In regards to 30°F temp rise at 5 GPM ... think about how high of a flow-rate that is (filling a 5 gallon bucket in one minute!) and how often anyone runs that much flow. Maybe people who have those multi-head body showers. Myself, I rarely run higher than 3 GPM for anything, and my showers are less than 2 GPM.

In regards to your electric service, yes. I believe your Watt rating should be 19.2K ... 19,200 watts. That'd be 80 amps @ 240 volts. 2x50 amp breakers supports 100 amps, which is the typical 20% "buffer" on circuit size. It's unlikely you can run the tankless without upgrading the service. Your power feed couldn't handle running the tankless plus air conditioning or electric space heating (if that's what you have) plus other household operations (lighting, cooking, electric clothes dryer, etc.). You'd also have to beef-up the wiring on the water heater circuit.

squirrelheaven

I am a big proponent of gas tankless in the right applications, but really not a proponent of whole house electric tankless. Besides there being No Payback on electric tankless systems, the chart that you are looking at shows the problem: Without major upgrades to the electrical service(Panel and all wires from the street) you can't get enough water flow in the winter.

You are kind of going about the chart backwards... normally, you determine the required temp rise in the winter (in your case, 105-35=70*) and then you find how many gpm the heater will produce. That heater will only produce 2.0 gpm @65* temp rise. That's not enough for a shower (2.5 gpm) when the water is 40*, let alone any colder.

I understand you want to get rid of your water tank... but it's not worth this.

My hardwoods were just watered down again. Plumber left the pressure relief line *out of the drain pipe when the washer drain was cleared (first time I've ever seen the PRV tripped). All the water really ruined the flooring this time. Parts of it are mush and severely buckled. I so want that thing out of here, and the closet space for a sink to handle washer drain backups, too. Very disappointing. I've been dreaming of this solution to water appliances on the living level for a few years now. On to tile floors! This stuff just can't weather the storm.

I read that a standard tub faucet runs about 5gpm. I was looking to take quite a few hot baths! but I could have run it at 3gpm if that would have been sufficient. But even those numbers don't work. Doesn't make sense to me.

I did figure the required temp rise first and the chart didn't even get close which was why I was confused.

Thanks for your help. Next house, I guess.

So does the info I found mean my service panel is 125 AMP service?

Here is the sizing info I found on Steibel. Originally, I'd thought about just getting the 36 model, as the temp-rise looked good and would seem to cover anything. Only a $900 unit. But after reading about the service panel requirements here, I was also checking that out in the specs, which brought me to a more doable looking 24 Plus, same unit their sizing chart recommends.

I just don't understand how such a low temp-rise can be used. I also was reading about someone in VT who has the Steibel Electric 29 with no problems.

This might be helpful info for others:

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The general sizing charts (the first chart) are very misleading. You really need to do the minimal math to determine gpm and temp rise in the winter. The second chart is really difficult to figure out.

But if you want 3 gpm (a shower is 2.5 gpm) in the winter when your temp rise will be between 65 and 70 degrees, you need a 29 Plus running at 240 volts. That would pull 120 amps - the entire rated capacity of your box. You would probably be looking at a 400 amp service at that point.

If you wanted the ability to run a shower and a lav faucet at the same time in the winter, you would have to have the largest unit which pulls 150 amps. BTW: these numbers don't change between brands of electric tankless heaters. This is basic physics and there's nothing that a company can do in the design of these electric heaters to change the amount of power needed to heat a given amount of water by a given temperature rise.

Keep in mind that 3.5 gpm is fairly minimal. It would work for a single person - but not a family since it wouldn't even power 2 showers.

It takes a HUGE amount of current to heat water on-demand. Electric whole-house might be okay in warm weather areas, but not in cold.

I see. Pretty amazing. I had no idea from the other reading I've done here and there over the last couple of years that the temp-rise was so limited on a lot of these units, or that the panel service would be an issue.

Good to know the numbers are a constant between mfrs and not design considerations across companies.

I think the 36 would make sense for me, not much more than the 29, even though this place is small and they often house only one person. But redoing the service panel is pretty expensive as I recall. I think they upgraded a lot of these units to 200 amps during a recent rehab, but I've had mine a long time and have the original panel. Don't remember the cost my electrician threw out when we were talking about upgrading the panel a number of months ago, but I don't recall it as a small project.

Thanks for your help : )

My 50g hot water heater seems to be is misbehaving (PRV), so I've got it turned off. I've been turning it on only when needed to do some chores, and good hot water becomes available pretty quickly. While the main reason for my wanting a tankless was water damage and closet space, cost-wise, I'm thinking about using this tank in an on-demand fashion. It seems a waste to keep it on all the time since it heats up so quickly.

Is this a bad idea for the unit? Bad on the components, operation, or something else? Bacteria even?

Tankless heaters have used as a marketing idea that they will increase efficiency by not having hot water sitting around in a tank all day not being used. It's an easy concept for consumers to grasp, but it's not true.

On an electric heater, the difference in the energy factor is very small between a tank and a tankless heater. A tank heater will be in the area of 0.93 and a tankless might be 0.95 - not enough difference to talk about and certainly not enough to provide payback. That represents the amount of heat lost through the walls of a well insulated tank.

With gas heaters it's different. The vast majority of the efficiency gain is in the lowered flue temps. Second is not having the heat loss through the center of the tank.

For your situation, what you should know is that a properly installed and operating electric tank heater is extremely reliable and efficient. Get yours replaced or repaired and the TPV properly routed to the drain so that it doesn't cause damage. Since you have lost confidence in your heater, I would just replace it rather than repair. Make sure that you are addressing thermal expansion issues that might have caused the TPV to open.

If you want more space, consider a stacking washer and dryer and eliminating the sink. The front loading LG's are full size (huge inside) stack and don't use much water - eliminating the overflowing washtub issue. That would be a cheaper way to get space than a tankless heater and they are great machines.

Thanks, Jake. Actually, if I were to get a tankless I could get that utility sink in the closet so any washer drain overflow problems would be contained in the sink. Right now it just goes into a drain pipe. When the drain clogs, it goes over onto my living area hardwoods. It's a terrible setup. I do have the stack w/d. This w-heater is probably 12 years old, so I'm looking to replace the unit. I was hoping to eliminate both the water tank and the washer drain problem by getting the tankless. Savings on my electric bill, if any, was only a side benefit. I was so looking forward to converting but the 400 amp service panel would certainly appear to be a no-go. I also really liked the idea that a tankless unit would last 20 or more years. I may just tile the lower level at some point, instead.

On the electric bill front, is it ok to leave the water heater off most of the time? Any problems associated with that?

There's no harm in turning off the water heater, there's nothing really to be gained by it either. It won't save money, it's just that you will have to wait for hot water.

The reason that I mentioned the front loading washers is because they use so little water that they don't have the same draining problems. They aren't as likely to overwhelm the drain.

Do get your current water heater replaced though. If you know it is having a problem, you don't want to press your luck. It would be too easy to forget to turn it off.

would a smaller unit work for you? If you live alone and don't take long showers? We put one like this in our cabin and it works perfectly for us. We have front loading washer, a dishwasher, and are just careful to not run everything at once and take short showers, etc. I don't know if it would buy you the space you need, but at 31" it might fit nicely under a bathroom vanity.

Here is a link that might be useful: 20 gallon water heater

It would be too easy to forget to turn it off.

You are so right.

Oly, there was a smaller one in here when I first moved in and I upgraded due to showering. I can't remember the size. Sure would be nice to have a small one like that though. Not sure I want to risk getting a too-small unit for my needs, however. Wish I could remember the other one's size.

Jake, you are the two Jakes? I'm wondering why keeping the unit turned off most of the time would *not conserve electricity. Esp since I keep reading how heating the water 24/7 is one reason to go tankless (even if the high electric draw of the tankless renders it a moot point).

I had the unit on this morning and, surprisingly, am still getting warm water. Not sure how long to heat for a shower, as my shower is out of commission right now.

Yes, sorry, should have made that clear that I have two accounts... the password is cached, but I don't remember what it is so I have a different account at work and home.

very little heat is lost through the insulation on an electric heater. That's why the energy factors on an electric tank is very close to an electric tankless.

So you're saying it doesn't spend much time needing to reheat the water during the day, then.

That is correct. Little heat is lost through the insulation on an electric heater. The newer ones have 2 inches of foam, I believe, giving them an R rating of about 14.

What's more, the heat that is lost from the time you turn it off to the time that you turn it on is expended again in bringing the water back up to temperature. The only real savings comes from the period of time that the water in the tank is cooler than it would otherwise have been - a lowered temperature differential- that causes less heat to be lost.

Bottom line - electric heaters have a low standby heat loss and there's little benefit to turning them off during the day. If you were to be gone for a week, that would be different.

Well why do they make such a big deal about it then, lol!!

Thanks for all your help, Jake : )

For electric whole house, there is an advantage of continuous hot water (albeit at a high price in an upgraded electrical service) and a smaller unit. Those aren't compelling for me - esp considering the low gpm these units can heat. That's why I don't recommend them.

For gas units, the story is very different. The savings is ~38% for a standard tankless and almost 50% for a condensing. That's a considerable savings. That is in addition to continuous hot water and space savings. A gas unit can supply a more reasonable number of gpm's also. Like an electric tankless, there can be an issue of not having the gas service to supply, particularly the largest, gas tankless.