We recommend that you use the Direct Vent Kit for your Takagi Water Heater as this can save you (On average) 15% on your heating bills vs a non-direct vented Water Heater.
There are primary and secondary loops and the primary is the loop attached to the Takagi. The secondary loops are the high and the low temperature heating loops.
To get one high temperature from the Takagi you need to use a primary loop and then a secondary loop to feed your system.
To get one low temperature from the Takagi you use a primary and secondary and wire as below.
There are various methods for wiring but the best method is to shut off the primary circulator based on the return of the low temperature water so as to not short cycle the Takagi.
To get your heat and your domestic hot water from the Takagi you can use many methods. The best method is to use an indirect water heating tank driven off the high temperature loop.