What should the Temperature Set Points be for Hot Water Heaters in Series?
What should the Temperature Set Points be for Hot Water Heaters in Series?
(OP)
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.
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





RE: What should the Temperature Set Points be for Hot Water Heaters in Series?
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
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RE: What should the Temperature Set Points be for Hot Water Heaters in Series?
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
RE: What should the Temperature Set Points be for Hot Water Heaters in Series?
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: Eng-Tips.com Forum Policies
RE: What should the Temperature Set Points be for Hot Water Heaters in Series?
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.
RE: What should the Temperature Set Points be for Hot Water Heaters in Series?
REDARRIN2 - I will look into that controller, we are using AO Smith Hot Water Heaters.