What should the Temperature Set Points be for Hot Water Heaters in Series?

[AndrewJYoung]

Question:
Is there a standard practice for setting the trip points for hot water heater in series?

Description:
Three 150 gallon hot water heaters plumbed in series. Circ pumps capable of roughly 150 gpms.

The set points
Tank 1 135 - 145
Tank 2 130 - 140
Tank 3 125 - 135

The thought process is that tank 1 will do all the work unless tank 1 can't keep up with the demand. In which case tank 2 turns on and if requried tank 3. Is circulating the water at 150 gpm enough to keep tank 2 and tank 3 from turning on? Any help would be appreciated.

Andrew J. Young ME

http://about.me/ajy

"Complexity is easy; simplicity is the result of great effort." ~Lawrence J. Kamm

 

[IRstuff]

Doesn't that put more stress on the first water heater?

Your scenario doesn't seem to make sense to me. When drawing from a water heater, you don't draw to empty; every gallon out is replaced with a gallon from the inlet. That means that the instant you pull water from tank1, makeup water enters the system, so if you draw 50 gallons, your circulation pump will even disperse the inlet water to all three tanks.

TTFN
faq731-376

 

[AndrewJYoung]

It does put the majority of the load on the first heater, but that is the desired effect. I probably should have mentioned this system is on a boat. By putting the majority of the load on the first heater, it will fail long before the second and third heater. If the first heater fails while at sea, it gives the engineer time to order a replacement and get it shipped to the next port without running the risk of the other two heaters failing because they are still relatively fresh.

What would you suggest IRstuff for the setup? Parallel?

The thought process is that when water isn't being used it just circulates. The hot water loops are long enough that by the time the water makes one full trip the temperature will have dropped enough to require additional heat. Instead of plumbing the heaters in parallel requiring all three heaters to turn on I've decided to plumb them in series so only one heater has to turn. I think I'm looking for a fluid mixing equation that I can use to determine if the water temperature will drop enough by tank 3 requiring it to kick on?

Andrew J. Young ME

http://about.me/ajy

"Complexity is easy; simplicity is the result of great effort." ~Lawrence J. Kamm

 

[IRstuff]

I'm just saying that any time there's a demand for hot water, all three heaters are going to turn on anyway. By design, your temperature setpoints ensure that when water is drawwn from the first tank, it's pulling in water that's 5°F colder, so the first heater must turn on.

The second tank must now draw water from the third, which is also 5°F colder, so the second heater must turn on.

The third tank must now draw on makeup water, which is presumably at least 5°F colder, so it also turns on.



TTFN
faq731-376

 

[PEDARRIN2]

Pipe them in parrallel and set a controller to alternate lead/lag.

Look at a water heater manufacturer like AO Smith and they have piping diagrams to show how to pipe it.

Alternating the lead/lag and even bringing on a second heater if the load requires is commonly done.

Otherwise you are going to overuse one heater and underuse the others.

The recirc pump can be controlled by an aquastat and the calculation for how much flow is required to maintain a certain temperature loss is well documented.

 

[AndrewJYoung]

IRSTUFF - its actually the other way around, the make up water enters Tank 1

REDARRIN2 - I will look into that controller, we are using AO Smith Hot Water Heaters.