DHWS low temp

OK, hopefully I can explain this and it will make at least a little bit of sense.

 
 

In 2009, we had 3 dorms remodeled with similar designs/specs. Two of the dorms are currently experiencing issues where we can’t get the Domestic Hot Water Storage tanks up to their setpoints of 120F. The third dorm has no problem holding 120f in the storage tanks at any time of day. Each dorm has 2 250 gallon hot water storage tanks.

 
 

Here is the setup in each dorm. We have a commercial NG water heater that feeds the tanks during the summer and when our central steam boilers are not operating. When the boilers are active, a shell and tube heat exchanger feeds the tanks. Circulator pumps are cycled depending on which source is being used.

 
 

The basic setup is:

 
 
 

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In the one dorm that seems to be working well, the hot line coming from the steam HX and the water heater enter the storage tank about 15 inches above where the line pulling from the tank is. In the two that aren’t working well, the lines are at the same height.

 
 
 
 

Below is what the working dorm tank piping looks like combined with what I suspect is happening in the other two dorms. It seems like the water that is dumped into the tank is getting pulled right out and back into the heat exchanger or water heater. The incoming and outgoing temps in these lines are within 1 degree of each other while the tank temperature is around 15 degress cooler.

 
 

My question is, would simply piping the hot side coming from the heat exchanger/water heater to an inlet in the tanks about a foot or so higher solve the problem or is there something I’m missing? I may try slowing down the flow into the tanks this week and see if that helps, but there’s only ball valves on the tanks and I don’t want to wear them down to a point where they eventually won’t seal.

 
 
 

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DHWS low temp

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