp, which then draws hot water out of the tank even faster, until you're running your shower on 100% hot water.

You can avoid this by running hotter temps - 120F seems to be plenty for back-to-back showers, but 3 showers in a row start pulling the outlet temps down. If you get a mixing valve on the water heater, you can safely run temps higher than 120F - at the expense of slightly lower efficiency due to additional heat loss over time and slighly lower efficiency of the heat pump. Alternatively you can run in one of the modes that will kick on the resistive elements, but if they kick on too often, you're probably better off running higher temps.
 

ReddykwRun

Member
Jun 5, 2019
187
150
LA -Lower Alabama
I'm very interested in replacing our 12-year-old gas water heater with one of these HP Rheem units. Is it possible to force the resistive heat elements to stay off? Am I reading your graph correctly in that the unit draws less than 500W of power when heating? What is the amount of power draw should both resistive heating elements kick in? What is the breaker size required? The Rheem website claims only 1 back-to-back shower, but you've reported doing at least two. Looking for feedback on your experience with this.

Thanks!
The model I have has four options available. Pure Heat Pump , Hybrid, Electric Heater (conventional) and Vacation (for you northern folks that want to keep the tank from freezing but not use much power)
 

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