Water heater, parallel vs series

[frodo]

link I am referencing.
http://www.texasinspector.com/files/Water-Heater-Plumbing.pdf

Parallel Installation

Parallel installation uses heaters that are identical in both BTU input and storage capacity. Normally, parallel installation is used when there is a demand for large quantities of hot water over a short period of time, such as filling a large garden tub or back to back showers. Equipment being equal, two or more heaters connected in parallel will deliver more hot water than the same heaters connected in series. In a parallel configuration, the hot water demand is taken equally from each unit. For every gallon of hot water drawn from each unit,.....see link

Series Installation

Series installation uses heaters that are not identical in both BTU input and storage capacity. An example might be when an expansion or addition is made in an application that will require additional hot water. For example, you add a new bathroom to your home. The existing system will not handle the demand and a new water heater must be installed. The new heater is not identical to the existing heater. In this example, series installation may be appropriate.............see link

I understand the difference, I have installed both systems residential and commercial settings
My question has to do with the "WHY" Of the Series installation being for tanks that are not identical in both BTU input and storage capacity

If 2 - elec. 40's are sitting next to each other, what would be the reason one should be a different BTU and size? Is the reasoning simply efficiency of savings of cost to operate?

looking for a logical debate,

 

[WorthFlorida]

Parallel, you'll know when one water heater failed as it cools down. Series, you may never know.
Parallel, both water heaters the on/off cycle will be nearly identical. Series, the first water heater will do most of the heating.
Parallel, heat recovery would be the same in heavy demand use compared to a single water heater. Series, the second unit would have a faster heat recovery rate since the water is pre warmed.

Other than when you have two different sized or types of water heater, not much difference with either series or parallel connection when needed more volume of water. Series is easier to install and the second unit can be closer to the fixtures needing hot water.

Two 40 gallon water heaters will give you faster heat recovery compared to one 80 gallon tank.

When high demand will be the case, such as to feed a large spa tub within the home or too many teenagers, there could be several choices. There have been a few discussions on this forum and many favor using a thermostatic mixing valve with one heater set to 140 degrees. It is very easy to install at lower cost than a second unit. For residential use, two water heaters may not be possible due to space and/or the supply of gas or the electric panel would not be able to meet the additional load.

 

[Sponsor]

 

[Jadnashua]

The flow restrictions on different sized WH are likely to be different, so getting them balanced may be difficult. Extra elbows, pipe lengths, etc. will add to those differences, not including internal ones on different tanks. Series, balance doesn't matter.

I don't see why you'd get less hot water in series versus parallel. In parallel, the outlet temp well decline fairly evenly. In series, it would stay hotter longer before things started to cool off - you'd get the entire capacity of the second one, and most of the first before it started to cool off. When in parallel, they'd both start to cool off fairly evenly.

 

[Phog]

I don't see why you'd get less hot water in series versus parallel.

The wording in the originally quoted passage is poor. By "more hot water" from parallel, they meant a higher flowrate. Not a bigger drawdown -- of course you actually get a slightly larger total drawdown with series when compared to parallel, all else equal.

 

[WorthFlorida]

As an example, when drawing 2 gallons a minute, parallel will be 1 gallon per minute from each unit, series would be 2 gallons from the first unit. The second one has not started to cool down and the parallel set only 1/2 of the water is drawn from each.
As I see it would be nearly the same temperature drop rate from either setup excluding recovery rate. It still 80 gallons of stored heated water.

It’d be nice to have it set up in a lab to test any difference. I’m sure someone has.

 

[frodo]

thank you for your responses
All of them have been informative,

What if a circulating line was added from the bottom of the 2nd tank. circ the water out to the house then into the top of the first wh
Do you think the circ line would provide tempered water in the 1st tank with the circulated water from tank 2

 

[WorthFlorida]

No. The most it will do is radiate the heated water into the living space.

 

[Phog]

As I see it would be nearly the same temperature drop rate from either setup excluding recovery rate. It still 80 gallons of stored heated water.

It’d be nice to have it set up in a lab to test any difference. I’m sure someone has.

In a typical fast drawdown for a single hot water tank, for example when filling a bathtub, you don't get 100% of the tank's rated volume at full temperature. You get more like 85% before things turn lukewarm, after this point the cold incoming water tempers the outlet temperature down quickly.

With parallel tanks, assuming they're both balanced, you'll get that same 85% out of each tank. But with series, you get 100% out of the first tank and 85% out of the second tank. So, a little more hot water to fill up the tub.

At least this is the theory, although I've also never tested it either!

 

[Master Plumber Mark]

Series is always best because of the future possibility of one of the heaters rusting shut at one of the nipples if it is set up in parallell
... Then one heater does most of the work --- it might not be an issue for a few years but no one maintains this because it is difficult to strip them down and inspect them .....

also if one of the heaters goes bad it does not matter what brand you put back in because in Series it will draw through both of the units...... You will never match up a heater to function right if it is set up in parallell 8 years down the road.

Parallell is also pain in the ass attempting to get both heaters to draw perfectly off of them.... you can spend the whole day attempting to plumb them equally, and it is still almost impossible to know how well balanced it is going to draw off of them unless you want to throw some flow meters onto both hot outlets on the heaters..... which is taking things to the stupid ridiculous level.....



series always performs better