DHW on FT-399 (Indirect Water Heater Heater Operation) 

Dip switch 3 on printed circuit board we can select the following:

ON: Smart priority:  Boiler pump and DHW pump on at the same time until firing rate  is > (greater than) 80%, then boiler pump is OFF.

OFF: Regular priority:   Boiler pump is off as soon as DHW calls identical to conventional DHW priority.

For primary/secondary piping, the DHW pump pulls off and return to the boiler loop.  The boiler pump can run as an injection pump to the main loop based on Smart priority.  If the contractor select Regular priority, the boiler pump will be off.  (see page 44/45 in the Boiler IO Manual.

For single boiler systems, the DHW pump and piping could come off the top, and space heating (boiler pump) with primary/secondary piping could come off the bottom connections.

On low DHW demands, both the DHW and boiler pumps will run on smart priority.  As the DHW load increases, eventually the Smart priority will drop out the boiler pump.  Once the firing rate drops below 50%, the boiler pump comes back on , assuming we still have a CH call.

For multiple boiler systems, pipe DHW off 1 or 2 boilers, and use individual boiler pumps and a LLH to have multiple CH zones. (See 46/47) in Boiler IO Manual.

Parameter # 39 is used to select type of temperature sensing utilized for indirect water heater,  either an aquastat (AQU) or a sensor (SENS) .