Thread: Dual Water Heaters

Bought a previously owned home and have a question about my dual water heaters which has me puzzled.

Both heaters are the same size and age and appear to be connected in series. When flushing the tanks, the water draining from the one with the cold water supply coming into from the street-side, was cold or at best barely lukewarm. Whereas, the water draining from the other tank was hot. I was expecting both to drain hot water. I checked the thermostat setting on the first tank and found both upper and lower heaters set at 125F. Could it be that one tank has an inoperable heating element or is what I experienced normal?

Appreciate your help and comments.

Originally Posted by Kenneth Wantzloeben

Bought a previously owned home and have a question about my dual water heaters which has me puzzled.

Both heaters are the same size and age and appear to be connected in series. When flushing the tanks, the water draining from the one with the cold water supply coming into from the street-side, was cold or at best barely lukewarm. Whereas, the water draining from the other tank was hot. I was expecting both to drain hot water. I checked the thermostat setting on the first tank and found both upper and lower heaters set at 125F. Could it be that one tank has an inoperable heating element or is what I experienced normal?

Appreciate your help and comments.

Setting up water tanks in series is not a common practice in my area but here are a few things you can check out. First, you might have a look to see if you have a separate breaker that is turned off at the panel. Second, if you have hard water, your element may be coated with mineral deposits. Check the element and the sacrificial anode to verify that its still there on the first tank. This might explain why the first tank is operating slower than the second as it would get more mineral deposits on the element. And finally it just might be that element is shot on the first tank. All the best. Hope this helps.

Dual water heaters may be connected in series or in parallel ... the the plumbers who install them will each their reason for connecting in series or connecting in parallel.

I prefer in series, some prefer in parallel. There is no "correct" answer, just like there is no 'correct' answer to 'Which came first, the chicken or the egg?' (I could debate either as being correct, or incorrect, basically using the same argument for either side, just applying it differently).

You have an inoperative element - usually due to the element itself rather than the thermostat. If there is only lukewarm water it may be the bottom element that's not functioning. You can check this with an amp meter if you're familiar with their use and working with live wires.

PS, you might want to lower the temp of the first tank. Once you get it working, it can be used as a preheater, or tempering tank, warming the water before it goes to tank #2 to be heated hot.

Just to let you all know Kennith is not being rude by not answering. I had answered his question on Monday, he responded with a thanks, someone added to check for separate breakers, to which I responded again. Then things went a little crazy; my second post did not make it to the thread, I sent it again and then both posts appeared. I tried to removed my second post and everything back to the first post went away!

Originally Posted by Eric Barker

You have an inoperative element - usually due to the element itself rather than the thermostat. If there is only lukewarm water it may be the bottom element that's not functioning. You can check this with an amp meter if you're familiar with their use and working with live wires.

Is it true that a standard residential water heater will not heat at all if the top element is not functioning?

Originally Posted by Dave Ruth

Is it true that a standard residential water heater will not heat at all if the top element is not functioning?

Kinda


Added in edit
While trying to explain this to someone I discovered I was wrong.
I do not like to delete a post, but I also do not want to leave incorrect information for someone to read. and be misinformed.

Originally Posted by Dave Ruth

Is it true that a standard residential water heater will not heat at all if the top element is not functioning?

The current is only transferred to the bottom element if the top temperature sensor is satisfied (e.g. 120 deg.). So if the water is already hot the bottom element can keep it hot at the top of the tank. If the water temp at the top of the tank falls below the sensor trip point the current is routed to the top element, and if it is open, there is no more heating of the water.

Originally Posted by Dave Ruth

Is it true that a standard residential water heater will not heat at all if the top element is not functioning?

Yes.

The water heater first heats the water in the top of of the tank by energizing the top element. When the water in the top of the tank reaches the temperature setting of the top thermostat the top thermostat switches over to the bottom thermostat, which is already calling for power to the lower element but no power is coming down to the lower element until the upper thermostat is satisfied and clicks off ('on' to the bottom element).

When the bottom element is energized the water in the bottom of the water heater is heated.

With the top element being bad, the water in the top of the water heater is not heated, thus the upper thermostat is always in the top element heating mode.

If a water heater only produces a limited amount of hot water, that is an indication that the top element is working but that the bottom element is not working - only the water in the top of tank gets heated ... and thus runs out of hot water fast.

Rick C., why do you say "Kinda"? Is there something new out there in water heaters?

Originally Posted by Jerry Peck

Yes.

Rick C., why do you say "Kinda"? Is there something new out there in water heaters?

Originally Posted by Vern Heiler

The current is only transferred to the bottom element if the top temperature sensor is satisfied (e.g. 120 deg.). So if the water is already hot the bottom element can keep it hot at the top of the tank.

Not a very likely scenario, as in "how did the water get to temp with an open element", but it does answer Rick C's "kinda".

Originally Posted by Vern Heiler

Not a very likely scenario, as in "how did the water get to temp with an open element", but it does answer Rick C's "kinda".

True, except that might only work when the top element first 'goes bad'. As soon as hot water is used or the water in the top of the tank cools and needs to be re-heated ... ain't gonna be no more hot water as the bottom element is not going to come back on.

Jerry asked

Rick C., why do you say "Kinda"? Is there something new out there in water heaters?

Dave's question was...

Originally Posted by Dave Ruth

Is it true that a standard residential water heater will not heat at all if the top element is not functioning?

To witch, I answered "Kinda".
Because, it's not completely accurate to say "a standard residential water heater will not heat at all if the top element is not functioning".
As Vern explained, and you know, if hot water use is low, the water heater could continue to provide hot water for days, weeks, even months, without any apparent problem, even when the top element is burnt out.

Originally Posted by Rick Cantrell

Because, it's not completely accurate to say "a standard residential water heater will not heat at all if the top element is not functioning".
As Vern explained, and you know, if hot water use is low, the water heater could continue to provide hot water for days, weeks, even months, without any apparent problem, even when the top element is burnt out.

Vern did not explain it, and a water heater will not continue to heat water regardless how little use there is with a burned out top element - not based on anything I have been told or learned ... please explain how the bottom element will heat the water when the top element is not working - I would like to be 'unconfused' about this.

Originally Posted by Jerry Peck

Vern did not explain it, and a water heater will not continue to heat water regardless how little use there is with a burned out top element - not based on anything I have been told or learned ... please explain how the bottom element will heat the water when the top element is not working - I would like to be 'unconfused' about this.

As was typing it out, step by step, I discovered a flaw in my thinking.
So if the top element burns out the water heater will be inoperative.
Thanks for making me try to explain it to you or I would not have gone through it step by step and discovered the flaw.

Originally Posted by Rick Cantrell

As was typing it out, step by step, I discovered a flaw in my thinking.
So if the top element burns out the water heater will be inoperative.
Thanks for making me try to explain it to you or I would not have gone through it step by step and discovered the flaw.

As I said "Not a very likely scenario, as in how did the water get to temp with an open element".

If the top element were to open after the water at the top of the tank was at temp (120 deg.) the bottom element would keep the water at the top of the tank at temp until there is a volume large enough to draw down the temp at the top of the tank below the differential of the bi-metalic strip/disk. Once that happens, end of hot water.

As far as the how this can be: The top element has a double pole double through contact. Once the temp causes the contacts to transfer the current to the bottom element the bottom element control is the sole controller of heat and will maintain 120 at the top of the tank.

Originally Posted by Vern Heiler

The top element has a double pole double through contact.

element does not have contacts but upper t-stat does. switching mechanism is single pole double throw although t-stat itself is double pole

Originally Posted by Dave Ruth

element does not have contacts but upper t-stat does. switching mechanism is single pole double throw although t-stat itself is double pole

Both legs of the 240 v supply are transferred from the top element t-stat to the bottom element t-stat. The top element t-stat (corrected from "element") is double pole double through, the bottom t-stat is double pole single through.

The top switch sends two legs of the 240 (two poles) to one of two destinations (upper or lower) and the bottom t-stat connects or breaks the two legs of the 240 (two poles) to the element. (one through).

Originally Posted by Vern Heiler

As I said "Not a very likely scenario, as in how did the water get to temp with an open element".

If the top element were to open after the water at the top of the tank was at temp (120 deg.) the bottom element would keep the water at the top of the tank at temp until there is a volume large enough to draw down the temp at the top of the tank below the differential of the bi-metalic strip/disk. Once that happens, end of hot water.

As far as the how this can be: The top element has a double pole double through contact. Once the temp causes the contacts to transfer the current to the bottom element the bottom element control is the sole controller of heat and will maintain 120 at the top of the tank.

Vern,

A water heater which is not used cools down and needs to be reheated ... except that the top element will not operate and thus the water heater will not reheat the water ... and the bottom element will not come on, end result being (even with no use) the tank is holding unheated water (cold water).

Originally Posted by Jerry Peck

Vern,

A water heater which is not used cools down and needs to be reheated ... except that the top element will not operate and thus the water heater will not reheat the water ... and the bottom element will not come on, end result being (even with no use) the tank is holding unheated water (cold water).

Jerry, after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element, which then keeps the water at the bottom element at the set temp (which should be the same as the top), and because heated water rises it keeps the water at the top of the tank above the temperature differential that would cause the top t-stat to switch power back to the top element. The bottom t-stat/element keeps the temperature of all of the water in the tank at the set temp and the only time the top element is brought back into the game is when the volume of water used is great enough to overwhelm the capacity of the bottom element.

Originally Posted by Vern Heiler

Jerry, after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element, which then keeps the water at the bottom element at the set temp (which should be the same as the top), and because heated water rises it keeps the water at the top of the tank above the temperature differential that would cause the top t-stat to switch power back to the top element. The bottom t-stat/element keeps the temperature of all of the water in the tank at the set temp and the only time the top element is brought back into the game is when the volume of water used is great enough to overwhelm the capacity of the bottom element.

Vern,

I see where your process is missing the step which happens: (bold and underlined)
- after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element,
- which then keeps the water at the bottom element at the set temp (which should be the same as the top),

The correct process is:
- after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element,
- which then heats the water at the bottom element at the set temp (which should be the same as the top),
- once the bottom element heats the water to the temperature set on the thermostat the bottom thermostat shuts the bottom element off ... the bottom element does not "keep" heating the water in the bottom of the tank.

This is the process: (I will start after the initial filling and heating of the water heater and a tank which is not being used, such as overnight when everyone is sleeping)
- 1) The tank has reheated from activities and use of the evening.
- 2) The heated water begins to cool after having been reheated in step 1).
- 3) The water temperature in the tank drops to where the top thermostat clicks the top element on.
- 4) The top element heats the top water to the temperature to the set point of the top thermostat and the top thermostat shuts the top element off and clicks the connection down to the bottom element.
- 5) The bottom thermostat senses the temperature of the bottom water and is setting there waiting for power from the top element
- 6) The bottom thermostat is already calling for the bottom element but just now gets power down from the top element and the bottom element starts to heat the bottom water.
- 7) The bottom element heats the bottom water to the temperature set on the bottom thermostat and the bottom thermostat click off.
- 8) The water heater is not full of hot water and the process begins again, except that the top element is now burned out.
- 9) Step 4) does not happen because the top element is burned out.
- 10) The water heater does not progress past trying to achieve step 4) ... power never makes it back down to the bottom thermostat.
- 11) The water heater is now holding a tank of cold water.

Originally Posted by Jerry Peck

Vern,

I see where your process is missing the step which happens: (bold and underlined)
- after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element,
- which then keeps the water at the bottom element at the set temp (which should be the same as the top),

The correct process is:
- after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element,
- which then heats the water at the bottom element at the set temp (which should be the same as the top),
- once the bottom element heats the water to the temperature set on the thermostat the bottom thermostat shuts the bottom element off ... the bottom element does not "keep" heating the water in the bottom of the tank.

This is the process: (I will start after the initial filling and heating of the water heater and a tank which is not being used, such as overnight when everyone is sleeping)
- 1) The tank has reheated from activities and use of the evening.
- 2) The heated water begins to cool after having been reheated in step 1).
- 3) The water temperature in the tank drops to where the top thermostat clicks the top element on.
- 4) The top element heats the top water to the temperature to the set point of the top thermostat and the top thermostat shuts the top element off and clicks the connection down to the bottom element.
- 5) The bottom thermostat senses the temperature of the bottom water and is setting there waiting for power from the top element
- 6) The bottom thermostat is already calling for the bottom element but just now gets power down from the top element and the bottom element starts to heat the bottom water.
- 7) The bottom element heats the bottom water to the temperature set on the bottom thermostat and the bottom thermostat click off.
- 8) The water heater is not full of hot water and the process begins again, except that the top element is now burned out.
- 9) Step 4) does not happen because the top element is burned out.
- 10) The water heater does not progress past trying to achieve step 4) ... power never makes it back down to the bottom thermostat.
- 11) The water heater is now holding a tank of cold water.

Jerry, where do you think the hot water in step 7) goes? Do you think it just sits at the bottom of the tank or does it head straight for the top?

Originally Posted by Vern Heiler

Jerry, where do you think the hot water in step 7) goes? Do you think it just sits at the bottom of the tank or does it head straight for the top?

Vern,

The entire tank is hot, top and bottom, the heat in the bottom gradually migrates to the top (as you know, heat rises), however, at the same time, the water at the top is cooling along with the water at the bottom, the top thermostat will click in calling for the top element and the bottom element will not longer have power.

The bottom element is not going to heat the entire tank to full temperature, was not designed or intended to, the bottom element is only designed and intended to heat the lower portion of the tank after the top portion of the tank has already been heated by the top element.

You think you can keep the entire tank hot enough with just the bottom element to keep the water at the top hot enough to not click in the top thermostat?

And I described it with no hot water usage ... use even just a little bit of hot water and the top thermostat will click the top element on (which removes power from the bottom element).

Originally Posted by Vern Heiler

Both legs of the 240 v supply are transferred from the top element t-stat to the bottom element t-stat. The top element t-stat (corrected from "element") is double pole double through, the bottom t-stat is double pole single through.

The top switch sends two legs of the 240 (two poles) to one of two destinations (upper or lower) and the bottom t-stat connects or breaks the two legs of the 240 (two poles) to the element. (one through).

Standard upper t/stat only switches one leg, other leg is physically connected on upper t/stat (after reset) but is always energized to bottom regardless of whether upper t/stat is opened or closed, so I said that switching mechanism is spdt

Originally Posted by Jerry Peck

Vern,

The entire tank is hot, top and bottom, the heat in the bottom gradually migrates to the top (as you know, heat rises), however, at the same time, the water at the top is cooling along with the water at the bottom, the top thermostat will click in calling for the top element and the bottom element will not longer have power.

The bottom element is not going to heat the entire tank to full temperature, was not designed or intended to, the bottom element is only designed and intended to heat the lower portion of the tank after the top portion of the tank has already been heated by the top element.

You think you can keep the entire tank hot enough with just the bottom element to keep the water at the top hot enough to not click in the top thermostat?

And I described it with no hot water usage ... use even just a little bit of hot water and the top thermostat will click the top element on (which removes power from the bottom element).

You think you can keep the entire tank hot enough with just the bottom element to keep the water at the top hot enough to not click in the top thermostat? Yes

And I described it with no hot water usage ... use even just a little bit of hot water and the top thermostat will click the top element on (which removes power from the bottom element). Which is why there is a dip tube to send the cold water to the bottom.

Originally Posted by Dave Ruth

Standard upper t/stat only switches one leg, other leg is physically connected on upper t/stat (after reset) but is always energized to bottom regardless of whether upper t/stat is opened or closed, so I said that switching mechanism is spdt

You are right. I did think both legs were switched but only one is.

As for the operation regarding when the lower element is used:
Whirlpool Standard Electric Water Heater

Eventually the cold water mixes with the hot, lowering the temperature to below the lower thermostat setting and the bottom element is energized. If enough water is drawn to cool the upper third of the tank, the upper thermostat will send power to the upper element first. When the upper third of the tank is heated, power will again be switched to the lower element (see figure 4).

Originally Posted by Vern Heiler

You think you can keep the entire tank hot enough with just the bottom element to keep the water at the top hot enough to not click in the top thermostat? Yes

And I described it with no hot water usage ... use even just a little bit of hot water and the top thermostat will click the top element on (which removes power from the bottom element). Which is why there is a dip tube to send the cold water to the bottom.

Vern,

You will need to do better than that ... check your water heater, use some hot water, see which element comes on (it will be the top one), then let if set overnight and connected a logging meter to the top and bottom elements and see which one comes on to reheat the tank as it cools (it will be the top one).

Provide something of substance to show what you are saying, that the bottom element will keep the tank hot enough to not click the top element on (that is the critical part as that shuts off power to the bottom thermostat), with or without hot water usage.

Originally Posted by Jerry Peck

Vern,

You will need to do better than that ... check your water heater, use some hot water, see which element comes on (it will be the top one), then let if set overnight and connected a logging meter to the top and bottom elements and see which one comes on to reheat the tank as it cools (it will be the top one).

Provide something of substance to show what you are saying, that the bottom element will keep the tank hot enough to not click the top element on (that is the critical part as that shuts off power to the bottom thermostat), with or without hot water usage.

Jerry,

Did you read Whirlpools explanation of how it works?
Whirlpool Standard Electric Water Heater

Originally Posted by Vern Heiler

Jerry,

Did you read Whirlpools explanation of how it works?
Whirlpool Standard Electric Water Heater

Go ahead Jerry and have a small serving of crow, I did and it doesn't go down so bad if you don't chew and swallow quickly

Originally Posted by Vern Heiler

Jerry,

Did you read Whirlpools explanation of how it works?
Whirlpool Standard Electric Water Heater

Vern,

You mean this part?
"If the upper element burns out, the water heater will cease to function because the upper thermostat will never be satisfied and power will never be switched to the lower element."

Thank you for documenting what I have been explaining to you.

Originally Posted by Jerry Peck

Vern,

You mean this part?
"If the upper element burns out, the water heater will cease to function because the upper thermostat will never be satisfied and power will never be switched to the lower element."

Thank you for documenting what I have been explaining to you.

You have very selective reading skills:
Eventually the cold water mixes with the hot, lowering the temperature to below the lower thermostat setting and the bottom element is energized. If enough water is drawn to cool the upper third of the tank, the upper thermostat will send power to the upper element first. When the upper third of the tank is heated, power will again be switched to the lower element (see figure 4).

Originally Posted by Vern Heiler

You have very selective reading skills:
Eventually the cold water mixes with the hot, lowering the temperature to below the lower thermostat setting and the bottom element is energized. If enough water is drawn to cool the upper third of the tank, the upper thermostat will send power to the upper element first. When the upper third of the tank is heated, power will again be switched to the lower element (see figure 4).

Vern,

I don't have selective reading skills ... but I also don't stop when I think I found something that supports what I think - as you appear to have done - you need to keep reading the ENTIRE article and you will see that the substantial source you provided (yes, it was a good one) says what I have been telling you and what you keep refusing to comprehend:
"If the upper element burns out, the water heater will cease to function because the upper thermostat will never be satisfied and power will never be switched to the lower element."

Just what part of the following do you not understand?
- "If the upper element burns out,
- the water heater will cease to function
- because the upper thermostat
- will never be satisfied and
- power will never be switched to the lower element."

YOU provide the source (very good source too) and YOU provided the article (very good article too) yet YOU refuse to read and understand what it states.

Vern, no, 'tis not me who is selectively reading that article ... 'tis you.

Jerry, remain on topic:

Originally Posted by Vern Heiler

As I said "Not a very likely scenario, as in how did the water get to temp with an open element".

If the top element were to open after the water at the top of the tank was at temp (120 deg.) the bottom element would keep the water at the top of the tank at temp until there is a volume large enough to draw down the temp at the top of the tank below the differential of the bi-metalic strip/disk. Once that happens, end of hot water.

As far as the how this can be: The top element has a double pole double through contact. Once the temp causes the contacts to transfer the current to the bottom element the bottom element control is the sole controller of heat and will maintain 120 at the top of the tank.

If you do not have selective reading, maybe its comprehension? I did not read what you stated in step 3) in the Whirlpool explanation!

Originally Posted by Jerry Peck

Vern,

I see where your process is missing the step which happens: (bold and underlined)
- after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element,
- which then keeps the water at the bottom element at the set temp (which should be the same as the top),

The correct process is:
- after the top element has heated the water to the set temp the current is sent to the bottom t-stat/element,
- which then heats the water at the bottom element at the set temp (which should be the same as the top),
- once the bottom element heats the water to the temperature set on the thermostat the bottom thermostat shuts the bottom element off ... the bottom element does not "keep" heating the water in the bottom of the tank.

This is the process: (I will start after the initial filling and heating of the water heater and a tank which is not being used, such as overnight when everyone is sleeping)
- 1) The tank has reheated from activities and use of the evening.
- 2) The heated water begins to cool after having been reheated in step 1).
- 3) The water temperature in the tank drops to where the top thermostat clicks the top element on.
- 4) The top element heats the top water to the temperature to the set point of the top thermostat and the top thermostat shuts the top element off and clicks the connection down to the bottom element.
- 5) The bottom thermostat senses the temperature of the bottom water and is setting there waiting for power from the top element
- 6) The bottom thermostat is already calling for the bottom element but just now gets power down from the top element and the bottom element starts to heat the bottom water.
- 7) The bottom element heats the bottom water to the temperature set on the bottom thermostat and the bottom thermostat click off.
- 8) The water heater is not full of hot water and the process begins again, except that the top element is now burned out.
- 9) Step 4) does not happen because the top element is burned out.
- 10) The water heater does not progress past trying to achieve step 4) ... power never makes it back down to the bottom thermostat.
- 11) The water heater is now holding a tank of cold water.

Originally Posted by Jerry Peck

Dual water heaters may be connected in series or in parallel ... the the plumbers who install them will each their reason for connecting in series or connecting in parallel.

I prefer in series, some prefer in parallel. There is no "correct" answer, just like there is no 'correct' answer to 'Which came first, the chicken or the egg?' (I could debate either as being correct, or incorrect, basically using the same argument for either side, just applying it differently).

Jerry,

Interesting following this post regarding upper and lower t-stats, but In regards to series or parallel. Why do you prefer series connection?? I do not know of any reason why one would prefer series over parallel for domestic water heater connections.

Here are some reasons why series is not a good idea.

1. Series connection increases pressure drop of piping system.
2. Series connection puts more burden on the first unit, which usually fails first.
3. Additional valves are required for piping arrangement with series.

Originally Posted by Ken Amelin

Jerry,

Interesting following this post regarding upper and lower t-stats, but In regards to series or parallel. Why do you prefer series connection?? I do not know of any reason why one would prefer series over parallel for domestic water heater connections.

Here are some reasons why series is not a good idea.

1. Series connection increases pressure drop of piping system.
2. Series connection puts more burden on the first unit, which usually fails first.
3. Additional valves are required for piping arrangement with series.

You get more hot water out of tanks in series. The first tank preheats the water which feeds the second tank, if the first tank preheats the water close to, or at, the temperature of the second tank then you have twice the hot water available.

Parallel tanks give almost twice the hot water, then run out of hot water at the same time as there is no preheating action.

Originally Posted by Jerry Peck

You get more hot water out of tanks in series. The first tank preheats the water which feeds the second tank, if the first tank preheats the water close to, or at, the temperature of the second tank then you have twice the hot water available.

Parallel tanks give almost twice the hot water, then run out of hot water at the same time as there is no preheating action.

To add to the list of reasons: If one water heater is turned off to conserve energy in a series configuration the first heaters energy source is turned off, the none heating tank now tempers the water temp by bringing it closer to the ambient temp of the tank. With parallel tanks, one has to be valved off as well as turning off the energy source. With one tank valved off no water circulates through that tank and the rotten egg syndrome can develop.

Originally Posted by Jerry Peck

You get more hot water out of tanks in series. The first tank preheats the water which feeds the second tank, if the first tank preheats the water close to, or at, the temperature of the second tank then you have twice the hot water available.

Parallel tanks give almost twice the hot water, then run out of hot water at the same time as there is no preheating action.

You don't get more hot water out of a series arrangement, if fact you run out sooner, as you only have the the upper portion of the of the second tank for immediate supply, (like a hot tub)

As soon as you start to empty the first tank into the second you have a mixing action which tempers the water.

But you do get twice the hot water in parallel arrangement you have both upper sections of each tank at the correct temperature with greater volume and greater capacity.

Originally Posted by Ken Amelin

You don't get more hot water out of a series arrangement, if fact you run out sooner, as you only have the the upper portion of the of the second tank for immediate supply, (like a hot tub)

As soon as you start to empty the first tank into the second you have a mixing action which tempers the water.

Right, and you will get one entire tank's worth of hot water and then some. Remember, as soon as water starts to flow, the first water heater begins heating up the water in the first tank while the second water heater begins heating up the water in the second tank, preheating the water in the first tank will allow it to take longer for the water to 'get cold' during use.

But you do get twice the hot water in parallel arrangement you have both upper sections of each tank at the correct temperature with greater volume and greater capacity.

Right, you get 1/3 and 1/3 = 2/3 (twice as much, yes, but still not a full tank's worth).

As I said, there are plumbers who are absolutely positive that series is better (I agree with them) and there are plumbers who are absolutely positive that parallel is better (apparently you agree with them).

One would need to go through all the different scenarios which could exist and so extensive testing of each of those conditions to definitively determine which is better and provides 'more hot water', however ...

... Vern posted a very good reason to install in series ... and that very good reason should be enough to end the debate in favor of series installation of two water heaters.

With either series or parallel arrangements, and thermostats in both tanks set to the same temperature, and all the water heated to the thermostat cutout temperature, there will be two tanks full of hot water. I do not understand how parallel or series will make any difference to the amount of hot water available to be drawn off, or to the recovery time (the power into the tanks is the same). With parallel, the hot-water maximum flow rate might be somewhat higher than with series as there is less pipe friction, but if it is the hot water will run out sooner, because the volume of hot-water available is the same with either arrangement.

My personal preference would be for the parallel arrangement, if only for the reason that when a tank springs an unexpected leak, as they do, a (reduced) supply of hot water is still available while waiting for a plumber to come and install a replacement for the failed tank. Tanks typically seem to fail at the beginning of a holiday or some other inconvenient time and having redundancy built into the system makes it easier and cheaper to repair.

Alternatively, if you swear by series, the addition of two pipe runs and extra shutoffs will allow tanks to be used normally in series and switched to parallel to allow for replacement of either one of the tanks without cutting off hot water, per attached diagram.

Originally Posted by Ken Amelin

You don't get more hot water out of a series arrangement, if fact you run out sooner, as you only have the the upper portion of the of the second tank for immediate supply, (like a hot tub)

As soon as you start to empty the first tank into the second you have a mixing action which tempers the water.

But you do get twice the hot water in parallel arrangement you have both upper sections of each tank at the correct temperature with greater volume and greater capacity.