Piping multiple water heaters c/w a Control Panel 1

[EnOm]

Hi
I am asking for some information regarding the best way pipe work for 4 water heaters that are coupled with a control panel. The setup it as follows:

[ul]
[li]The 4 water heaters are supposed to be 3 duty, and 1 alternate.[/li]
[li]The all have an equal capacity of 300L.[/li]
[li]There is a re-circulation pump attached, it feeds into the cold water supply pipe, before the first water heater.[/li]
[/ul]


I would have just used "First-In, Last Out" arrangement, but the control panel is confusing me. I was thinking of foregoing the control panel, but my supervisor said he wanted it. I do realize that there is are many options of how to set the pipe work. We aren't responsible for executing it, but I need some pointers so I can at least suggest a decent layout.
Thank you

Best Regards.

 

[DRWeig]

Well, I can't tell what the water heaters are for, and I have no feel for your process. But I feel like doing some typing anyway.

As you mentioned, there are various ways to work it.

Are the water heaters fired with gas, steam, electricity? That can make a difference in the way I'd do it. With gas or steam, energy efficiency can be either greater or smaller at part load. With electric heaters it doesn't matter.

Looking at heater energy efficiency and maintenance only (no downstream piping losses, and no accounting for mixing losses), here's where I'd start:

Gas: Simple natural-draft or forced-draft burners get far less efficient at part load, especially when proportionally controlled. Condensing type burner/stack arrangements get more efficient at part load. So, if natural draft, I'd want my controls to arrange the online heaters so that firing rates are maximized (minimum number of heaters working). If they're condensing-boiler types, I'd want to keep the firing rates as low as possible (maximum number of heaters working).

Steam: Usually like natural-draft burners, less efficient at part loads. Minimize the number of heaters online and maximize the firing rates.

Electric: Equally efficient at all loads.

For maintenance purposes, regardless of fuel, I'd want to rotate in a new lead-lag arrangment on accumulated hours for each heater, to equalize wear and tear. That's pretty self-explanatory, and it should be implemented no matter what firing order you choose.

If you want some suggestions more directly applicable to your project, please give us:

Make and model of heaters
Fuel type
Piping diagram
Description of the end-use load. Is fairly constant, or does it vary with time? How big is the water demand? At what temperature? Is the end use temperature controlled by a mixing valve, or does the load receive whatever is in the heater.
What is the purpose of the circulating pump? Is it to reduce wait times at bathroom sinks, or is it the primary pump for some sort of process loop? Or is it something else?


Best to you,

Goober Dave

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[EnOm]

DRWeig,
Thank you for your response. My application is not very complex and that is why I did not mention too many specifics but you are correct, my post is lacking some details. It is not a process or anything, it is a rather large home, it has something like 18 toilets. However here is the information I have at the moment.
By new lead-lag you mean the lead heater role will jump between the heaters right?

-The heaters are electric.
-Make and model has not yet been determined.
-Fuel type: NA
-Piping diagram. None atm.
-The end use load shouldn't vary too much. Probably a peak demand period in the morning when people start waking up.
-The water demand is estimated at 180 GPH. I reached this by using the tables is ASPE handbooks.
-The end use temperature is controlled by a mixing valve.
-The circulating pump is to reduce wait time at the outlets.

My main source of confusion is the following:
The heaters have a thermostat the turns the heater on and off according to the temperature of the water inside. This along with (First-In, Last Out) piping would result in somewhat balanced run time hours, based on thermostat control alone.
If the water heaters are turning on and off based on hot water demand (cold water entering the heaters is what turns them on), the only role I can imagine a control panel performing is closing and opening some valves to allow water in and out of the water heaters designated in the control sequence.
I could be mistaken since since I don't have a lot of experience. Your input is highly appreciated.

How this relates to piping is I need to have an idea of how the heaters will operate to size the pipes according to the expected flows in and out of the heaters.

Best Regards.

 

[PEDARRIN2]

You do not want a standby water heater. You can have a redundant heater if each heater is sized for 33% of the load, but you do not want a domestic water heater just standing there not being used. The water in the unused heater would become stagnant and there would be too much risk of bacterial growth.

I would recommmend using all four heaters at the same time and piping them either in a reverse return or a parallel fashion to make sure all heaters are used simultaneously. The thermostat on each heater will control the on/off so there is no need for a control panel. Why does your supervisor want it?

 

[EnOm]

Thank you for the response PEDARRIN2.
I think there is a small conflict in your statement. You wrote "You can have a redundant heater if each heater is sized for 33% of the load", but then you followed it with "you do not want a domestic water heater just standing there not being used". Isn't a redundant heater a heater that is not being used? Can you please elaborate on this bit?

I also agree with your recommendation. My supervisor wants a control panel because he heard about them and assumes they have some added benefit. He is more of an electrical engineer so he doesn't know a lot of details about this topic. Can you mention a case where a control panel is required so I can use it in my argument when I discuss the issue with him again?

Thank you
Best Regards

 

[PEDARRIN2]

I see where I didn't explain what I meant very clearly.

To me, a redundant water heater is one whose contribution is not needed to meet 100% of the load, but is still being used.

You have 4 heaters - if one is redundant, the other three make up 100% of the load, so with 4 heaters you have 133% of total load. This allows for the entire load being covered while a water heater is being maintenanced.

Where I have seen control panels on water heaters is for gas "boilers" where the equipment have no integral storage and just produce hot water. They each have a pump which circulate between the boilers and a storage tank. The control panel determines which boiler comes on and which goes off to minimize run time of each.

If you water heaters are of this type, but are electric, i would assume the manufacturer has a control panel which would act this way. The manufacturers of gas systems have this control panel.

 

[DRWeig]

Hi EnOm,

I can't add much to PEDARRIN2's comments for your situation. Nicely phrased.

Best to you,

Goober Dave

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