https://www.rheem.com/tankless-electric-water-heater-sizing-tool/

Your dishwasher has its own heating element and only uses cold water.

https://www.energydepot.com/RPUres/library/TLwaterheater.asp

How do you know her DW model has a built-in heater that can heat water from cold? Most in the US require 120F at the inlet.

An electric unit will require a lot of amps, 120a in some models. You have to have a large enough service for that.

They all have built in heating elements. How do you think they dry the dishes?

Branson4020,

Some dishwashers do not have assured water heating. The heating element may operate during wash and/or rinse periods but only to help maintain the water temperature, there is no thermostat to delay cycle progress to insure a specific temp is reached. These types of dishwashers cannot work properly on a cold water supply. Some models that have a thermostat may limit heating time and also not work properly on cold water. It's rare that a dishwasher in the U.S. is connected to a cold water supply.

Hi Kristen Lynne,

Please reconsider an electric tankless water heater. They require huge amounts of current to work - and even then they will have very limited capacity. What's more, they won't save you any money over a tank water heater.

The cost of the electrical service upgrade, and potentially the cost of peak electrical usage, can make these very expensive.

Now, to answer your question:

You add up the things that will be used at the same time:

Kitchen sink is 2 gpm

Bathroom sink is 1 gpm

washer 2 gpm

shower 2.5 gpm

Tub 4-5 gpm

Then you look at the temp rise. We use 105 as the design temp, because that's what a shower needs. So, to take water from say 60 degrees to 105 is a 45 degree temp rise.

Then we look at the ratings of the water heater to see which unit will match.

So, for example, you say that it' s likely you will run two showers at the same time (2x2.5=5gpm) @ 45 degree rise: That would require a 36 kw (150 amps)

Here is a good chart: Note, they tend to under-estimate gpm in most of the fixtures. They assume they are all new and meeting the most recent water flow restrictions.

Also note that any electric heater has the same numbers - there isn't a heater that's going to get you more GPM for less electricity, for example, as long as you are talking a standard electric heater.

Lastly, the information about the dishwasher is incorrect: Most dishwashers are plumbed to hot water only AND have a heater. The heater helps to boost / maintain water temps... but aren't usually designed to heat the water from "cold". There are a few exceptions.

dadoes,

Well I'll be damned - I just checked my dishwasher and its plumbed to the hot water side. I always assumed the heating element did it all. In any case, the Rheem tankless calc tool ignores the dishwasher.

Again, that Rheem tool underestimates GPM on most faucets. Most shower heads in use are 2.5 gpm, for example.

My DW uses hot water except for heating the water for the pot wash cycle and of course for the drying cycle. Tankless works best where the tank is close to the use point that is why they work well in Europe in small spaces and in NA more than one unit is often the best. I rarely wash clothes in cold water because I find they do not get clean. The electric versus gas depens on what is the least expensive choice inyour area where I live electricity is cheaper tan gas and we have an 80 gal. rug electric HW tank works great and I have never run out of hot water. For our house we would need at least 3 tankless units

Jake, where did you see a requirement for a shower or tub?

Thanks all!!! I didn’t realize these tankless units need more power compared to a regular tank water heater. But “duh” it makes total sense. I think we’ve maxed out our electric panel. :/ We did have a gas line added a couple years ago and only have a gas stove and gas fireplace insert. I did have them go ahead and upgrade the size of the pipes in the ground so we could add more gas appliances if we want to. Maybe I will investigate that option instead.

The main reason we want to replace this tank is it’s already and older unit, so we don’t expect it to last much longer, and it takes up almost the entire closet where it’s located. We could REALLY use that extra storage space.

I initially steered away from the gas units because (1) it would cost more to have the plumber run a gas line for it, and (2) I’ve been told they cost more than the electric units.

I will be contacting a local Plumber to get some quotes. Thanks all!

And, no we don’t have any showers or tubs on this unit. Our two full bathrooms have their very own water heater! (Old house quarks).

Jim,

Since 1992 the federally mandated max gpm is 2.5 gpm on a shower head. In some states it's 2.0 and recently (2018) California has gone with 1.8. Some people use 1.8 even when they don't have to... but we generally design around 2.5 unless we know it's something else.

Tubs are not flow restricted and I've always used 4-5 gpm as the number.

Edit: Jim, I misunderstood your question...

I didn't know if she needed a shower or tub - I just used that information for my example. She would do her own calculations given her situation.

Kristen Lynne,

The issue with tankless is that they must heat the water very quickly. That requires very large amounts of energy at one time - where a tank heater can heat it slowly so it uses less input, but over a longer period of time.

A gas tankless will use considerably less overall energy than a standard gas water heater - not so much because it's "tankless" and doesn't have standby losses, but because it's a sealed combustion with much lower exhaust gas temps- and so less heat is wasted.

An electric heater is already almost 100% efficient. Standby losses in an electric heater are only about 3% - and so an electric tankless never saves much money.

If it's likely your only need is 2 -3 gpm then a much smaller unit could work - but even then I think you would be in trouble with the electrical service since you've maxed out your panel.

Consider a much smaller tank unit - such as a 15 gal.

@Jake The Wonderdog - Would it be code compliant to consider getting a smaller/shorter tank unit like you suggest and then build a (very solid/sturdy) stand to gain floor storage space? The primary reason I want to switch to a tankless is to regain (much needed) storage space in the small closet where this is located.

I think that's a reasonable and cost effective solution - much more so than a gas tankless.

I don't know the code implications - other than I know that gas tank heaters are required to be elevated in garages (to avoid combustion of potentially flammable gasses near the floor) - so there's some precedence.

Yes, that is all I could find online regarding codes for elevating hot water tanks. I will discuss the option with my plumber. Thanks for the tips!