Rheem instantaneous whole house water heater


I’ve had Sense for almost 2 years and it still hasn’t detected my water heater. Since I t uses up to 30,000 watts when it kicks on I’d really like to see what it’s costing me.


WOW! My electric water heater uses 4,500 watts. Is yours an electric on-demand unit?


30kw seems crazy! Is that just the very short duration initial surge?


They only run for seconds to minutes. They actually cost the same or less than tank style as you don’t heat what you don’t use


A number of the higher flow on-demand electric water heaters draw that much or more, although only during high demand usage.

It’s one reason why on-demand electric hasn’t caught on, VERY high peak demand on home electric service, despite the savings of not maintaining a tank full of hot water 24x7.


This would be a prime candidate for a “Sense Saves” situation if one could compare instantaneous vs. tank-based water heating in the real world. I looked for those metrics when I replaced my water heater but couldn’t find them, although I didn’t have a choice of instantaneous because the high current draw would be beyond my supply capacity but there must be many situations were instantaneous heating saves: infrequent use being the primary candidate.
Given the likely constant high relative current I assume this would be easy for the Sense engine to parse? It must be a 208/240V supply. Sense did have trouble in the early days with my 4.2kW tank being identified. I believe was related to “phase awareness” for multi-phase devices. @d_hansenjr, did you talk to support?


The cost for actually heating the water is the same whether using 4,500 watts or 30,000 watts. It’s basically 3.4 btu for each watt.
What makes instant cheaper is the storage of heated water and related “heat loss”.
No matter what you do there will always be heat loss but much less using on demand.


I recently looked into on-demand also, for a summer camp with a leaking standard hot water tank and very limited space for the replacement. New water heaters are larger diameter than 23 year old (certainly no complaint on its reliability) ones, which got us thinking about alternatives.

Unfortunately, we’d have to replace the service entrance and the panel. Even one of the smaller on-demand units would require a 200 amp panel. Between the panel, the on-demand heater, power company fees, and the labor we were looking at over $3,000. So we gave up on the idea. We’d need a HUGE amount of “light use” savings to pay that back, particularly at upstate NY power prices of 7 cents/kwh.


Each size water heater usually comes in 3 sizes. Regular, short and tall. Say you need a 50 gallon, look at the tall and it will have a smaller footprint.


@samwooly1 That’s kinda-sorta true … certainly on the watts-to-BTUs but the EF of an average new tank is around .95 and good instantaneous is up to .99 so already instantaneous is a “no brainer”. Typically this was dismissed as an “insignificant gain” by industry. When you factor in tank heat loss you guess the average difference must be more than 10% so it starts to get interesting. Of course the main reason instantaneous doesn’t take hold, one would guess, is that it puts much more peak demand on the utility and your local supply = more dangerous. A hybrid of the system of a small tank and low flow instantaneous would perhaps be a more practical solution for many as are heat-pump powered systems. I’m still waiting for the day my fridge compressor can dump heat to my water tank!


Hey @andy I read “Summer Camp” and I’m thinking SOLAR. LOL.
Come on … you can do it!

1 Like

The person to talk to is @steve, this is his area of expertise.
Most homes aren’t wired for the instant yet but getting there. Heat loss is a major issue and where I have recirculating hot water, I’ve dealt with and spent a lot of time researching in my design and installion of the system.
If I had to do it again I would use a heat pump water heater with auxiliary heat. The power factor on those is unbelievable at over 3 to 1


I needed a smaller heat-pump water heater than is currently made in the U.S. to go that route. I got excited about a retrofit of something like the nyle (as featured on This Old House) but it seems to have dropped off the face of the earth. @steve, help??

1 Like

Hear pump water heaters are larger capacity for a reason. Because they heat so much slower it takes hours to reheat. The extra capacity makes sure there will be enough water for showers and such.


Yes indeed, but our now leaking water heater was a tall too, and the new ones are almost 3” wider.

Nothing to do with Sense of course, but do appreciate the community assistance.


((-;). That’s (e.g. “the day my fridge compressor can dump heat”) kind of what provides something like 97-98% of my hot water now. The waste heat from the geothermal heat pump, whether heating or cooling, is dumped into the domestic hot water tank, raising it to 155-160 degrees on really cold days. I do have a mixing valve to provide a constant 120 degrees to the house for safety, and the high “free” tank temperature guarantees lots of hot water.

Even in the summer, which is “down hill” for geothermal (doesn’t have to work very hard), it provides almost all of the hot water and in the winter the water heater doesn’t run at all.

I do wish Sense was able to accurately monitor all this, but unfortunately the same geothermal system uses a “noisy” variable frequency/constant pressure pump, which seriously messes up Sense’s algorithms.

1 Like

Living in a city apartment I have often wondered if piping my downstairs neighbors ceiling could be classified as “geothermal”. GeothermaLOL?