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Aquabooster is the name of your hot water system . It's a step up from just running a coil . The coil will deliver constant hot water if the flow rate though the coil is under a certain rate with the same incoming temperature of fresh water . The water that heats up the coil is also used for heating the home. Durning the winter the water enters the house is cooler and boiler water is sent out to heat the home . The tank will give you storage for good even temperature hot water and volume for a shower..

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I have a 40 gallon electric Hot Water tank being supplied from a tankless coil in a Weil McLain WGTO4 boiler. There is a standard mixing valve of the hot water output of this tank. I connected an elapsed timer to the 220 VAC input to the tank to check how ofter the tank heaters ran and used electricity. I was pleasantly surprised that the tank heaters rarely turned on. The tankless coil in the boiler is supplying hot water (various temperatures) to the cold water inlet of the tank and the hot water outlet of the tank is controlled by the mixing valve. I am very happy with this setup; We now have a steady supply of hot water at a constant safe temperature. The original boiler piping only had a ball valve to mix the hot water from the coil and the valve was closed. So, you had a spurt of cold water in the shower followed by a spurt of scalding hot water from the coil and then cooler water until the boiler kicked in and raised the boiler temp to provide warmer hot water. And, we are now able to turn off the oil burner/boiler for 6 months of the year and use our HVAC heat pump for house heating in the Spring and Fall and the electric hot water tank for DHW.

Also, I am thinking about adding a circulator and check valve to the loop between the boiler tankless coil and the hot water tank and converting the boiler control to a cold start setup. Not sure of the savings to be gained from a cold start setup.

I piped the from the tankless coil to the cold water inlet then from the drain valve port I put a brass Tee fitting so I could put the drain valve back and have a place to pipe to the circulator and from the circulator to the cold inlet of the tankless coil. That way the heat from the boiler entered at about 6" from the bottom and exited to the bottom drain valve opening. this put all the hot water at the bottom. That way gravity would do the job of letting the hot water rise to the top. I'm not sure that is the best way to pipe that design, but it worked for my customer.

The reason for only three tappings on your tank is that you have something we used to call a "range boiler". they would be piped to a coil that was installed in a coal furnace or coal steam boiler and would absorb heat from the coal then thermo-syphon to the tank. The center taping is for a control well in the event you might have a circulator pump moving the water.

There are a few different way to pipe an aqua booster , with pros and cons .

Three pipes yes , the third ..(middle) pipe with the circulator, pumps out toward the coil and the return heated water is using the cold water pipe back to the tank. From what I can see…

I noticed an open flow valve tap . Throw a plug in the open tap for safety …

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is why only 3 ports on the Aquabooster. Better than a tankless by itself. Worse than an indirect with cold start, or another more efficient means of DHW.