Why would you want to do that ? What is the point of having 2 tanks when they can only heat 50% of their capacity at one time ?

In many water heaters unhooking an element in each one wouldn't work because only one element comes on at a time. You could either have one tank heating at a time, or you would be overloading the circuit. IN some heaters both elements come on simultaneously, so your theory could work. Either way you could just get lower wattage heating elements, and that could be made to work in your scenario( depending on circuit capacity of course).

OR of course- run another wire!

"OR of course- run another wire!"

You are going to need two separate circuits no matter what you try with the heating elements.

Equipment must be installed according to the manufacturer's instructions.

Here is a paste directly from a HWT installation manual:

"Electrical Requirements

Use proper size solid copper wire.
Use a UL approved strain relief.
Connect ground wire to green ground screw.
Failure to do so can result in
death, fire, or electrical shock.
If you lack the necessary skills required to properly install the electrical wiring to this water heater, do not proceed
but have a qualified electrician perform the installation.

When making the electrical connections, always make sure:

The electrical supply has the proper overload fuse or breaker protection.
Wire sizes and connections comply with all applicable codes.
Wiring enclosed in approved conduit (if required by local codes).

The water heater and electrical supply are properly grounded. "

I didn't leave anything out and I don't see anything there that says it has to be a dedicated circuit. Some might. There is def. wiggle room. You could put in elements with half the wattage on both tanks and meet the requirements of the existing wiring/breaker if you were installing two of THESE tanks.

I don't understand the logic of running two water heaters at half wattage. Could you please explain what you are trying to accomplish.

My guess is he needs more capacity. This will work, the water just takes longer( obviously) to come up to temp. I have seen something similar before and it seemed to work fine for the application.

Our code also states that the water heating appliance must
be installed on a metel pan with provided means of drain or pump.

Water heaters are set up with SPDT thermostats, so that only one element operates at a time. Normally the top element heats first, so that hot water is immediately available at the top hot water outlet. When the top thermo opens, it actually supplies power to the lower thermo, allowing the bottom element to be energized so the rest of the water in the tank can be heated.

In theory, you could probably hack something up so that this scheme were extended to a second water heater - 4 thermostats and 4 elements in all. It would actually sort of make sense IF the heaters were plumbed in series.

However, such an installation wouldn't be following the WH manufacturer's installation instructions, and thus would probably be considered a code violation.

regarding ontariojer's copy of the instructions... I've seen those exact instructions. Usually the circuit requirements are written elsewhere. They usually specify a certain ampacity. I haven't yet seen one requiring a dedicated circuit, but if it requires a certain size circuit, then consumes the maximum by itself, this sorta implies that the circuit will be dedicated.

I partly agree with pharkus- the ampacity requirements are separate, because of so many different element options using the same tank. As far as IMPLYING that the circuit is dedicated, thats like saying if you CAN plug a portable heater into a circuit, and that heater draws 15 amps, you should only have one outlet(not a duplex!)on each circuit.
The job of the breaker is to protect the wire, if you change out the elements to a wattage that the wire and breaker is suitable for, and the instructions don't EXPLICITLY say you need a dedicated circuit, then why couldn't you run two tanks?

As to DavidR's post, that is true for many, if not most tanks. Some do however have both top and bottom come on at the same time. Changing the elements out to different wattage ones is fine however, since the same tanks come with a variety of different wattages anyway! 240 vs 208 vs 120 for example. All of these can are available using the exact same tank.

You don't plumb them in series however, you should set them up on a manifold so you try to get equal draw on each tank.

Maybe this post should be called Dueling water heaters- en garde!

I like that. good idea.

ontariojer, if'n the end user has a problem with his two electric space heaters tripping the breaker, said end user can easily unplug one.

Not so with a permanently-installed appliance.

"This appliance WILL consume the maximum amount allowed for a 30A circuit, and is required to be connected to exactly a 30A circuit".

How does that NOT imply that connecting something else to the same circuit would be stupid?

"I didn't leave anything out and I don't see anything there that says it has to be a dedicated circuit."

Better look through the entire installation instructions.

The circuit requirements are NOT in the installation section, but normally right at the front of the instructions.

You are not going to save any power by trying to change how the heating elements are used and sequenced.

It will take the same number of kW-hrs to heat the water.

Put the heaters in cascade on the water side.

Parallel tends to have balancing problems if the tanks do not empty at the EXACT same rate (the rate can be adjusted by throttling the input of the tanks to try and make the flow out of each tank match, but it rarely works at more than a single flow rate.
.

Implying something is not the same as requiring something.

ME: but if it requires a certain size circuit, then consumes the maximum by itself, this sorta implies that the circuit will be dedicated.

ONTARIOJER: Implying something is not the same as requiring something.

... Debatable, but if we accept it, then on the same basis, implying that something WILL be is not the same as implying that it is REQUIRED to be.

What I'm saying is: if you lower the wattage to half on each element, you can still be within the max capacity, and as long as the tanks don't explicitly say it must be a dedicated circuit, then it should be fine.

It's the same as you said- If you plug in a 15A heater on a 15A circuit, you have implicitly filled the circuit ampacity and you shouldn't plug in anything else(yeah, like that happens), but you CAN plug in two 7.5 amp heaters, since you have up to twelve outlets. That's what I mean by Implied vs. required. I'm sorry if this isn't coming out as clearly as I want.

I get the feeling a lot on this forum that people are arguing about two different things. Does anyone else feel that way?

Brickeyee

If the guy wants to save power, I agree. However, if he wants more water available then:

two tanks x half the wattage in each tank= same amperage = twice the time = twice the water heated= twice the KwH

Obviously his recovery time is going to suck, but if he has needs for a lot of hot water in a short time frame, with large gaps in between, this could work for him. Always following Manuf. instructions and local codes of course. I would like to add that one should always hire a licensed electrician if you are not competent and confidant and/or that is required in your area.

from the comments..
running my 80gal, 240v, 4500w water heater on 120v would be BAD!..but the sticky wicket is..its been running that way for 3 years..never out of hot water..and a lower electric bill.
why? we're on a demand meter system. there is a measure of the Peak KW..and a fee, something like 3.73 per KW peak. YES $3.73..but its only the highest KW..then KWH are 4.5 cents instead of 8ish or so..so its best it we keep the KW to a min..absolute min..so a large tank of heated water that very well insulated. ideally heated off peak only. hum sound like 2 tanks heated on 120 v instead of 240v..at 1/4 the KW rate. to get half you have to series the 2 heaters..I don't think putting 2 heater in series is a good idea. this might lead to grounding problems..

-dkenny

If you mean in series electrically, that won't work very well.

You'd only get heating of the water when the water was cold in both tanks and all the thermostats were closed. As soon as one tank was hot and shut off, both tanks would be off.

Also, even if it did work, the effect of connecting two 240v heaters in series across a 240v supply would be the same as operating them separately on two 120v supplies - one-quarter the power requirement, not one-half.

To use your 120v scheme, you could use two 120v circuits, or a multiwire circuit. Again, the power consumed by each WH would be one-quarter of that consumed by the same WH powered by 240 volts.

Operating a 240v appliance on 120v is an interesting way to limit demand. I don't see anything unsafe about it, though technically it would probably be a code violation since you're not following the manufacturer's installation instructions. I think you'd have a tough time convincing a pro electrician to install a water heater that way. :)

Indeed, one of the houses I lived in and worked on was wired that way. It wasn't to limit demand, though - the last guy was just an idiot. The water heater and washing machine were both on one 120V circuit, and the whole thing was turned off by the "OIL BURNER SAFETY SWITCH" at the top of the basement stairs. There wasn't an oil burner anywhere to be found, btw.

Wiring the units in series won't cause "grounding problems", but it won't work very well either.

Usually when you have a dual hot water heater setup, you hook them up in series(tank 1 feeds tank 2,tank 2 feeds house)the idea is that you will have pretty much a tank and a half of hot water before it starts to get cold,as the 2nd tank is being fed hot water from the 1st tank, it will never get cold water unless you are just running them empty.I usually only see this in huge houses.

"Usually when you have a dual hot water heater setup, you hook them up in series(tank 1 feeds tank 2,tank 2 feeds house)..."

This is correct.

It is just about impossible to get the parallel tanks to empty at the exact same rate.